Abstract
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.
Highlights
A destructive earthquake of Mj 7.3 (Mw 7.0) occurred in the Kumamoto area, southwest Japan, at 01:25 (JST), April 16, 2016, which corresponds to the mainshock of the Kumamoto earthquake sequence (Fig. 1a, b) (Japan Meteorological Agency (JMA) 2016a, b)
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
Coseismic crustal deformation detected by InSAR (Geospatial Information Authority of Japan GSI 2016) and distribution of related seismicity (National Research Institute for Earth Science and Disaster Resilience NIED 2016) indicate that the central to northeastern part of the Futagawa– Hinagu fault zone (FHFZ) ruptured during the mainshock
Summary
A destructive earthquake of Mj 7.3 (Mw 7.0) occurred in the Kumamoto area, southwest Japan, at 01:25 (JST), April 16, 2016, which corresponds to the mainshock of the Kumamoto earthquake sequence (Fig. 1a, b) (Japan Meteorological Agency (JMA) 2016a, b). A large earthquake of Mj 6.5 (Mw 6.2) occurred at 21:26 (JST), April 14, 2016, about 28 h before the mainshock and is interpreted as the largest foreshock of the earthquake sequence (Fig. 1b) (JMA 2016a, b; GSI 2016; NIED 2016). We found that surface ruptures appeared associated with the largest foreshock and enlarged after the mainshock at several sites along the Sugito et al Earth, Planets and Space (2016) 68:170. Another large earthquake of Mj 6.4 (Mw 6.0) occurred at 00:03, April 15, 2016 (Fig. 1b) (JMA 2016a, b; GSI 2016; NIED 2016). We do not think that this earthquake produced surface ruptures along the FHFZ, because the epicenter of this earthquake was several kilometers southwest of the foreshock/mainshock-related ruptures and no surface rupture has been identified in the epicentral area of this earthquake
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