SUMMARY

Understanding earthquake processes and crustal deformation requires knowledge of the stress concentration process in the crust. With the enhancement of observation networks, it has become possible to consider in detail the relationships between localized deformation and seismic activity in island arcs and the process of stress concentration. According to previous studies, inelastic deformation in localized weak zones in the crust is considered to play an important role in the stress concentration process. Kyushu, located in southwest Japan, has a 20–30 km band-like active seismic activity and an enclosed aseismic zone. In particular, a part of the seismic active region called the Beppu-Simahara Graben, which is dominated by north–south extensional deformation, is characterized by high seismic activity and a remarkable aseismic zone. We identified the relationship between inelastic deformation and stress concentration processes in this area by using analyses of geodetic and seismic data. The results inverted from both the strain rate field obtained by the geodetic observations and the deviatoric stress field estimated from focal mechanism data reveal a large inelastic deformation zone ($\sim {10^{ - 7}} \,\mathrm{ yr}^{-1}$) beneath the area of active seismicity. From comparison with previous works, the inelastic deformation zone in the lower crust may correspond to an area with high temperature and/or fluid. This may suggest that inelastic deformation is in progress in the area where the strength of lower crustal rocks has reduced due to the presence of geothermics and/or fluids. Furthermore, we confirmed that this inelastic deformation causes stress concentrations of up to $10\,\,{\rm{kPa}}\,\,{\rm{yr}}^{-1}$ in the upper crust. These results show that stress concentration occurs locally in the upper crust, above the inelastic deformation zone in the weakened lower crust, owing to the presence of geothermal and/or fluid; this stress concentration induces seismic activity and crustal deformation.

中文翻译：

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从地震和GNSS数据推断日本九州岛地震带及其周围地区的非弹性应变率和应力场

概要

了解地震过程和地壳变形需要了解地壳中的应力集中过程。随着观测网络的增强，有可能详细考虑岛弧中局部变形和地震活动与应力集中过程之间的关系。根据以前的研究，地壳局部薄弱区域的非弹性变形被认为在应力集中过程中起着重要作用。位于日本西南部的九州有20-30公里的带状活跃地震活动和一个封闭的抗震带。特别是，地震活动区的一部分称为Beppu-Simahara Graben，主要由南北向伸展变形控制，其特点是地震活动高，抗震区明显。通过对大地测量和地震数据的分析，我们确定了该地区的非弹性变形与应力集中过程之间的关系。从大地观测获得的应变率场和从震源机制数据估计的偏应力场得到的结果都得到了反转，结果显示出较大的非弹性变形区（$ \ sim {10 ^ {-7}} \，\ mathrm {yr} ^ {-1} $）在地震活动区域下方。与以前的工作相比，下地壳中的非弹性变形区可能对应于具有高温和/或流体的区域。这可能表明由于地热和/或流体的存在，下地壳岩石的强度降低的区域正在进行非弹性变形。此外，我们证实了这种非弹性变形会导致上地壳的应力集中高达$ 10 \，\，{\ rm {kPa}} \，\，{\ rm {yr}} ^ {-1} $。这些结果表明，由于地热和/或流体的存在，应力集中在上地壳局部发生，而在弱化的下地壳中非弹性变形区上方。这种应力集中会引起地震活动和地壳变形。