Near-Field Across-Fault Seismic Ground Motions

Abstract There are many engineering applications that require an understanding of the nature of strong ground motions adjacent to and spanning across faults. Unfortunately, such near-field observations at distances less than 100 m of fault rupture are few and incomplete. In this study a 3D finite-difference method is used to simulate strong ground motions for a hypothetical M w  6.5 earthquake at sites within a few tens of meters of the fault to document the nature of strong ground motion at pairs of sites across the fault as a first step toward providing ground-motion input for engineering design applications. We employ several distributed slip kinematic models to examine ground-motion variability. We also examine the ground motions for fault scenarios ranging from vertical strike-slip to low-angle thrust faulting. The results show that the motions have two primary components: (1) far-field waves that undergo focusing and amplification due to finite-source rupture directivity and (2) near-field waves that are sensitive to the tectonic rebound, or fling, of the closest section of the fault to the recording stations. Both the far-field and near-field controlled motions result in nonstationary pulse-like velocity waveforms that have many implications for the design of engineered structures located close to or spanning faults.