Category Archives: geology

Earthquake Report: Mendocino fault Update #1

Today is one of my busiest days of the semester. I am administering a final and my two classes are presenting their video projects. Then, we had this M 6.5 earthquake in the wee hours and a M 7.7 (prelim USGS Mw) in the Solomons (I will post about this later).

Here is an update. For more background on the regional tectonics and the initial Earthquake Report, head here.

  • Here are the USGS websites for the main shock and larger aftershocks
  • 2016.12.08 14:47 UTC M 6.5
  • 2016.12.08 16:24 UTC M 2.4
  • 2016.12.08 16:32 UTC M 4.7
  • 2016.12.08 18:08 UTC M 2.9
  • and an earthquake that I will mention below
  • 2016.12.08 18:33 UTC M 4.3

There was an earthquake to the east on 2016.12.05. This M 4.3 seems to have occurred on a segment of the Mendocino fault. Here is my Earthquake Report for that earthquake. The M 4.3 was not a foreshock to the M 6.5 and probably did not affect the fault in that region.

UPDATE: 13:15 PST: figure notes:

  • I have prepared an updated map. I include some inset figures. I will update this report shortly as I need to get to my class to admin a final.
  • I include the USGS “Did You Feel It?” report maps for these two earthquakes on the right side of the map. I also include screen shots of the attenuation plots (these show how the ground motions diminish with distance from the earthquake).
  • In the upper left corner I include the first figure from Rollins and Stein (2010). In their paper they evaluate (as stated in the title), “Coulomb stress interactions among M ≥ 5.9 earthquakes in the Gorda deformation zone and on the Mendocino Fault Zone, Cascadia subduction zone, and northern San Andreas Fault.” In this figure they present the major earthquakes in this region from 1976-2010. We have had several large earthquakes since this paper was published, but they behaved very similarly to the ones discussed in this paper. Note the location of teh 1994 Mendocino fault earthquake. Today’s M 6.5 happened in a region very close to the 1994 earthquake (albeit further offshore than the ’94 quake).
  • In the lower left corner is a figure where Rollins and Stein (2010) show their modeling results for the 1994 earthquake. The colors represent changes in static stress imparted upon faults/crust from rupture on the Mendocino fault in 1994. The 1994 earthquake was significantly larger than today’s M 6.5 (e.g. an M 7.0 is 32 releases ~32 times more energy than an M 6.0), so the static stress changes from today’s earthquakes could be assumed to be less than from the ’94 earthquake. Red designates faults/crust with an increased stress and blue designates a decreased stress. The left panel shows the model results when they generate bilateral rupture (the earthquake starts in one location and slips on either side of the fault) and the right panel shows the model results from unilateral rupture (the fault nucleates at one spot, but instead, slips to only one side of the fault). Note that the increased stresses extend about 1/4 to 1/2 a degree of longitude. This equates to about 15 to 30 miles or 28 to 56 km. The CSZ and SAF are at a much further distance than this, so would not “feel” this earthquake. However, there could be dynamic changes in stress (dynamic triggering), but they would be small and would not last very long (basically the time that the seismic waves are traveling through an area). SO, the fact that we have not had any earthquakes in these other regions suggests that the threat of dynamic triggering are over.

Below I plot the seismicity from the past week, with color representing depth and diameter representing magnitude (see legend). I use the USGS Quaternary fault and fold database for the faults.

I also include the shaking intensity contours on the map for the two earthquakes. Note how these MMI contours are quite different between the two earthquakes.

These contours are based upon the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.

I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.

I also include a figure from Rollins and Stein (2010). This figure shows how the 1994 earthquake (similar to today’s M 6.5) imparted stresses on the adjacent faults and crust.


  • UPDATE: I added a version of the Rollins and Stein (2010) figure of the regional earthquakes. I include their figure caption as blockquote below.

  • Tectonic configuration of the Gorda deformation zone and locations and source models for 1976–2010 M ≥ 5.9 earthquakes. Letters designate chronological order of earthquakes (Table 1 and Appendix A). Plate motion vectors relative to the Pacific Plate (gray arrows in main diagram) are from Wilson [1989], with Cande and Kent’s [1995] timescale correction.

  • Here is the Rollins and Stein (2010) figure that is in the report above. I include their figure caption as blockquote below.

  • Coulomb stress changes imparted by our models of (a) a bilateral rupture and (b) a unilateral eastward rupture for the 1994 Mw = 7.0 Mendocino Fault Zone earthquake to the epicenters of the 1995 Mw = 6.6 southern Gorda zone earthquake (N) and the 2000 Mw = 5.9 Mendocino Fault Zone earthquake (O). Calculation depth is 5 km.

UPDATE 13:15 PST: Notes about the seismicity from the past month or so.

Here is something that I thought was interesting about this series of earthquakes in the past week or so. There were a couple earthquakes along the Blanco fracture zone that are too distant to impart changes in static stress on the CSZ, MF, or SAF (though people always want to seek relations where there are none). There was also a M 4.3 earthquake at the mouth of the Mattole River, near Petrolia. This is also a very very small earthquake that does not implicate other fault zones. However, following the M 6.5 today, there was an aftershock (at least 2) near the M 4.3. SO, I thought that perhaps these happened in a region that had seen an increase in static coulomb stress following the M 4.3. They are very close to the M 4.3. So, it seems reasonable that they were ready to go when the M 6.5 happened.

    References

  • Atwater, B.F., Musumi-Rokkaku, S., Satake, K., Tsuju, Y., Eueda, K., and Yamaguchi, D.K., 2005. The Orphan Tsunami of 1700—Japanese Clues to a Parent Earthquake in North America, USGS Professional Paper 1707, USGS, Reston, VA, 144 pp.
  • Chaytor, J.D., Goldfinger, C., Dziak, R.P., and Fox, C.G., 2004. Active deformation of the Gorda plate: Constraining deformation models with new geophysical data: Geology v. 32, p. 353-356.
  • Dengler, L.A., Moley, K.M., McPherson, R.C., Pasyanos, M., Dewey, J.W., and Murray, M., 1995. The September 1, 1994 Mendocino Fault Earthquake, California Geology, Marc/April 1995, p. 43-53.
  • Geist, E.L. and Andrews D.J., 2000. Slip rates on San Francisco Bay area faults from anelastic deformation of the continental lithosphere, Journal of Geophysical Research, v. 105, no. B11, p. 25,543-25,552.
  • Irwin, W.P., 1990. Quaternary deformation, in Wallace, R.E. (ed.), 1990, The San Andreas Fault system, California: U.S. Geological Survey Professional Paper 1515, online at: http://pubs.usgs.gov/pp/1990/1515/
  • McLaughlin, R.J., Sarna-Wojcicki, A.M., Wagner, D.L., Fleck, R.J., Langenheim, V.E., Jachens, R.C., Clahan, K., and Allen, J.R., 2012. Evolution of the Rodgers Creek–Maacama right-lateral fault system and associated basins east of the northward-migrating Mendocino Triple Junction, northern California in Geosphere, v. 8, no. 2., p. 342-373.
  • Nelson, A.R., Asquith, A.C., and Grant, W.C., 2004. Great Earthquakes and Tsunamis of the Past 2000 Years at the Salmon River Estuary, Central Oregon Coast, USA: Bulletin of the Seismological Society of America, Vol. 94, No. 4, pp. 1276–1292
  • Rollins, J.C. and Stein, R.S., 2010. Coulomb stress interactions among M ≥ 5.9 earthquakes in the Gorda deformation zone and on the Mendocino Fault Zone, Cascadia subduction zone, and northern San Andreas Fault: Journal of Geophysical Research, v. 115, B12306, doi:10.1029/2009JB007117, 2010.
  • Stoffer, P.W., 2006, Where’s the San Andreas Fault? A guidebook to tracing the fault on public lands in the San Francisco Bay region: U.S. Geological Survey General Interest Publication 16, 123 p., online at http://pubs.usgs.gov/gip/2006/16/
  • Wallace, Robert E., ed., 1990, The San Andreas fault system, California: U.S. Geological Survey Professional Paper 1515, 283 p. [http://pubs.usgs.gov/pp/1988/1434/].

Earthquake Report: Mendocino fault!

I was awake and just logging into my laptop, still in bed, when I first felt some movement. The movement was slight and not impulsive, so I thought it was a small earthquake. Then the shaking reappeared. This is when I started counting. one one-thousand, two one-thousand…. twenty one-thousand. The S-Wave lasted about 20 seconds. I thought back to the 2010 earthquake that lasted about that long and it was a M 6.5 earthquake. SO, I immediately thought this was probably a mid M 6 earthquake. However, the shaking was subdued. So, it could be a larger earthquake further away. I logged into social media and people were already contacting me. A friend felt it shake for 2 minutes in southern Oregon (so I thought it might be a large earthquake on the Blanco fracture zone, especially since there were a couple up there recently).

UPDATE: Here is my Earthquake Report Update #1

I checked the USGS website here and saw that it was closer to me (Manila, CA), along the Mendocino fault. At first it was a M 6.8, but the location and magnitude changed to an M 6.5.

This earthquake appears to have occurred along the Mendocino fault, a right-lateral (dextral) transform plate boundary. This plate boundary connects the Gorda ridge and Juan de Fuca rise spreading centers with their counterparts in the Gulf of California, with the San Andreas strike-slip fault system. Transform plate boundaries are defined that they are strike-slip and that they connect spreading ridges. In this sense of the definition, the Mendocino fault and the San Andreas fault are part of the same system. This earthquake appears to have occurred in a region of the Mendocino fault that ruptured in 1994. See the figures from Rollins and Stein below. More on earthquakes in this region can be found in Earthquake Reports listed at the bottom of this page above the appendices.

The San Andreas fault is a right-lateral strike-slip transform plate boundary between the Pacific and North America plates. The plate boundary is composed of faults that are parallel to sub-parallel to the SAF and extend from the west coast of CA to the Wasatch fault (WF) system in central Utah (the WF runs through Salt Lake City and is expressed by the mountain range on the east side of the basin that Salt Lake City is built within).

