Electrical Resistivity Structure Around the Atotsugawa Fault, Central Japan, Revealed by a New 2‐D Inversion Method Combining Wideband‐MT and Network‐MT Data Sets
Electrical Resistivity Structure Around the Atotsugawa Fault, Central Japan, Revealed by a New 2‐D Inversion Method Combining Wideband‐MT and Network‐MT Data Sets
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DOI：
10.1029/2020jb020904
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发表时间：
2021-02
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影响因子：
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通讯作者：
Y. Usui;M. Uyeshima;T. Ogawa;R. Yoshimura;N. Oshiman;S. Yamaguchi;H. Toh;H. Murakami;K. Aizawa;Toshiya Tanbo;Y. Ogawa;T. Nishitani;S. Sakanaka;M. Mishina;H. Satoh;T. Goto;T. Kasaya;T. Mogi;Y. Yamaya;I. Shiozaki;Y. Honkura
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文献类型：
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作者：
Y. Usui;M. Uyeshima;T. Ogawa;R. Yoshimura;N. Oshiman;S. Yamaguchi;H. Toh;H. Murakami;K. Aizawa;Toshiya Tanbo;Y. Ogawa;T. Nishitani;S. Sakanaka;M. Mishina;H. Satoh;T. Goto;T. Kasaya;T. Mogi;Y. Yamaya;I. Shiozaki;Y. Honkura
Atotsugawa断层是日本最活跃的断层之一，断层处的应变积累被认为是由富含流体的下地壳中的一个地壳剪切带引起的。为了识别剪切带并调查下地壳中含水流体的起源，我们在断层周围部署了一个网络MT测量以及一个传统的宽带MT测量，并结合两个MT数据集进行了反演。在反演中，通过修改传统的反演算法，我们精确地表示了网络MT测量的千米级偶极子，以提供对电阻率结构的约束。在研究区下地壳中，Atotsugawa断层、Ushikubi断层和高山-Oppara断层带下方存在局部导电异常。将我们的电阻率结构与地震速度结构进行比较，我们解释说，下地壳导体是具有高度连通流体的局部韧性剪切带。我们认为，局部化的韧性剪切带是造成沿活动断裂沿着应变积累的主要原因。此外，在俯冲菲律宾海板片及其向下延伸的地幔楔中，还发现了高导部分，这可能是菲律宾海板片和太平洋板片脱水的流体所致。大面积导电区的存在支持了以前的地震和地球化学研究的建议，即Atotsugawa断层周围的下地壳流体起源于俯冲板。
The Atotsugawa fault is one of the most active faults in Japan, and the strain accumulation at the fault is considered to be caused by an aseismic shear zone in the fluid‐rich lower crust. To identify the shear zone and investigate the origin of the aqueous fluid in the lower crust, we deployed a Network‐MT survey in addition to a conventional wideband‐MT survey around the fault and performed an inversion combining both the MT data sets. In the inversion, by modifying a conventional inversion algorism, we accurately represented kilometer‐scale dipoles of the Network‐MT measurement to provide constraints on the electrical resistivity structure. In the lower crust under the study area, there are localized conductive anomalies below the Atotsugawa fault, the Ushikubi fault, and the Takayama‐Oppara fault zone. Comparing our electrical resistivity structure with the seismic velocity structure, we interpreted that the lower‐crustal conductors are localized ductile shear zones with highly connected fluid. We considered that the localized ductile shear zones are responsible for the strain accumulation along the respective active faults. In addition, in the mantle wedge above the subducting Philippine Sea slab and its downward extension, a highly conductive portion is detected, which may be attributed to the fluid dehydrated from the Philippine Sea slab and/or the Pacific slab. The existence of the large conductive area supports the suggestion of previous seismic and geochemical studies that the fluid of the lower crust around the Atotsugawa fault originated from subducting slabs.