Category Archives: cascadia

Earthquake Report: Explorer plate

Last night I had completed preparing for class the next day. I was about to head to bed. I got an email from the Pacific Tsunami Warning Center notifying me that there was no risk of a tsunami due to an earthquake with a magnitude M 6.6. I noticed it was along the Sovanco fault, a transform fault (right-lateral strike-slip). Strike slip faults can produce tsunami, but they are smaller than tsunami generated along subduction zones. The recent M = 7.5 Donggala Earthquake in Sulawesi, Indonesia is an example of a tsunami generated in response to a strike-slip earthquake (tho coseismic landslides may be part of the story there too).

I thought I could put together a map in short time as I already had a knowledge base for this area (e.g. earthquake reports from 2017.01.07 and 2016.03.18). However, as I was creating base maps in Google Earth, before I completed making a set (the posters below each take 4 different basemaps displayed at different transparencies), there was the M 6.8 earthquake. Then there was the M 6.6 earthquake. I had to start all over. Twice. Heheh.

This region of the Pacific-North America plate boundary is at the northern end of the Cascadia subduction zone (CSZ). To the east, the Explorer and Juan de Fuca plates subduct beneath the North America plate to form the megathrust subduction zone fault capable of producing earthquakes in the magnitude M = 9 range. The last CSZ earthquake was in January of 1700, just almost 319 years ago.

The Juan de Fuca plate is created at an oceanic spreading center called the Juan de Fuca Ridge. This spreading ridge is offset by several transform (strike-slip) faults. At the southern terminus of the JDF Ridge is the Blanco fault, a transtensional transform fault connecting the JDF and Gorda ridges.

At the northern terminus of the JDF Ridge is the Sovanco transform fault that strikes to the northwest of the JDF Ridge. There are additional fracture zones parallel and south of the Sovanco fault, called the Heck, Heckle, and Springfield fracture zones.

The first earthquake (M = 6.6) appears to have slipped along the Sovanco fault as a right-lateral strike-slip earthquake. Then the M 6.8 earthquake happened and, given the uncertainty of the location for this event, occurred on a fault sub-parallel to the Sovanco fault. Then the M 6.5 earthquake hit, back on the Sovanco fault.

So, I would consider the M 6.6 to be a mainshock that triggered the M 6.8. The M 6.5 is an aftershock of the M 6.6.

Based upon our knowledge of how individual earthquakes can change the stress (or strain) in the surrounding earth, it is unlikely that this earthquake sequence changed the stress on the megathrust. Over time, hundreds of these earthquakes do affect the potential for earthquakes on the CSZ megathrust. But, individual earthquakes (or even a combination of these 3 earthquakes) do not change the chance that there will be an earthquake on the CSZ megathrust. The chance of an earthquake tomorrow is about the same as the chance of an earthquake today. Day to day the chances don’t change much. However, year to year, the chances of an earthquake get higher and higher. But of course, we cannot predict when an earthquake will happen.

So, if we live, work, or play in earthquake country, it is best to always be prepared for an earthquake, for tsunami, and for landslides.

Below is my interpretive poster for this earthquake


I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 6.5 in one version.

I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.

I include the earthquake mechanisms for 2 special earthquakes that happened in the past two decades along this plate boundary system. In 2001 the M 6.8 Nisqually earthquake struck the Puget Sound region of Washington causing extensive damage. This earthquake was an extensional earthquake in the downgoing JDF plate. The damage was extensive because the earthquake was close to an urban center, where there was lots of infrastructure to be damaged (the closer to an earthquake, the higher the shaking intensity).

In 2012 was a M = 7.8 earthquake along the northern extension of the CSZ. The northern part of the CSZ is a very interesting region, often called the Queen Charlotte triple junction. There are some differences than the Mendocino triple junction to the south, in northern California. There continues to be some debate about how the plate boundary faults are configured here. The Queen Charlotte is a right lateral strike slip fault that extends from south of Haida Gwaii (the large island northwest of Vancouver Island) up northwards, where it is called the Fairweather fault. There are several large strike-slip earthquakes on the Queen Charlotte/Fairweather fault system in the 20th century. However, the 2012 earthquake was a subduction zone fault, evidence that the CSZ megathrust (or some semblance of this subduction zone) extends beneath Haida Gwaii (so the CSZ and QCF appear to over lap).

  • 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 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 include the slab 2.0 contours plotted (Hayes, 2018), 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.li>

    Magnetic Anomalies

  • In the map below, I include a transparent overlay of the magnetic anomaly data from EMAG2 (Meyer et al., 2017). As oceanic crust is formed, it inherits the magnetic field at the time. At different points through time, the magnetic polarity (north vs. south) flips, the north pole becomes the south pole. These changes in polarity can be seen when measuring the magnetic field above oceanic plates. This is one of the fundamental evidences for plate spreading at oceanic spreading ridges (like the Gorda rise).
  • Regions with magnetic fields aligned like today’s magnetic polarity are colored red in the EMAG2 data, while reversed polarity regions are colored blue. Regions of intermediate magnetic field are colored light purple.
  • We can see the roughly northeast-southwest trends of these red and blue stripes in the JDF and Pacific plates. These lines are parallel to the ocean spreading ridges from where they were formed. The stripes disappear at the subduction zone because the oceanic crust with these anomalies is diving deep beneath the Sunda plate (part of Eurasia), so the magnetic anomalies from the overlying North America plate (and accretionary prism) mask the evidence for the JDF plate.

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • 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). I place a blue star in the general location of today’s seismicity.
  • In the upper left corner is a map showing the plate boundary faults associated with the northern CSZ and to the north (including the Queen Charlotte fault; Braunmiller and Nabalek, 2002). I place a red star in the general location of today’s seismicity. These earthquakes occurred in the region east of the Explorer rift. This region of the world still contains some major tectonic mysteries and this is quite exciting. This shows the Winona Block as a microplate between the Pacific and North America plates, north of the Explorer plate. The Winona Block is labeled “WIN BLOCK” on the map. Note that there are two spreading ridges on the western and central part of this block. It is possible that the Explorer ridge-rift system extends into the Winona Block to form a third spreading ridge in the Winona Block.
  • In the lower left corner is a map from Dziak (2006). Dziak (2006) used bathymetric and seismologic data to evaluate the faulting in the region and discussed how the Explorer plate is accommodating a reorganization of the plate boundary.
  • Here is the map with a month’s seismicity plotted.


  • Here is the map with a century’s seismicity plotted.


  • Here is a video showing the earthquake epicenters for the period of 1900-2017 for USGS earthquakes with magnitudes M ≥ 5.0. Here is a link to the embedded video below (2.5 MB mp4). Note how the earthquakes that happen between the northern terminus of the JDF Ridge and the southern terminus of the Queen Charlotte fault form a wide band (not a stepwise patter that might reflect steps in ridges and spreading centers). This pattern is key to unravelling the mysteries of the western Explorer plate.
    • Here is the map with the seismicity from 1900-2017 plotted. These are USGS earthquakes with magnitudes M ≥ 7.0 for this time period. I include the moment tensors from the 2012 and 2013 earthquakes (the only earthquakes for this time period that have USGS moment tensors). The 2012 earthquake generated a tsunami. I discuss the 2012 “Haida Gwaii” earthquake here.