The three main faults in the region north of San Francisco are the SAF, the MF, and the Bartlett Springs fault (BSF). I also place a graphical depiction of the USGS moment tensor for this earthquake. The SAF, MF, and BSF are all right lateral strike-slip fault systems. There are no active faults mapped in the region of Sunday’s epicenter, but I interpret this earthquake to have right-lateral slip. Without more seismicity or mapped faults to suggest otherwise, this is a reasonable interpretation.

The Cascadia subduction zone is a convergent plate boundary where the Juan de Fuca and Gorda plates subduct norteastwardly beneath the North America plate at rates ranging from 29- to 45-mm/yr. The Juan de Fuca and Gorda plates are formed at the Juan de Fuca Ridge and Gorda Rise spreading centers respectively. More about the CSZ can be found here.

Below I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I use the USGS Quaternary fault and fold database for the faults.

I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.

I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.

This is a preliminary report and I hope to prepare some updates as I collect more information.

    I have placed several inset figures.

  • In the upper right corner is a map of the Cascadia subduction zone (CSZ) and regional tectonic plate boundary faults. This is modified from several sources (Chaytor et al., 2004; Nelson et al., 2004)
  • Below the CSZ map is an illustration modified from Plafker (1972). This figure shows how a subduction zone deforms between (interseismic) and during (coseismic) earthquakes. Today’s earthquake did not occur along the CSZ, so did not produce crustal deformation like this. However, it is useful to know this when studying the CSZ.
  • To the left of the CSZ map is the USGS Did You Feel It felt report map. This map is based upon reports submitted by real people. Note how the felt reports extend beyond the modeled estimates of MMI shaking as represented by the MMI contours on the map.
  • In the lower left corner is a figure from Dengler et al. (1995) that shows focal mechanisms from earthquakes in this region, along the Mendocino fault. Today’s earthquake is near the 1994 earthquake.
  • To the right of the Dengler et al. (1995) figure, I present a photo I took of the seismograph observed in Van Matre Hall on the Humboldt State University campus. This seismograph is operated by the HSU Department of Geology.
  • In the upper left corner is a figure from Rollins and Stein (2010). In their paper they discuss how static coulomb stress changes from earthquakes may impart (or remove) stress from adjacent crust/faults.


  • Here is a map from Rollins and Stein, showing their interpretations of different historic earthquakes in the region. This was published in response to the January 2010 Gorda plate earthquake. The faults are from Chaytor et al. (2004). The 1980, 1992, 1994, 2005, and 2010 earthquakes are plotted and labeled. I did not mention the 2010 earthquake, but it most likely was just like 1980 and 2005, a left-lateral strike-slip earthquake on a northeast striking fault.

  • Here is a large scale map of the 1994 earthquake swarm. The mainshock epicenter is a black star and epicenters are denoted as white circles.

  • Here is a plot of focal mechanisms from the Dengler et al. (1995) paper in California Geology.

  • In this map below, I label a number of other significant earthquakes in this Mendocino triple junction region. Another historic right-lateral earthquake on the Mendocino fault system was in 1994. There was a series of earthquakes possibly along the easternmost section of the Mendocino fault system in late January 2015, here is my post about that earthquake series.


    References

  • Atwater, B.F., Musumi-Rokkaku, S., Satake, K., Tsuju, Y., Eueda, K., and Yamaguchi, D.K., 2005. The Orphan Tsunami of 1700—Japanese Clues to a Parent Earthquake in North America, USGS Professional Paper 1707, USGS, Reston, VA, 144 pp.
  • Chaytor, J.D., Goldfinger, C., Dziak, R.P., and Fox, C.G., 2004. Active deformation of the Gorda plate: Constraining deformation models with new geophysical data: Geology v. 32, p. 353-356.
  • Dengler, L.A., Moley, K.M., McPherson, R.C., Pasyanos, M., Dewey, J.W., and Murray, M., 1995. The September 1, 1994 Mendocino Fault Earthquake, California Geology, Marc/April 1995, p. 43-53.
  • Geist, E.L. and Andrews D.J., 2000. Slip rates on San Francisco Bay area faults from anelastic deformation of the continental lithosphere, Journal of Geophysical Research, v. 105, no. B11, p. 25,543-25,552.
  • Irwin, W.P., 1990. Quaternary deformation, in Wallace, R.E. (ed.), 1990, The San Andreas Fault system, California: U.S. Geological Survey Professional Paper 1515, online at: http://pubs.usgs.gov/pp/1990/1515/
  • McLaughlin, R.J., Sarna-Wojcicki, A.M., Wagner, D.L., Fleck, R.J., Langenheim, V.E., Jachens, R.C., Clahan, K., and Allen, J.R., 2012. Evolution of the Rodgers Creek–Maacama right-lateral fault system and associated basins east of the northward-migrating Mendocino Triple Junction, northern California in Geosphere, v. 8, no. 2., p. 342-373.
  • Nelson, A.R., Asquith, A.C., and Grant, W.C., 2004. Great Earthquakes and Tsunamis of the Past 2000 Years at the Salmon River Estuary, Central Oregon Coast, USA: Bulletin of the Seismological Society of America, Vol. 94, No. 4, pp. 1276–1292
  • Rollins, J.C. and Stein, R.S., 2010. Coulomb stress interactions among M ≥ 5.9 earthquakes in the Gorda deformation zone and on the Mendocino Fault Zone, Cascadia subduction zone, and northern San Andreas Fault: Journal of Geophysical Research, v. 115, B12306, doi:10.1029/2009JB007117, 2010.
  • Stoffer, P.W., 2006, Where’s the San Andreas Fault? A guidebook to tracing the fault on public lands in the San Francisco Bay region: U.S. Geological Survey General Interest Publication 16, 123 p., online at http://pubs.usgs.gov/gip/2006/16/
  • Wallace, Robert E., ed., 1990, The San Andreas fault system, California: U.S. Geological Survey Professional Paper 1515, 283 p. [http://pubs.usgs.gov/pp/1988/1434/].

Earthquake Report: Japan!

While I was returning from my research cruise offshore of New Zealand, there was an earthquake offshore of Japan in the region of the 2011.01.11 M 9.0 Tohoku-Oki Earthquake. Japan is one of the most seismically active regions on Earth. Below is a series of earthquake reports for the region of Japan. Here is the USGS website for this M 6.9 earthquake.

Here is my interpretive poster for the extensional earthquake that is in the upper North America plate. This earthquake has a shallow depth and produced a small tsunami run-up. I include two versions: (1) the first one has seismicity from the past 30 days and (2) the second one includes earthquakes with magnitudes M ≥ 5.5. The second map is useful to view the aftershock region of the 2011.03.11 M 9.0 earthquake. The M 9.0 Tohoku-Oki Earthquake was a subduction zone earthquake, while this M 6.9 earthquake is a shallow depth extensional earthquake. I label the location of the 1944 Tonanki and 1946 Nankai subduction zone earthquakes (both M 8.1). These earthquakes spawned decades of research that continues until this day. I discuss the recurrence of earthquakes in this region of Japan in my earthquake report here.

I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.

I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.

I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault. The hypocentral depth plots this close to the location of the fault as mapped by Hayes et al. (2012). So, the earthquake is either in the downgoing slab, or in the upper plate and a result of the seismogenic locked plate transferring the shear strain from a fracture zone in the downgoing plate to the upper plate.

    Inset Figures

    I include some inset figures. Here is some information about them. Below I include the original figures with the figure captions as blockquotes.

  • In the upper right corner is a map showing the tectonics of the region (Kurikami et al., 2009). I include this map below.
  • In the lower right corner is a figure from the USGS that shows seismicity along the subduction zone that forms the Japan trench.
  • To the left of the cross section shows a low angle oblique view of the plate configuration in this region (from AGU).
  • In the upper left corner is a comparison of the USGS “Did You Feel It?” report maps. The map on the right is from the M 9.0 Tohoku-Oki earthquake and the map on the left is from this M 6.9 earthquake.



  • Here is the upper figure showing the tectonic setting (Kurikami et al., 2009). I include their figure caption as a blockquote.

  • Active faults in southwest Japan from the Active Fault Research Centre’s active fault database (http://www.aist.go.jp/RIODB/activefault/cgi-bin/index.cgi). The faults are color coded by sense of movement (green = dextral; blue = normal, red = reverse, yellow = sinistral).

  • Here is another figure showing the tectonic setting (Kurikami et al., 2009). I include their figure caption as a blockquote.

  • Current tectonic situation of Japan and key tectonic features.

  • The upper slope of the accretionary prism for this part of the subduction zone that forms the Japan trench has well developed normal faults. Tsuji et al. (2013) present seismic reflection profiles that for this region. I present their figure and include their figure citation below as a blockquote. The first figure is a map showing the locations of the cross sections and the locations of sites with direct observations of sea floor surface displacements (surface ruptures).

  • Index maps for the 2011 Tohoku-oki earthquake in the Japan Trench (JCG, JAMSTEC, 2011). (a) Blue and white contour lines are subsidence and uplift, respectively, estimated from tsunami inversion (Fujii et al., 2011), with contour intervals of 0.5 m (subsidence) and 1.0 m (uplift).Blue arrows indicate dynamic seafloor displacements observed at seafloor observatories (Kido et al., 2011; Sato et al., 2011). Red lines are locations of seismic profiles (SR101, MY101, and MY102) shown in Fig. 2. Stars indicate diving sites and are labeled with dive numbers of pre-earthquake observations (blue numerals) and post-earthquake observations in 2011 (red numerals) and in 2012 (orange numerals). Background heatflow values measured before the 2011 earthquake are displayed as colored dots (Yamano et al.,2008; Kimura et al., 2012). (b) Enlarged map around the diving sites, corresponding to the yellow rectangle in panel (a). Red dashed lines indicate seafloor traces of normal faults (i.e.,ridge structures). Yellow dashed lines indicate estimated locations of the backstop interface. The white dashed line indicates the boundary of the area of significant seafloor uplift (49 m uplift)and also the tsunami generation area (Fujii et al., 011), corresponding to the reddish-brown area in panel (a). Observations made during the post-earthquake dives are described in panel(b).


