Stress conditions and seismicity around the rupture zone of the mainshock of the 2016 Kumamoto earthquake sequence in Kyushu, southwest Japan

The 2016 Kumamoto earthquake sequence in Kyushu included a foreshock (Mw 6.2) on 14 April and the mainshock (Mw 7.0) on 16 April, both of which were caused by fault ruptures at the intersection of the Futagawa and Hinagu fault zones. However, not all sections of the two fault zones were ruptured during the mainshock; in particular, although the northernmost (Takano–Shirahata) section of the Hinagu fault zone ruptured, the rupture did not propagate to southern sections of the fault zone. We examined fault geometry, geological structure, and seismicity around the fault zones, and conducted numerical Coulomb stress change and slip tendency analyses to investigate rupture conditions around the source faults. Fault geometry and slip tendencies indicated that before the foreshock, the source fault (the Futagawa section of the Futagawa fault zone) was favorable for rupture. Seismicity analysis showed that, before the foreshock, a remarkable low-seismicity zone existed where the rupture ultimately terminated. A material or structural boundary in the fault zones probably caused this anomalous seismicity and affected rupture propagation during the mainshock. Coulomb stress change analysis indicated a positive stress change in the Hinagu section just after the mainshock. In addition, the strikes of the optimum slip planes were parallel to that of the Hinagu section just after the mainshock, and slip tendencies in the section increased at that time. These results suggest that the Hinagu section was brought closer to failure just after the mainshock. In contrast, later in the post-mainshock period, the strikes of the optimum slip planes were oblique to that of the Hinagu section and the slip tendencies of the section were low. These results suggest that the favorability for a future rupture of the Hinagu section has declined since 2016 Kumamoto earthquake sequence.