Near-source ground motion along strike-slip faults: Insights into magnitude saturation of PGV and PGA

Empirical data suggest that peak ground acceleration (PGA) and peak ground velocity (PGV) saturate as a function of magnitude for large magnitude ruptures close to the fault. Because data are sparse in the near-source region of large magnitude events, we have explored this question by simulating large magnitude strike-slip earthquakes. We use kinematic simulations to generate ground motion for a strike-slip fault that has a large aspect ratio (length/width). We consider both homogeneous or heterogeneous rupture models. We find that close to the fault along strike profiles of PGV and PGA increase to a maximum at a certain epicentral distance and then decrease to an asymptotic level beyond this distance. Critical factors for predicting ground motion are the position of an observer along strike, the depth of the hypocenter below the top of the fault, and the ratio of rupture velocity to shear-wave velocity. To understand the cause of the amplitude variation of along strike profiles of PGV and PGA, we use the isochrone method and the concept of the critical point to investigate how the geometry and kinematic parameters interact to produce the computed ground motion. We construct a predictor based on the critical point that does well in predicting the position of the maximum of PGV and PGA for stations close to the fault. For heterogeneous rupture models we find that the behavior is more complex though the general observation that along strike profiles of PGV and PGA increase to a maximum and then decrease still holds. This has implications for empirical attenuation relationships that essentially average the ground motion for all stations along strike with the same distance to the fault.