Category Archives: geology

Earthquake in the Fiji Region!

There was a large earthquake in the Fiji region yesterday (Saturday my time). The epicenter is far from the major convergent plate boundary. Here is the USGS page for this earthquake.

Here is a map at the global scale. The epicenter is marked by a red circle, just South of Samoa and East of Fiji. This earthquake is related to the subduction zone associated with the Tonga Trench. The moment tensor shows an east-northeast striking compressional solution, due to principal axis compression in the n-nw/s-se direction. Prior to looking at the moment tensor, I was expecting to see an extensional solution at this depth. The cross section of seismicity is sourced from earthquakes designated by the purple line.

Here is a regional scale map showing the plate configuration.

This is a local scaled map showing the complexity of the spreading ridges and transform faults to the east of this earthquake swarm. The Modified Mercali Intensity Contour 3.5 is plotted as a light blue circle. Epicenters are plotted by color in relation to depth.

Here is the regional map showing the slab contours. The depth of this earthquake (434 km) is close to, but above, the slab depth (500 km). If one looks at the cross section of historic seismicity, it appears that the slab is possibly bending upwards. Perhaps there is some compression in the upper plate here, causing the compressional moment tensor.

Here is another view of the slab, generated using P-wave tomography. Doug Weins discusses his work in this region. “Red and blue colors denote slow and fast velocities, respectively, and the velocity perturbation scale is shown at the bottom.”

Interestingly, deep focus earthquakes take up ~66% of the deep earthquakes globally. From this paper, we can see that the slab contour may change strike in the region of yesterday’s earthquake.

Richards et al., 2011 also show bends in the downgoing slab. There is some controversy about the configuration of the slab in this region. They show a detached slab just above the main port (more Star Wars), above the main slab.

The New Hebrides subduction zone dips to the east and turns into a transform fault just west of yesterdays earthquake. This map shows the profile for the above cross section from Richards et al. (2102)

This figure shows Richards et al. Figure 4, that displays their interpretation of how the plates came to be configured here. The Australia plate detached and collided with the Pacific slab about 4 million years ago.

Here is the USGS Open File poster for the region (Benz et al., 2010). Hypocenters are plotted as cross sections to show the geometry of the subducting slabs.


As one might expect, an earthquake at this depth, given this magnitude, would not generate strong ground motions at the surface. The pager, an estimate of human and infrastructural losses, reflects this low likelihood of damage.

Here is a primer about Focal Mechanisms from the USGS.

    References:

  • Hayes, G. P., D. J. Wald, and R. L. Johnson (2012), Slab1.0: A three-dimensional model of global subduction zone geometries, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524.
  • Benz, H.M., Herman, Matthew, Tarr, A.C., Furlong, K.P., Hayes, G.P., Villaseñor, Antonio, Dart, R.L., and Rhea, Susan, 2011, Seismicity of the Earth 1900–2010 eastern margin of the Australia plate: U.S. Geological Survey Open-File Report 2010–1083-I, scale 1:8,000,000.
  • Richards, S., Holm., R., Barber, G., 2011. When slabs collide: A tectonic assessment of deep earthquakes in the Tonga-Vanuatu region, Geology, v. 39, pp. 787-790.
  • Yu, W. and Wen, L., 2012. Deep-Focus Repeating Earthquakes in the Tonga–Fiji Subduction Zone, BSSA, v. 102, no. 4, pp. 1829-1849

Earthquake offshore of El Salvador!

We just had a large magnitude earthquake offshore of El Salvador. Here is the USGS web page for the earthquake. Based on the magnitude and depth, it is not expected to have triggered a tsunami that would reach the coast of CA, OR, nor WA.

Here is a map that shows the region. The epicenter is plotted in green (the initial depth was set at 70 km, so it plotted in green. the depth was updated to 40 km, so will plot in yellow in other maps.).

