5:00 PM - 5:15 PM
[J02-4-03] Anatomy of the source zones of large earthquakes in Japan
We use high-resolution seismic tomography to study the detailed 3-D structure in the source zones of large earthquakes in Japan to clarify their causal mechanisms. To image the Tohoku megathrust zone, we collected a large number of arrival-time data of suboceanic events (M 3.5-6.0) in the Tohoku forearc under the Pacific Ocean. The suboceanic events are relocated precisely using local arrival times of P and S waves and sP depth phase, and many outer-rise events near the Japan Trench were located by a portable OBS network. Then we inverted the arrival-time data to determine tomographic images of the megathrust zone beneath the Tohoku forearc. The obtained Vp and Vs images are similar to each other, and they show significant velocity variations in the megathrust zone. The 2011 Tohoku megathrust earthquake (Mw 9.0) and its major foreshocks and aftershocks (M > 6.0) occurred in high-velocity (high-V) areas of the megathrust zone, which may represent rigid asperities where the subducting Pacific plate and the overriding Okhotsk plate were strongly coupled. A prominent high-V area with large coseismic slip extends from the Tohoku mainshock hypocenter to the Japan Trench, which may represent the mainshock asperity. We also determined detailed tomographic images in the source zone of the 2011 Iwaki crustal earthquake (M 7.0) and the Fukushima nuclear power plant (FNPP) area. Prominent low-velocity and high Poisson's ratio zones are revealed beneath the Iwaki source area and the FNPP, which may reflect fluids released from the dehydration of the subducting Pacific slab. Our results suggest that the Iwaki earthquake was triggered by the ascending fluids from the Pacific slab dehydration and the stress variation induced by the 2011 Tohoku mainshock. The similar structures beneath the Iwaki source area and the FNPP suggest that the security of the FNPP site should be strengthened to withstand potential large earthquakes in the future.