Near-Source Strong Pulses During Two Large MJMA 6.5 and MJMA 7.3 Events in the 2016 Kumamoto, Japan, Earthquakes

Extremely large ground motions with strong pulses were observed near source faults during two large M JMA 6.5 and M JMA 7.3 events of the 2016 Kumamoto earthquakes in Japan. To investigate the mechanisms for generation of near-source strong pulses during both events, we first performed strong ground motion simulations in a broadband frequency range between 0.2 and 10 Hz using the empirical Green’s function method. For both the M 6.5 and M 7.3 events, strong motion generation area (SMGA) source models were prepared to simulate the ground motions, which were able to reproduce well the characteristics of the observed ground motions in and around the source areas. We also conducted ground motion simulations based on hypothetical simple source models to study the effect of rupture directivity on near-source strong ground motions. The principal findings regarding rupture directivity effects on near-source strong ground motions are as follows: (1) During the M 6.5 event, the forward and upward rupture directivities from two strike-slip SMGAs caused two distinct strong pulses in the fault normal and parallel components, respectively. (2) During the M 7.3 event, the upward rupture directivity along the fault dip direction from the strike-slip SMGA, including a small normal-slip, caused a strong pulse in both the fault parallel and normal components.