This map shows the historic earthquakes as grey circles. These earthquakes are largely associated with the subduction zone here, where the Cocos plate is subducting northeastwardly beneath the Carribean plate to form the Middle America trench.

This map shows the modeled intensity using the Modified Mercali Intensity Scale (MMI, a measure of the ground shaking). The MMI has a scale of I – XII.

This is the same map, zoomed in for a larger scale view of the area.

Here is a view of how the intensity attenuates (diminishes as the energy is absorbed by the earth’s crust) with distance from the earthquake. The blue dots are real data, with medians plotted as brown dots. One can see that there is considerable variation in the intensity for individual measurements. This can be due to a variety of reasons (variation in the earth’s crust, site conditions, etc.). The green and orange lines are model estimates of intensity attenuation given two models from Atkinson and Wald, 2007 (using Ground Motion Prediction Equations, GMPEs, based on regression through data from earthquakes in California and Central-Eastern US, respectively). The California regression seems to fit the data better than the Central/Eastern US model.

This map has the intensity plotted as contours, which allows one to see how the intensity varies across the region in relation to the geography and place names.

Based on the location, depth, and moment tensor shown below, I interpret this to be a normal earthquake. This earthquake is the result of extension in the downgoing plate, possibly from the bending (i.e. a bending moment normal fault).

This map shows the depth contours to the slab (the top of the downgoing Cocos plate). These slab contours were developed by Gavin Hayes who works at the USGS. Here is a web page that has links to the other slabs Hayes and his colleagues have developed. They are largely based on their interpretations of historic seismicity.

This is the pager (version 1) that shows modeled estimates of human and infrastructural damage from this earthquake. These pager estimates are revised as more seismological information is analyzed. The loss estimates assist aid agencies make plans on how much assustance they may need to send to the region (in the form of food, medical, money, etc.).

Here is the Did You Feel It map, showing observations based upon the DYFI survey tool. The results are plotted using the same color range for the MMI scale.

Here is the USGS poster for the tectonics of the region:

Here is a profile showing Ranero et al. (2003) interpretation of the structure of the subduction zone here. Look at the upper panel and note how the top of the slab bends downward. This causes extension that may be responsible for today’s earthquake. This is figure 8.14 from Mann et al., 2007.

Here is a map that shows preliminary tsunami wave height estimates from Here.

Here is a table of estimates for the tsunami wave height.

Here is a primer about Focal Mechanisms from the USGS.

References:
Hayes, G. P., D. J. Wald, and R. L. Johnson (2012), Slab1.0: A three-dimensional model of global subduction zone geometries, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524.
Mann, Paul, Robert D. Rogers, and Lisa Gahagan. “Overview of plate tectonic history and its unresolved tectonic problems.” Central America: Geology, resources and hazards 1 (2007): 201-237.
Ranero, C., Morgan, P., McIntosh, K. & Reichert, C.: Bending-related faulting and mantle serpentinization at the Middle America trench. Nature 425 (2003), pp.367–373.

Earthquake in Guam!

Last night (Pacific Time) there was a M 6.7 earthquake in the western Pacific near Guam. Here is the USGS page for the earthquake. The depth is about 137 km, so it is unlikely to generate a tsunami. Also, the depth reduces the likelihood of damage from ground shaking.

Here is a map showing the global tectonics. The epicenter is west of the Marianas Trench, which is formed by subduction of the Pacific plate beneath the Phillipine plate. Note there is a spreading ridge east of the epicenter.

Here is a map showing the global tectonics along with the historic seismicity.

Here is a regional map showing the epicenter as it relates to Guam (Hagatna) and Saipan.

Here is a regional map showing the historic earthquakes.

This map shows the modeled Modified Mercalli Shaking Intensity (contours), along with the Did You Feel It results (colors on the islands). The MMI contours are generated by a computer model. The DYFI data are the results of observations made by real people.

This is the Moment Tensor from the USGS website. This shows that the earthquake is a “normal” earthquake, caused by extension in the Earth’s crust. Based on the location of this earthquake, it is probably located in the downgoing Pacific plate.

