Stress induced shear wave splitting in Volcanic Regions: Observations and Modelling of Mt. Asama, Japan

Active volcanoes are dynamic systems that perturb the regional stress field in which they exist, by manner of both the weight of the volume of the mountain and the movement of large masses of magma under pressure. The resulting change in the crustal stress field is hypothesized to affect its seismic properties; in particular the degree and orientation of shear wave birefringence. Initial work into furthering the understanding of the link between volcanism-induced crustal stress and shear wave splitting is presented here. The focus of research is on the 2004 and 2008 eruptions of Mt. Asama in Japan. Previous work has shown that a roughly E-W trending dike, with 0.8m of opening, accompanied the 2004 eruption. This deformation has been modelled with finite element methods and we will constrain the surface deformation with existing GPS data. Methods to model the effects of the resultant stress field on shear wave splitting are laid out, and splitting measurements from earthquake data collected over the 2008 eruption are also presented, continuing on from previous work by Savage et al. [2010]. Preliminary measurements show changes in splitting fast direction at a few stations that coincide with GPS baselines and eruption times in 2008, but the relations are not as straightforward as they were for the larger 2004 eruptions.