Crustal Deformation and Fault Models of the 2016 Kumamoto Earthquake Sequence: Foreshocks and Main Shock

Abstract

We explored crustal deformation associated with the foreshocks and the main shock of the 2016 Kumamoto earthquake sequence. We conducted kinematic-Global Navigation Satellite System analysis for the foreshocks, and succeeded in separately retrieving the coseismic crustal deformation for the two M6-class events that occurred nearly contemporaneously (within 3 h). Our fault model shows that the first seismic event occurred in the northern part of the Takano-Shirahata segment of the Hinagu Fault, while the second occurred in the southern part of the segment. For the main shock, we mapped the widely distributed ground displacements in and around the Futagawa Fault zone by conducting an Interferometric Synthetic Aperture Radar analysis. The obtained displacement field shows clear displacement boundaries linearly along the Futagawa and the Hinagu faults, across which the sign of the displacement component turns to the opposite, suggesting that the two faults were intimately involved with the main shock. The previously known fault trace of the Futagawa Fault terminates at the western edge of the Aso Caldera, but the intense deformation implying fault ruptures clearly appears within the caldera. Our fault model suggests that the main rupture occurred on the Futagawa Fault in a right-lateral fashion with normal faulting. The rupture on the Futagawa Fault extends into the Aso Caldera, and the fault plane dips oppositely toward the southeast, suggesting that the rupture propagates eastward on a conjugate fault against the main fault. The rupture on the Hinagu Fault shows a right-lateral fault motion on a plane dipping west.