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0 * I <br />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 recurrence interval of 300 years. Since the last large event was in 1906, the probability <br />that the magnitude 8 event will occur in the next 30 years was estimated at less than <br />10% 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 135 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. (1997), Idriss (1991;1995), Boore et al (1997), Campbell (1997), and <br />Abrahamson and Silva (1997) attenuation relations for rock site conditions. For the <br />Boore et al. (1997) relationship a typical "rock" shear wave velocity of 620 m/sec was <br />used. The average horizontal peak accelerations for the MPE ground motions are listed <br />in Table 2. <br />The CRCV and San Andreas faults both produce MPE peak accelerations of 0.058 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 magnitude 6.5 earthquake at a distance of 62 km and <br />the San Andreas event is for a magnitude 8.0 earthquake at a distance of 135 km. The <br />response spectra for these two events are shown in Figure 2. The response spectra for <br />the San Andreas fault is larger than the response spectrum for the CRCV at all periods <br />except in the range 0.03 to 2.0 seconds. In this period range the two spectra are quite <br />similar, with the CRCV spectrum being slightly higher. I recommend using the San <br />