Near-Source Ground Motion Variability from M~6.5 Dynamic Rupture Simulations

Abstract

We study ground-motion variability from dynamic rupture simulations, with a particular focus on PGA-values exceeding 1g in moderate earthquakes. Such extreme ground motions are generally observed near the ruptured fault, and it is expected that they are dominated by source effects. We investigate earthquake source characteristics and near-source ground motion using spontaneous dynamic rupture simulations of earthquakes of ~M 6.5. We generate a large number of dynamic source models with stochastic initial stress distribution for three classes of faulting (thrust, normal and strike slip) with buried and surface rupturing faults. Stress and frictional strength consider two extreme cases of normal stress, 1) depth-dependent, and 2) depth-independent. This diversity of rupture models generates a broad range of scenarios for evaluating near-source ground motion variability and for identifying the causes for extreme ground motion. Comparing PGV and PSA of our simulation with empirical GMPE, we find an increased variability in the near-field of the rupture. The consistent saturation of these quantities predicted by GMPE is not obvious in our calculations. Rather, there are significant ground-motion reductions near the source for buried faults, but considerable increase for surface rupturing earthquakes. Extreme ground motions appear to be correlated with faulting that breaks the free surface, with largest shaking levels for strike-slip ruptures. For buried ruptures, thrust faulting earthquakes generate the strongest ground motion, normal faulting the weakest. In general, simulated ground motion are consistent with GMPE’s at distance >5km, but not at short distance.