Verification of high rupture speed of the 1990 Luzon earthquake, Philippines from Taiwan seismic network observations

Abstract On July 16, 1990, a Mw 7.7 earthquake struck central Luzon, Philippines. The event produced more than 100 km of surface rupture along the Philippine Fault system. Seismologists conducted detailed investigations of the overall source rupture process using data from the global seismic network. However, analyses of multiple rupture asperities and rupture directivity based on teleseismic data revealed inconsistencies in estimated rupture velocities. In this study, we incorporate additional observations from regional seismic networks to examine the rupture characteristics of the Luzon earthquake and to evaluate its rupture velocity. By examining seismic array data, we identify two distinct zones of concentrated energy release on the fault plane. Given the similarity in wave propagation paths and focal mechanisms between the mainshock and its aftershocks, we select seismograms from aftershocks recorded by the seismic network as empirical Green’s functions to investigate the source rupture of the Luzon earthquake. To reproduce the envelope patterns of P- and S-wave arrivals in the mainshock waveforms, we perform simulations of seismic wave travel times. The results indicate that a relatively high rupture velocity (approaching the S-wave velocity in the source region) is required to match the observations. Furthermore, a literature review of post-earthquake investigations and recent source studies suggests that the Luzon earthquake exhibits several distinctive source characteristics, resembling rupture behavior associated with supershear velocities. We therefore infer that during the rupture process of the Luzon earthquake, rupture velocity in a limited portion of the fault may have exceeded the shear-wave velocity.