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The largest historical earthquake on the North Coast segment was the 1906,San <br /> Francisco Earthquake with a moment magnitude of 7.8 (Well and Coppersmith, 1994). <br /> This earthquake was due to simultaneous rupture of the North Coast and Peninsula <br /> segments. <br /> The 1988 U. S. Geological Survey Working Group on earthquake probabilities in <br /> California estimated the expected magnitude on the North Coast segment as M=8 with <br /> a return period of 300 years. Since the last large event was in 1906, the probability that <br /> the magnitude 8 event will occur in the next 30 years was estimated at less than 10% <br /> with good confidence. Using the Working group procedure, I computed the <br /> probability that the North Coast segment will experience a magnitude 8 earthquake <br /> during the next 100 years to be about 20%. This probability is lower than the chance of <br /> 1/3 in 100 years discussed above. Although it is more conservative, there is a <br /> precedence for using a magnitude 8 event on the San Andreas. <br /> I recommend using an MPE of magnitude 8.0 at a distance of 124 km for the San <br /> Andreas Fault. <br /> MPE Ground Motions <br /> The horizontal peak ground acceleration (PGA) is estimated for the MPE for each fault. <br /> The PGA is estimated by averaging the median peak acceleration predicted by the <br /> Sadigh et al. (1993), Idriss (1991), Boore et al (1994), and Abrahamson and Silva (1993) <br /> attenuation relations for rock (Class A is used for Boore et al). -The average horizontal <br /> peak accelerations for the MPE ground motions are listed in Table 2. <br /> The CRCV and San Andreas faults both produce MPE peak accelerations of 0.06g at the <br /> site. Although these two faults have the same MPE acceleration, the events are quite <br /> different: the CRCV event is for a mangitude 6.4 earthquake at a distance of 46 km and <br /> the San Andreas event is for a magnitude 8.0 earthquake at a distance of 124 km. The <br /> response spectra for these two events are shown in Figure 2. At periods greater than 0.2 <br /> seconds the San Andreas event yields the largest spectral accelerations., <br /> I recommend using a time history representative of the ground motion from a distant <br /> magnitude 8.0 earthquake. One alternative is to use the Seed-Idriss M=8 time history <br /> built into SHAKE. A second alternative is use the Pasadena recording of the 1952 Kern <br />