The Quaternary Tectonics of Central Kyushu and the 2016 Kumamoto Earthquake: From a Multifaceted Viewpoint Combining Geology, Seismology, and Geodesy

Abstract

The 2016 Kumamoto earthquake occurred in the tectonically complex central Kyushu area where several forcing factors such as the subducting Philippine Sea plate, the Median Tectonic Line and the Nankai forearc sliver, the spreading Okinawa trough, and the migrating volcanic front are involved. NeogeneQuaternary tectonics of central Kyushu are revisited by integrating geological, seismological, and geodetical approaches. Deformation histories of the Futagawa and Hinagu fault zones, the source faults of the Kumamoto earthquake, are also established in an attempt to explain the relationship between geologic structures and rupture processes of the earthquake. The results show that present-day tectonics surrounding central Kyushu are considered to have originated in the last 1 Ma or younger, as a transtensional tectonic zone (Central Kyushu Shear Zone) characterized by combined dextral faults and rift zones (or volcanoes) . Reflecting spatiotemporal variations of the crustal stress field and rift activity, the Futagawa and Hinagu fault zones show multi-stage deformation throughout the NeogeneQuaternary periods: normal faulting to dextral faulting for the Futagawa fault zone and sinistral to dextral faulting for the Hinagu fault zone. Those diverse histories of stress and strain fields in central Kyushu possibly led to the complexities of fault geometry and rupture process of the Kumamoto earthquake.

An Ancient >200 m Cumulative Normal Faulting Displacement Along the Futagawa Fault Dextrally Ruptured During the 2016 Kumamoto, Japan, Earthquake Identified by a Multiborehole Drilling Program

“…The Futagawa fault ruptured by dextral strike-slip motion during the 2016 Kumamoto earthquake mainshock, which might suggest a considerable difference between the paleotectonic stress environment that caused normal faulting and the current tectonic stress state. Oohashi et al (2020) concluded that the Futagawa fault zone showed multistage deformation throughout the Neogene-Quaternary from normal faulting to dextral faulting, reflecting significant changes in the tectonic stress state. Our observations of the large cumulative normal faulting displacement are consistent with the interpretation by Oohashi et al (2020).…”

“…Oohashi et al (2020) concluded that the Futagawa fault zone showed multistage deformation throughout the Neogene-Quaternary from normal faulting to dextral faulting, reflecting significant changes in the tectonic stress state. Our observations of the large cumulative normal faulting displacement are consistent with the interpretation by Oohashi et al (2020).…”

“…The Futagawa fault was surveyed and first defined as a normal fault with a 70-100 m offset dipping north-northwest (Oohashi et al, 2020;Watanabe & Ono, 1969). However, this fault clearly ruptured in a dominantly strikeslip faulting regime during the mainshock of the 2016 Kumamoto earthquake (Shirahama et al, 2016).…”

Overview of the Special Issue “Crustal Dynamics (Part II): Toward the Integration of Geology and Geophysics”

“…A study by Oohashi et al (2020a) is the result of a close collaboration among geologist, geodesist, and seismologist, and presents an integrated view of the Quaternary tectonics of central Kyushu obtained from field-based structural analyses of geology and instrumentalbased stress/strain analyses of geophysics. The BeppuShimabara graben, which has long been debated from geological viewpoints, is rein-terpreted from the on-going tectonic process revealed by the latest geophysical observations.…”