Other Report Pages

Some Relevant Discussion and Figures

  • Here is the general tectonic map of the region (Braunmiller and Nabalek, 2002). Today’s earthquakes happened in a place that suggest the Explorer ridge extends further to the north into the Winona Block. Below I include the text from the original figure caption in blockquote.

  • Map of Explorer region and surroundings. Plate boundaries are based on Riddihough’s [1984] and Davis and Riddihough’s [1982] tectonic models. Solid lines are active plate boundaries (single lines are transform faults, double lines are spreading centers, barbed lines are subduction zones with barbs in downgoing plate direction). The wide double line outlines the width of the Sovanco fracture zone, and the dots sketch the Explorer-Winona boundary. Plate motion vectors (solid arrows) are from NUVEL-1A [DeMets et al., 1994] for Pacific-North America motion and from Wilson [1993] for Pacific-Juan de Fuca and Juan de Fuca-North America motion. Open arrows are Explorer relative plate motions averaged over last 1 Myr [Riddihough, 1984] (in text, we refer to these most recent magnetically determined plate motions as the ‘‘Riddihough model’’). Winona block motions (thin arrows), described only qualitatively by Davis and Riddihough [1982], are not to scale. Abbreviations are RDW for Revere-Dellwood- Wilson, Win for Winona, FZ for fault zone, I for island, S for seamount, Pen for peninsula.

  • Here is the larger scale figure that shows the details of the plate boundary in this region (Braunmiller and Nabalek, 2002). Below I include the text from the original figure caption in blockquote.

  • Close-up of the Pacific-Explorer boundary. Plotted are fault plane solutions (gray scheme as in Figure 3) and well-relocated earthquake epicenters. The SeaBeam data are from the RIDGE Multibeam Synthesis Project (http://imager.ldeo.columbia.edu) at the Lamont-Doherty Earth observatory. Epicenters labeled by solid triangles are pre-1964, historical earthquakes (see Appendix B). Solid lines mark plate boundaries inferred from bathymetry and side-scan data [Davis and Currie, 1993]; dashed were inactive. QCF is Queen Charlotte fault, TW are Tuzo Wilson seamounts, RDW is Revere-Dellwood-Wilson fault, DK are Dellwood Knolls, PRR is Paul Revere ridge, ER is Explorer Rift, ED is Explorer Deep, SERg is Southern Explorer ridge, ESM is Explorer seamount, SETB is Southwest Explorer Transform Boundary, SAT is Southwestern Assimilated Territory, ESDZ is Eastern Sovanco Deformation Zone, HSC is Heck seamount chain, WV is active west valley of Juan de Fuca ridge, MV is inactive middle valley.

  • This is the figure that shows an interpretation of how this plate boundary formed over the past 3 Ma (Braunmiller and Nabalek, 2002). Below I include the text from the original figure caption in blockquote.

  • Schematic plate tectonic reconstruction of Explorer region during the last 3 Myr. Note the transfer of crustal blocks (hatched) from the Explorer to the Pacific plate; horizontal hatch indicates transfer before 1.5 Ma and vertical hatch transfer since then. Active boundaries are shown in bold and inactive boundaries are thin dashes. Single lines are transform faults, double lines are spreading centers; barbed lines are subduction zones with barbs in downgoing plate direction. QCF is Queen Charlotte fault, TW are the Tuzo Wilson seamounts, RDW is Revere-Dellwood-Wilson fault, DK are the Dellwood Knolls, ED is Explorer Deep, ER is Explorer Rift, ERg is Explorer Ridge, ESM is Explorer Seamount, SOV is Sovanco fracture zone, ESDZ is Eastern Sovanco Deformation Zone, JRg is Juan de Fuca ridge, and NF is Nootka fault. The question mark indicates ambiguity whether spreading offshore Brooks peninsula ceased when the Dellwood Knolls became active (requiring only one independently moving plate) or if both spreading centers, for a short time span, where active simultaneously (requiring Winona block motion independent from Explorer plate during that time).

  • Below I include some inset maps from Audet et al. (2008 ) and Dziak (2006). Each of these authors have published papers about the Explorer plate. Dziak (2006) used bathymetric and seismologic data to evaluate the faulting in the region and discussed how the Explorer plate is accommodating a reorganization of the plate boundary. Audet et al. (2008 ) use terrestrial seismic data to evaluate the crust along northern Vancouver Island and present their tectonic map as part of this research (though they do not focus on the offshore part of the Explorer plate). I include these figures below along with their figure captions. Today’s earthquakes happened at the northwestern portion of these maps from Dziak (2006).
  • Dziak, 2006

  • This map shows the shape of the seafloor in this region and there is an inset map that shows the major fault systems here.

  • Bathymetric map of northern Juan de Fuca and Explorer Ridges. Map is composite of multibeam bathymetry and satellite altimetry (Sandwell and Smith, 1997). Principal structures are labeled: ERB—Explorer Ridge Basin, SSL—strike-slip lineation. Inset map shows conventional tectonic interpretation of region. Dashed box shows location of main figure. Solid lines are active plate boundaries, dashed line shows Winona-Explorer boundary, gray ovals represent seamount chains. Solid arrows show plate motion vectors from NUVEL-1A (DeMets et al., 1994) for Pacific–North America and from Wilson (1993) for Pacific–Juan de Fuca and Juan de Fuca–North America. Open arrows are Explorer relative motion averaged over past 1 m.y. (Riddihough, 1984). Abbreviations: RDW—Revere-Dellwood-Wilson,Win—Winona block, C.O.—Cobb offset, F.Z.—fracture zone. Endeavour segment is northernmost section of Juan de Fuca Ridge.

  • This map shows the line work Dziak (2006) used to delineate the structures shown in the bathymetric map.

  • Structural interpretation map of Explorer–Juan de Fuca plate region based on composite multibeam bathymetry and satellite altimetry data (Fig. 1). Heavy lines are structural (fault) lineations, gray circles and ovals indicate volcanic cones and seamounts, dashed lines are turbidite channels. Location of magnetic anomaly 2A is shown; boundaries are angled to show regional strike of anomaly pattern.

  • This map shows the seismicity patterns (this matches the patterns in the animation above).

  • Earthquake locations estimated using U.S. Navy hydrophone arrays that occurred between August 1991 and January 2002. Focal mechanisms are of large (Mw>4.5) earthquakes that occurred during same time period, taken from Pacific Geoscience Center, National Earthquake Information Center, and Harvard moment-tensor catalogs. Red mechanism shows location of 1992 Heck Seamount main shock.

  • Here Dziak (2006) shows how they interpret that this plate boundary is being reconfigured with time. Like the rest of the adjacent plate boundary (Queen Charlotte/Fairweather, Cascadia, San Andreas), there is an overall dextral (right-lateral) shear couple between the North America and Pacific plates. Some of the existing structures represent the orientation of faults from an earlier strain field. Eventually through going faults will align with the band of seismicity in the above map and above animation. At least, that is one hypothesis. Seems reasonable to me, given the very short record of earthquakes.

