Japan Geoscience Union Meeting 2015

Presentation information

International Session (Oral)

Symbol S (Solid Earth Sciences) » S-IT Science of the Earth's Interior & Techtonophysics

[S-IT05] Hard-Rock Drilling: Oceanic Lithosphere to Island Arc Formation and Beyond

Wed. May 27, 2015 11:00 AM - 12:45 PM 303 (3F)

Convener:*Yoshihiko Tamura(R & D Center for Ocean Drilling Science, Japan Agency for Maine-Earth Science and Technology), Yildirim Dilek(Department of Geology & Environmental Earth Science Miami University), Tomoaki Morishita(School of Natural System, Colleage of Science and Technology, Kanazawa University), Takashi Sano(Department of Geology and Paleontology, National Museum of Nature and Science), Natsue Abe(R&D Center for Ocean Drilling Science Japan Agency for Marine-Earth Science and Technology), Susumu Umino(Department of Earth Sciences, Kanazawa University), Eiichi TAKAZAWA(Department of Geology, Faculty of Science, Niigata University), Shigeaki Ono(Japan Agency for Marine-Earth Science and Technology), Katsuyoshi Michibayashi(Institute of Geosciences, Shizuoka University), Chair:Natsue Abe(R&D Center for Ocean Drilling Science), Shigeaki Ono(Japan Agency for Marine-Earth Science and Technology), Katsuyoshi Michibayashi(Institute of Geosciences, Shizuoka University)

11:00 AM - 11:15 AM

[SIT05-12] Earthquake activity in the Pacific plate near the Japan Trench axis after the 2011 Tohoku-Oki Earthquake

*Koichiro OBANA1, Gou FUJIE1, Yasuyuki NAKAMURA1, Shuichi KODAIRA1 (1.JAMSTEC)

Keywords:Intra-plate earthquake, horst and graben, normal faulting, OBS

Intra-plate normal-faulting earthquakes near oceanic trenches likely associate with bending of the incoming/subducting plates. Focal mechanisms of the intra-plate normal-faulting earthquakes in trench-outer slope area suggest tensional stress at shallow depths, which may promote infiltration of seawater several tens of kilometers into the oceanic lithosphere. Recent seismic structural studies in the trench-outer slope area present seismic velocity changes in the oceanic plate approaching the trench accompanied by the development of bending-related faults cutting the oceanic crust (e.g., Fujie et al., 2013, Grevemeyer et al., 2007). However, details on hypocenter locations, especially in depths, of outer-trench normal-faulting earthquakes and relation to the crustal structures have not been well understood due to less frequent activity than inter-plate earthquakes and lack of near-field observations. After the 2011 Tohoku-Oki Earthquake (Mw 9.0), shallow normal-faulting seismicity has been active in the incoming/subducting Pacific plate near the Japan Trench (e.g., Asano et al., 2011). To investigate the stress state in the incoming/subducting Pacific plate near the trench axis and relations between earthquakes and crustal structures, we have conducted a series of ocean bottom seismograph (OBS) observations near the trench axis since the occurrence of the 2011 Tohoku-Oki earthquake. These OBS observations provide accurate hypocenter locations and focal mechanisms of earthquakes occurred in the Pacific plate. Earthquakes with a normal-faulting focal mechanism occurred at depths of shallower than 40 km beneath the outer slope of the Japan Trench. The normal-faulting earthquakes in the oceanic crust coincide with normal-faults cutting the oceanic crust and forming horst and graben structures. The hypocenter distributions and T-axes directions suggest earthquakes activity along pre-existing structures in the oceanic crust in addition to the trench-parallel normal faults. Both the pre-existing structures, such as fracture zones, and trench-parallel normal faults formed in the trench outer slope area could act as faults of the shallow normal-faulting earthquakes. Furthermore, the normal-faulting earthquakes occurred at deeper depths compared with the OBS observations before the 2011 earthquake by Hino et al. (2009). The 2011 Tohoku-Oki Earthquake likely changed the stress state in the Pacific plate. These observations suggest that stress regime in the oceanic lithosphere, which could change in temporal and spatial, and both pre-existing and newly created faults in the oceanic crust are important factor to understand the hydration of the oceanic plate prior to the subduction.