Japan Geoscience Union Meeting 2021

Presentation information

[J] Oral

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

[S-CG54] Ten years from the 2011 Tohoku Earthquake: A milestone of Solid Earth Science

Sun. Jun 6, 2021 10:45 AM - 12:15 PM Ch.17 (Zoom Room 17)

convener:Ryota Hino(Graduate School of Science, Tohoku University), Shuichi Kodaira(Research Institute of Marine Geodynamics, Japan Agency for Marine-Earth Science and Technology), Toru Matsuzawa(Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University), Takeshi Iinuma(National Research and Development Agency Japan Agency for Marine-Earth Science and Technology), Chairperson:Shuichi Kodaira(Research Institute of Marine Geodynamics, Japan Agency for Marine-Earth Science and Technology), Ryota Hino(Graduate School of Science, Tohoku University)

11:00 AM - 11:15 AM

[SCG54-08] Inputs to the Japan Trench subduction zone and its implications for the subduction zone processes

★Invited Papers

*Gou Fujie1, Shuichi Kodaira1, Yasuyuki Nakamura1, Seiichi Miura1 (1.Japan Agency for Marine-Earth Science and Technology)

Keywords:oceanic plate, Tohoku earthquake, petit-spot, seismic survey

The megathrust earthquakes in plate subduction zones, such as the 2011 Tohoku earthquake, occur along the boundary between the overriding plate and the subducting oceanic plate. Therefore, the nature of the incoming oceanic plate, such as surface topography, thermal state, lithology, and the water content, affects various processes in subduction zones.
However, the oceanic plate had been poorly studied before. One of the reasons was the difficulties in conducting extensive observations on the oceanic plate because the oceanic plate is generally far from land. However, with the advancement of the various studies in the plate subduction zone, the importance of revealing the nature of the incoming oceanic plate, as the inputs to the subduction zones, has been come to recognized.

Since 2009, We have conducted extensive controlled-source seismic surveys on the oceanic Pacific plate off northeastern Japan arc in order to reveal the nature and spatial variations of the inputs to the Japan Trench subduction zone. We have conducted eight 2-D wide-angle seismic reflection and refraction surveys using ocean bottom seismometers (OBSs) and a large volume airgun array of R/V Kairei and R/V Kaimei of JAMSTEC. The OBS spacing was basically 6-km and we have deployed about 400 OBSs in total. Along all the OBS survey lines, we have obtained multi-channel seismic (MCS) reflection data using a 6-km-long hydrophone streamer cable. In addition, we collected MCS data along more than a hundread of densely alighned short survey lines across the Japan Trench using a relatively small MCS reflection survey system.

Based on these large amount of seismic data, we have revealed systematic structural evolution of the oceanic plate prior to subduction due to the plate bending faulting. In addition, we found the oceanic plate is not laterally so uniform as previously thought. One of the notable spatial variations is the sediment thickness. In most area on the Pacific plate off northeastern Japan arc, the thickness of the sediments is roughly 400 meters. However, there are several areas with extremely thin sediment cover, e.g., ninety percent of the sediment seems to be missing. We investigated the detailed seismic structure beneath the thin sediment cover and found that the areas of thin sediments are heavily disturbed by recent magmatic activities related to the petit-spot.

Intriguingly, the along-trench variations in the sediment thickness on the incoming oceanic plate are well correlates with the extension of the large coseismic slip areas of the 2011 Tohoku earthquake. Therefore, we propose that post-spreading volcanic activities on the oceanic plate before subduction are factors controlling the size and the distribution of the interplate earthquakes after subduction through the disturbance and thermal metamorphism of the sediments.