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.
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
Here we find an earthquake swarm in western Iran. The convergence between the Afica and Indo-Australia plates with the Eurasia plates is evidenced by the subduction zone along Sumatra and Java, the uplift of the Himalayas, the fold and thrust belt in Iran (where this swarm is located), and subduction in the Mediterranean (and uplift of the Alps). Here is the USGS page for the largest magnitude earthquake of this swarm.
Here is the Mww moment tensor from the largest magnitude (M 6.2), supporting the interpretation of a northwest striking compressional earthquake. This is consistent with the northwest trending fold and thrust belt shown in the aerial imagery below.
This is a global scale map showing the plate boundaries in red and the epicenters in orange:
This is a larger scale map showing the regional fold and thrust belt in western Iran.
The USGS “pager” is an estimate of damage to people and their infrastructure. This estimate is generated by overlaying population information and building inventories with an estimate of ground shaking generated by a ground motion prediction model. This is only an estimate that is useful for emergency managers to use to determine the scope of any aid that might be offered or sent to the region.
Here is the USGS map of the plate boundaries and historic seismicity.
This image is a primer for us who want to learn more about focal mechanisms and moment tensors. Moment tensors and focal mechanisms are calculated in different ways, but their graphical depiction is largely the same. We may use this graphic to help us interpret the moment tensor of the M 6.2 earthquake shown above>
There have been a nice suite of aftershocks to help us interpret these data. There were several aftershocks less than or equal to M = 6. These occurred within hours of the mainshock. They also occurred northwest of the mainshock. This suggested that the fault that ruptured strikes west-northwest, based on the moment tensors. I mentioned this spatial relation on the first page about this earthquake swarm here. There is more information about the mainshock and tsunami measurements on that first page.
Here is the mainshock moment tensor:
We now have some more aftershocks, including a slightly larger magnitude earthquake M = 6.3 further northwest from the main swarm. This earthquake occurred ~7 hrs after the main shock. At first I thought I might consider this a triggered earthquake (not an aftershock). Following are a couple figures I put together to explain how I interpret this swarm.
Here is a map that shows the epicenters, the magnitudes, and the time the earthquakes were recorded. The USGS moment tensor for the M = 7.9 earthquake is in green. The USGS moment tensor for the M = 6.3 earthquake is in orange.
In this map I placed lines that are oriented for my preferred fault plane solution from each of the moment tensors. I center the red strike line on the epicenter for each earthquake. There is some spread to the strike (neither of the two largest earthquakes align perfectly with their partner’s strike).
This map shows a strike line that is the average between the strike lines for the two largest earthquakes. This is far from being quantitative, but it shows a little about how if we consider the different sources of error for each measurement (aleatory uncertainty) and what we understand about how these moment tensors represent the real earthquake(s) (epistemic uncertainty), it is reasonable to interpret these earthquakes to be along the same fault (or at least a synthetic or subparallel fault).
And here is a primer on focal mechanisms. The moment tensor plots are made differently than are focal mechanisms (they use tensors instead of first motions). However, these moment tensor plots use the same graphical representation as focal mechanism plots (i.e. beach balls). Basically, each focal mechanism results in a graphic that shows two possible fault plane solutions (the criss-cross separating the colors, orange or green in this case). The regions of color represent earth that has “bulged out” (compression) and the white represents earth that has contracted (tension) or “bulged in.” This graphic explains this further and click on the graphic to see it full size. This graphic is from here at the USGS. If you want to see a primer on moment tensors, here is the USGS page on them.
These are deep earthquakes and are clearly not on the subduction zone fault. Also, the moment tensors show mostly strike slip (the M 7.9 slightly extensional-oblique). If we start thinking about forearc slivers (“strain partitioning,” generally in the upper plate, where a subduction zone fault is not perpendicular to the convergence motion, a strike slip fault system will form to accommodate the margin parallel strain). This earthquake swarm appears to be rupturing a forearc sliver in the lower plate. We saw another potential analogy of this in the southwest Pacific earlier this year (the earthquake swarm in the Solomon Islands. I will need to see if anyone has documented (philosophically or otherwise) such a phenomenon. Most forearc slivers that have been documented are in the upper plate (the great Sumatra fault is a great example of a forearc sliver). The Aleutians form blocks that may rotate in response to oblique convergence.
This map shows historic earthquakes in pink. Today’s earthquake is on the eastern boundary of the 1965 M 8.7 subduction zone earthquake. Note the plate motion vectors in white. Note how the plate motion vector nearest the Rat Islands is not perpendicular to the margin (the subduction zone fault).
Here is a graphic showing what a forearc sliver might look like (from GSA).