  • Tectonic model of Explorer plate boundaries. Evidence presented here is consistent with zone of shear extending through Explorer plate well south of Sovanco Fracture Zone (SFZ) to include Heck, Heckle, and Springfield seamounts, and possibly Cobb offset (gray polygon roughly outlines shear zone). Moreover, Pacific– Juan de Fuca–North American triple junction may be reorganizing southward to establish at Cobb offset. QCF—Queen Charlotte fault.

  • From Audet et al. (2008), here is another view of the fault system in this part of the plate boundary.

  • Identification of major tectonic features in western Canada. BP—Brooks Peninsula, BPfz—Brooks Peninsula fault zone, NI— Nootka Island, QCTJ—Queen Charlotte triple junction. Dotted lines delineate extinct boundaries or shear zones. Seismic stations are displayed as inverted black triangles. Station projections along line 1 and line 2 are plotted as thick white lines. White triangles represent Alert Bay volcanic field centers. Center of array locates town of Woss. Plates: N-A—North America; EXP—Explorer; JdF—Juan de Fuca; PAC—Pacific.

  • Speaking of the Queen Charlotte/Fairweather fault system, here is another map that shows the tectonics of this region. Hyndman (2015) shows the region where the 2012 Haida Gwaii earthquake ruptured. I include two more figures below. This figure Below I include the text from the original figure caption in blockquote.

  • The Queen Charlotte fault (QCF) zone, the islands of Haida Gwaii and adjacent area, and the locations of the 2012 Mw 7.8 (ellipse), 2013 Mw 7.5 (solid line), and 1949 Ms 8.1 (dashed) earthquakes. The along margin extent of the 1949 event is not well constrained.

  • This map shows the main and aftershocks from the 2012 Haida Gwaii earthquake sequence (Hyndman, 2015). This 2012 sequence is interesting because, prior to these earthquakes, it was unclear whether the fault along Haida Gwaii was a strike-slip or a thrust fault. For example, Riddihough (1984) suggests that there is no subduction going on along the Explorer plate at all. Turns out it is probably both. When this 2012 earthquake happened, I took a look at the bathymetry in Google Earth and noticed the Queen Charlotte Terrace, which looks suspiciously like an accretionary prism. This was convincing evidence for the thrust fault earthquakes. Below I include the text from the original figure caption in blockquote.

  • Aftershocks of the 2012 Mw 7.8 Haida Gwaii thrust 13 earthquake (after Cassidy et al., 2013). They approximately define the rupture area. The normal-faulting mechanisms for two of the larger aftershocks are also shown. Many of the aftershocks are within the incoming oceanic plate and within the overriding continental plate rather than on the thrust rupture plane.

  • This is a great version of this figure that shows how there are overlapping subduction (thrust) and transform (strike-slip) faults along the Haida Gwaii region (Hyndman, 2015). Below I include the text from the original figure caption in blockquote.

  • Model for the 2012 Mw 7.8 earthquake rupture and the partitioning of oblique convergence into margin parallel motion on the Queen Charlotte transcurrent fault and nearly orthogonal thrust convergence on the Haida Gwaii thrust fault.

  • Here is a figure that shows two ways of interpreting the Queen Charlotte triple junction region (Kreemer et al., 1998). Note the 1900-2017 seismicity map above, which supports the interpretation in the right panel (B). Something of trivial nature is that this article is from the pre-computer illustration era (see the squiggly hand drawn arrow in the right panel B). Below I include the text from the original figure caption in blockquote.

  • (A) Major tectonic features describing the micro-plate model for the Explorer region. The Explorer plate (EXP) is an independent plate and is in convergent motion towards the North American plate (NAM). V.I. D Vancouver Island; PAC D the Pacific plate; JdF D the Juan the Fuca plate. The accentuated zone between the Explorer and JdF ridges is the Sovanco transform zone and the two boundary lines do not indicate the presence of faults but define the boundaries of this zone of complex deformation. (B) The key features of the pseudo-plate model for the region are a major plate boundary transform fault zone between the North American and Pacific plates and the Nootka Transform, a left-lateral transform fault north of the Juan the Fuca plate.

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:
  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

  • Here is another way to look at these beach balls.
  • 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. The following three animations are from IRIS.
  • Strike Slip:

    Compressional:

    Extensional:

  • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.

  • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

  • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.

  • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.


    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.
  • Braunmiller, J. and Nabelek, J., 2002. Seismotectonics of the Explorer region in JGR, v. 107, NO. B10, 2208, doi:10.1029/2001JB000220, 2002
  • 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.
  • Audet, P., Bostock, M.G., Mercier, J.-P., and Cassidy, J.F., 2008., Morphology of the Explorer–Juan de Fuca slab edge in northern Cascadia: Imaging plate capture at a ridge-trench-transform triple junction in Geology, v. 36, p. 895-898.
  • Clarke, S. H., and Carver, G. C., 1992. Late Holocene Tectonics and Paleoseismicity, Southern Cascadia Subduction Zone, Science, vol. 255:188-192.
  • Dziak, R.P., 2006. Explorer deformation zone: Evidence of a large shear zone and reorganization of the Pacific–Juan de Fuca–North American triple junction in Geology, v. 34, p. 213-216.
  • Flück, P., Hyndman, R. D., Rogers, G. C., and Wang, K., 1997. Three-Dimensional Dislocation Model for Great Earthquakes of the Cascadia Subduction Zone, Journal of Geophysical Research, vol. 102: 20,539-20,550.
  • Heaton, f f., Kanamori, F. F., 1984. Seismic Potential Associated with Subduction in the Northwest United States, Bulletin of the Seismological Society of America, vol. 74: 933-941.
  • Hyndman, R. D., and Wang, K., 1995. The rupture zone of Cascadia great earthquakes from current deformation and the thermal regime, Journal of Geophysical Research, vol. 100: 22,133-22,154.
  • Keemer, C., Govers, R., Furlong, K.P., and Holt, W.E., 1998. Plate boundary deformation between the Pacific and North America in the Explorer region in Tectonophysics, v. 293, p. 225-238.
  • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. doi:10.7289/V5H70CVX
  • McPherson, R. M., 1989. Seismicity and Focal Mechanisms Near Cape Mendocino, Northern California: 1974-1984: M. S. thesis, Arcata, California, Humboldt State University, 75 p
  • 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
  • Plafker, G., 1972. Alaskan earthquake of 1964 and Chilean earthquake of 1960: Implications for arc tectonics in Journal of Geophysical Research, v. 77, p. 901-925.
  • Riddihough, R., 1984. Recent Movements of the Juan de Fuca Plate System in JGR, v. 89, no. B8, p. 6980-6994.
  • Wang, K., Wells, R., Mazzotti, S., Hyndman, R. D., and Sagiya, T., 2003. A revised dislocation model of interseismic deformation of the Cascadia subduction zone Journal of Geophysical Research, B, Solid Earth and Planets v. 108, no. 1.

