Surface fault ruptures associated with the 14 April foreshock (Mj 6.5) of the 2016 Kumamoto earthquake sequence, southwest Japan

The 2016 Kumamoto earthquake sequence was a rare event worldwide in that the surface ruptures associated with the largest foreshock (Mj 6.5) of 21:26 (JST), 14 April ruptured again during the mainshock (Mj 7.3) of 01:25 (JST), 16 April. The 14 April Mj 6.5 earthquake produced 6-km-long surface ruptures along the central portion of the Futagawa–Hinagu fault zone (FHFZ). The mainshock produced 31-km-long surface ruptures along the central to northeastern part of the FHFZ. Field observations and eyewitness accounts documented that the offsets of the ruptures associated with the 14 April foreshock became larger after the 16 April mainshock, suggesting that the same portion of the fault ruptured to the surface twice in the Kumamoto earthquake sequence. The 6-km-long surface ruptures associated with the largest foreshock are located near a geometric bend of the FHFZ characterized by ~50° change in strike. The epicenter of the mainshock is also located near the bend. These observations imply that the Kumamoto earthquake sequence was initiated due to a stress concentration on the bend of the FHFZ, and the mainshock was initiated approximately at the same place about 28 h after the largest foreshock. This foreshock/mainshock sequence of the Kumamoto earthquake is not successive events on the adjacent different fault zones, because the 6-km-long surface ruptures of the largest foreshock are part of the 31-km-long surface ruptures of the mainshock.