Here is a primer about Focal Mechanisms from the USGS.

This is the Pager Page, which shows the probability of damage to people and their infrastructure. These are generated by a computer model and are updated regularly following an earthquake as more seismological information is used to refine the earthquake shaking/intensity models. This one is version 2 and probably wont be updated.

Here is the regional tectonic map produced by the USGS. Historic earthquakes, tectonic plate boundaries, and downgoing plate slab contours are plotted.

Here is a map that shows the bathymetry (the shape and depths of the seafloor) in this region, as determined by Smith and Sandwell at Scripps Institute of Oceanography. Depth contours are 500 meters.

Here is a regional map showing the depth contours of the dowgoing Pacific plate, as generated by Gavin Hayes from the USGS.

References:
Hayes, G. P., D. J. Wald, and R. L. Johnson (2012), Slab1.0: A three-dimensional model of global subduction zone geometries, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524.
Smith, W. H. F., and D. T. Sandwell, Global seafloor topography from satellite altimetry and ship depth soundings, Science, v. 277, p. 1957-1962, 26 Sept., 1997.

Napa Earthquake Update

There have been a swarm of geologists running around Napa taking photos and documenting evidence from the earthquake swarm on Sunday. Here is my first post about this earthquake swarm, with the M 6.0 earthquake being the largest magnitude earthquake.

Mike Oskin and his students have been diligent in their reporting. Most of their observations have been posted to Oskin’s twitter account. Here is a google earth kmz file that Oskin posted to his twitter feed. They followed the surface trace of the fault, which is represented by the red line. There are notes and photos that are documented by yellow push pin icons. Here is a map with the epicenters of the swarm and the Oskin kmz observations overlain. Download the kmz to see the photos.

Here is the channel 2 photo database of damage.

Here I post a couple of the photos taken by Alex Moran. Many of them have Chad Trexler in them for scale. These are both posted on Oskin’s kmz file.

White Cliff Drive: Offset curb with tape measure for scale. Photo by Alex Morelan.

23 cm right lateral along Oak Rock Dr. Photo by Alex Morelan.

Here is an updated map showing all the aftershocks as of today at noon local time.

Here is a photo (Lori Dengler) of the seismograph as recorded in Van Matre Hall at Humboldt State University, Dept. of Geology.

This is a map from the California Geological Survey that shows the faults in the region.

Here is a kmz file of a map produced by Wesling and Hanson as part of their USGS NEHRP final report. Here is a map showing their map underlain by the USGS faults and the epicenter plots.

Here is a photo from the AP:

Here is a photo from SFGate:

Napa Earthquake!

Social media was abuzz today about the M 6.0 earthquake in Napa. Here is the USGS webpage for the M 6.1 earthquake. This earthquake, and the aftershocks, have epicenters that lie between the Rogers Creek and Green Valley fault systems. These two major fault systems are inboard/east of the San Andreas fault, each accommodating a portion of the Pacific-North America plate boundary relative motion rate. Today’s earthquake swarm appears to plot near the West Napa fault (WNF) and east of the Carneros fault (CF). The WNF is not mapped as far south as this earthquake swarm, but the fault system likely extends south, as possibly evidenced by today’s swarm.

Here is a map showing the faults in the San Francisco bay area. Epicenters from today’s swarm appear in the center of the map as orange dots. The largest dot is the M 6.0 epicenter. There is also a swarm of earthquakes to the northwest, just south of Clear Lake. These are related to the geothermal activity in the Geysers region. Faults are plotted with color representing the relative age of the most recent movement. Younger to older = red, orange, yellow, blue.

The faults in this region are mostly related to the strike-slip (transform) relative motion along the San Andreas fault system. Faults are sub-parallel to the SAF system and have a similar sense of motion as the SAF (right lateral, or dextral). Here is a moment tensor that shows two possible fault plane interpretations (either northwest striking right lateral or northeast striking left lateral). Given that the SAF is nw striking right lateral, the most reasonable interpretation of this moment tensor is nw striking right lateral.

