Foreshock migration preceding the 2016 Mw 7.0 Kumamoto earthquake, Japan

We investigated the spatiotemporal evolution of an earthquake sequence following a series of large shallow intraplate earthquakes, including a M-w 6.2 foreshock and M-w 7.0 main shock, in the Kumamoto area of Kyushu, SW Japan. To more precisely characterize the evolution of the earthquake sequences, we applied a matched filter technique to continuous waveform data, using template events obtained via a double-difference relocation algorithm. Migrations of seismicity fronts along the directions of fault strike and dip are clearly seen, starting immediately after the M-w 6.2 foreshock. These migrations are interpreted to result from aseismic slip triggered by the foreshock, propagating toward the nucleation point of the subsequent M-w 7.0 main shock rupture. When combined with static stress changes induced by the M-w 6.2 foreshock, it is likely that stress transfer from both aseismic and seismic slip during the foreshock sequence loaded stress onto the main shock rupture faults, bringing them closer to failure.