Presentation Information
[T14-O-1][Invited] Seismotectonics, volcanism and subduction dynamics in Kyushu: New insights from seismic tomography
*Dapeng ZHAO1 (1. Tohoku Univ.)
【ハイライト講演】 九州地域はフィリピン海プレートが沈み込む成熟した島弧であり、沖縄トラフと中央構造線の会合部であり、火山弧と別府－島原地溝が交差する、世界でも特異な地域である。趙大鵬博士はこれまで、地震波トモグラフィーを用いて九州地域の地下構造、プレートや流体と地震火山活動の関係など数々の重要な指摘をしてきた。趙博士による九州の地震火山活動とフィリピン海プレートの沈み込ダイナミクスは必見である。 ※ハイライト講演とは．．．
Keywords:
Kyushu,earthquake,volcano,subduction zone,seismic tomography
Kyushu is a mature and typical subduction zone where the Philippine Sea (PHS) plate is subducting beneath the Eurasian plate along the Nankai Trough and the Ryukyu Trench at a rate of 4-5 cm/year, causing strong seismic and volcanic activities. In the past decade we have used seismological methods to study seismotectonics, volcanism and subduction dynamics of the Kyushu region. The main findings are summarized as follows.
Significant structural heterogeneities in the source zone of the 2016 Kumamoto earthquake (M7.3) are revealed by high-resolution seismic velocity and attenuation tomography (Wang et al. 2017, 2018; Zhao et al. 2018). Its mainshock and major foreshock and aftershocks took place in a high-velocity and high-Q zone in the upper crust, whereas low-velocity and low-Q anomalies exist in the lower crust and uppermost mantle, suggesting that fluids ascending from the mantle wedge affected the generation of the Kumamoto earthquake sequence.
Spatial and temporal variations of the stress field are revealed in the Kumamoto source zone (Yu et al. 2019). The friction coefficient of the seismogenic faults is estimated to be relatively small (~0.4), indicating that the active faults in northern-central Kyushu are weak. The fault weakening may be caused by fluids beneath the source area and arc magma under the nearby Aso active volcano.
The geometry of the subducting PHS slab beneath Kyushu is estimated by using local seismicity, local earthquake tomography, converted and reflected waves, and receiver functions. Teleseismic tomography has revealed that the PHS slab has subducted aseismically down to ~450 km depth beneath the Tsushima Strait (Zhao et al. 2021). The PHS slab exhibits significant lateral variations in its lithospheric age from 20 to 42 Myr in and around Kyushu (Hua et al. 2018).
The Beppu-Shimabara graben in northern-central Kyushu was produced by the joint effect of three factors: northward extension of the opening Okinawa Trough, westward extension of the Median Tectonic Line, and hot and wet upwelling flows in the mantle wedge beneath active volcanoes (Zhao et al. 2018)
The generation of large crustal earthquakes in Kyushu is affected by structural heterogeneities in the crust and upper mantle, in particular, arc magma and fluids associated with dehydration reactions of the PHS slab. Large earthquakes do not strike anywhere randomly, but only anomalous areas that can be detected by geophysical imaging (Zhao et al. 2018, 2021).
References
Hua, Y., D. Zhao et al. (2018). Age of the subducting Philippine Sea slab and mechanism of low-frequency earthquakes. Geophys. Res. Lett., 45, 2303-2310.
Wang, H., D. Zhao et al. (2018). Crustal tomography of the 2016 Kumamoto earthquake area in West Japan using P and PmP data. Geophys. J. Int., 214, 1151-1163.
Wang, Z., D. Zhao et al. (2017). Seismic attenuation tomography of the source zone of the 2016 Kumamoto earthquake (M7.3). J. Geophys. Res., 122, 2988-3007.
Yu, Z., D. Zhao et al. (2019). Stress field in the 2016 Kumamoto earthquake (M7.3) area. J. Geophys. Res., 124, 2638-2652.
Zhao, D., K. Yamashita, G. Toyokuni (2018). Tomography of the 2016 Kumamoto earthquake area and the Beppu-Shimabara graben. Sci. Rep., 8, 15488.
Zhao, D., J. Wang, Z. Huang, X. Liu (2021). Seismic structure and subduction dynamics of the western Japan arc. Tectonophysics 802, 228743.