Rupture Speed Signatures of Near-fault Particle Motion in Large Strike-slip Earthquakes

Earthquake rupture propagation speed is an essential source factor that largely controls hazard and risk. However, measuring rupture speeds of natural earthquakes is often challenging and ambiguous. Near-fault seismic waveforms (recorded within several km) are believed to have high capability for resolving rupture process. In this study, we probe the feasibility of using near-fault data signatures to directly infer rupture speeds in continental strike-slip earthquakes. To thoroughly understand near-fault features, we synthesize the near-fault seismic waves for kinematic source models on a strike-slip fault under different rupture speeds in a 3D medium. We identify the dependence of velocity waveform and particle motion on rupture speed in both amplitude and shape. In addition, we compare our results with the analytical solution with steady-state constant rupture speed. The discrepancies between the kinematic model and the analytical model indicate the contribution of radiation from different configurations. With inspecting the near-fault dataset of eight M>7 strike-slip earthquakes, we find that instead of dealing with the velocity waveforms with multiple high-frequency spikes, the features of the particle motion shape are easier to identify. Then we apply the particle-motion-based criterion to identify signatures associated with supershear, subshear, and other complexities such as multiple rupture fronts and initial-stage rupture phase. Our study highlights the further application of near-fault seismic data in studying earthquake sources.