Numerical simulation of near-fault ground strains and rotations from actual earthquakes with predominantly dip-slip mechanisms

In the absence of records of near-fault ground strains and rotations from strong earthquakes, deterministic physics-based simulations have become an important tool for characterizing these motions in the low-frequency range (e.g., < 1.0 Hz). Building on a previous study of near-fault ground strains and rotations from actual strike-slip ruptures conducted by the authors, this article investigates the spatial and temporal characteristics of such motions generated by actual earthquakes with predominantly dip-slip mechanisms. This is achieved by performing forward ground-motion simulations of the 1994 M_w 6.7 Northridge, the 1989 M_w 6.9 Loma Prieta, and the 1985 M_w 8.1 Michoacan earthquakes using previously published finite-fault rupture models. For each considered seismic event, time histories of ground strains and rotations are generated at near-fault recording stations and at a dense grid of observation points. This is accomplished by finite differencing translational motions simulated at very closely spaced stations using a kinematic modeling approach. The simulation results show large-amplitude axial strain, shear strain, and rocking in the near-fault region. For the considered earthquakes, the maximum peak ground strain over all grid points is of the order of ~ 100–250 μstrain, whereas the maximum peak ground rocking ranges from ~ 100 to ~ 200 μrad. The attenuation characteristics of peak ground strains and rotations differ for the considered seismic events and depend on the component of interest and the rupture distance. Finally, peak ground rocking can be reasonably estimated from peak vertical ground velocity using a properly selected scaling factor despite the significant variability of the latter in the near-fault region. Filtering out the very low frequencies of ground motion (< 0.1 Hz), including the static offset, significantly affects the scaling factor.