Source location and mechanism analysis of an earthquake triggered by the 2016 Kumamoto, southwestern Japan, earthquake

The 2016 Kumamoto earthquake (Mw 7.0) occurred in the central part of Kyushu Island, southwestern Japan, on April 16, 2016. The mainshock triggered an event of maximum acceleration 700 gal that caused severe damage to infrastructure and thousands of homes. We investigate the source location of the triggered event, and the timing of large energy release, by employing the back-projection method for strong-motion network data. The optimal location is estimated to be [33.2750°, 131.3575°] (latitude, longitude) at a depth of 5 km, which is 80 km northeast of the epicenter of the mainshock. The timing is 33.5 s after the origin time of the mainshock. We also investigate the source mechanism by reproducing observed displacement waveforms at a near-source station. The waveforms at smallersized events, convolved with the source time function of a pulse width 1 s, are similar to the signature of the observed waveforms of the triggered event. The observations are also reproduced by synthetic waveforms for a normal-fault mechanism and a normal-fault with strike-slip components at the estimated locations. Although our approach does not constrain the strike direction well, our waveform analysis indicates that the triggered earthquake occurred near the station that observed the strong motions, primarily via a normal-fault mechanism or a normal-fault with strike-slip