Return to the Earthquake Reports page.

Earthquake Report: 1700 Cascadia subduction zone 317 year commemoration

Today (possibly tonight at about 9 PM) is the birthday of the last known Cascadia subduction zone (CSZ) earthquake. There is some evidence that there have been more recent CSZ earthquakes (e.g. late 19th century in southern OR / northern CA), but they were not near full margin ruptures (where the entire fault, or most of it, slipped during the earthquake).

I have been posting material about the CSZ for the past couple of years here and below are some prior Anniversary posts, as well as Earthquake Reports sorted according to their region along the CSZ. Below I present some of the material included in those prior reports (to help bring it all together), but I have prepared a new map for today’s report as well.


On this evening, 317 years ago, the Cascadia subduction zone fault ruptured as a margin wide earthquake. I here commemorate this birthday with some figures that are in two USGS open source professional papers. The Atwater et al. (2005) paper discusses how we came to the conclusion that this last full margin earthquake happened on January 26, 1700 at about 9 PM (there may have been other large magnitude earthquakes in Cascadia in the 19th century). The Goldfinger et al. (2012) paper discusses how we have concluded that the records from terrestrial paleoseismology are correlable and how we think that the margin may have ruptured in the past (rupture patch sizes and timing). The reference list is extensive and this is but a tiny snapshot of what we have learned about Cascadia subduction zone earthquakes. Brian Atwater and his colleagues have updated the Orphan Tsunami and produced a second edition available here for download and here for hard copy purchase (I have a hard copy).

Here is a map of the Cascadia subduction zone, modified from Nelson et al. (2006). The Juan de Fuca and Gorda plates subduct norteastwardly beneath the North America plate at rates ranging from 29- to 45-mm/yr. Sites where evidence of past earthquakes (paleoseismology) are denoted by white dots. Where there is also evidence for past CSZ tsunami, there are black dots. These paleoseismology sites are labeled (e.g. Humboldt Bay). Some submarine paleoseismology core sites are also shown as grey dots. The two main spreading ridges are not labeled, but the northern one is the Juan de Fuca ridge (where oceanic crust is formed for the Juan de Fuca plate) and the southern one is the Gorda rise (where the oceanic crust is formed for the Gorda plate).


Today I prepared this new map showing the results of shakemap scenario model prepared by the USGS. I prepared this map using data that can be downloaded from the USGS website here. Shakemaps show what we think might happen during an earthquake, specifically showing how strongly the ground might shake. There are different measures of this, which include Peak Ground Acceleration (PGA), Peak Ground Velocity (PGV), and Modified Mercalli Intensity (MMI). More background information about the shakemap program at the USGS can be found here. One thing that all of these measures share is that they show that there is a diminishing of ground shaking with distance from the earthquake. This means that the further from the earthquake, the less strongly the shaking will be felt. This can be seen on the maps below. The USGS prepares shakemaps for all earthquakes with sufficiently large magnitudes (i.e. we don’t need shakemaps for earthquakes of magnitude M = 1.5). An archive of these USGS shakemaps can be found here. All the scenario USGS shakemaps can be found here.

I chose to use the MMI representation of ground shaking because it is most easily comparable for people to understand. This is because MMI scale is designed based upon relations between ground shaking intensity and observations that people are able to make (e.g. how strongly they felt the earthquake, how much objects in their residences or places of business responded, how much buildings were damaged, etc.).

The MMI ground motion model 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. More on the MMI scale can be found here and here.


Here is the USGS version of this map. The outline of the fault that was used to generate the ground motions that these maps are based upon is outlined in black.


I prepared an end of the year summary for earthquakes along the CSZ. Below is my map from this Earthquake Report.

  • Here is the map where I show the epicenters as circles with colors designating the age. I also plot the USGS moment tensors for each earthquake, with arrows showing the sense of motion for each earthquake.
  • I placed a moment tensor / focal mechanism legend in the lower left corner of the map. 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.
  • In some cases, I am able to interpret the sense of motion for strike-slip earthquakes. In other cases, I do not know enough to be able to make this interpretation (so I plot both solutions).

    I include some inset figures in the poster.

  • In the upper left 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 lower right of the Cascadia map and cross section is a map showing the latest version of the Uniform California Earthquake Rupture Forecast (UCERF). Let it be known that this is not really a forecast, and this name was poorly chosen. People cannot forecast earthquakes. However, it is still useful. The faults are colored vs. their likelihood of rupturing. More can be found about UCERF here. Note that the San Andreas fault, and her two sister faults (Maacama and Bartlett Springs), are orange-red.
  • To the upper right of the Cascadia map and cross section is a map showing the shaking intensities based upon the USGS Shakemap model. Earthquake Scenarios describe the expected ground motions and effects of specific hypothetical large earthquakes. The color scale is the same as found on many of my #EarthquakeReport interpretive posters, the Modified Mercalli Intensity Scale (MMI). The latest version of this map is here.
  • In the upper right corner I include generalized fault map of northern California from Wallace (1990).
  • To the left of the Wallace (1990) 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 lower right 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.


This figure shows how a subduction zone deforms between (interseismic) and during (coseismic) earthquakes. We also can see how a subduction zone generates a tsunami. Atwater et al., 2005.


Here is a version of the CSZ cross section alone (Plafker, 1972).


Here is an animation produced by the folks at Cal Tech following the 2004 Sumatra-Andaman subduction zone earthquake. I have several posts about that earthquake here and here. One may learn more about this animation, as well as download this animation here.

Here is a graphic showing the sediment-stratigraphic evidence of earthquakes in Cascadia. Atwater et al., 2005. There are 3 panels on the left, showing times of (1) prior to earthquake, (2) several years following the earthquake, and (3) centuries after the earthquake. Before the earthquake, the ground is sufficiently above sea level that trees can grow without fear of being inundated with salt water. During the earthquake, the ground subsides (lowers) so that the area is now inundated during high tides. The salt water kills the trees and other plants. Tidal sediment (like mud) starts to be deposited above the pre-earthquake ground surface. This sediment has organisms within it that reflect the tidal environment. Eventually, the sediment builds up and the crust deforms interseismically until the ground surface is again above sea level. Now plants that can survive in this environment start growing again. There are stumps and tree snags that were rooted in the pre-earthquake soil that can be used to estimate the age of the earthquake using radiocarbon age determinations. The tree snags form “ghost forests.


Here is a photo of the ghost forest, created from coseismic subsidence during the Jan. 26, 1700 Cascadia subduction zone earthquake. Atwater et al., 2005.


Here is a photo I took in Alaska, where there was a subduction zone earthquake in 1964. These tree snags were living trees prior to the earthquake and remain to remind us of the earthquake hazards along subduction zones.


This shows how a tsunami deposit may be preserved in the sediment stratigraphy following a subduction zone earthquake, like in Cascadia. Atwater et al., 2005. If there is a source of sediment to be transported by a tsunami, it will come along for the ride and possibly be deposited upon the pre-earthquake ground surface. Following the earthquake, tidal sediment is deposited above the tsunami transported sediment. Sometimes plants that were growing prior to the earthquake get entombed within the tsunami deposit.


