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Watanabe, Tsuyoshi; Hiratsuka, Shinya*; Shimada, Koji
no journal, ,
Fluid has a significant influence on the seismogenic process because it reduces the fracture strength of rocks. Fluid migration also affects the long-term safe confinement function of a geological disposal systems. To clarify the existence and the migration channels of such fluid, shear wave splitting analysis is a useful method, which can estimate the anisotropy related to cracks and fluid in the crust. We conducted the shear wave splitting analysis in Iwaki area, northeast Japan, where seismic activity was increased and anomalous discharge of hot springs was observed about one month after the 2011 Tohoku Earthquake (Mw 9.0). As a result, we showed that the crack orientations turn to generally arc-parallel direction in Iwaki area, and additionally, that the spatial distribution of anisotropic intensity is relatively high at a seismic site near the hot springs with anomalous discharge. This suggests that fluid from deep underground gather in the crust in steady-state.
Murakami, Osamu; Watanabe, Tsuyoshi
no journal, ,
In order to reveal the crustal anisotropy structure around Mt. Ontake, we applied shear-wave splitting analysis to the seismic records observed at the permanent seismic stations. We obtained two shear-wave splitting parameters at each event-station pair: the fast shear-wave polarizations, and time delays between the arrival times of the fast shear-wave and the ones of the slow shear-wave. Although some stations show the fast shear-wave polarizations near parallel to the direction of the maximum regional compressional stress, others show the fast shear-wave polarizations near perpendicular to the direction of the maximum regional compressional stress.