    Reflection seismic profiles obtained in the central part of tsunami source area(line MY102 in panels f–h), at its northern edge (line MY101 in panels c–e), and its outside (line SR101 in panels a,b). Original profiles of (a) line SR101, (c) line MY101, and (f) line MY102. Composite seismic reflection profiles with geological interpretations of(b) line SR101,(d) line MY101, and (g) line MY102 (Tsuji et al.,2011). Red arrows in panel (d) and (g) indicate seafloor displacements (Ito et al.,2011; Kido et al.,2011; Sato et al.,2011). Enlarged profiles around (e) Site 2W on line MY101, and (h) Site 3W on line MY102.

  • Here is a figure from Tsuji et al. (2013) that shows some images of the seafloor. These show views of ruptured sea floor.

  • (a) Diving tracks on seafloor bathymetry at Site 2W. Stars indicate locations of seafloor photographs displayed in panels (b)–(f). (b) Photograph of an open fissure representative of those commonly observed after the earthquake. (d) An open fissure was observed during post-earthquake observations where (c) no fissure had been before the earthquake.(g,h) Photographs taken in (g) 2011 and (h) 2012 showing the heat flow measurements being made at the same location by SAHF probe.


    (a) Diving tracks on seafloor bathymetry at Site 1E. The white dashed line indicates the location of the interpreted fault. Stars indicate locations of seafloor images displayed in panels(b)–(f).(b) Photograph of an open fissure representative of those commonly observed after the earthquake. (d) Open fissure seen during post-earthquake observations where (c) a clam colony (1 m wide) was observed before the earthquake. (e,f) Photographs taken in (e) 2011 and (f) 2012,showing the heatflow measurements at the same location by SAHF probe. (g) Dive track on seafloor bathymetry at Site 3E. The star indicates the location of (h) a seafloor photograph showing a steep cliff.

  • Here is an explanation for the extension generated during the 2011 earthquake.

  • Schematic images of coseismic fault ruptures and the tsunami generation model (a) at the northern edge (and outside) and (b) in the central part of the tsunami source area. Soft slope sediments covering the continental crust are not shown in these images. (a) Collapse of the continental framework occurred mainly at the backstop interface north of the large tsunami source area. (b) Anelastic deformation around the normal fault allowed large extension of the overriding plate in the tsunami source area.

  • These are some observations posted by the Pacific Tsunami Warning Center.

Earthquake Report: New Zealand Post # 01

This is the first of several posts about a complex earthquake series that happened along the northern end of the South Island in New Zealand. I was at sea on the R/V Tangaroa collecting piston cores offshore along the Hikurangi subduction zone this month. While I was at sea, there was a large earthquake, probably along one of the upper plate faults in this region. I present a simple interpretive poster below and will follow up with several more posts as I find more time between my other responsibilities (I have been gone at sea for two weeks, so I have lots of catch up work to do). This earthquake series is in a complicated part of the Earth where a subduction plate boundary turns into a transform plate boundary. There was a tsunami warning for the nearby coasts, but not for a global tsunami.

Geonet is a website in New Zealand that is a collaboration between the Earthquake Commission and GNS Science. Here is the website at Geonet where one can find the most up to date observations and interpretations about this M 7.8 Kaikoura Earthquake series.

  • Below is a map that I prepared that shows the earthquakes (magnitude M ≥ 2.5) for the month of November as green circles (diameter represents earthquake magnitude). I also plot earthquakes with magnitudes M ≥ 5.5 from the period of 1950-2016. These are from the USGS NEIC, so the regional network run in New Zealand may have a larger number of earthquakes. I present two maps, one with a 250 m resolution bathymetric grid as a base and one with a Google Earth satellite based map as a base. This is not an official GNS nor NIWZ figure, but they were major supporters of the TAN163 cruise that I participated on, so we can attribute the core data to these organizations.
  • I placed the moment tensors for the larger earthquakes during this time period since the M 7.8 earthquake. The main earthquake is a compressional earthquake, probably on an upper plate fault. The M 7.8 earthquake triggered slip on other thrusts and some strike-slip faults in the region. Surface deformation measured using Interferometric Synthetic Aperture Radar (INSAR) is localized, supporting the upper plate rupture interpretation. Slip on the thrust during the 7.8 is estimated to be about 10 meters, which is also the maximum slip on some of the strike-slip fault systems. There have been some excellent photographs of the fault rupture (I will include these in a later post). Most of the large earthquakes are strike-slip, but there are some connecting faults that show thrust mechanisms. I also show the regions of different faults that have been observed to have surface ruptures.
  • After the earthquake, we changed our plans to conduct some post-earthquake response analyses. We collected additional cores to search for sedimentary evidence of the M 7.8 earthquake. We also collected sub-bottom profile and bathymetric data to search for seafloor exposed fault rupture. The cores we collected for our general study are shown as red cross-dots. The cores we collected as the earthquake are plotted as yellow cross-dots.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
    • Inset Figures

      I include some inset figures. Here is some information about them. Below I include the original figures with the figure captions as blockquotes.

    • In the upper right corner is a map from NIWA (Phil Barnes). This map shows some of the major faults in this region. I placed the observed fault offsets on this map as orange lines. This map is on the NIWA website, but I will find the Barnes publication this came from and post that in a follow up web page.
    • In the lower right corner is a map from Geological and Nuclear Sciences (GNS) in Māori: Te Pū Ao. This map shows the regional faults and where there have been observations of surface rupture. The coseismic (during the earthquake) Global Positioning System (GPS) observations. Earthquakes are also plotted.
    • To the left of that is a figure from the Geospatial Information Authority of Japan (GSI). This is a summary figure showing modeled uplift as compared to InSAR analysis results. Note how localized the deformation is. I will present and discuss the analyses that went into this figure in a follow-up report.
    • To the left of that is a generalized tectonic map of the region.



    Here are the USGS websites for the large earthquakes plotted in the map above.

  • 20161113 M 7.8 11:02 USGS
  • 20161113 M 6.5 11:32 USGS
  • 20161113 M 6.1 11:52 USGS
  • 20161113 M 6.2 13:31 USGS
  • 20161113 M 5.7 19:28 USGS
  • 20161114 M 6.5 00:34 USGS
  • 20161114 M 5.4 01:30 USGS
  • 20161114 M 5.8 06:47 USGS
  • 20161115 M 5.4 01:34 USGS
  • 20161115 M 5.4 06:30 USGS
  • 20161118 M 5.1 14:22 USGS
  • 20161122 M 5.9 00:19 USGS
  • 20161122 M 5.3 00:19 USGS
  • 20161122 M 5.0 19:38 USGS
  • Here is an image of the seismograph as recorded by the Humboldt State University, Department of Geology, Baby Benioff seismometer.

  • Here is the USGS Seismicity of the Earth map for this region (Benz et al., 2010). Click on the map for the pdf of this report (66 MB pdf). Here is 10 MB jpg file.

  • Initial recon overflights report that there have been over 100,000 landslides from this earthquake. Below is a map that shows a different analysis that people are conducting. There will be more.

  • Here is one of the early InSAR results from COMET. I will present more of these is a follow-up report.

  • Here is a cool comparison animated gif showing before and after the earthquake. This is an animated gif showing photos taken by Casey Miln and Andrew Spencer.

  • Casey Miln

  • Andrew Spencer

    Videos

  • Here is a video of the Kekerengu fault rupture. This is the yt link for the embedded video below. Here is an mp4 link.
  • Here is a video of the Papatea fault rupture. This is the yt link for the embedded video below. Here is an mp4 link.
  • Here is a video of the Papatea fault and uplifted coast. This is the yt link for the embedded video below.
  • Here is a video of the earthquake and aftershocks. This is the yt link for the embedded video below.
  • Here is a video showing a simulation of the M 7.8 Kaikoura earthquake. This is the yt link for the embedded video below.
  • Here is a video of the road to Kaikoura. This is the yt link for the embedded video below.

Earthquake Report: El Slavador!

While waiting to board my plane from Sydney (AU) to SFO, I put together an interpretive poster for the M 7.0 earthquake offshore of El Salvador from today. I will spend more time later adding more information, including some background material about this subduction zone and some of the materials that I include in this poster. Generally, I use the same type of data that I include in my other earthquake posters. I will add that information here later (need to board my plane now). Here is the USGS website for this earthquake.

I have some maps that show the historic earthquakes in this region that I will upload later (probably as an update to this initial report).

Here is my interpretive poster for the extensional earthquake that is in the downgoing (subducting) Cocos plate. This earthquake has a shallow depth and produced a small tsunami run-up.

I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.

I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely. Based on the moment tensor and my knowledge of the tectonics of this region, I interpret this earthquake to have had a right lateral strike slip motion along an east-west fault.

    Inset Figures

    I include some inset figures. Here is some information about them. Below I include the original figures with the figure captions as blockquotes.

  • In the upper right corner is a map from Funk et al. (2009) that shows focal mechanisms for earthquakes in this region. These data are from the Harvard Centroid Moment Tensor (CMT) catalog for all earthquakes from 1976 to 2007. The region outlined in dashed red lines shows the limit for the Funk et al. (2009) summary figure in the lower right corner and the map and cross section on the left side of the poster.
  • In the lower right corner is a tectonic summary figure from Funk et al. (2009). The authors suggest that there is a forearc sliver in this region. A forearc sliver is a large plate boundary scale strike-slip fault bounded block that is formed due to strain partitioning. When the relative plate motion at a subduction zone are not perpendicular to the megathrust fault, the motion that is perpendicular is accommodated by the megathrust fault. The plate motion that is not perpendicular to the subduction zone fault is accommodated by strike-slip faults parallel to the strike of the subduction zone fault. The authors place red arrows showing the relative motion along the plate bounding fault along the eastern boundary of this forearc sliver (these are called forearc sliver faults for obvious reasons). A classic example of a forearc sliver fault is the Sumatra fault along the Sunda subduction zone. Forearc sliver faults do not always bound a block like this and are not always parallel to the plate boundary. The Cascadia subduction zone has a series of forearc sliver faults offshore, but these are formed oblique to the plate boundary. For the case in cascadia, the plate margin parallel strain is accommodated by rotating blocks, not rigid blocks. As these blocks rotate in response to this shear couple, they rotate and form strike-slip faults between the blocks (forming “bookshelf” faults). Another plate where relative oblique motion creates rotating blocks is along the Aleutian subduction zone.
  • On the left part of the poster I include a map and some cross sections (Funk et al., 2009). The map shows epicenters from 1995-2003. The locations of the cross sections B, C, D, and E are designated by red lines. Arc volcanoes plotted in the cross sections are also shown on the map. The cross sections show the hypocenters from the earthquakes plotted on the map.
  • To the right of these Funk et al. (2009) figures, I include a subset of figures from Benz et al. (2010). There is a map that shows USGS epicenters with dots colored by depth and magnitude represented by circle diameter. There is also a cross section for this region, just to the northwest of El Salvador. Cross section B-B’ shows the earthquake hypocenters along a profile displayed on the map. Note how the subduction zone dip steepens to the northeast.


