In this study, the rupture processes of the foreshock and main shock of the 2016 Kumamoto earthquake were investigated based on the inversion of strong ground motions. Strong motion records in the near-source region were mainly utilized because the authors were interested in the generation mechanism of damaging ground motions in the near-source region. Empirical Green’s functions (EGFs) were applied to avoid uncertainty in the subsurface structure model. The results clearly indicate that, for the main shock, a region of significantly large slip and slip velocity existed approximately 15 km northeast of the hypocenter. However, no “asperity” was observed between the hypocenter and Mashiki. Thus, it is not appropriate to conclude that the large-amplitude pulse-like ground motion in Mashiki was generated by the forward-directivity effect associated with the rupture of an asperity. For the foreshock, the result suggested two regions of large slip velocity: one is approximately 3 km northeast of the rupture starting point on the shallower part of the fault; the other is near the hypocenter. The damaging ground motions during the foreshock in Mashiki could be attributed to the forward-directivity effect associated with the rupture of these asperities. Based on the results, simplified source models were developed for the foreshock and main shock for the purpose of estimating strong ground motions at sites of engineering interest.

Key Words: the 2016 Kumamoto earthquake, rupture process, forward directivity, characterized source model