The NOAA/NWS/Pacific Tsunami Warning Center has updated their animation of the simulation of the 1700 “Orphan Tsunami.”

Source: Nathan C. Becker, Ph.D. nathan.becker at noaa.gov


Below are some links and embedded videos.

  • Here is the yt link for the embedded video below.
  • Here is the mp4 link for the embedded video below. (2160p 145 mb mp4)
  • Here is the mp4 link for the embedded video below. (1080p 145 mb mp4)
  • Here is the text associated with this animation:

    Just before midnight on January 27, 1700 a tsunami struck the coasts of Japan without warning since no one in Japan felt the earthquake that must have caused it. Nearly 300 years later scientists and historians in Japan and the United States solved the mystery of what caused this “orphan tsunami” through careful analysis of historical records in Japan as well as oral histories of Native Americans, sediment deposits, and ghost forests of drowned trees in the Pacific Northwest of North America, a region also known as Cascadia. They learned that this geologically active region, the Cascadia Subduction Zone, not only hosts erupting volcanoes but also produces megathrust earthquakes capable of generating devastating, ocean-crossing tsunamis. By comparing the tree rings of dead trees with those still living they could tell when the last of these great earthquakes struck the region. The trees all died in the winter of 1699-1700 when the coasts of northern California, Oregon, and Washington suddenly dropped 1-2 m (3-6 ft.), flooding them with seawater. That much motion over such a large area requires a very large earthquake to explain it—perhaps as large as 9.2 magnitude, comparable to the Great Alaska Earthquake of 1964. Such an earthquake would have ruptured the earth along the entire length of the 1000 km (600 mi) -long fault of the Cascadia Subduction Zone and severe shaking could have lasted for 5 minutes or longer. Its tsunami would cross the Pacific Ocean and reach Japan in about 9 hours, so the earthquake must have occurred around 9 o’clock at night in Cascadia on January 26, 1700 (05:00 January 27 UTC).

    The Pacific Tsunami Warning Center (PTWC) can create an animation of a historical tsunami like this one using the same too that they use for determining tsunami hazard in real time for any tsunami today: the Real-Time Forecasting of Tsunamis (RIFT) forecast model. The RIFT model takes earthquake information as input and calculates how the waves move through the world’s oceans, predicting their speed, wavelength, and amplitude. This animation shows these values through the simulated motion of the waves and as they race around the globe one can also see the distance between successive wave crests (wavelength) as well as their height (half-amplitude) indicated by their color. More importantly, the model also shows what happens when these tsunami waves strike land, the very information that PTWC needs to issue tsunami hazard guidance for impacted coastlines. From the beginning the animation shows all coastlines covered by colored points. These are initially a blue color like the undisturbed ocean to indicate normal sea level, but as the tsunami waves reach them they will change color to represent the height of the waves coming ashore, and often these values are higher than they were in the deeper waters offshore. The color scheme is based on PTWC’s warning criteria, with blue-to-green representing no hazard (less than 30 cm or ~1 ft.), yellow-to-orange indicating low hazard with a stay-off-the-beach recommendation (30 to 100 cm or ~1 to 3 ft.), light red-to-bright red indicating significant hazard requiring evacuation (1 to 3 m or ~3 to 10 ft.), and dark red indicating a severe hazard possibly requiring a second-tier evacuation (greater than 3 m or ~10 ft.).

    Toward the end of this simulated 24-hours of activity the wave animation will transition to the “energy map” of a mathematical surface representing the maximum rise in sea-level on the open ocean caused by the tsunami, a pattern that indicates that the kinetic energy of the tsunami was not distributed evenly across the oceans but instead forms a highly directional “beam” such that the tsunami was far more severe in the middle of the “beam” of energy than on its sides. This pattern also generally correlates to the coastal impacts; note how those coastlines directly in the “beam” have a much higher impact than those to either side of it.

    Offshore, Goldfinger and others (from the 1960’s into the 21st Century, see references in Goldfinger et al., 2012) collected cores in the deep sea. These cores contain submarine landslide deposits (called turbidites). These turbidites are thought to have been deposited as a result of strong ground shaking from large magnitude earthquakes. Goldfinger et al. (2012) compile their research in the USGS professional paper. This map shows where the cores are located.


    Here is an example of how these “seismoturbidites” have been correlated. The correlations are the basis for the interpretation that these submarine landslides were triggered by Cascadia subduction zone earthquakes. This correlation figure demonstrates how well these turbidites have been correlated. Goldfinger et al., 2012.


    This map shows the various possible prehistoric earthquake rupture regions (patches) for the past 10,000 years. Goldfinger et al., 2012. These rupture scenarios have been adopted by the USGS hazards team that determines the seismic hazards for the USA.


    Here is an update of this plot given new correlations from recent work (Goldfinger et al., 2016).


    Here is a plot showing the earthquakes in a linear timescale.


    I combined the plot above into another figure that includes all the recurrence intervals and segment lengths in a single figure. This is modified from Goldfinger et al. (2012).


Earthquake Report: 2016 Summary Cascadia

Here I summarize the seismicity for Cascadia in 2016. I limit this summary to earthquakes with magnitude greater than or equal to M 4.0. I reported on all but two of these earthquakes. I put this together a couple weeks ago, but wanted to wait to post until the new year (just in case that there was another earthquake to include).

I prepared a 2016 annual summary for Earth here.

    I include summaries of my earthquake reports in sorted into three categories. One may also search for earthquakes that may not have made it into these summary pages (use the search tool).

  • Magnitude
  • Region
  • Year

Earthquake Summary Poster (2016)

  • Here is the map where I show the epicenters as circles with colors designating the age. I also plot the USGS moment tensors for each earthquake, with arrows showing the sense of motion for each earthquake.
  • I placed a moment tensor / focal mechanism legend in the lower left corner of the map. 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.
  • In some cases, I am able to interpret the sense of motion for strike-slip earthquakes. In other cases, I do not know enough to be able to make this interpretation (so I plot both solutions).

    I include some inset figures in the poster.

  • In the upper left 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 lower right of the Cascadia map and cross section is a map showing the latest version of the Uniform California Earthquake Rupture Forecast (UCERF). Let it be known that this is not really a forecast, and this name was poorly chosen. People cannot forecast earthquakes. However, it is still useful. The faults are colored vs. their likelihood of rupturing. More can be found about UCERF here. Note that the San Andreas fault, and her two sister faults (Maacama and Bartlett Springs), are orange-red.
  • To the upper right of the Cascadia map and cross section is a map showing the shaking intensities based upon the USGS Shakemap model. Earthquake Scenarios describe the expected ground motions and effects of specific hypothetical large earthquakes. The color scale is the same as found on many of my #EarthquakeReport interpretive posters, the Modified Mercalli Intensity Scale (MMI). The latest version of this map is here.
  • In the upper right corner I include generalized fault map of northern California from Wallace (1990).
  • To the left of the Wallace (1990) 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 lower right 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.



