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Seismic velocity structure at the southern termination of the 2016 Kumamoto Earthquake rupture, Japan
Earth, Planets and Space ( IF 3.362 ) Pub Date : 2020-10-07 , DOI: 10.1186/s40623-020-01276-1
Yasuhira Aoyagi , Haruo Kimura , Kazuo Mizoguchi

The earthquake rupture termination mechanism and size of the ruptured area are crucial parameters for earthquake magnitude estimations and seismic hazard assessments. The 2016 Mw 7.0 Kumamoto Earthquake, central Kyushu, Japan, ruptured a 34-km-long area along previously recognized active faults, eastern part of the Futagawa fault zone and northernmost part of the Hinagu fault zone. Many researchers have suggested that a magma chamber under Aso Volcano terminated the eastward rupture. However, the termination mechanism of the southward rupture has remained unclear. Here, we conduct a local seismic tomographic inversion using a dense temporary seismic network to detail the seismic velocity structure around the southern termination of the rupture. The compressional-wave velocity ( V p) results and compressional- to shear-wave velocity ( V p/ V s) structure indicate several E–W- and ENE–WSW-trending zonal anomalies in the upper to middle crust. These zonal anomalies may reflect regional geological structures that follow the same trends as the Oita–Kumamoto Tectonic Line and Usuki–Yatsushiro Tectonic Line. While the 2016 Kumamoto Earthquake rupture mainly propagated through a low- V p/ V s area (1.62–1.74) along the Hinagu fault zone, the southern termination of the earthquake at the focal depth of the mainshock is adjacent to a 3-km-diameter high- V p/ V s body. There is a rapid 5-km step in the depth of the seismogenic layer across the E–W-trending velocity boundary between the low- and high- V p/ V s areas that corresponds well with the Rokkoku Tectonic Line; this geological boundary is the likely cause of the dislocation of the seismogenic layer because it is intruded by serpentinite veins. A possible factor in the southern rupture termination of the 2016 Kumamoto Earthquake is the existence of a high- V p/ V s body in the direction of southern rupture propagation. The provided details of this inhomogeneous barrier, which are inferred from the seismic velocity structures, may improve future seismic hazard assessments for a complex fault system composed of multiple segments.

中文翻译:

2016 年日本熊本地震破裂南端的地震速度结构

地震破裂终止机制和破裂面积大小是地震震级估计和地震危险性评估的关键参数。2016 年日本九州中部发生的 Mw 7.0 熊本地震沿先前公认的活动断层、二川断层带东部和日奈古断层带最北端破裂了 34 公里长的区域。许多研究人员认为,阿苏火山下方的岩浆房终止了向东破裂。然而,南向破裂的终止机制仍不清楚。在这里,我们使用密集的临时地震网络进行局部地震层析成像反演,以详细描述破裂南端附近的地震速度结构。纵波速度 (Vp) 结果和纵横波速度 (Vp/Vs) 结构表明上地壳中存在几个 E-W-和 ENE-WSW 趋势的带状异常。这些带状异常可能反映了与大分-熊本构造线和臼杵-八代构造线具有相同趋势的区域地质结构。虽然 2016 年熊本地震破裂主要通过沿 Hinagu 断层带的低 V p / V s 区域(1.62-1.74)传播,但在主震震源深度的地震南端与 3 公里-直径高 V p/ V s 身体。在低和高 V p/ V s 区域之间的 E-W 向速度边界上,发震层的深度有一个快速 5 公里的台阶,与六国构造线很好地对应;这一地质边界很可能是造成地震层错位的原因,因为它被蛇纹岩脉侵入。2016 年熊本地震南部破裂终止的一个可能因素是在南部破裂传播方向存在高 V p/ V s 体。从地震速度结构推断出的这种非均匀屏障的详细信息可能会改善未来对由多个片段组成的复杂断层系统的地震危险性评估。
更新日期:2020-10-07
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