Japan Geoscience Union Meeting 2015

Presentation information


Symbol S (Solid Earth Sciences) » S-CG Complex & General

[S-CG57] Structure, evolution and dynamics of mobile belts

Thu. May 28, 2015 9:00 AM - 10:45 AM IC (2F)

Convener:*Toru Takeshita(Department of Natural History Sciences, Graduate School of Science, Hokkaido University), Hiroshi Sato(Earthquake Prediction Research Center, Earthquake Research Institute, The University of Tokyo), Koichiro Obana(Research and Development Center for Earthquake and Tsunami, Japan Agency for Marine-Earth Science and Technology), Takuya NISHIMURA(Disaster Prevention Research Institute, Kyoto University), Yukitoshi Fukahata(Disaster Prevention Research Institute, Kyoto University), Aitaro Kato(Graduate School of Environmental Studies, Nagoya University), Jun Muto(Department of Earth Sciences, Tohoku University), Katsushi Sato(Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University), Shuichi Kodaira(Institute for Research on Earth Evolution Japan Agency for Marine-Earth Science and Technology), Takeshi Sagiya(Disaster Mitigation Research Center, Nagoya University), Tatsuya Ishiyama(Earthquake Research Institute, University of Tokyo), Makoto MATSUBARA(National Research Institute for Earth Science and Disaster Prevention), Yasutaka Ikeda(Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo), Chair:Tadashi Yamasaki(National Institute of Advanced Industrial Science and Technology)

9:15 AM - 9:30 AM

[SCG57-11] 3-D Resistivity imaging of source regions of the Iwaki normal faulting sequences

*Makoto UYESHIMA1, Yasuo OGAWA2, Masahiro ICHIKI3 (1.Earthquake Research Institute, The University of Tokyo, 2.Volcanic Fluid Research Center Tokyo Institute of Technology, 3.Graduate School of Science, Tohoku University)

Keywords:3-D resistivity structure, source region of the Iwaki earthquake, localized crustal fluids

Following the 2011 Tohoku-Oki earthquake, M9.0, several areas of the inland Japan were activated due to significant change of the stress field. Among all, intense swarm-like seismicity associated with shallow normal faulting was induced in Ibaraki and Fukushima prefectures in the boundary area between Kanto and Tohoku districts, Japan. In order to elucidate a high-resolution model of crustal resistivity structure in this region and to get insights on causes of those induced earthquakes, MT surveys were performed in Jan. 2012 and from Dec. 2013 to Jan. 2014, by using Phoenix and Metronix Wideband MT instruments.
After estimating impedance tensors and induction vectors with the aid of the BIRRP code (Chave and Thomson, 2004), a 3-D phase tensor (PT) and induction vector (IV) inversion code was applied to the dataset. In order to investigate the influence of the initial model on the final structural model, we did several inversion runs with initial resistivity values ranging from 20 to 2000 Ohm-m. All the inversion runs could get respective final models with RMS of around 2. Although some differences in the final models are detected, overall characteristics and scales (in length and intensity) are similar for all the final models. Generally, induced earthquakes are distributed in the higher electrical resistivity zones. We delineated a separate low-resistivity anomaly directly beneath the hypocenter of the largest earthquake in the sequence (the M7.0 Iwaki earthquake), indicating crustal fluids in this region. Together with previously obtained seismic image (Kato et al., 2013), we hypothesize that strong crust underwent structural failure due to the infiltration of crustal fluids into the seismogenic zone from deeper levels, or stress accumulation on the edge of the isolated weak portion, causing the Iwaki earthquake.