  • Here is the focal mechanism summary map from Funk et al. (2009). The authors depict the forearc sliver as a shaded red region. Note the strike-slip focal mechanisms in the region on the northeast of the forearc sliver.

  • Tectonic setting of Central America displayed on satellite topography and bathymetry from Sandwell and Smith (1997). Subduction of the Cocos plate beneath the Caribbean plate occurs along the Middle America Trench. Global positioning system (GPS)–based plate velocities are relative to a fixed Caribbean plate, and focal mechanisms are from the Harvard Centroid Moment Tensor (CMT) catalog for all events from 1976 to 2007. Northwestward arc-parallel translation of the Central America forearc sliver (red shading) occurs at 7–8 mm yr–1 on Nicoya Peninsula of Costa Rica (Norabuena et al., 2004) and 14 mm yr–1 in Nicaragua (DeMets, 2001). The boundary between the Caribbean and North American plate occurs along the left-lateral strike-slip Swan Islands fault zone (SIFZ), Polochic fault (PF), and Motagua fault (MF).

  • Here is the seismicity map and cross sections for the region to the southeast of El Salvador. The subduction zone in the El Salvador region is depicted by cross section B. Note that the subduction zone has a low angle dip in the shallow region of the fault, then steepens to 55 degrees to the east.


  • (A) Earthquake locations are from the National Earthquake Information Center (NEIC) and, in Nicaragua, were recorded by the local Nicaragua network from 1995 to 2003 operated by the Instituto Nicaraguense de Estudios Territoriales (INETER ). (B–E) Earthquake profiles are perpendicular to the Middle America Trench (MAT) and extend to the interior volcanic highlands of Central America. These profiles merge all earthquakes within a 50-km-wide swath along each transect. Seismic activity beneath the volcanic front in Nicaragua is more evident because of more data from local stations of the Nicaraguan seismic network. These shallow crustal earthquakes commonly occur within the upper 30 km of the crust and are concentrated within ~25 km of the active Central America volcanic front (CAVF).

  • Here is the summary figure from Funk et al. (2009). This map shows the detailed fault mapping the authors prepared for their manuscript. Their observations include field mapping and seismic profiles. There are several parts of this forearc sliver fault system that show how the strike-slip system bends and steps. There are restraining bends (where the s-s fault generates compression) and releasing bends (where the s-s fault generates extension). These regions are colored red and green, respectively. In the Marabios En Exchelon segment of this map there are some plate margin obvlique extensional fault bounded basins. These appear to be formed by bookshelf style faulting.

  • Regional tectonic map of Central America emphasizing key structures described in this paper. The El Salvador fault zone (ESFZ) is characterized by a broad right-lateral shear zone accommodating transtensional motion that results in multiple pull-apart basins . A major transition zone occurs in the Gulf of Fonseca, where strike-slip fault zones along the Central American forearc sliver change strike from dominantly east-west strikes in El Salvador to northwesterly strikes in Nicaragua. A proposed restraining bend connects faults mapped in the Gulf of Fonseca with fault scarps deforming Cosiguina volcano and faults of the Central America volcanic front north of Lake Managua . Diffuse and poorly exposed faults parallel to the Central America volcanic front in northern Nicaraguan segment are inferred to represent a young fault boundary in which right-lateral shear is accommodated over a broad zone. This model proposes a young en echelon pattern of strike-slip and secondary faults based on secondary extensional features and fi ssure eruptions along the Marabios segment of the Central America volcanic front. Lake Managua and the Managua graben are interpreted to occur at a major releasing bend in the trend of the Nicaraguan depression and are marked by the curving surface trace of the Mateare fault interpreted from aeromagnetic data. Subsequent right-lateral strike-slip motion related to translation of the Central America forearc sliver may occur along these reactivated normal faults. The Lake Nicaragua segment of the Central America volcanic front is bounded by a normal fault (LNFZ—Lake Nicaragua fault zone) offsetting the Rivas anticline, the southeastward continuation of this normal fault into Costa Rica (CNFZ—Costa Rica fault zone), and a synthetic normal fault (SRFZ—San Ramon fault zone) that we discovered in our survey of Lake Nicaragua. Transverse faults (MFZ—Morrito fault zone, JMFZ—Jesus Maria fault zone) strike approximately east-west across the Central America volcanic front. North-south–trending rift zones are abundant in El Salvador but less common in Nicaragua and may also be controlled by regional east-west extension affecting the northwestern corner of the Caribbean plate.

  • Here is a map and cross section of seismicity that was written about an earthquake pair that happened in 2001 from Martínez-Díaz et al. (2004). In January 2001 there was a M 7.7 earthquake in the downgoing Cocos plate. In February there was a M 6.6 earthquake in the Caribbean plate, which was probably triggered by the January quake. The triggered M 6.6 quake was a strike-slip earthquake on along the plate boundary forearc sliver, the El Salvador fault zone (ESFZ). I include other figures below.
    • Here are the USGS websites for the major earthquakes discussed by Martínez-Díaz et al. (2004).
    • 1982.06.19 M 6.2 El Salvador (may have triggered an earthquake in 1986)
    • 2001.01.13 M 7.7 El Salvador
    • 1982.06.19 M 6.6 El Salvador (may have been triggered by M 7.7)


    A: Geodynamic setting of study area. Line within box indicates trace of cross sections B and C. Plate motion direction and rates are shown by arrows (data from DeMets, 2001). B: Cross section passing through coordinates 13.938N, 88.788W. Focal mechanisms were taken from Harvard Centroid Moment Tensor (CMT) and U.S. Geological Survey–National Earthquake Information Center (USGS-NEIC) catalogues (period 1977–February 2003). Gray dots are hypocenters from USGSNEIC database (Ms >2.5). Dashed lines represent estimation of upper and lower bounds of seismogenic zone of subducted slab. C: Cross section of 2001 seismic sequence of El Salvador following two main shocks of January and February. Hypocenters are from Servicio Nacional de Estudios Territoriales catalogue and focal mechanisms are from Harvard CMT and USGS-NEIC catalogues.

  • Here is a map that shows focal mechanisms for large earthquakes in the region from 1977-2001. The ESFZ is visible on the shaded relief map (the fault creates enough topography to be shown at the scale of kms).

  • Radar–Shuttle Radar Topography Mission image of El Salvador (courtesy of Jet Propulsion Laboratory) with historical destructive earthquakes (white circles) and instrumental epicenters (Ms >2.5, period 1977–2001) from U.S. Geological Survey–National Earthquake Information Center (USGS-NEIC) catalogue (small dots). Small focal mechanisms are from events with Mw >5.5 (period 1977–2001, Harvard Centroid Moment Tensor database). Large mechanisms are from Buforn et al. (2001). Inset: map of faults extracted from Geological Map of El Salvador (Bosse et al., 1978). ESFZ—El Salvador fault zone.

  • Here is a map that shows how the afershocks (of the M 6.6) were limited to two segments of the ESFZ.

  • A: Aftershocks sequence of February 2001 (Mw 6.6) event projected on Radar image of El Salvador fault zone (ESFZ). SM—San Miguel; SV—San Miguel volcano; IL—Lago Ilopango; RS—rupture segment; JC—Jucuapa; RL—Rio Lempa; RG—Rio Grande. B: Oblique view of digital elevation model of volcanic arc with ESFZ trace.

  • Here are some figures that show the modeled changes in static coulomb stress generated by earthquakes in 1982 and 2001. Regions that get increased in stress following an earthquake are plotted in red and regions that have lowered levels of stress are in blue. We do not know what the state of stress is on any fault at any given time. The changes in stress in these regions are very small compared to the stresses that faults release during earthquakes (so, the possibility that a fault my be triggered by these small changes is also very small). Generally, a fault would need to be at a high state of stress to be triggered by an earthquake in this manner.
  • Panels A, B, and C are maps showing coulomb stress changes for the 1982 series and the 2001 series. Panel D is a cross section for the 2001 sequence. Note how the 1986 and Feb 2001 faults are in regions of increased stress. Also note how the ESFZ in regions adjacent to the Feb 2001 earthquake have increased stress.

  • A–C: Models of change in static Coulomb failure stress (CFS) generated after events of 1982, January 2001, and February 2001 projected on map of faults. Black line represents El Salvador fault zone (ESFZ). D: Model for 2001 event is shown in cross section. E: Joint model with CFS change generated by three major events. Rectangles represent surface ruptures. Dotted rectangles represent hypothetical ruptures.

  • Finally, here are the hypocenters shown with a low angle oblique view. The upper figure shows aftershocks for the Jan 2001 earthquake and the lower figure shows the aftershocks for the Feb earthquake.

  • Fault planes of main shocks fitted to aftershocks of the first three days, with the epicentres and outline of El Salvador. A. Representation for the 13 January Mw 7.7 event. The fault trace is subparallel to the coast. B. Representation for the 13 February Mw 6.6 event.

