A 3D electrical resistivity model around the focal zone of the 2017 southern Nagano Prefecture earthquake (MJMA 5.6): implications for relationship between seismicity and crustal heterogeneity
A 3D electrical resistivity model around the focal zone of the 2017 southern Nagano Prefecture earthquake (MJMA 5.6): implications for relationship between seismicity and crustal heterogeneity
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DOI：
10.1186/s40623-018-0950-1
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发表时间：
2018-11-21
影响因子：
3
通讯作者：
Adachi, Mamoru
中科院分区：
文献类型：
--
作者：
Ichihara, Hiroshi;Kanehiro, Junna;Adachi, Mamoru
日本中部Ontake火山东南侧下方的震群区为研究地震活动、火山过程和地壳流体之间的相互作用提供了重要机会。2017年6月25日，在Ontake群地区发生了M5.6地震，地球化学和地球物理研究表明，来自下地壳和地幔的孔隙流体通道影响了断层破裂。为了弄清电阻率分布（反映孔隙流体、蚀变沉积物和温度），在这次地震余震区周围的35个地点测量了音频和宽带大地电磁数据。基于这些观测到的大地电磁数据的三维电阻率反演模型显示了以下主要特征：（1）两个导电带（C-1和C-2）位于泉水之下，同位素研究表明流体来自地幔或下地壳;（2）余震震源位于这两个导电带（C-1和C-2）之间的电阻区。地震活动性与电导率之间的关系表明，C-1和C-2导体可以解释为相互关联的孔隙流体，高温和/或沉积物在非弹性条件下。此外，5.6级地震的断层破裂位于中央电阻和导电C-2带之间的边界附近，表明应力积累与岩石、温度和/或孔隙流体分布的非均匀性有关。如果在地震区普遍观察到这些特征，则对电阻率结构的调查可能有助于估计潜在地震的震级和评估风险。
Seismic swarm areas below the southeast flank of Ontake volcano, central Japan, provide an important opportunity to study interactions between seismicity, volcanic processes and crustal fluid. On June 25, 2017, an M5.6 earthquake occurred in the Ontake swarm area where geochemical and geophysical studies suggest that pore fluid pathways from the lower crust and mantle affect fault rupture. To clarify the electrical resistivity distribution (that reflects pore fluids, altered sediments and temperature), audio-frequency and broadband magnetotelluric data were measured at 35 sites around the aftershock area of this earthquake. A 3D resistivity inversion model based on these observed magnetotelluric data shows the following key features: (1) two conductive zones (C-1 and C-2) underlie springs where isotope studies indicate fluids of mantle or lower crustal origin and (2) aftershock hypocentres locate in a resistive area between these two aseismic conductive zones (C-1 and C-2). The relationship between seismicity and conductivity suggests that the C-1 and C-2 conductors can be interpreted as interconnected pore fluid, high temperature and/or sediment under aseismic elastic conditions. In addition, the fault rupture of the M5.6 earthquake was located near the boundary between the central resistive and conductive C-2 zone, indicating stress accumulation associated with heterogeneity of rock, temperature and/or pore fluid distribution. If these features are observed generally in seismic areas, surveys of resistivity structure could contribute to estimating the magnitude of potential earthquakes and evaluation of risk.