The great Fort Tejon earthquake of January 9, 1857 (magnitude 7.9) was the last major earthquake in this region. It ruptured a 370 kilometer (220 mile) segment of the San Andreas Fault and produced 10.0 meters (33 feet) of offset in this area. Based on the average rate of fault movement calculated in problem 1b, estimate how many years of accumulated strain were released during that earthquake. (Note: This answer is based on a very simplistic assumption.)
The following questions are based on Map T-25a, Figure 37-3, a detailed topographic map of the San Andreas Fault, Figure 37-5, a stereogram of the San Andreas Fault in the Carrizo Plain National Monument, and Figure 37-6, a portion of the “McKittrick Summit, California,” quadrangle. “Wallace Creek” is an offset stream that has been labeled for you on Figure 37-6.
Most of the movement between the Pacific Plate side and the North American Plate side of the San Andreas Fault is released abruptly after years of accumulating strain—at which time the fault ruptures with a displacement of several meters.
Further, it appears that different segments of the fault remain locked for different periods of time. In a general way, the size of an earthquake expected to occur along a segment of the fault system relates to the length of time the fault accumulates strain before rupturing. The longer the strain accumulates, the larger the earthquake when the fault finally ruptures.
What will be an ideal response?
about 280 years of accumulated strain
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