Influence of Fault Rupture Velocity on Directivity Effect in Near-Fault Ground Motion

ABSTRACT Considering the destruction caused by the directivity effect of near-fault ground motion, finite difference models with different source modes are established to explain the directivity effect mechanism, the directivity characteristics of ground motion in the 2016 Kumamoto earthquake are analyzed, and the influences of fault rupture velocity (Vr) on the directivity effect are analyzed. The following conclusions can be made: (1) The directivity effect is mainly caused by the superposition of energy in the forward direction of the fault rupture, which is most significant when the Vr approaches the shear wave velocity (Vs). (2) The ground motion caused by the 2016 Kumamoto earthquake has a distinctive directivity. (3) The Vr is a critical factor influencing the directivity effect, the directivity effect enhances with increasing Vr. INTRODUCTION Near-fault ground motions have gained more attention since the 1994 Northridge earthquake, the 1995 Kobe earthquake, and the 1999 Chi-Chi earthquake (Alavi & Krawinkler, 2004; Bray & Rodriguez-Marek, 2004; Hall et al., 1995; Kalkan & Kunnath, 2006; P. G. Somerville, 2003). The ground motion in the near-fault area is significantly affected by the fault rupture mechanism, rupture process, site location, and permanent ground displacement (X. Chen & Wang, 2020; Y. Zhang et al., 2022). The ground motion in near-fault areas may differ greatly from the ground motion far from the fault areas, which is the near-fault effect of ground motion (Sun et al., 2020; Wu et al., 2021; Yang et al., 2023). The near-fault effect is generally expressed in two aspects: one is the difference with the ground motion characteristics in the area far from the fault, such as the difference in ground motion waveform, spectrum, and duration. The other aspect is the characteristics of ground motion distribution in the near-fault area, such as the distribution characteristics in peak ground velocity (PGV), peak ground acceleration (PGA), and response spectrum amplitude (Farrugia et al., 2017; Jinjun Hu et al., 2011; Mo et al., 2022). Near-fault ground motions are usually characterized by long-period velocity pulses, which have significant destructive effects on large structures and also increase the risk of landslides, debris flows and other disasters (Bray & Rodriguez-Marek, 2004; Lu et al., 2021; Quaranta et al., 2022; P. G. Somerville, 2003; P. G. Somerville et al., 1997; Song et al., 2018). The directivity effect of fault rupture propagation is one of the primary causes of velocity pulse (Bray & Rodriguez-Marek, 2004; P. G. Somerville et al., 1997; Wang et al., 2002).