  • Here is a map that shows earthquakes from the NEIC for the region from 1900 to 2016, for magnitudes greater than or equal to M 6.0. The color of the dot represents the hypocentral depth of the earhquake. Note how the earthquakes fenerally get deeper along with the subduction zone megathrust. Some do not for earthquakes in the Caribbean plate and along the Swan Islands fault zone. The lower map shows earthquakes M ≥ 7.0.
  • Here are the USGS queries I used for the maps below.
    • Earthquakes from 1900-2016 for magnitudes M ≥ 6.0 USGS
    • Earthquakes from 1900-2016 for magnitudes M ≥ 7.0 USGS



  • Here is the “seismicity of the Earth” USGS series poster for this region. Click on the thumbnail below for the pdf version (13 MB pdf).

  • Here are the tsunami model results from the Pacific Tsunami Warning Center. Note how the observations in the text report below are generally consistent with this forecast.

  • Here is the National Tsunami Warning Center (NTWC) tsunami travel-time map.

  • Here are the tsunami observations.

    References:

  • Benz, H.M., Tarr, A.C., Hayes, G.P., Villaseñor, Antonio, Furlong, K.P., Dart, R.L., and Rhea, Susan, 2011, Seismicity of the Earth 1900–2010 Caribbean plate and vicinity: U.S. Geological Survey Open-File Report 2010–1083-A, scale 1:8,000,000.
  • Funk, J., Mann, P., McIntosh, K., and Stephens, J., 2009. Cenozoic tectonics of the Nicaraguan depression, Nicaragua, and Median Trough, El Salvador, based on seismic-reflection profiling and remote-sensing data in GSA Bulletin, v. 121, no. 11/12, p. 1491-1521.
  • Martínez-Díaz, J.A., Álvarez-Gómez, J.A., Benito, B., and Hernández, D., 2004. Triggering of destructive earthquakes in El Salvador in GSA Bulletinm, v. 32., no. 1, p. 65-68.

Earthquake Report: Laytonville (northern CA)!

This morning there was another earthquake in northern CA between the San Andreas (SAF) and Maacama faults (MF). This region has been active for the past few years, with earthquakes in the M 3-4 range. Most recently, there was a M 3.8 really close to today’s M 4.1. These earthquakes may indicate the possibility of an unmapped fault. There have been earthquakes in 2000, 2014, and 2015 that align along strike (of a possible fault that is sub-parallel to the SAF/MF systems). Here is the USGS website for today’s M 4.1 earthquake.

The San Andreas fault is a right-lateral strike-slip transform plate boundary between the Pacific and North America plates. The plate boundary is composed of faults that are parallel to sub-parallel to the SAF and extend from the west coast of CA to the Wasatch fault (WF) system in central Utah (the WF runs through Salt Lake City and is expressed by the mountain range on the east side of the basin that Salt Lake City is built within).

The three main faults in the region north of San Francisco are the SAF, the MF, and the Bartlett Springs fault (BSF). Here is a map that shows Saturday’s epicenter as a red star. I also place a graphical depiction of the USGS moment tensor for this earthquake. The SAF, MF, and BSF are all right lateral strike-slip fault systems. There are no active faults mapped in the region of Saturday’s epicenter, but I interpret this earthquake to have right-lateral slip. Without more seismicity or mapped faults to suggest otherwise, this is a reasonable interpretation.

Below I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I use the USGS Quaternary fault and fold database for the faults. I outlined the Vizcaino Block, which many interpret to be a prehistoric subduction zone accretionary prism from a time before the San Andreas existed.

I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.

I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely. Based on the moment tensor and my knowledge of the tectonics of this region, I interpret this earthquake to have had a right lateral strike slip motion along an east-west fault.

    I include some inset figures in the poster.

  • In the upper right corner I include a map from McLaughlin et al. (2012) that shows the regional faulting.
  • In the lower right corner I include generalized fault map of northern California from Wallace (1990).
  • In the lower left corner I include a map that shows the seismicity for this region since 1960, including earthquakes with magnitudes greater than or equal to M 1.5. I have labeled some of the significant earthquake swarms, with magnitudes ranging from M 3-4. Here is the USGS search that I used to create this map.
  • To the right of the seismicity map is a figure that shows the evolution of the San Andreas fault system since 30 million years ago (Ma). This is a figure from the USGS here.
  • In the upper left corner I include the Earthquake Shaking Potential map from the state of California. This is a probabilistic seismic hazard map, basically a map that shows the likelihood that there will be shaking of a given amount over a period of time. More can be found from the California Geological Survey here. I place a yellow star in the approximate location of today’s earthquake.


  • Earlier this month (a couple days ago), there was an earthquake in this region. Below is my interpretive poster for that earthquake. Here is my Earthquake Report.

  • Earlier this year, there was an earthquake in this region. Below is my interpretive poster for that earthquake. Here is my Earthquake Report.

  • Last year there was an earthquake in this region. Below is my interpretive poster for that earthquake. Here is my Earthquake Report.

  • I place a map shows the configuration of faults in central (San Francisco) and northern (Point Delgada – Punta Gorda) CA (Wallace, 1990). Here is the caption for this map, that is on the lower left corner of my map. Below the citation is this map presented on its own.
  • Geologic sketch map of the northern Coast Ranges, central California, showing faults with Quaternary activity and basin deposits in northern section of the San Andreas fault system. Fault patterns are generalized, and only major faults are shown. Several Quaternary basins are fault bounded and aligned parallel to strike-slip faults, a relation most apparent along the Hayward-Rodgers Creek-Maacama fault trend.


  • About 75% of the relative plate motion is accommodated along the SAF and its synthetic sister faults in the northern CA region. The rest of the plate boundary motion is accommodated along the Eastern CA shear zone and Walker Lane, along with the Central Nevada Seismic Belt, and the Wasatch fault systems. In Northern CA, there is about 33-37 mm/yr strain accumulated on the SAF plate boundary system. About 18-25 mm/yr is on the SAF, 8-11 mm/yr on the MF, and 5-7 mm/yr on the Bartlett Springs fault system (Geist and Andrews, 2000).
  • Here is a map from McLaughlin et al. (2012) that shows the regional faulting. I include the figure caption as a blockquote below.

  • Maps showing the regional setting of the Rodgers Creek–Maacama fault system and the San Andreas fault in northern California. (A) The Maacama (MAFZ) and Rodgers Creek (RCFZ) fault zones and related faults (dark red) are compared to the San Andreas fault, former and present positions of the Mendocino Fracture Zone (MFZ; light red, offshore), and other structural features of northern California. Other faults east of the San Andreas fault that are part of the wide transform margin are collectively referred to as the East Bay fault system and include the Hayward and proto-Hayward fault zones (green) and the Calaveras (CF), Bartlett Springs, and several other faults (teal). Fold axes (dark blue) delineate features associated with compression along the northern and eastern sides of the Coast Ranges. Dashed brown line marks inferred location of the buried tip of an east-directed tectonic wedge system along the boundary between the Coast Ranges and Great Valley (Wentworth et al., 1984; Wentworth and Zoback, 1990). Dotted purple line shows the underthrust south edge of the Gorda–Juan de Fuca plate, based on gravity and aeromagnetic data (Jachens and Griscom, 1983). Late Cenozoic volcanic rocks are shown in pink; structural basins associated with strike-slip faulting and Sacramento Valley are shown in yellow. Motions of major fault blocks and plates relative to fi xed North America, from global positioning system and paleomagnetic studies (Argus and Gordon, 2001; Wells and Simpson, 2001; U.S. Geological Survey, 2010), shown with thick black arrows; circled numbers denote rate (in mm/yr). Restraining bend segment of the northern San Andreas fault is shown in orange; releasing bend segment is in light blue. Additional abbreviations: BMV—Burdell Mountain Volcanics; QSV—Quien Sabe Volcanics. (B) Simplifi ed map of color-coded faults in A, delineating the principal fault systems and zones referred to in this paper.

  • Here is the figure showing the evolution of the SAF since its inception about 29 Ma. I include the USGS figure caption below as a blockquote.

  • EVOLUTION OF THE SAN ANDREAS FAULT.

    This series of block diagrams shows how the subduction zone along the west coast of North America transformed into the San Andreas Fault from 30 million years ago to the present. Starting at 30 million years ago, the westward- moving North American Plate began to override the spreading ridge between the Farallon Plate and the Pacific Plate. This action divided the Farallon Plate into two smaller plates, the northern Juan de Fuca Plate (JdFP) and the southern Cocos Plate (CP). By 20 million years ago, two triple junctions began to migrate north and south along the western margin of the West Coast. (Triple junctions are intersections between three tectonic plates; shown as red triangles in the diagrams.) The change in plate configuration as the North American Plate began to encounter the Pacific Plate resulted in the formation of the San Andreas Fault. The northern Mendicino Triple Junction (M) migrated through the San Francisco Bay region roughly 12 to 5 million years ago and is presently located off the coast of northern California, roughly midway between San Francisco (SF) and Seattle (S). The Mendicino Triple Junction represents the intersection of the North American, Pacific, and Juan de Fuca Plates. The southern Rivera Triple Junction (R) is presently located in the Pacific Ocean between Baja California (BC) and Manzanillo, Mexico (MZ). Evidence of the migration of the Mendicino Triple Junction northward through the San Francisco Bay region is preserved as a series of volcanic centers that grow progressively younger toward the north. Volcanic rocks in the Hollister region are roughly 12 million years old whereas the volcanic rocks in the Sonoma-Clear Lake region north of San Francisco Bay range from only few million to as little as 10,000 years old. Both of these volcanic areas and older volcanic rocks in the region are offset by the modern regional fault system. (Image modified after original illustration by Irwin, 1990 and Stoffer, 2006.)

    • Here is a map that shows the shaking potential for earthquakes in CA. This comes from the state of California here.
    • Earthquake shaking hazards are calculated by projecting earthquake rates based on earthquake history and fault slip rates, the same data used for calculating earthquake probabilities. New fault parameters have been developed for these calculations and are included in the report of the Working Group on California Earthquake Probabilities. Calculations of earthquake shaking hazard for California are part of a cooperative project between USGS and CGS, and are part of the National Seismic Hazard Maps. CGS Map Sheet 48 (revised 2008) shows potential seismic shaking based on National Seismic Hazard Map calculations plus amplification of seismic shaking due to the near surface soils.


