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The 2008 Mogul, Nevada Earthquake

external image clip_image002.jpg Unlike most earthquakes that occur over a period of time whether it is a couple minutes, a couple hours, or even over a day the Mogul or Washoe County, Nevada experienced over 5,000 earthquakes [See Figure 1 and 2 below] between the months of February 2008 through November 2008, although between February and April when more than 1,000 occurred. These earthquakes ranged from .7 magnitudes to 4.7 magnitudes on the Richter scale and are referred to as earthquake swarms.
external image clip_image004.jpg Earthquake swarms are events where local area experiences sequences of many similar sized earthquakes striking in a relatively short period of time. They tend to be centered along a rift, which an area of Earth’s crust that is being pulled part as the massive tectonic plates move in opposite directions.
external image clip_image006.jpg In Reno, Nevada the Pacific tectonic plate is shearing away from the North American plate along the San Andreas Fault, meaning the Reno area is expanding in an East and West direction. Yet the earthquakes have not been associated directly with the San Andreas Fault, meaning the bedrock faults surrounding the Reno area is what is causing the earthquake swarms. What is so unusual about the Mogul earthquake swarms is that where the swarms are occurring, there are no known bedrock faults that can be matched up on the maps.
Three factors help determine what size magnitude an earthquake is; one is how long the fault is, two is how deep the fault penetrates, and three how far the fault slips in the process. Since there are no known faults surrounding the area, the next step would be to look at the depth of the Mogul swarms. If you look at Figure 3 to the right, which charts exactly where all the swarms have occurred, you can see that there were almost all of the swarms occurred right below the surface and or on the surface, while only a slim few were below -10 km. Lastly, the swarms were believed to be caused by shallow slipping of the faults, making it a strain release rather than surface rupture. The foreshock activity was confined to a 1 to 2 km long striking structure that came from the shallowest earthquakes over time the quakes are nearly parallel to but less than 1 km East of the initial zone.
As you can see from Figure 4, the areas in blue felt little to no action, while the areas in red felt the most of the swarms shaking. Residents admit to a violent shake that only lasted a brief wile, and resulted in minor damages to some houses, such as cracks in walls, or plaster and paint damage. Rocks fell and blocked a water flume, sending water flowing into backyards, and only one injury was ever reported. A shoulder dislocation occurred due to a bookshelf falling, but no other known injuries were ever reported. The most costly of all the damage that occurred was that of building structural damage, or that of the wine store that lost $3,500 in damages due to un-earthquake friendly storage cellar.
 Image generated by GPL Ghostscript (device=pnmraw)
Image generated by GPL Ghostscript (device=pnmraw)
While it is still not known what caused the massive swell of earthquake swarms in the Mogul area, the swarms have certainly made an impact on the media, and residents in that area. It also made seismologists want to explore this area further, and develop maps to the new fault lines. Over the last year the swarms have tapered off, and the mystery of the 2008 Mogul, Nevada swarms continues.

References:
http://articles.latimes.com/2008/may/01/nation/na-quakes1

http://www.seismo.unr.edu/feature/2008/mogul.html

www.iris.edu/hq/esreg/priv/download_file/103

http://freerepublic.com/focus/f-news/2006663/posts

http://www.agiweb.org/geotimes/july08/article.html?id=nn_reno.html

http://www.nbmg.unr.edu/EQ/MogulEQ/documents/dePolo_Mogul-Somersett_EQ_Report.pdf