Here is a map from Krutikov et al. 2008 (Active Tectonics and Seismic Potential of Alaska, Geophysical Monograph Series 179 Copyright 2008 by the American Geophysical Union. 10.1029/179GM07)
Note that there are blocks that are rotating to accomodate the oblique convergence. There are also margin parallel strike slip faults that bound these blocks. These faults are in the upper plate, but may impart localized strain to the lower plate, resulting in strike slip motion on the lower plate (my arm waving part of this). Note how the upper plate strike-slip faults have the same sense of motion as these deeper earthquakes.
Here is something cool too… This animation was created by IRIS here. The seismologic record of the M 7.9 earthquake as recorded at many operating seismometers is compiled to show the vertical motion across the United States. Red is up and blue is down. Click on the map to view the animation.
This one is large and fairly deep. Here is the USGS web page for this earthquake. There are several aftershocks, the largest so far is M 6. Today’s earthquakes are located along the Aleutian subduction zone, nearby the Bolan Ridge (the horseshoe shaped ridge to the north of the earthquake epicenters).
Here is the moment tensor, which shows this to be an oblique earthquake mechanism. This earthquake is possibly related to the internal deformation in the downgoing slab, which appears to be extensional in this location.
Here is a regional map showing the shaking intensity (Modified Mercalli Scale).
Here is a local map showing the shaking intensity and some of the island names.
This map shows historic earthquakes in pink. Today’s earthquake is on the eastern boundary of the 1965 M 8.7 subduction zone earthquake.
This is a different map of the historic earthquakes (a better resolution than the one above, but with fewer earthquake slip patches plotted).
Here is the list of recorded wave heights reported by:
NWS NATIONAL TSUNAMI WARNING CENTER PALMER AK
435 PM AKDT MON JUN 23 2014
Here are some aftershocks plotted. They align with the northwest striking moment tensor.
Here are some more aftershocks plotted. I left the earlier map since it shows how the largest aftershocks are aligned with each other.
Major triggered earthquake along strike with the west-northwest trend of the main shock and largest aftershocks. This swarm looks like it has reactivated a ridge-parallel fault. The magnetic anomalies are sub-parallel to this trend (look at the historic earthquake slip map above to see the magnetic anomalies).
According to the pager report, there is low exposure (not many people, nor their infrastructure, were in region of strong shaking determined from the computer simulations). Note the large probability for <1 fatality and <1 million USD loss.
All rightee… We have a number of aftershocks that are lighting up the potential fault that ruptured a couple days ago. Here is my page for this earthquake. Here is the USGS page.
The aftershocks are aligned with the north-south transform fault system (named the Colba Ridge) that separates the Cocos plate to the West and the Nazca plate to the East. Here is the local map with historic epicenters in gray and the mainshock and aftershock epicenters plotted as orange circles. Note that many of the historic epicenters also align along this fracture zone.
Here is a map with only the recent epicenters in orange.
Here is an updated moment tensor (Mwb). With these aftershocks, we can better interpret this as rupture on a N-S striking right-lateral fault.
There are 5 moment tensors on the USGS page. For those looking for a refresher on focal mechanisms, here is the USGS page that describes them.
Here is the USGS poster that describes the historic seismicity and plate tectonic setting.
Here is a more comprehensive view of the local and regional tectonics in this region. This is a compilation by Harley M. Benz and others.
Good sized earthquake offshore Panama! Here is the USGS page.
This is probably an earthquake on an oblique high angle reverse fault, based on this early moment tensor. The hypocentral depth is pretty shallow too.
This looks related to the seismicity that plots along the North-South transform fault system that separates the Cocos and Nazca plates. However, it is near a triple junction and could be related to the East-West transform fault, or the subduction zone that strikes to the northwest. Very interesting.
Here is a regional map showing historic epicenters in grey. Today’[s earthquake is plotted in orange.
Here is a local map showing the same historic quakes, just zoomed in more.
This shows that the shaking was probably felt throughout coastal Panama.
Here is the USGS regional tectonic map:
Here is the PAGER report, that indeed shows there were many people who probably felt this earthquake.
Scott Burns, emeritus at Portland State and a student of Peter Birkeland, wrote an excellent article about the OSO Slide. I made a few animations and armchair interpretations about this slide shortly after it happened (here).
Scott discusses the geologic history and how heavy rainfall was a likely co-conspirator that led to the triggering of this horribly destructive landslide. Here is the article.
This is a general location map:
Here is an animation from David George and Dick Iverson at the USGS:
As a reminder, here is a map showing the historic landslides in the area:
Here is the geologic mapping done at the 1:24,000 scale published by the Washington State Department of Natural Resources. Click on the map below to see the entire map. Below is a clip of the map in the region of the OSO Landslide of 2014. Click here to see this inset map in a new browser window. Note the location of a dotted strike slip fault in the region of the OSO slide.