    Cascadia subduction zone: General Overview

  • Cascadia’s 315th Anniversary 2015.01.26
  • Cascadia’s 316th Anniversary 2016.01.26
  • Earthquake Information about the CSZ 2015.10.08


The big player this year was an M 6.5 along the Mendocino fault on 2016.12.08. Here I present an inventory of 8 earthquakes with M ≥ 5.0. There are a few additional earthquakes with smaller magnitudes that are of particular interest.

Please visit the #EarthquakeReport pages for more information about the figures that I include in the Earthquake Report interpretive posters below.


    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: Petrolia (CA)

This morning there was a good shaker that was widely felt across the region. I did not feel it. I was probably driving at the time, or grading papers, which can have the same sense-deadening effect. Here is the USGS website for this M 4.3 earthquake. The earthquake occurred in an interesting part of the world, in the region of the Mendocino triple junction (MTJ) where the Cascadia subduction zone (CSZ), the San Andreas fault (SAF), and the Mendocino fault (MF) congregate. I was going to write the word “meet,” but I am not convinced that these plate boundary faults actually meet.

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. There are two focal mechanisms for this earthquake and I include both of them on the interpretive poster below. Based on the moment tensor and my knowledge of the tectonics of this region and using the v. 2 focal mechanism, I interpret this earthquake to have had a right lateral strike slip motion along an east-west fault. However, it is equally likely that this was a northeast striking thrust fault earthquake as suggested by the v. 1 focal mechanism.

    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 USGS figure that shows the evolution of these plate boundary systems.
  • Above the USGS figure is a map that shows more details about the evolution of the MTJ region for the last 12 Ma (million years). This is from a paper by McLaughlin et al. (2012).


  • 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.

  • 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: Mendocino fault!

Well, I felt that one. The shaking lasted about 5-7 seconds in Manila, CA (where I live). Here is the USGS website for this M = 5.6 earthquake. 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 lower left corner is a map of the Cascadia subduction zone and regional tectonic plate boundary faults. This is modified from several sources (Chaytor et al., 2004; Nelson et al., 2004)
  • Above the CSZ map is an illustration from Atwater et al. (2005). This figure shows how a subduction zone deforms between (interseismic) and during (coseismic) earthquakes. We also can see how a subduction zone generates a tsunami. Atwater et al., 2005.
  • In the upper right 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.
  • In the lower right corner is an image from an introductory textbook I found several years ago. I believe this is from Pearson Higher Ed.


For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:

Here is a map of the Cascadia subduction zone, modified from Nelson et al. (2004). The Juan de Fuca and Gorda plates subduct norteastwardly beneath the North America plate at rates ranging from 29- to 45-mm/yr. Sites where evidence of past earthquakes (paleoseismology) are denoted by white dots. Where there is also evidence for past CSZ tsunami, there are black dots. These paleoseismology sites are labeled (e.g. Humboldt Bay). Some submarine paleoseismology core sites are also shown as grey dots. The two main spreading ridges are not labeled, but the northern one is the Juan de Fuca ridge (where oceanic crust is formed for the Juan de Fuca plate) and the southern one is the Gorda rise (where the oceanic crust is formed for the Gorda plate).



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.


    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.

Earthquake Report: Bayside (northern California): Update #1

So, I put together another map with today’s earthquake in context with the historic seismicity and some other factors. Now the USGS magnitude is M = 4.7 and there is a moment tensor for this earthquake (that looks very similar to the focal mechanism, which is not always the case.). Here is my initial earthquake report here.

Below is a map showing the Northern California Earthquake Data Center (NCEDC) seismicity plotted. Today’s M 4.7 earthquake is plotted as a yellow star. This earthquake is similar to other earthquakes plotted in this region.

    Here are the data plotted on the map.

  • Northern California Earthquake Data Center Double Differenced earthquake epicenters, using the Northern California Earthquake Catalog (1984-2014). These epicenters are located by using the double difference method. Basically, earthquakes from a similar region are processed in such a way that, because they are in a similar region it is assumed that the seismic waves/rays travel through the same material (i.e. with the same seismic velocity). With this assumption, their positions can be better determined. These better positions are better relative to each other, but not in an absolute way. Here is an overview of the double difference method from Lamont Doherty. There is a software program that people use to process seismic data for this method (HypoDD).
  • These earthquake epicenters are plotted vs depth with color and magnitude with circle diameter.
  • I plot the depth to the slab in purple. These lines represent an estimate of the depth of the Cascadia subduction zone fault (McCrory et al., 2006).
  • I also plot the current USGS active fault and fold database. The offshore fault map is incomplete, but has been remapped by Dr. Chris Goldfinger and will be released by the USGS in the coming months. I cannot plot the new faults until it is officially released. These faults are in red and then I also plot the faults used by the USGS national seismic hazard map team in black.
  • On the eastern part of the map one may observe the non-volcanic tremor interpreted by the Pacific Northwest Seismic Network. These data can be downloaded by anyone. There is also a great online interface that lets one create animations. These tremor are basically small earthquakes that are not as resolvable on seismographs, so they cannot be located like regular earthquakes. Because of this, these tremor locations are only epicenters (no depth information).
  • The background data are topographic data and bathmetric data compiled by Dr. Jason Chaytor when he was working at the Active Tectonics Lab at Oregon State University.


    I also include some inset figures.

  • In the upper left corner I place a map of the Cascadia subduction zone. This map shows the Cascadia subduction zone, along with other major plate boundary faults in the region (Gorda Rise, Mendocino fault, San Andreas fault). The Juan de Fuca and Gorda plates subduct norteastwardly beneath the North America plate at rates ranging from 29- to 45-mm/yr. Sites where evidence of past earthquakes (paleoseismology) are denoted by white dots. Where there is also evidence for past CSZ tsunami, there are black dots. These paleoseismology sites are labeled (e.g. Humboldt Bay). Some submarine paleoseismology core sites are also shown as grey dots. The two main spreading ridges are not labeled, but the northern one is the Juan de Fuca ridge (where oceanic crust is formed for the Juan de Fuca plate) and the southern one is the Gorda rise (where the oceanic crust is formed for the Gorda plate). The map also shows the interpretation of faults that are part of the internally deforming Gorda plate. These faults within the Gorda plate are responsible for the large damaging earthquakes in 1980, 2005, and 2010 (others also in 2014, and 2015).
  • In the upper right corner I place a figure from Rollins and Stein (2010) that shows their interpretations for some earthquakes in this 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.
  • In the lower left corner I place a figure from Chaytor et al. (2004) that shows their interpretation of the tectonics of the Gorda plate based upon high resolution bathymetric data (showing the shape of the seafloor).
  • I also include the moment tensor and a moment tensor legend. 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.

Here is my initial earthquake report map as presented in the first earthquake report here.


Here is the seismic record from Jaime Wayne’s Netquake Seismometer. Here is a link to the netquake page. The seismometer is located near Orick.


