The origin of a strong, near-fault velocity pulse during a surface-breaking earthquake

Abstract:

Ground motions with strong (greater than a few m/s), long-period (a few seconds) velocity pulses have been reported in recently earthquakes, and potentially pose significant hazard for tall buildings and large infrastructures. Yet, the physical origin of such velocity pulses are not well understood. Here, using the simulations of spontaneous dynamic rupture, we examine the origin of a large velocity pulse recorded at several near-fault stations during the 2016 Mw7.1 Kumamoto (Japan) earthquake. We show that a set of near-fault waveform data as well as seismologically estimated radiated energy and seismic moment can be well reproduced by a relatively simple, dynamic rupture model with uniform along-strike pre-stress and frictional properties. Our results suggest that a large, long-period velocity pulse observed at station 93048 located 650 m away from the surface trace of the Futagawa fault is caused by the interaction of the main rupture front and that reflected due to Earth's free surface in near-fault regions. The amplitude of a velocity pulse is the largest in the fault-parallel component of ground motions, which is confined within a few km from the fault trace. The duration of the velocity pulse is controlled by the arrival of stopping phases originating from one edge of the fault. Such generic mechanism suggests that the generation of strong, long-period, pulse-like ground motions is a common occurrence for any surface-breaking earthquakes. Therefore, we conclude that the dynamic interaction of propagating rupture with Earth’s free surface, which is not currently incorporated in ground-motion estimation studies, is an important physical characteristics for predicting near-fault ground motions.