Re-evaluation of the regional tectonic stress fields and faulting regimes in central Kyushu, Japan, behind the 2016 Mw 7.0 Kumamoto Earthquake

To re-evaluate the regional tectonic stress fields in central Kyushu, Japan, the region in which the 2016 Kumamoto Earthquakes occurred on 14 April 2016 (Mw 6.2) and 16 April (Mw 7.0), the faulting regimes in central Kyushu were analyzed using the focal mechanisms of this earthquake sequence. Results show that almost all of the focal mechanisms of the earthquakes occurring along the active Futagawa-Hinagu fault zone fall into two spatial faulting regimes: a strike-slip (SS) regime along this fault zone and a pure normal faulting (NF) regime without or with minor strike-slip component in the northern part of this fault zone. In terms of the relationship between the two horizontal principal stresses acting on Kyushu Island, these two regimes are regarded as a set of tectonics stress fields. The highly accumulated strain energy along this fault zone and asymmetrically unbalanced stress condition for the maximum horizontal principal stress acting on the east-west sides of the crustal blocks in this area expected from a pair of these two regimes might explain the relatively large number of aftershocks following the Kumamoto Earthquake as compared to other recent inland earthquakes in the Japanese Islands. From the results of the present analyses, it is considered that the regional stress field of Honshu Island could be extended to Kyushu Island and that the kinematics of the Philippine Sea Plate may have been affecting the stress field in Kyushu since the late Miocene.