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

Wed. May 27, 2015 2:15 PM - 4:00 PM A06 (APA HOTEL&RESORT TOKYO BAY MAKUHARI)

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:Eiji Kurashimo(Earthquake Research Institute, the University of Tokyo)

2:45 PM - 3:00 PM

[SCG57-03] Seismic structure in the southern Ryukyu Trench subduction zone

*Yojiro YAMAMOTO1, Tsutomu TAKAHASHI1, Yuka KAIHO1, Yasushi ISHIHARA1, Ryuta ARAI1, Ayako NAKANISHI1, Koichiro OBANA1, Seiichi MIURA1, Shuichi KODAIRA1, Yoshiyuki KANEDA2 (1.JAMSTEC, 2.Nagoya University)

In the Ryukyu Trench subduction zone, many large earthquakes occurred historically. Recent seismic and geodetic studies indicate that the occurrence of very low frequency earthquake [Ando et al., 2012] and slow slip events [Heki and Kataoka, 2008; Nishimura, 2014] in the southern Ryukyu subduction zone. In addition, the result of offshore geodetic observation showed interplate coupling occurs near the trench [Nakamura et al., 2010], where plausible seismogenic zone of the 1771 Yaeyama earthquake (Mw 8.0) is located [Nakamura, 2009]. These results suggest that the interplate coupling is not so weak and it is possible for the large interplate earthquake to occur in this region. However, not only the fault plane geometry of past large earthquakes but also the local seismic structure is uncertain due to the sparse seismic observation network. To investigate the hypocenter distribution and the subducted plate geometry, we have conducted the passive seismic observation using 6 land stations and 30 ocean bottom seismographs (OBSs) from Nov. 2013 to Mar. 2014, as a part of “Research project for compound disaster mitigation on the great earthquakes and tsunamis around the Nankai trough region”.
We performed a seismic tomography to estimate the hypocenter location and plate geometry by using a part of obtained data, although the data picking is still in progress. The initial P-wave model was established by referring the result of active source survey [Arai et al., 2014], and the initial S-wave model was calculated by assuming a Vp/Vs value of 1.73. As the initial S-wave velocity model did not include the low-velocity sediment layer just beneath the OBSs, we calculated a station correction value for the S-wave arrival data by using the differential times of arrivals between PS converted waves and direct P-waves.
Preliminary result shows northwestern dipping hypocenter distribution and low velocity layer in the forearc region. We interpreted this layer as the subducted oceanic crust. In that case, most of earthquakes located within the oceanic crust and the uppermost oceanic mantle, and the dip angle of plate boundary gradually increased from 10 degrees near the trench axis to 30 degrees beneath the island arc. We will add more data and estimate more detail relationship between earthquake location and plate geometry.