Abstract
Field investigations and analyses of satellite images and aerial photographs reveal that the 2016 Mw 7.1 (Mj 7.3) Kumamoto earthquake produced a ∼40-km surface rupture zone striking NE-SW on central Kyushu Island, Japan. Coseismic surface ruptures were characterized by shear faults, extensional cracks, and mole tracks, which mostly occurred along the pre-existing NE-SW-striking Hinagu–Futagawa fault zone in the southwest and central segments, and newly identified faults in the northeast segment. This study shows that (i) the Hinagu–Futagawa fault zone triggered the 2016 Kumamoto earthquake and controlled the spatial distribution of coseismic surface ruptures; (ii) the southwest and central segments were dominated by right-lateral strike-slip movement with a maximum in-site measured displacement of up to 2.5 m, accompanied by a minor vertical component. In contrast, the northeast segment was dominated by normal faulting with a maximum vertical offset of up to 1.75 m with a minor horizontal component that formed graben structures inside Aso caldera; (iii) coseismic rupturing initiated at the jog area between the Hinagu and Futagawa faults, then propagated northeastward into Aso caldera, where it terminated. The 2016 Mw 7.1 Kumamoto earthquake therefore offers a rare opportunity to study the relationships between coseismic rupture processes and pre-existing active faults, as well as the seismotectonics of Aso volcano.
Highlights
The Mw 7.1 (Mj 7.3) Kumamoto earthquake occurred on 16 April 2016 (Fig. 1), resulting in extensive damage and more than 50 deaths on Kyushu Island, Japan
The seismic source inversions for the earthquake sequences of M >6 foreshocks and Mw 7.1 main shock show that (i) the total length of fault ruptured zone is up to 40∼50 km and (ii) the southwestern segment of the seismogenic fault was dominated by right-lateral strikeslip mechanism and the northeastern segment had a combination feature of strike-slip and normal faults (Asono and Iwata 2016; Kubo et al 2016). These seismic results are consistent with the field observations that (i) the total length of coseismic surface rupture zone is up to ∼40 km, (ii) the surface deformation in the southwestern segment of the rupture zone is dominated by strike-slip displacement, and (iii) the northeastern segment of the rupture zone was characterized by normaldominated displacement that formed graben structures extending ∼10 km within the west-southwest side of Aso caldera (Lin et al 2016)
Interferometric Synthetic Aperture Radar (InSAR) data were used in this study for comparing the deformation features of ground surfaces along the coseismic surface ruptures observed in the field and detected by the observation data acquired in April 2016 before and after the earthquake, that were released by Geospatial Information Authority of Japan (2016)
Summary
The Mw 7.1 (Mj 7.3) Kumamoto earthquake occurred on 16 April 2016 (Fig. 1), resulting in extensive damage and more than 50 deaths on Kyushu Island, Japan. Seismogenic fault was dominated by right-lateral strikeslip mechanism and the northeastern segment had a combination feature of strike-slip and normal faults (Asono and Iwata 2016; Kubo et al 2016) These seismic results are consistent with the field observations that (i) the total length of coseismic surface rupture zone is up to ∼40 km, (ii) the surface deformation in the southwestern segment of the rupture zone is dominated by strike-slip displacement, and (iii) the northeastern segment of the rupture zone was characterized by normaldominated displacement that formed graben structures extending ∼10 km within the west-southwest side of Aso caldera (Lin et al 2016). In order to determine the motion of the seismogenic fault, ground deformation, and relationships between coseismic surface ruptures and pre-existing faults, our survey group conducted a 10-day field study of structural features, beginning 1 day after the main shock. I discuss the relationships between surface ruptures and pre-existing active faults, as well as their seismotectonic implications
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