      References

    • Geist, E.L. and Andrews D.J., 2000. Slip rates on San Francisco Bay area faults from anelastic deformation of the continental lithosphere, Journal of Geophysical Research, v. 105, no. B11, p. 25,543-25,552.
    • Irwin, W.P., 1990. Quaternary deformation, in Wallace, R.E. (ed.), 1990, The San Andreas Fault system, California: U.S. Geological Survey Professional Paper 1515, online at: http://pubs.usgs.gov/pp/1990/1515/
    • McLaughlin, R.J., Sarna-Wojcicki, A.M., Wagner, D.L., Fleck, R.J., Langenheim, V.E., Jachens, R.C., Clahan, K., and Allen, J.R., 2012. Evolution of the Rodgers Creek–Maacama right-lateral fault system and associated basins east of the northward-migrating Mendocino Triple Junction, northern California in Geosphere, v. 8, no. 2., p. 342-373.
    • Stoffer, P.W., 2006, Where’s the San Andreas Fault? A guidebook to tracing the fault on public lands in the San Francisco Bay region: U.S. Geological Survey General Interest Publication 16, 123 p., online at http://pubs.usgs.gov/gip/2006/16/
    • Wallace, Robert E., ed., 1990, The San Andreas fault system, California: U.S. Geological Survey Professional Paper 1515, 283 p. [http://pubs.usgs.gov/pp/1988/1434/].

Earthquake Report: Maule, Chile!

This morning there was a moderate sized earthquake along the megathrust in Chile. Here is the USGS website for this M 6.4 earthquake. Based upon the hypocentral depth, and the moment tensor, this appears to be along the megathrust fault (an “interface” event). This is a very seismically active region of the world, with a number of Great (M ≥ 8.0) earthquakes in recent years.

Below is my interpretive poster. I have plotted the epicenters (using the USGS earthquake feed kml) for the past 30 days with magnitudes 2.5 or greater, with color representing depth. I also include the slab depth contours from Hayes et al. (2012). These are the depth contours for the fault interface of the subduction zone. Today’s hypocentral depth is 90.8 km and the Hayes et al. (2012) slab contour in this region is between 80 and 100 km. I include the USGS moment tensor for this earthquake. I also plot the rupture length regions of historic earthquakes for this subduction zone in green (Beck et al., 1998). I present the patches for the 2010 and 2015 subduction zone earthquakes outlined in white dashed lines. Today’s earthquake happened in the region of the subduction zone that is “down-sip” from the 2010 earthquake. The 2010 earthquake would probably have loaded the fault in this region, so this M 6.4 earthquake may be related to the 2010 earthquake. However, it has been over 5 years since the 2010 earthquake. That being said, there are still aftershocks from the 2011 Tohoku-Oki earthquake (so it seems possible).

I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.

I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely. Based on the moment tensor and my knowledge of the tectonics of this region, I interpret this earthquake to have had a right lateral strike slip motion along an east-west fault.

    I include some inset figures in the poster.

  • In the upper left corner I place a cross section from Melnick et al. (2006). This shows the prehistoric earthquake history on the left and a cross section of the subduction zone on the right. This cross section is in the region of the 2010 subduction zone earthquake.
  • In the upper right corner I include the space-time diagram from Beck et al. (1998 ) showing the along strike length of prehistoric earthquakes in the central subduction zone. The map above shows these prehistoric rupture strike lengths as green lines (labeled with green labels). The 2015 earthquake series ruptured past the southern boundary of the 1943 earthquake and about 30% into the 1922 earthquake region. There is a small gap between the 2010 and 2015 earthquake series, which aligns with the Juan Fernandez Ridge (a fracture zone in the Nazca plate; von Huene et al., 1997; Rodrigo et al., 2014). This fracture zone appears to be a structural boundary to earthquake slip patches (subduction zone segmentation), at least for some earthquakes. Beck et al. (1998 ) show that possibly two earthquakes ruptured past this boundary (1730 A.D. and possibly 1647 A.D., though that is queried). This segment boundary appears to be rather persistent for the past ~500 years
  • In the lower right corner, I include a time-space diagram from Moernaut et al. (2010). This diagram extends further south than the Beck et al. (1998) figure.
  • In the lower left corner I include an inset map from the USGS Seismicity History poster for this region (Rhea et al., 2010). There are two seismicity cross sections here as well, with their locations plotted on the map. The USGS plot these hypocenters along these two cross sections and I include those.


  • In February of this year, there was a M 6.3 earthquake near the coast which caused considerable damage. Below is my interpretive poster for that earthquake and here is my report.

  • In September through November of 2015, there was a M 8.3 earthquake further to the north. Below is my interpretive poster for that earthquake and here is my report, where I discuss the relations between the 2010, 2015, and other historic earthquakes in this region. Here is my report from September.

  • Here is a space time diagram from Beck et al. (1998 ). The 2015 earthquake occurs in the region of the 1943 and 1880 earthquakes. I updated this figure to show the latitudinal extent of the 2010 and 2015 earthquakes.

  • Here is the space-time diagram from Moernaut et al., 2010. I include their figure caption below in blockquote.

  • Fig.: Setting and historical earthquakes in South-Central Chile. Data derived from Barrientos (2007); Campos et al. (2002); Melnick et al.(2009)

  • The September 2015 earthquake series inspired me to compile some information on the historic tsunami in this region. Here is my report on those tsunami. Below I present my figure and an animation that compares these three tsunami from 1960, 2010, and 2014.
  • These three maps use the same color scale. There is not yet a map with this scale for the 2015 tsunami, so we cannot yet make the comparison.

  • Here is an animation of these three tsunami from the US NWS Pacific Tsunami Warning Center (PTWC). This is the YouTube link.

  • Here is the cross section of the subduction zone just to the south of this Sept/Nov 2015 swarm (Melnick et al., 2006). Below I include the text from the Melnick et al. (2006) figure caption as block text.

  • (A) Seismotectonic segments, rupture zones of historical subduction earthquakes, and main tectonic features of the south-central Andean convergent margin. Earthquakes were compiled from Lomnitz (1970, 2004), Kelleher (1972), Comte et al. (1986), Cifuentes (1989), Beck et al. (1998 ), and Campos et al. (2002). Nazca plate and trench are from Bangs and Cande (1997) and Tebbens and Cande (1997). Maximum extension of glaciers is from Rabassa and Clapperton (1990). F.Z.—fracture zone. (B) Regional morphotectonic units, Quaternary faults, and location of the study area. Trench and slope have been interpreted from multibeam bathymetry and seismic-reflection profiles (Reichert et al., 2002). (C) Profile of the offshore Chile margin at ~37°S, indicated by thick stippled line on the map and based on seismic-reflection profiles SO161-24 and ENAP-017. Integrated Seismological experiment in the Southern Andes (ISSA) local network seismicity (Bohm et al., 2002) is shown by dots; focal mechanism is from Bruhn (2003). Updip limit of seismogenic coupling zone from heat-fl ow measurements (Grevemeyer et al., 2003). Basal accretion of trench sediments from sandbox models (Lohrmann, 2002; Glodny et al., 2005). Convergence parameters from Somoza (1998 ).

  • In March 2015, there was some seismicity in this September/November 2015 earthquake slip region. I put together an earthquake report about those earthquake of magnitudes M = 5.0-5.3. I speculate that the 1922 earthquake region is a seismic gap. Note that this September/November 2015 earthquake region is along the southern portion of the seismic gap that I labeled on the map below. Dutchsinse can kiss my 4$$.
  • Here is a map that shows the recent swarm of ~M = 5 earthquakes. There are moment tensors for the earthquakes listed below, some recent historic subduction zone earthquakes. I placed the general along-strike distance for older historic earthquakes in green (and labeled their years). The largest earthquake ever recorded, the Mw = 9.5 Chile earthquake, had a slip patch that extends from the south of the map to just south of the 2010 earthquake swarm. The 2010 and 2014 earthquake swarm epicenters are plotted as colored circles, while most other historic earthquake epicenters are plotted as gray circles. Note how this March 2015 swarm is at the northern end of the 1922/11/11 M 8.3 earthquake. At the bottom of this page, I put a USGS graphic about what these moment tensor plots (beach balls) tell us about the earthquakes.

  • Hundreds of people died as a result of the 1922 earthquake. The USGS has more news reports about the 1922 earthquake here. There were also reports of a tsunami over 9 meters. So we know that this segment of the fault can produce large earthquakes and tsunami. However, it has been about a century since the last Great subduction zone earthquake in this region of the fault.

References:

Earthquake Report: Italy Update #1!

Here is an update to the #EarthquakeReport for the M 6.6 earthquake that hit Italy early this morning my time. Ironically, I was preparing a report for earthquakes in the western Pacific for my class when this M 6.6 earthquake hit and I did not notice the USGS email because I was so engaged with the western Pacific report. Here is the interpretive poster for that region over the past week or so.

This region has been experiencing a series of large earthquakes since August 2016, possibly culminating in this M 6.6 earthquake. These earthquakes may load adjacent faults in the region, so this may not be over. Given the series of earthquakes in the region further to the north (from 1916-1920), this may not be over. Stay tuned and stay safe!

The M 6.1 earthquake happened following the M 5.5 earthquake, so people had already been staying outside of their houses. This is thought to be why the casualty number was lower than expected for the M 6.1 earthquake. While the damage estimates are likely to be closer to the bar for both the M 6.1 and M 6.6 earthquakes, the casualty list is also thought to be lower for the M 6.6 earthquake for the same reason.

Below is a poster that shows epicenters from 2008 – 2016. I have included moment tensors for the largest megnitude earthquakes and outlined the region that has had earthquakes in the two periods (2009 & 2016). I also include the fault database from the Instituto Nazionale di Geofisica e Vulcanologia (the Database of Individual Seismogenic Sources, DISS v. 3.; DISS Working Group, 2015). There is a DISS legend to the right of the moment tensor legend. The Mt Vettore fault is considered a “Debated Seismogenic Source” in this database and is the blue fault line on the northern part of the 2016 rupture region.

I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely. The tectonics of this region has many normal (extensional) faults, which explain the extensional moment tensor.

    I include some inset figures and maps.

