This image showing a portion of the San Andreas Fault along the San Francisco Peninsula was taken by the UAVSAR instrument on NASA's Gulfstream III research aircraft. The narrow body of water running diagonally along the fault from upper left to lower right is the Crystal Springs Reservoir, which provides the primary source of water for San Francisco. Credit: NASA
California is often known more for its earthquakes than anything, and no wonder. The San Andreas Fault, what is known as a transform fault, runs for about 800 through much of the length of California. A transform fault results when two tectonic plates are sliding past one another. Some of the largest and most damaging earthquakes in the US in the last couple of hundred years have involved the San Andreas Fault. Part of its potential to cause such great damage is that two of California’s major cities, San Francisco and Los Angeles, are located along of portion of the fault.
The San Andreas Fault has been monitored continuously for seismic activity for many years. One of the more popular locations for monitoring is in a part of the fault near the town of Parkfield, California, where magnitude 6.0 earthquakes have occurred relatively consistently every 22 years. The last major activity at this location was in 2004. During 2004 and 2005 the San Andreas Fault Observatory at Depth (SAFOD) was constructed. It included a bore hole a little over 2 miles deep to enable better recording of earthquake activity.
On July 10 Berkeley Seismological Laboratory researchers Robert Nadeau and Aurélie Guilhem published the results of analyzing seismic activity in the Parkfield area in Science. Their conclusions are based on analyzing data from 2002 through 2009, including data from SAFOD, once it was completed. Their data were compiled from 76 monitoring stations around Parkfield.
Tremor rates have been elevated over the past six years, which is considered a sign that a large earthquake could be imminent. In spite of the increased number of tremors, the stress on the fault has been increasing, rather than decreasing. In their paper, Nadeau and Guilhem say that “the persistent changes in tremor suggest that stress is now accumulating more rapidly beneath this part of the San Andreas Fault, which ruptured in the moment magnitude 7.8 Ft. Tejon earthquake of 1857.”
That this area has escaped a major magnitude 7.0+ earthquake since 1857 does not bode well. “This segment is now fully locked,” according to Nadeau and Guilhem, and the fault was predicted to produce another big earthquake sometime between 1942 and 1999. Well, that means we are 10 years overdue and counting..
For more info:
- USGS map of recent earthquakes in California, continuously updated
- Nonvolcanic Tremor Evolution and the San Simeon and Parkfield, California, Earthquakes (Science paper, subscription required)
- USGS Earthquake Hazards Program
- Berkeley Seismological Laboratory
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