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Spontaneous Rupture Simulations of Mw7.7 Earthquakes on the Southern San Andreas Fault

Kim B. Olsen, Steven M. Day, Jean B. Minster, Yifeng Cui, Amit Chourasia, David A. Okaya, Philip J. Maechling, & Thomas H. Jordan

Published 2008, SCEC Contribution #1148

Previous numerical simulations (TeraShake1) of large (Mw 7.7) southern San Andreas fault earthquakes predicted localized areas of strong amplification in the Los Angeles area associated with directivity and wave-guide effects from northwestward-propagating rupture scenarios. The TeraShake1 source was derived from inversions of the 2002 Mw 7.9 Denali, Alaska, earthquake. That source was relatively smooth in its slip distribution and rupture characteristics, owing both to resolution limits of the inversions and simplifications imposed by the kinematic parameterization. New simulations (TeraShake2), with a more complex source derived from spontaneous rupture modeling with small-scale stress-drop heterogeneity, predict a similar spatial pattern of peak ground velocity (PGV), but with the PGV extremes decreased by factors of 2-3 relative to TeraShake1. The TeraShake2 source excites a less coherent wavefield, with reduced along-strike directivity accompanied by streaks of elevated ground motion extending away from the fault trace. The source complexity entails abrupt changes in direction and speed of rupture correlated to changes in slip-velocity amplitude and waveform, features that might prove challenging to capture in a purely kinematic parameterization. Despite the reduced PGV extremes, northwest-rupturing TeraShake2 simulations still predict entrainment by basin structure of a strong directivity pulse, with PGVs in Los Angeles and San Gabriel basins that are much higher than predicted by empirical methods. Significant areas of those basins have predicted PGV above the 2% probability of exceedance (POE) level relative to current attenuation relationships (even when the latter includes a site term to account for local sediment depth), and wave-guide focusing produces localized areas with PGV at roughly 0.1-0.2% POE (about a factor of 4.5 above the median). In contrast, at rock sites in the 0-100 km distance range, the median TeraShake2 PGVs are in very close agreement with the median empirical prediction, and extremes nowhere reach the 2% POE level. The rock-site agreement lends credibility to some of our source-modeling assumptions, including overall stress drop level and the manner in which we assigned dynamic parameters to represent the mechanical weakness of near-surface material. Future efforts should focus on validating and refining these findings, assessing their probabilities of occurrence relative to alternative rupture scenarios for the southern San Andreas fault, and incorporating them into seismic hazard estimation for southern California.

Olsen, K. B., Day, S. M., Minster, J. B., Cui, Y., Chourasia, A., Okaya, D. A., Maechling, P. J., & Jordan, T. H. (2008). Spontaneous Rupture Simulations of Mw7.7 Earthquakes on the Southern San Andreas Fault. Bulletin of the Seismological Society of America, 98, 1162-1185. doi: 10.1785/0120070148.