  • In the upper left corner is a map that shows seismicity for this region on maps and cross sections (Boncio et al., 2004). I placed orange stars in the approximate location of the October, 2016 earthquakes. These earthquakes happen to appear right on the center of cross section b (the lowermost cross section on the right). It appears that these earthquakes are rupturing along the Mt. Vettore fault.
  • In the upper right corner I include a map and cross section compiled by Istituto Nazionale di Geofisica e Vulcanologia (INGV). The map and cross section are from Pierantoni et al. (2013). The cross sectional focal mechanism is located in the approximate location on the cross section. Stars designate the epicentral locations for the recent seismicity on the map. This hypothesis for which structures these earthquakes are appearing on is consistent with my interpretation shown on the Boncio et al. (2004) map and cross section.
  • In the lower right corner is another map from INGV that shows the mapped faults in this region and the location of hte 2016.10.30 M 6.6 earthquake, along with other October 2016 seismicity. Observations of potential surface rupture have occurred in the region labeled “Surface faulting.”
  • In the lower left corner is another map from INGV that shows epicenters discriminated by time (August = blue, 72 hours = yeallow, 24 hours = orange, 1 hour = red). Note the overlap not seen in my main map. The local seismic network that INGV uses shows many more earthquakes than the global network contributing to the USGS database.
  • Along the base of the poster I include “USGS Did You Feel It?” maps for the 4 largest earthquakes plotted on this map.


  • Here are some views of the damage from news organizations.
  • BBC
  • BBC
  • BBC
  • As a remiinder, this region is in a seismically hazardous region of Italy. Here is the 10% probability of exceedance map (for 50 yrs) from INGV.

  • This is another view of the seismic hazard in Europe (Giardini et al., 2013).

  • Boncio et a. (2004) present a remarkable assessment of the seismic hazard in this region based on a 3-D model for seismogenic sources. I present some of their figures below. I include their original figure captions as blockquotes.
  • This map shows a detailed view of the normal faults in the region. Today’s earthquake is in the region shown in box 3, east of the Umbra Valley.

  • Digital elevation model of central Italy with active normal faults of the Umbria-Marche-Abruzzo Apennines and parameters of active stress tensors obtained by inversion of focal mechanisms of background microseismicity (1), aftershock sequences (2, 3, 4, 5) or striated active faults in seismic areas (6); stress data from Brozzetti and Lavecchia (1994), Boncio and Lavecchia (2000a) and Pace et al. (2002a); the stress axes are given as trend (first three numbers) and plunge (last two numbers).

  • This map shows an even more detailed and large scale view of the faults and seismicity in this region. Today’s earthquakes align to the north of Norcia, approximately along the cross section labeled “sec B.” The two cross sections are in the lower right part of the figure, with section B the lowermost cross section. Today’s earthquake may be on the AF2, the C-NFs (Colfiorito-Norcia fault systems), or MVf (Mt. Vettore fault). The AF2 fault is a proposed low angle detachment fault. These types of faults are controversial in that there are arguments about whether they are seismogenic or not. This year’s Pacific Cell Friends of the Pleistocene field trip in Panamint Valley presented research results that attempted to address this question. In Panamint Valley there are faults that have similar configurations as these faults in Italy.

  • Geological cross sections from seismic reflection profiles across the Gubbio, Gualdo T. and Colfiorito seismic areas (from Boncio et al., 1998; Boncio and Lavecchia, 2000b); epi- and hypocentral distribution of back-ground microseismicity recorded in the Umbria-Marche Apennines and rheological profiles (strength envelopes in critical stress difference, σ1–σ3) built for two different thermal contexts (50 and 40 mW/m2 surface heat flow, see Figure 3 for location); the depth of the brittle-plastic transition on rheological profiles is indicated by arrows; the used rheological parameters are indicated: crustal layering is from DSS data; A (empirical material constant), n (stress exponent) and E (activation energy) are creep parameters; ´ε = longitudinal strain rate (calculated by balancing of a regional geologic section; Figure 5 in Boncio et al., 2000); see text for further details.

  • This map shows a smaller scaled view (than the above figure) with focal mechanisms and cross sections (with structural interpretations). Hypocenters are also plotted on these cross sections. Today’s earthquakes are just south of cross section b. (earthquakes happened here in 1997)

  • Epicentres of the major seismic sequences of the last twenty years (Gubbio, 1984; Colfiorito, 1997; Norcia, 1979; Sangro, 1984) plus three small seismic sequences in the L’Aquila area (1992, 1994 and 1996); seismotectonic sections and rheological profiles built according to the local thermal context. The dashed line (sections ‘a’ and ‘b’) represents the AF low-angle extensional detachment; arrows in seismotectonic sections indicate the maximum depth-extent of the activated seismogenic faults as suggested by the best defined aftershock volume; rheological parameters as in Figure 7; in the southern Abruzzo area, creep strengths for geologic and geodetic longitudinal strain rates are compared (geologic strain rate calculated from data of Galadini and Galli, 2000; geodetic strain rate from D’Agostino et al., 2001); seismological data from Amato et al. (1998); Boncio (1998); Boncio et al. (2004); Cattaneo et al. (2000); De Luca et al. (2000); Deschamps et al. (1984); Ekstrom et al. (1998); Haessler et al. (1988); Harvard CMT database at www.seismology.harvard.edu.

Earthquake Report: Italy!

There was just another earthquake in Italy. This one is a larger magnitude M = 6.6. This region has been especially seismically active since August 2016.

This earthquake is north of the region that had an M 6.3 earthquake in 2009 that led to an interesting (putting it nicely) interaction between scientists, public employees/politicians, and the legal system. Basically, several seismologists were sentenced to prison. More on this is found online, for example, here and here.

Below is my interpretive map that shows the epicenter, along with the shaking intensity contours. These contours use the Modified Mercalli Intensity (MMI) scale. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. There is a legend for MMI intensities in the upper part of the interpretive poster below.

I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely. The tectonics of this region has many normal (extensional) faults, which explain the extensional moment tensor.

    I include some inset figures and maps.

  • In the upper left corner I include estimates of damage to people (possible fatalities) and their belongings from the Rapid Assessment of an Earthquake’s Impact (PAGER) report. More on the PAGER program can be found here. An explanation of a PAGER report can be found here. PAGER reports are modeled estimates of damage. On the top is a histogram showing estimated casualties and on the right is an estimate of possible economic losses. This PAGER report suggests that there will be quite a bit of damage from this earthquake (and casualties). This earthquake has a high probability of damage to people and their belongings.
  • In the upper right corner is a map showing the faulting mapped in the region surrounding and including Italy (Billi et al., 2006). There is a convergent plate boundary along the eastern part of Italy (part of the Alpide belt, a convergent boundary that extends from the Straits of Gibraltar to Australia). This fault system dips westward and is onshore in the south, but extends offshore into the Adriatic Sea in central-northern Italy. In the central part of Italy is a series of north-northwest striking extensional faults. It is these extensional (normal) faults that are responsible for the damaging seismicity in this region of central Italy. This includes the 1915, 1997, 2009, and 2016 earthquakes.
  • To the right of the Billi et al. (2006) fault map is a plot showing the seismicity from the last year. Today’s earthquakes are plotted as orange circles and the epicenters from August are plotted as gray circles.
  • To the left of this map is a figure that shows the median PGA (Peak Ground Acceleration, units of g where g = 9.8 m/s2) that has a 10% probability of exceedance (PE) in the next 50 years. This model assumes a Vs30 greater than 800 m/s. Vs is the average seismic velocity in the upper 30 meters. Vs30 is a proxy used for global to regional estimates of seismic hazard.I include their original figure captions as blockquote. This is from Stucchi et al., 2011.
  • In the lower right corner are two panels with results from the USGS “Did You Feel It?” website. The upper panel shows results from felt reports. The circles are colored vs. MMI intensity. The lower panel is a plot that shows these reports as their MMI values vary with distance from the earthquake (the horizontal axis). There are Ground Motion Prediction Equations that are empirical models (that model shaking intensity vs. distance) that are used to estimate shaking for this earthquake. The output from this model is the source of data for the shakemaps and the PAGEr damage estimates. Note how the attenuation relations (how the seismic energy is absorbed with distance from the earthquake) fit the green line (GMPE relations for lithosphere and earthquakes in California).
  • In the lower left corner is a map that shows seismicity for this region on maps and cross sections. I placed orange stars in the approximate location of the October, 2016 earthquakes. These earthquakes happen to appear right on the center of cross section b (the lowermost cross section on the right). It appears that these earthquakes are rupturing along the Mt. Vettore fault.


  • Here is the USGS PAGER alert.

    Here are some other maps that might help. (well, one so far)

  • This Billi et al. (2006) map shows some of these west dipping normal faults in central Italy, just south of the Apennines.

  • There are some excellent maps and figures from a study from 2004 (Boncio et al, 2004). This material was posted on twitter here.
  • Below is my interpretive poster from the earthquakes from a few days ago. For more information about the figures displayed on this poster, here is the complete report for this swarm.

  • Here is a map that shows the earthquakes from the past few months, for magnitudes greater than or equal to M = 2.5. Note the M 6.6 earthquake (the largest orange circle) is between the 2016.08.23 M 6.2 earthquake (the largest gray circle in the south) and the 2016/10/28 M 6.2 earthquake (the largest yellow circle in the north).

  • Boncio et a. (2004) present a remarkable assessment of the seismic hazard in this region based on a 3-D model for seismogenic sources. I present some of their figures below. I include their original figure captions as blockquotes.
  • This map shows a detailed view of the normal faults in the region. Today’s earthquake is in the region shown in box 3, east of the Umbra Valley.

  • Digital elevation model of central Italy with active normal faults of the Umbria-Marche-Abruzzo Apennines and parameters of active stress tensors obtained by inversion of focal mechanisms of background microseismicity (1), aftershock sequences (2, 3, 4, 5) or striated active faults in seismic areas (6); stress data from Brozzetti and Lavecchia (1994), Boncio and Lavecchia (2000a) and Pace et al. (2002a); the stress axes are given as trend (first three numbers) and plunge (last two numbers).

