Tomography of the 2016 Kumamoto earthquake area and the Beppu-Shimabara graben

Abstract

Detailed three-dimensional images of P and S wave velocity and Poisson’s ratio (σ) of the crust and upper mantle beneath Kyushu in SW Japan are determined, with a focus on the source area of the 2016 Kumamoto earthquake (M 7.3) that occurred in the Beppu-Shimabara graben (BSG) where four active volcanoes and many active faults exist. The 2016 Kumamoto earthquake took place in a high-velocity and low-σ zone in the upper crust, which is surrounded and underlain by low-velocity and high-σ anomalies in the upper mantle. This result suggests that, in and around the source zone of the 2016 Kumamoto earthquake, strong structural heterogeneities relating to active volcanoes and magmatic fluids exist, which may affect the seismogenesis. Along the BSG, low-velocity and high-σ anomalies do not exist everywhere in the upper mantle but mainly beneath the active volcanoes, suggesting that hot mantle upwelling is not the only cause of the graben. The BSG was most likely formed by joint effects of northward extension of the Okinawa Trough, westward extension of the Median Tectonic Line, and hot upwelling flow in the mantle wedge beneath the active volcanoes.

Stress Field in the 2016 Kumamoto Earthquake (M 7.3) Area

“…This σ 3 direction is parallel to the spreading direction at the Okinawa Trough (Figure ; Tada, ). We agree to the notion that the extensional stress regime in central Kyushu is attributed to the rollback of the PHS plate and the northward extension of the Okinawa Trough (Matsumoto, Nakao, et al, ), as well as the existence of hot and wet mantle upwelling under the Beppu‐Shimabara graben (Zhao et al, ). The PHS plate is located southeast of the Kyushu Island (Figure ) and drives the extension on Kyushu by its rollback, which has induced the back‐arc spreading at the Okinawa Trough (Kamata & Kodama, ).…”

“…The σ 3 axes are stably determined in the three areas, being oriented NNW‐SSE and nearly horizontally with plunges of 2–7° (Figure ). The dominant direction of the σ 3 axis is roughly parallel to the spreading direction at the Okinawa Trough (Figure a; Tada, ), indicating ongoing extensional tectonics at present (Matsumoto, Nakao, et al, ; Zhao et al, ). The stress ratio also shows spatial variations (Figure ).…”

“…Significant seismic velocity variations in the crust and upper mantle beneath the Kumamoto source area have been revealed by many tomographic studies (e.g., Liu & Zhao, , ; J. Wang & Zhao, ; Z. W. Wang et al, ; Yu et al, ; Zhao et al, ). Our results show that the 3‐D velocity model leads to a decrease of the number of inconsistent polarity data and so a better stress inversion result (Figure ), similar to the study of the stress field in the 1994 Northridge earthquake area (Zhao et al, ).…”

“…One reason for the weak faults may be the effect of arc magma and fluids ascending from the mantle wedge to the crust (e.g., Nimiya et al, ; Z. W. Wang et al, ; Yagi et al, ; Yoshida et al, ; Zhao et al, ). Recent tomographic studies show that the 2016 Kumamoto earthquake occurred in a strong patch (with a low attenuation and a high velocity) in the upper crust but was underlain by arc magma and fluids in the lower crust and upper mantle wedge (with a high attenuation and a low velocity; H. Wang et al, ; Z. W. Wang et al, ; Zhao et al, ).…”

“…The average horizontal slip rate of the Futagawa‐Hinagu fault zone has been ~0.88 mm/year in the late Quaternary (Tsutsumi & Okada, ). Obviously, the nucleation and rupture processes of the 2016 Kumamoto earthquake were related to the Futagawa‐Hinagu fault zone (Asano & Iwata, ; Kato et al, ), whereas several recent studies have suggested that the earthquake generation was also affected by arc magma and fluids in the Kyushu subduction zone (e.g., Lin et al, ; Z. W. Wang et al, ; Yagi et al, ; Yoshida et al, ; Zhao et al, ).…”