Physics Based Probabilistic Seismic Hazard Calculations for Southern California

Abstract eng:
Deterministic source and wave propagation effects such as rupture directivity and basin response can have a significant impact on near-fault ground motion levels, particularly at longer shaking periods. CyberShake, as part of the Southern California Earthquake Center’s (SCEC) Community Modeling Environment, is developing a methodology that explicitly incorporates these effects within seismic hazard calculations through the use of physics-based 3D ground motion simulations. To calculate a waveform-based probabilistic hazard curve for a site of interest, we begin with Uniform California Earthquake Rupture Forecast, Version 2 (UCERF2) and identify all ruptures (excluding background seismicity) within 200 km of the site of interest. We convert the UCERF2 rupture definition into multiple rupture variations with differing hypocenter location and slip distribution, which results in about 400,000 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 (e.g., spectral acceleration) are then extracted from these synthetics and combined with the original rupture probabilities to produce probabilistic seismic hazard curves for the site. Thus far, we have produced hazard curves for spectral acceleration at a suite of periods ranging from 3 to 10 seconds at about 20 sites in the Los Angeles region, with the ultimate goal being the production of full 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.

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