  • This map shows an even more detailed and large scale view of the faults and seismicity in this region. Today’s earthquakes align to the north of Norcia, approximately along the cross section labeled “sec B.” The two cross sections are in the lower right part of the figure, with section B the lowermost cross section. Today’s earthquake may be on the AF2, the C-NFs (Colfiorito-Norcia fault systems), or MVf (Mt. Vettore fault). The AF2 fault is a proposed low angle detachment fault. These types of faults are controversial in that there are arguments about whether they are seismogenic or not. This year’s Pacific Cell Friends of the Pleistocene field trip in Panamint Valley presented research results that attempted to address this question. In Panamint Valley there are faults that have similar configurations as these faults in Italy.

  • Geological cross sections from seismic reflection profiles across the Gubbio, Gualdo T. and Colfiorito seismic areas (from Boncio et al., 1998; Boncio and Lavecchia, 2000b); epi- and hypocentral distribution of back-ground microseismicity recorded in the Umbria-Marche Apennines and rheological profiles (strength envelopes in critical stress difference, σ1–σ3) built for two different thermal contexts (50 and 40 mW/m2 surface heat flow, see Figure 3 for location); the depth of the brittle-plastic transition on rheological profiles is indicated by arrows; the used rheological parameters are indicated: crustal layering is from DSS data; A (empirical material constant), n (stress exponent) and E (activation energy) are creep parameters; ´ε = longitudinal strain rate (calculated by balancing of a regional geologic section; Figure 5 in Boncio et al., 2000); see text for further details.

  • This map shows a smaller scaled view (than the above figure) with focal mechanisms and cross sections (with structural interpretations). Hypocenters are also plotted on these cross sections. Today’s earthquakes are just south of cross section b. (earthquakes happened here in 1997)

  • Epicentres of the major seismic sequences of the last twenty years (Gubbio, 1984; Colfiorito, 1997; Norcia, 1979; Sangro, 1984) plus three small seismic sequences in the L’Aquila area (1992, 1994 and 1996); seismotectonic sections and rheological profiles built according to the local thermal context. The dashed line (sections ‘a’ and ‘b’) represents the AF low-angle extensional detachment; arrows in seismotectonic sections indicate the maximum depth-extent of the activated seismogenic faults as suggested by the best defined aftershock volume; rheological parameters as in Figure 7; in the southern Abruzzo area, creep strengths for geologic and geodetic longitudinal strain rates are compared (geologic strain rate calculated from data of Galadini and Galli, 2000; geodetic strain rate from D’Agostino et al., 2001); seismological data from Amato et al. (1998); Boncio (1998); Boncio et al. (2004); Cattaneo et al. (2000); De Luca et al. (2000); Deschamps et al. (1984); Ekstrom et al. (1998); Haessler et al. (1988); Harvard CMT database at www.seismology.harvard.edu.

  • Here is a poster that shows the seismic hazard for Europe (Giardini et al., 2013).

  • Here is a more detailed seismic hazard map for Italy (Stucchi et al., 2011). This shows the median PGA (Peak Ground Acceleration, units of g where g = 9.8 m/s2) that has a 10% probability of exceedance (PE) in the next 50 years. This model assumes a Vs30 greater than 800 m/s. Vs is the average seismic velocity in the upper 30 meters. Vs30 is a proxy used for global to regional estimates of seismic hazard.I include their original figure captions as blockquote.

  • The seismic hazard map showing the PGA distribution with 10% probability of exceedance in 50 years, computed on hard ground (VS30 > 800 m=s).

Earthquake Report: Mendocino fault!

Yesterday there was an earthquake along the eastern extension of the Mendocino fault system. This magnitude M = 4.1 earthquake (here is the USGS website for this earthquake) is a small magnitude, but it was widely felt. I was in Manila (CA) at the time, so I am surprised that I did not feel it. I was in the bath at the time, so maybe my shampooing was too energetic?

This earthquake appears to have occurred along the Mendocino fault, a right-lateral (dextral) transform plate boundary. This plate boundary connects the Gorda ridge and Juan de Fuca rise spreading centers with their counterparts in the Gulf of California, with the San Andreas strike-slip fault system. Transform plate boundaries are defined that they are strike-slip and that they connect spreading ridges. In this sense of the definition, the Mendocino fault and the San Andreas fault are part of the same system. This earthquake appears to have occurred in a region of the Mendocino fault that ruptured in 1994. See the figures from Rollins and Stein below.

Below is my interpretive map that shows the epicenter, along with the shaking intensity contours. These contours use the Modified Mercalli Intensity (MMI) scale. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.

I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely. Based on the moment tensor and my knowledge of the tectonics of this region, I interpret this earthquake to have had a right lateral strike slip motion along an east-west fault.

    I have placed several inset figures.

  • In the upper right corner is a map of the Cascadia subduction zone (CSZ) and regional tectonic plate boundary faults. This is modified from several sources (Chaytor et al., 2004; Nelson et al., 2004)
  • Below the CSZ map is an illustration modified from Plafker (1972). This figure shows how a subduction zone deforms between (interseismic) and during (coseismic) earthquakes. Today’s earthquake did not occur along the CSZ, so did not produce crustal deformation like this. However, it is useful to know this when studying the CSZ.
  • In the lower left corner is a map from Rollins and Stein (2010), showing their interpretations of different historic earthquakes in the region. This was published in response to the January 2010 Gorda plate earthquake. Today’s earthquake is near the 1983 earthquake.
  • Above the Rollins and Stein figure are two USGS plots. The upper plot shows a map displaying the “Did You Feel It?” felt reports. The color scale is the same as for the MMI legend in the upper left corner. The lower plot shows how the shaking intensity attenuates (diminishes) with distance from the epicenter.


  • Here is a map from Rollins and Stein, showing their interpretations of different historic earthquakes in the region. This was published in response to the January 2010 Gorda plate earthquake. The faults are from Chaytor et al. (2004). The 1980, 1992, 1994, 2005, and 2010 earthquakes are plotted and labeled. I did not mention the 2010 earthquake, but it most likely was just like 1980 and 2005, a left-lateral strike-slip earthquake on a northeast striking fault.

  • Here is a large scale map of the 1983 earthquake swarm. The mainshock epicenter is a black star and epicenters are denoted as white circles. Note how the aftershocks trend slightly southeast in this region. Today’s swarm does the same (and the moment tensor also shows a slightly southeast strike). Note how the interpreted fault dips slightly to the north, which is the result of north-south compression from the relative northward motion of the Pacific plate.

  • Here is a large scale map of the 1994 earthquake swarm. The mainshock epicenter is a black star and epicenters are denoted as white circles.

  • Here is a plot of focal mechanisms from the Dengler et al. (1995) paper in California Geology.

  • In this map below, I label a number of other significant earthquakes in this Mendocino triple junction region. Another historic right-lateral earthquake on the Mendocino fault system was in 1994. There was a series of earthquakes possibly along the easternmost section of the Mendocino fault system in late January 2015, here is my post about that earthquake series.

  • The Gorda and Juan de Fuca plates subduct beneath the North America plate to form the Cascadia subduction zone fault system. In 1992 there was a swarm of earthquakes with the magnitude Mw 7.2 Mainshock on 4/25. Initially this earthquake was interpreted to have been on the Cascadia subduction zone (CSZ). The moment tensor shows a compressional mechanism. However the two largest aftershocks on 4/26/1992 (Mw 6.5 and Mw 6.7), had strike-slip moment tensors. These two aftershocks align on what may be the eastern extension of the Mendocino fault.
  • There have been several series of intra-plate earthquakes in the Gorda plate. Two main shocks that I plot of this type of earthquake are the 1980 (Mw 7.2) and 2005 (Mw 7.2) earthquakes. I place orange lines approximately where the faults are that ruptured in 1980 and 2005. These are also plotted in the Rollins and Stein (2010) figure above. The Gorda plate is being deformed due to compression between the Pacific plate to the south and the Juan de Fuca plate to the north. Due to this north-south compression, the plate is deforming internally so that normal faults that formed at the spreading center (the Gorda Rise) are reactivated as left-lateral strike-slip faults. In 2014, there was another swarm of left-lateral earthquakes in the Gorda plate. I posted some material about the Gorda plate setting on this page.
  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. Many of the earthquakes people are familiar with in the Mendocino triple junction region are either compressional or strike slip. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

  • This figure shows what a transform plate boundary fault is. Looking down from outer space, the crust on either side of the fault moves side-by-side. When one is standing on the ground, on one side of the fault, looking across the fault as it moves… If the crust on the other side of the fault moves to the right, the fault is a “right lateral” strike slip fault. The Mendocino and San Andreas faults are right-lateral (dextral) strike-slip faults. I believe this is from Pearson Higher Ed.


    References:

  • Atwater, B.F., Musumi-Rokkaku, S., Satake, K., Tsuju, Y., Eueda, K., and Yamaguchi, D.K., 2005. The Orphan Tsunami of 1700—Japanese Clues to a Parent Earthquake in North America, USGS Professional Paper 1707, USGS, Reston, VA, 144 pp.
  • Chaytor, J.D., Goldfinger, C., Dziak, R.P., and Fox, C.G., 2004. Active deformation of the Gorda plate: Constraining deformation models with new geophysical data: Geology v. 32, p. 353-356.
  • Dengler, L.A., Moley, K.M., McPherson, R.C., Pasyanos, M., Dewey, J.W., and Murray, M., 1995. The September 1, 1994 Mendocino Fault Earthquake, California Geology, Marc/April 1995, p. 43-53.
  • Nelson, A.R., Asquith, A.C., and Grant, W.C., 2004. Great Earthquakes and Tsunamis of the Past 2000 Years at the Salmon River Estuary, Central Oregon Coast, USA: Bulletin of the Seismological Society of America, Vol. 94, No. 4, pp. 1276–1292
  • Rollins, J.C. and Stein, R.S., 2010. Coulomb stress interactions among M ≥ 5.9 earthquakes in the Gorda deformation zone and on the Mendocino Fault Zone, Cascadia subduction zone, and northern San Andreas Fault: Journal of Geophysical Research, v. 115, B12306, doi:10.1029/2009JB007117, 2010.