Japan Geoscience Union Meeting 2019

Presentation information

[J] Poster

S (Solid Earth Sciences ) » S-SS Seismology

[S-SS15] Active faults and paleoseismology

Tue. May 28, 2019 1:45 PM - 3:15 PM Poster Hall (International Exhibition Hall8, Makuhari Messe)

convener:Mamoru Koarai(Earth Science course, College of Science, Ibaraki University), Takashi OGAMI(National Institute of Advanced Industrial Science and Technology), Ryosuke Doke(Hot Springs Research Institute of Kanagawa Prefecture), Hisao Kondo(Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology)

[SSS15-P15] Electrical resistivity structure from the surface to the seismogenic zone below the Gomura fault zone, Kyoto, Southwest Japan

*Satoru Yamaguchi1, Akira Mimura1, Kazuki Fukue1, Shigehiro Katoh2, Hideki Murakami3, Makoto Uyeshima4 (1.Department of Geosciences, Graduate School of Science, Osaka City University, 2.The Museum of Nature and Human Activities, Hyogo, 3.Science and Technology Unit, Research and Education Faculty, Kochi University, 4.Earthquake Research Institute, The University of Tokyo )

Keywords:Gomura fault, Electrical conductivity, active fault, Magnetotellurics

The Gomura Fault Zone in the Tango Peninsula, Kyoto, consists of three fault segments, the Go-seihou, Gomura, and Chuzenji Faults. They show different features in fault activity (average slip rates, the latest events etc.). The Go-seihou Fault is a very short in length (~2.8km long) with no clear displacement. Along the Gomura Fault, a clear surface rupture has appeared at the 1927 Kita-Tango earthquake. Cumulative geomorphological displacement is well recognized along the Chuzenji Fault, but no surface rupture appeared at the 1927 Kita-Tango earthquake. Conductivity structure can be one of the crucial elements to clarify subsurface structure of active faults. Furthermore, the Gomura fault zone is especially suitable for studying the relationships between conductivity structure and fault activity because three faults run nearly parallel within about 3km in simple geological setting.
We proposed the conductivity models of the Gomura Fault Zone combining the two conductivity models; one is the shallow model (0 – 1.5 km in depth) and the deep model (0 -12 km in depth). The shallow model is determined based on the result obtained using an audio-frequency magnetotelluric (AMT) method which is excellent in shallow part resolution. While, the deep model is determined based on the result using a wide-band magnetotelluric (WBMT) method which is suitable for wide- and deep-range survey.
The final model is characterized and interpreted as follows.
(1) The final model covers from the surface to a depth of 12 km including the epicenter of the 1927 Kita-Tango earthquake and three faults (the Go-seihou, Gomura and Chuzenji Faults).
(2) The final model is characterized by six conductive zones and one resistive zone.
(3) The fault plane of the Gomura Fault starts the hypocenter of the 1927 Kita-Tango earthquake, goes through seismogenic and conductive region, and reaches to the Gomura Fault.
(4) The clear conductive zones were detected below both the Gomura and Chuzenji Faults in the shallow model. The conductive zone below the Gomura Fault is wider and more conductive than that below the Chuzenji Fault. We interpreted these differences reflect the latest faulting event age: that of the Gomura Fault (~90 yrs. ago) is much younger than that of the Chuzenji Fault. The conductive zones are formed by intruding groundwater into a damage zone around the faults.
(5) No conductive zone is recognized below the Go-seihou Fault and the result indicates that the fault is a secondary fault.
(6) The bottom of the resistive zone can be interpreted as that of the Miyazu granite.