David Jacobson reports that in a manuscript accepted for publication yesterday, new evidence for 10 ground-rupturing earthquakes prior to 1857, along the Big Bend of the Southern San Andreas Fault was presented. The study, headed by Dr. Katherine Scharer at the USGS in Pasadena, examined the earthquake history of the San Andreas near Frazier Mountain in northeast Kern County. This area last ruptured in the 1857 M=7.7-7.9 Fort Tejon earthquake.
One of the reasons why this location is of importance is because in Southern California, the Big Bend, Carrizo, and Mojave sections of the San Andreas Fault accommodate 50 to 70% of plate motion. This means that the seismic hazard is high. While the Mojave section to the south ruptures on average, every 100-135 years, by knowing more about the overall region, more can be said about the extent and magnitude of past ruptures. Therefore, by piecing together the earthquake history along various sections of the San Andreas, it is the hope that the seismic hazard will be better understood.
In order to complete this study, over 30 trenches were excavated, in which the structural and sedimentological evolution of the area could be documented. The Frazier Mountain site along the San Andreas is a pull-apart basin, meaning that through successive ruptures over the last 1200 years, the basin has grown and been ripped apart numerous times, allowing for distinct earthquake horizons to form.
These horizons often appear as small tears, large cracks, and or folding. Because of this range, the degree of certainty as to whether a horizon has been found, varies. Nonetheless, at this site, 11 horizons dating back to 800 A.D. were identified. Such precise dating was accomplished by dating charcoal and plant remains found at each horizon.
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