CyberShake: A Physics-Based Seismic Hazard Model for Southern California

Robert W. Graves, Scott Callaghan, Ewa Deelman, Edward H. Field, Thomas H. Jordan, Gideon Juve, Carl Kesselman, Philip J. Maechling, Gaurang Mehta, Kevin R. Milner, David A. Okaya, Patrick Small, & Karan Vahi

Accepted 2010, SCEC Contribution #1426

CyberShake, as part of the Southern California Earthquake Center’s (SCEC) Community Modeling
Environment, is developing a methodology that explicitly incorporates deterministic source and wave
propagation effects within seismic hazard calculations through the use of physics-based 3D ground motion
simulations. To calculate a waveform-based seismic hazard estimate for a site of interest, we begin with
Uniform California Earthquake Rupture Forecast, Version 2.0 (UCERF2.0) and identify all ruptures within
200 km of the site of interest. We convert the UCERF2.0 rupture definition into multiple rupture variations
with differing hypocenter locations and slip distributions, resulting in about 415,000 rupture variations per
site. Strain Green Tensors are calculated for the site of interest using the SCEC Community Velocity Model,
Version 4 (CVM4), and then, using reciprocity, we calculate synthetic seismograms for each rupture variation.
Peak intensity measures are then extracted from these synthetics and combined with the original rupture
probabilities to produce probabilistic seismic hazard curves for the site. Being explicitly site-based,
CyberShake directly samples the ground motion variability at that site over many earthquake cycles (i.e.,
rupture scenarios) and alleviates the need for the ergodic assumption that is implicitly included in traditional
empirically based calculations. Thus far, we have simulated ruptures at over 200 sites in the Los Angeles
region for ground shaking periods of 2 s and longer, providing the basis for the first generation CyberShake
hazard maps. Our results indicate that the combination of rupture directivity and basin response effects can
lead to an increase in the hazard level for some sites, relative to that given by a conventional Ground Motion
Prediction Equation (GMPE). Additionally, and perhaps more importantly, we find that the physics-based
hazard results are much more sensitive to the assumed magnitude-area relations and magnitude uncertainty
estimates used in the definition of the ruptures than is found in the traditional GMPE approach. This reinforces
the need for continued development of a better understanding of earthquake source characterization and the
constitutive relations that govern the earthquake rupture process.

Citation
Graves, R. W., Callaghan, S., Deelman, E., Field, E. H., Jordan, T. H., Juve, G., Kesselman, C., Maechling, P. J., Mehta, G., Milner, K. R., Okaya, D. A., Small, P., & Vahi, K. (2010). CyberShake: A Physics-Based Seismic Hazard Model for Southern California. Pure and Applied Geophysics, (accepted).