Forward modeling of stress fields around Mt. Fuji and its implication for seismic anisotropy
Abstract:Seismic anisotropy is used to constrain stresses or geologic structures. In volcanic areas, distinguishing characteristics of the stress is important for understanding geologic processes at depths. We measure shear wave splitting of Mt. Fuji. In this area, the geophysical environment includes regional stress fields, the gravitational effect of the volcanic edifice, and stress perturbations caused by the 2011 Tohoku-Oki Earthquake and a Mw 5.9 event on 15 March 2011. Measurement of splitting can be a good indicator of the interaction of these factors. We used data from 2009 to 2012 at 24 seismic stations around the volcanic edifice.
The distribution of fast directions shows a radial pattern in the vicinity of the volcanic edifice while a pattern of stations far from the summit is parallel to the regional compression. We conducted forward modeling of stress fields around Mt. Fuji to test whether the contrast of patterns can be explained by stress. We estimated regional stresses from focal mechanisms by the methodology of Hardebeck and Michael (2006). And then, we calculated stress fields by gravitational effect as a consequence of the deformation of mass of volcanic edifice by solving a Boussinesq problem. We obtained horizontal directions of principal stresses as eigenvectors of stress tensors that sum both regional stress and gravitational effects. The estimation reproduced the radial pattern of seismic anisotropy near the volcano. It may be possible to constrain the absolute range of stresses if we estimate the influence of gravitational effect of topography and area of seismic anisotropy similtaneously.
Measurement results of splitting indicate temporal consistency of anisotropy around the volcanic edifice. Delay times did not show significant change of seismic anisotropy less than 0.025s. By contrast, calculated vp/vs ratios in the measured area suggest that some small change may exist. In previous geophysical observations of Mt. Fuji, possible perturbation of stresses or seismic velocities by the 2011 Tohoku-Oki Earthquake did exist (e.g., Fujita et al., 2013, Brenguier et al., 2014). These changes, however, are small and our lack of significant change of splitting is consistent with these former studies. Subtle change of splitting may be detected if events are restricted spatially enough.