This map shows the results of the USGS “did you feel it?” survey. I hope everyone who felt this earthquake filled out a form. Please do so here if you have not yet done so. The colors represent the Modified Mercalli Intensity Scale (MMI scale; a measure of the shaking intensity based upon observations people make about how strongly the earthquake shook).

This is a map showing an estimate of ground shaking (MMI scale) based upon computer modeling.

This is a map also showing the simulated intensity as MMI contours.

Here is a map showing the regional faulting in the area, along with the Sonoma Volcanics Wagner et al., 2011. These volcanic units represent the passage of the Mendocino triple junction through the bay area (~8-14 millions of years ago). The Carneros and West Napa faults are labeled CF and WNF respectively. There are plate motion arrows along the major faults reminding us that the relative motion of these faults is right lateral.

Here is a map that shows the swarm epicenters as they relate to the local geography. Napa Valley is to the north. The Carneros fault is not plotted since it is not part of the USGS fault and fold database (Dr. Rich Koehler reminded me that the USGS database is a Quaternary database, so it is probably because the CF has not displaced Quaternary age geologic units). The WNF fault system is mapped as yellow and blue lines along the western boundary of Napa Valley.

Here is another map from the Wagner et al., 2011 paper. This map shows how the CF and WNF systems relate to each other.Just south of the word “Napa,” the WNF lines stop at the edge of the Tertiary volcanics that are mapped in pink. South of that, the geology is mapped as Cenozoic sedimentary deposits. It is possible that the WNF continues south, but has not been found to displace the geologic units in that area.

Here is the USGS PAGER page that shows an automated estimate of damage to people and infrastructure. This is useful for govt. agencies who may be responsible to plan evacuations and assistance to internally displaced people.

Here is a map that shows the historic ruptures along the SAF and inboard fault systems (Smith and Sandwell, 2006). The GVF shows a rupture in 1858 and 1864. The Rogers Creek/Maacama fault sustem shows a rupture in 1898. The Hayward fault ruptured in 1868.

This is a map that shows the earthquake probabilities for the faults in the SF Bay area. This is part of the 2008 Uniform California Earthquake Rupture Forecast (UCERF). Compiled by USGS, Southern California Earthquake Center (SCEC), and the California Geological Survey (CGS), with support from the California Earthquake Authority. Here is a 10 MB high resolution version of the map. Here is a short document that discusses the Hayward fault earthquake of 1868 and what we might expect on the HF in the future.

Here is a photo database of damage from this earthquake.

Here is a summary of observations made by the media.

Here is a photo gallery documenting damage from the earthquake.

Smith, B. R., and D. T. Sandwell (2006), A model of the earthquake cycle along the San Andreas Fault System for the past
1000 years, J. Geophys. Res., 111, B01405, doi:10.1029/2005JB003703.

Wagner, D. L.; Saucedo, G. J.; Clahan, K. B.; Fleck, R. J.; Langenheim, V. E.; McLaughlin, R. J.; Sarna-Wojcicki, A. M.; Allen, J. R.; Deino, A. L., 2011. Geology, geochronology, and paleogeography of the southern Sonoma volcanic field and adjacent areas, northern San Francisco Bay region, California, Geosphere, 7: 658 – 683

1964 Alaska Earthquake

I just got back from a great Seismological Society Meeting in Anchorage, Alaska. The meeting was held there in part to celebrate the 50th anniversary of the 1964 Great Alaska Earthquake. I will post more online in the coming week. I present here a couple maps below, as well as a link to the USGS open file report that shows before and after photos in Anchorage.

Here is a map that shows the regional extent of the 1964 earthquake. Regions of coseismic uplift/subsidence are delineated by blue/red polygons.

Here is a map that shows the extent of historic earthquakes in southern Alaska. This is from a USGS open file report.

