Three‐dimensional modeling of near‐fault ground motions with nonplanar rupture models and topography: Case of the 2004 Parkfield earthquake

Abstract

We study the applicability of deterministic strong ground motion simulations at very near fault distances for a subvertical strike‐slip fault model corresponding to the 2004 M6 Parkfield, California, earthquake in the frequency range up to 1 Hz. Theoretical modeling under the assumptions of a planar rupture and 1‐D medium shows that as a consequence of the S wave radiation pattern, the particle motion for such close stations should be almost linear in the fault‐normal (FN) direction, having fault‐parallel (FP) and vertical (V) components of almost zero. However, as shown on the Parkfield earthquake recordings, observed particle motions are rather circular with peak velocities at FP and V components comparable to those at FN components. We investigate several realistic features that could explain this controversy, namely, nonplanar fault, realistic three‐dimensional (3‐D) medium, and the topography of the area. We test and quantify these hypotheses using discrete wave number and discontinuous Galerkin modeling methods applying 1‐D and 3‐D velocity structures, respectively, and two nonplanar rupture models. We compare the synthetic and observed particle motions and peak velocity ratios and conclude that deviations from a planar rupture geometry in reasonable bounds for the Parkfield fault and the influence of topography only partially explain the behavior of the observed seismograms. On the contrary, the heterogeneous 3‐D velocity structure significantly reduces the synthetic peak ratios to values closer to 1 and provides rather circular particle motions. Therefore, the 3‐D velocity model is crucial to obtain realistic estimates of ground motions at near‐fault distances and is more important than the detailed fault geometry or topography in the Parkfield area.