[S-CG63] Dynamics in mobile belts

[SCG63-13] Resistivity structure around the Atotsugawa fault system revealed by inversion schemes combining Wideband- and Network-MT methods

★Invited Papers

*Yoshiya Usui^1,2, Makoto Uyeshima¹, Tsutomu Ogawa¹, Ryokei Yoshimura³, Naoto Oshiman³, Satoru Yamaguchi⁴, Hiroaki Toh⁵, Hideki Murakami⁶, Koki Aizawa⁷, Toshiya Tanbo⁸, Yasuo Ogawa⁹, Tadashi Nishitani¹⁰, Shin'ya Sakanaka¹⁰, Masaaki Mishina¹¹, Hideyuki Satoh¹², Tada-nori Goto¹³, Takafumi Kasaya¹⁴, Toru Mogi¹⁵, Yusuke Yamaya¹⁶, Ichiro Shiozaki¹⁷, Yoshimori Honkura¹⁸ (1.Earthquake Research Institute, the University of Tokyo, 2.ITOCHU Techno-Solutions Corporation, 3.Disaster Prevention Research Institute, Kyoto University, 4.Graduate School of Science, Osaka City University, 5.Graduate School of Science, Kyoto University, 6.Natural Sciences Cluster-Science and Technology Unit, Research and Education Faculty, Kochi University, 7.Institute of Seismology and Volcanology, Faculty of Science, Kyushu University, 8.Tateyama Caldera Sabo Museum, 9.Volcanic Fluid Research Center, School of Science, Tokyo Institute of Technology, 10.Graduate School of International Resource Sciences, Akita University, 11.Graduate School of Science, Tohoku University, 12.Nuclear Regulation Department, Nuclear Regulation Authority, 13.Graduate School of Life Science, University of Hyogo, 14.Japan Agency for Marine–Earth Science and Technology, 15.Faculty of Science, Hokkaido university, 16.National Institute of Advanced Industrial Science and Technology, 17.Graduate School of Engineering, Tottori University, 18.Graduate School of Science and Engineering, Tokyo Institute of Technology)

Keywords:new inversion scheme, wideband and Network MT method, lower-crustal fluid, fault-zone conductor, ductile shear zone, upper mantle fluid

The Atotsugawa fault system (central Japan) in the Niigata-Kobe tectonic zone is one of the best field to research the physical mechanism of strain accumulation along earthquake faults because of its potential of causing large earthquakes (~M7.0), high deformation rate and existence of many geophysical studies.

In order to elucidate the strain accumulation mechanism around the Atotsugawa fault system, we investigated the resistivity structure around the fault system. To delineate a reliable resistivity structure from the upper crust to the uppermost mantle, we estimated the resistivity structure by combined inversion schemes of the wideband-magnetotelluric method and the Network-MT method. In the Network-MT method, metallic telephone cables several kilometers in length are used to measure electric potential differences.

By a two-dimensional combined inversion scheme, we revealed that, in the lower crust, there are localized conductive areas below the Atotsugawa fault, the Ushikubi fault and the Takayama-Oppara fault zone. We interpreted that the lower-crustal conductors are localized ductile shear zones with highly connected fluid, being responsible for the strain accumulation along the respective active faults just above the conductive areas. In addition, in the mantle wedge under the Atotsugawa fault system, a large conductive area was imaged, which may be attributed to the fluid dehydrated from the Philippine Sea slab and/or the Pacific slab.

In addition, so as to evaluate the effects of three-dimensional distortion to the resistivity structure, such as caused by the land-sea boundary at the north of the Atotsugawa fault system, we newly developed a three-dimensional inversion scheme combining the wideband-magnetotelluric method and the Network-MT method.

In this presentation, we review the results of the two-dimensional modeling and show an early result of the three-dimensional modeling.