Here is a link to the USGS open file report. Click on the image to get the 12 MB pdf.

Rolf Aalto: Large Natural Floodplains: Overlooked Links in the Global Source-to-sink Continuum

Rolf Aalto is giving a presentation at the HSU Geology Coloquium on Tuesday 4/15/2014 in Van Matre Hall, room 109, at 5 PM.

This is Dr. Aalto’s website at Exeter.


This is what Dr. Aalto is interested in:
Rolf researches rivers and erosion across 6 continents, including: South America (Beni, Mamore, Orinoco & Ucayali Rivers), North America (Sacramento-CA, Feather-CA & Salmon-ID Rivers & Rio Grande-NM), Australasia (Strickland & Fly Rivers PNG), Europe (Danube River Romania), and Asia (Mekong River, Cambodia & exploratory sites in China). He leads the Exeter Radiometry Lab, which features world-class analytical facilities for tracing and dating particle movement throughout a wide range of fluvial dispersal systems. He develops novel field surveying/sampling and laboratory techniques to quantify processes across a range of fluvial environments as well as working to enhance remotely sensed data (SRTM and Aster). His currently funded research projects include a Critical Zone Observatory (CRB-CZO), a NSF-Margins project studying fluvial and biogeochemical processes in Papua New Guinea, a NSF project studying the evolution of the Sacramento River system, a NERC project investigating the evolution of the Beni River System in Bolivia, and a NERC project studying the evolution of the Mekong River.

Rolf’s teaching focuses on the application of GIS, modelling, and laboratory methods to solving problems within River Basin Science. Students rate his modules highly, especially for ‘development‘, and graduates report employability exceeding 90%. He is delighted to lead 2nd year field trips to California and Washington State (USA), New Zealand, and South Africa.

Here is some background information from his website:
Rolf obtained his undergraduate degrees from UC Berkeley, where he was inspired to study fluvial processes in a module taught by Prof. William Dietrich (at that time working in Papua New Guinea), completing an honors thesis studying floodplain sedimentation in a specially designed flume. He completed a MSc degree at the University of Washington (Seattle), working with Prof. Thomas Dunne as a Research Assistant to calculate sediment fluxes along the Amazon River and writing a thesis on ”Discordance between suspended sediment diffusion theory and observed sediment concentration profiles in rivers.” While developing ideas for his PhD and seeking funding to pursue his research ambitions on tropical rivers, Rolf was awarded a NASA Earth System Science Fellowship. A scouting campaign to collect samples along rivers in Bolivia laid the framework for Rolf to write a major NSF research grant, culminating in his dissertation “Geomorphic Form and Process of Sediment Flux within an Active Orogen.”

Rolf next worked as a Post Doc at UC Berkeley, returning full circle to study fluvial processes in PNG. He was hired as Research Faculty at Washington with Prof. David Montgomery (a MacArthur ‘Genius‘), funded by a NASA Post Doctoral grant to investigate the SRTM dataset and a CALFED project on the Sacramento River. He was then promoted to Assistant Professor (he remains an Affiliate Associate Professor) at UW where he developed a laboratory and graduate program, wrote four successful NSF research proposals, and initiated new projects in Amazonia, Romania, California, PNG, Venezuela, Greenland, and SE Asia. Rolf has also consulted professionally since 1995 on a range of topics related to geomorphic hazards and river restoration (as a Licensed Engineering Geologist). To date he has written/co-written successful research proposals worth >$10.0 million USD and led/co-led the execution of this research by diverse international & interdisciplinary teams working across a wide range of logistically challenging environments throughout the world.

In 2007 Rolf joined Exeter’s internationally acclaimed River Basin Science group to further develop world-class analytical facilities for tracing and dating sediment movement throughout a wide range of fluvial dispersal systems. He collaborates extensively with Prof. Nicholas (physics-based models of large river systems), Prof. Quine (erosion and biogeochemical evolution of soils), and Dr. Aragao (vegetation and fire in the tropics).