In this map below (from a Mendocino fault earthquake on 2016/01/01), 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:

  • Chaytor, J.D., Goldfinger, C., Dziak, R.P., Fox, C.G., 2004. Active deformation of the Gorda plate: Constraining deformation models with new geophysical data. Geology 32, 353-356.
  • McCrory, P. A., Blair, J. L., Oppenheimer, D. H., and Walter, S. R., 2006. Depth to the Juan de Fuca slab beneath the Cascadia subduction margin; a 3-D model for sorting earthquakes U. S. Geological Survey
  • Nelson, A.R., Kelsey, H.M., Witter, R.C., 2006. Great earthquakes of variable magnitude at the Cascadia subduction zone. Quaternary Research 65, 354-365.
  • Rollins, J.C., 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 115, 19 pp.

Earthquake Report: Bayside (northern California)

Well, after installing a stilling basin for our new tide gage installation at Trinidad, CA, I was napping in my upstairs bedroom in Manila, CA. I was awakened by a short (2-3 second) short shaking earthquake. Turns out it was a M 4.8 earthquake east-southeast of my residence. Here is the USGS website for this earthquake. The depth is currently set at about 23 km, so it is near the megathrust, but is probably in the Gorda plate. There was an earthquake in this region last October, which had a different focal mechanism and was to the north a few kms.

#Update 1. I looked at the map at the bottom of this report. Today’s earthquake plots close to where the megathrust is estimated to be between 15 and 20 km (McCrory et al., 2006). So, I was correct that this earthquake is in the downgoing Gorda plate.

#Update 2. The map now has a moment tensor (blue) instead of a focal mechanism (orange). Now I am thinking that this could possibly be on an east-west fault since it is more aligned with the Mendocino fault. However, I am sticking with my initial interpretation as most of the earthquakes that we know about in the Gorda plate are northeast striking left-lateral strike slip faults.

    I put together this quick earthquake poster for this earthquake and have a few brief inset figures.

  • In the upper left corner I place a map of the Cascadia subduction zone. I discuss this figure below.
  • In the upper right corner I place three figures. These three maps each show a different measure of the ground shaking using the Modified Mercalli Intensity Scale. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here.
      From left to right:

    1. The “Did You Feel It?” map. This is a map that shows the ground shaking based upon peoples’ online reporting.
    2. The Shake Map. This map shows a computer modeled estimate of the ground shaking.
    3. The MMI contour map.
    4. In the lower right corner I show the attenuation with distance plot. This is a plot showing how the ground motions attenuate (lessen) with distance from the earthquake. The orange line is an estimate of the intensity of ground motions based on a numerical model. This numerical model is based on a regression of hundreds of earthquakes (distance vs. magnitude/intensity). These regressions form the basis for Ground Motion Prediction Equations (GMPEs). The blue dots are the actual observations made by real people (using the DYFI form that I posted above). These model based estimates of ground shaking intensity are used, especially for larger earthquakes, to determine what damage might be expected.
    5. I placed a moment tensor / focal mechanism legend in the upper right corner of the map. 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 suspect that this is probably a left lateral strike slip earthquake based upon the focal mechanism and our knowledge of the tectonics of the Gorda plate.


      Here is the record from the seismometer located across the hallway from the HSU Dept of Geology Office. The seismograph is located in Van Matre Hall. Photo Credit Dr. Mark Hemphill-Haley.


      Here I have a summary of earthquakes for this region (including an earthquake in the Explorer plate to the north).


      I present material about the Cascadia subduction zone for the Friends of the Arcata Marsh (FOAM) held on 7/22/16 at the Arcata Marsh Interpretive Center. This page has some supporting material from this presentation, including the digital presentation file. The material in this post is also found on this page here.


      Here is a map of the Cascadia subduction zone, modified from Nelson et al. (2004). The Juan de Fuca and Gorda plates subduct norteastwardly beneath the North America plate at rates ranging from 29- to 45-mm/yr. Sites where evidence of past earthquakes (paleoseismology) are denoted by white dots. Where there is also evidence for past CSZ tsunami, there are black dots. These paleoseismology sites are labeled (e.g. Humboldt Bay). Some submarine paleoseismology core sites are also shown as grey dots. The two main spreading ridges are not labeled, but the northern one is the Juan de Fuca ridge (where oceanic crust is formed for the Juan de Fuca plate) and the southern one is the Gorda rise (where the oceanic crust is formed for the Gorda plate).


      Here is a version of the CSZ cross section alone (Plafker, 1972). This shows two parts of the earthquake cycle: the interseismic part (between earthquakes) and the coseismic part (during earthquakes). Regions that experience uplift during the interseismic period tend to experience subsidence during the coseismic period.


      This figure shows how a subduction zone deforms between (interseismic) and during (coseismic) earthquakes. We also can see how a subduction zone generates a tsunami. Atwater et al., 2005.

      Here is an animation produced by the folks at Cal Tech following the 2004 Sumatra-Andaman subduction zone earthquake. I have several posts about that earthquake here and here. One may learn more about this animation, as well as download this animation here.

      This figure shows the regions that participate in this interseismic and coseismic deformation at Cascadia. Atwater et al., 2005. Black dots on the map show sites where evidence for coseismic subsidence has been found in coastal marshes, lakes, and estuaries.

      Here is a map showing a number of data sets. Seismicity is plotted versus depth (NCEDC). Tremor is plotted (Pacific Northwest Seismic Network). Vertical Deformation rates are plotted (unpublished). Slab depth contours (km) are plotted (McCrory et al., 2006). Fault locking zones are plotted (Wang et al., 2003; Burgette et al., 2009). Bob McPherson (Humboldt State University, Department of Geology) is currently working on a research paper where he will discuss how the seismicity reveals the location of the seismogenically locked fault zone.


      This map shows the various possible prehistoric earthquake rupture regions (patches) for the past 10,000 years. Goldfinger et al., 2012. These rupture scenarios have been adopted by the USGS hazards team that determines the seismic hazards for the USA.

        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.
      • Burgette, R. et al., 2009. Interseismic uplift rates for western Oregon and along-strike variation in locking on the Cascadia subduction zone in Journal of Geophysical Research, v. 114, B01408, doi:10.1029/2008JB005679
      • 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
      • Goldfinger, C., Nelson, C.H., Morey, A., Johnson, J.E., Gutierrez-Pastor, J., Eriksson, A.T., Karabanov, E., Patton, J., Gràcia, E., Enkin, R., Dallimore, A., Dunhill, G., and Vallier, T., 2012. Turbidite Event History: Methods and Implications for Holocene Paleoseismicity of the Cascadia Subduction Zone, USGS Professional Paper # 1661F. U.S. Geological Survey, Reston, VA, 184 pp.
      • McCrory, P. A., Blair, J. L., Oppenheimer, D. H., and Walter, S. R., 2006. Depth to the Juan de Fuca slab beneath the Cascadia subduction margin; a 3-D model for sorting earthquakes U. S. Geological Survey
      • Nelson, A.R., Kelsey, H.M., and Witter, R.C., 2006. Great earthquakes of variable magnitude at the Cascadia subduction zone: Quaternary Research, doi:10.1016/j.yqres.2006.02.009, p. 354-365.
      • Plafker, G., 1972. Alaskan earthquake of 1964 and Chilean earthquake of 1960: Implications for arc tectonics in Journal of Geophysical Research, v. 77, p. 901-925.
      • USGS Quaternary Fault Database: http://earthquake.usgs.gov/hazards/qfaults/
      • Wang, K., Wells, R., Mazzotti, S., Hyndman, R. D., and Sagiya, T., 2003. A revised dislocation model of interseismic deformation of the Cascadia subduction zone Journal of Geophysical Research, B, Solid Earth and Planets v. 108, no. 1.

