Seismic velocity measurements have revealed that the Tohoku-Oki earthquake affected velocity structures of volcanic zones far from the epicenter. Using a seismological method based on ambient seismic noise interferometry, we monitored the anisotropy in the Mount Fuji area during the year 2011, in which the Tohoku-Oki earthquake occurred (Mw = 9.0). Here we show that even at 400 km from the epicenter, temporal variations of seismic anisotropy were observed. These variations can be explained by changes in the alignment of cracks or fluid inclusions beneath the volcanic area due to stress perturbations and the propagation of a hydrothermal fluid surge beneath the Hakone hydrothermal volcanic area. Our results demonstrate how a better understanding of the origin of anisotropy and its temporal changes beneath volcanoes and in the crust can provide insight into active processes, and can be used as part of a suite of volcanic monitoring and forecasting tools.
We measure shear wave splitting and estimate stresses of Mount Fuji, Japan, to interpret anisotropic structure and its implication for geologic processes using local crustal earthquake seismograms from 2009 to 2012. The measured fast polarizations have preferred orientations at each station with mean values of delay times <0.15 s. We infer that the anisotropic structure is located at shallow depths (<4 km) from a lack of focal depth dependence of delay times. The fast polarization directions for stations within approximately 15 km of the summit of Mount Fuji show a radial pattern pointing toward the summit, while stations far from the summit exhibit fast polarization directions approximately parallel to the NW-SE compressional regional stress field. We infer that the symmetrical seismic anisotropic structure around the summit and the fast directions parallel to the regional compression observed at distant stations from the summit reflect interactions of the gravitational stresses and regional tectonics. Assuming stress control only, the spatial pattern of anisotropy can be fit by the interaction of gravitational with regional stresses if the regional maximum horizontal stress is 1.02 times lithostatic pressure (51.9 MPa at a depth of 2.0 km). If structural anisotropy also contributes to the radial pattern, then the regional maximum horizontal stress magnitude is not constrained.
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