Strong asymmetry in near-fault ground velocity during an oblique strike-slip earthquake revealed by waveform particle motions and dynamic rupture simulations

The 2022 Mw 7.0 Chihshang (Taiwan) earthquake, captured by almost a dozen near-fault strong-motion seismometers, high-rate GPS and satellite data, offers a rare opportunity to examine dynamic fault rupture in detail. Using dynamic rupture simulations, we investigate the particle motions recorded at near-fault strong-motion and 1 Hz GPS stations surrounding the main asperity. Some of these stations were as close as 250 m from the fault trace as determined by sub-pixel correlation of Sentinel-2 images. Our model reproduces the observed strong asymmetry in the ground motions on either side of the fault rupture, which results from along-dip spatial variability in rake angle on the steeply dipping fault (70°) at shallow depth (2 km). Observed near-fault, pulse-like fault-parallel ground velocity larger than fault-normal velocity can be explained by a model with a sub-shear rupture speed, which may be due to shallow rupture propagation within low-velocity material and to free surface reflections. In addition, we estimate a slip-weakening distance Dc of ~0.7-0.9m from strong-motion seismogram recorded at Station F073, which is located ~250 m from the fault rupture, and the results of dynamic rupture modeling. The inferred Dc is similar to other empirically derived estimates found for crustal earthquakes. These results have important implications for near-fault ground-motion hazard.