Science on Tap Humboldt 2016.04.06

I present material about the Cascadia subduction zone at the Humboldt Science on Tap held on 4/6/16 at Blondies in Arcata, CA. This page has some supporting material from this presentation, including the digital presentation file.

    This is the digital presentation

  • Here is the digital presentation (100 MB pptx)

    This is a video of the presentation

  • Here is the digital file of the embedded video below (350 MB mp4)
  • Here is the yt link for the embedded video below

    Here are some sources of information about the Cascadia subduction zone

  • For the 315th anniversary of the most recent full rupture CSZ earthquake I put together a summary of our state of knowledge about the CSZ and that 1700 A.D. Jan. 26 earthquake. 2015.01.26
  • The USGS (and others) put together an educational video about the CSZ. I post this video and other supporting information online here: 2015.10.08

Here is an educational video about Cascadia subduction zone earthquakes and tsunamis.

Here is a tsunami hindcast for the Jan 26, 1700 Cascadia subduction zone megathrust earthquake that may have ruptured all the way south to Humboldt Bay. This is the download link for the embedded video below (35 MB mp4).

Here is an animation showing the Holocene record of earthquakes along the Cascadia subduction zone (Goldfinger et al., 2012).

Here is an animation of a Cascadia subduction zone earthquake generated tsunami. This is the download link for the embedded video below (15 MB mp4).

Cascadia subduction zone: M 9.0 tsunami simulation video

NOAA Center for Tsunami Research just released an animation that shows a numerical simulation of what a tsunami may appear like when the next Cascadia subduction zone earthquake occurs. I present a summary about the CSZ tectonics on a 316th year commemoration page here. I include a yt link and embedded video and an mp4 embedded video and download link.

Here is a screenshot from the video:


Earthquake Report: Gorda!

There was just a pair of earthquakes near the Gorda rise. There was first an earthquake of magnitude M = 5.0, followed by a M = 4.9. The 5.0 appears to be within the GP, but the 4.9 seems to be at the eastern boundary of the ridge. These two earthquakes do not pose a tsunami risk for northern California (or elsewhere). Nor do they likely have an affect upon the likelihood of a Cascadia subduction zone earthquake.

**UPDATE: 2016.01.29 20:00 PST
The USGS has prepared a moment tensor for the M 5.0 earthquake. I have updated the map. It is clear now that this is an extensional (normal) earthquake. I include an updated map below the original map.
/end update

    Here are the USGS web pages for these two earthquakes

  • 2016.01.30 M 5.0
  • 2016.01.30 M 4.9

Below is an interpretive map showing these two earthquakes as red circles, as well as other seismicity for the past week. I include an inset map of the Cascadia subduction zone (after Chaytor et al., 2004; Nelson et al., 2004), an inset map from Chaytor et al. (2004), and from Rollins and Stein (2010). I explain these inset maps below.

I include labels for the plate boundary fault systems in the region and include generic focal mechanisms for these fault systems. There is a legend that shows how moment tensors can be interpreted. Moment tensors are graphical solutions of seismic data that show two possible fault plane solutions. One must use local tectonics, along with other data, to be able to interpret which of the two possible solutions is correct. The legend shows how these two solutions are oriented for each example (Normal/Extensional, Thrust/Compressional, and Strike-Slip/Shear). There is more about moment tensors and focal mechanisms at the USGS.

Based on the locations of these earthquakes, along with our knowledge of the local tectonics, I interpret the M 5.0 to possibly be a northeast striking left-lateral strike-slip fault earthquake. The M 4.9 could either be like that, or be an extensional earthquake (like the 2014.03.13 earthquake listed below). I summarized the regional tectonics of the Cascadia subduction zone recently here. The M 5.0 earthquake is similar to the earthquake “P” on the Rollins and Stein (2010) map.


**UPDATE: 2016.01.29 20:00 PST
Here is the updated map. Note the extensional moment tensor. This is aligned nicely with the ridges from the Gorda rise (which are visible in the large scale map at the bottom of this page.



/end update

There was an earthquake along the Gorda rise in 2014, much to the north of these two earthquakes. Here is my earthquake report for that 2014.03.13 M 5.2 earthquake.

Here is a map of the Cascadia subduction zone, modified from Nelson et al. (2006). The Juan de Fuca and Gorda plates subduct norteastwardly beneath the North America plate at rates ranging from 29- to 45-mm/yr. Sites where evidence of past earthquakes (paleoseismology) are denoted by white dots. Where there is also evidence for past CSZ tsunami, there are black dots. These paleoseismology sites are labeled (e.g. Humboldt Bay). Some submarine paleoseismology core sites are also shown as grey dots. The two main spreading ridges are not labeled, but the northern one is the Juan de Fuca ridge (where oceanic crust is formed for the Juan de Fuca plate) and the southern one is the Gorda rise (where the oceanic crust is formed for the Gorda plate).


Here is a map from Chaytor et al. (2004) that shows some details of the faulting in the region. The moment tensor (at the moment i write this) shows a north-south striking fault with a reverse or thrust faulting mechanism. While this region of faulting is dominated by strike slip faults (and most all prior earthquake moment tensors showed strike slip earthquakes), when strike slip faults bend, they can create compression (transpression) and extension (transtension). This transpressive or transtentional deformation may produce thrust/reverse earthquakes or normal fault earthquakes, respectively. The transverse ranges north of Los Angeles are an example of uplift/transpression due to the bend in the San Andreas fault in that region.


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 Januray 2010 Gorda plate earthquake. The faults are from Chaytor et al. (2004).


Here is a large scale map of the region for these two earthquakes. This is taken from Google Earth. The two largest orange circles (color represents depth < 33 km) are the M 5.0 and M 4.9, with diameter representing magnitude. The M 5.0 is clearly in the region where there are north-northeast striking faults.

I have Earthquake Reports for other CSZ related earthquakes here:

References:

  • Chaytor, J.D., Goldfinger, C., Dziak, R.P., Fox, C.G., 2004. Active deformation of the Gorda plate: Constraining deformation models with new geophysical data. Geology 32, 353-356.
  • Nelson, A.R., Kelsey, H.M., Witter, R.C., 2006. Great earthquakes of variable magnitude at the Cascadia subduction zone. Quaternary Research 65, 354-365.
  • Rollins, J.C., 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 115, 19 pp.