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.
Here is a great summary of this earthquake swarm.
Here is a link to a video taken from a drone.
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

Earthquake swarm in western Iran

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>