A magma chamber model beneath Unzen Volcano inferred from geodetic and seismic data using FEM

A supplying system of the magma beneath Unzen volcano has been proposed using both geodetic and seismic data simultaneously. Spatial variation of geodetic data is affected by change of magma body in both of shape and pressure. In most cases, relative stress field is able to be inferred from focal mechanisms of earthquakes. This changing also disturbs stress field in the region around the magma chamber. A model about shape and internal pressure of magma chamber is constrained by mean of both grounds deformation and focal mechanism. Unzen Volcano is the massive volcanic complex, located in the middle part of Kyushu Island, Japan, erupted accompanying by pyroclastic flows and formed a huge lava dome at the summit during 1990-1995. Based on geodetic data during period 1991-2004, a magma chamber model composed of four pressure sources was constructed by grid search method (Kohno et al., 2008). For the data with greater crustal deformation observed in 1992-93 and 1993-94, we applied finite element method (FEM) to model the magma chamber. Especially, shape of a pressure source at a depth of 7 km (C-source) was also estimated in this searching process. C-source during 1992-93 has an oblate spheroid with south-southwest dip oriented made good correlation with observation data. Regards only from ground deformation data, the length of the major axis is smaller than 3km. The shape sustains to an idea that magma ascends easily toward the shallower chamber. The best solution of C-source in 1993-94 was obtained as a prolate spheroid source with north dip oriented. This latter source is smaller size and dipping to opposite side with the previous one. Although they are separated at distance of 1 km, this is not far to call different magma chamber each other. Different shape indicated that inhomogeneous structure of magma chamber. From this arrangement of the sources, we considered a model that the magma chamber at a depth of about 7 km could be composed of small magma chambers like pocket. These shapes are parallel to distribution of seismic epicenters, which means there is some correlation with magma chamber. We also estimated deviatoric stress tensors for the focal region beneath Unzen Volcano using stress tensor inversion method through 1986-88 the deviatoric stress field in depth beneath Chijiwa bay, west next of Shimabara peninsula, drastically changed in the directions of principal stresses. It is interpreted that the phenomenon was caused by overpressure of numerous dikes having dimension long oriented in vertical direction. In the other period, that the principal stress did not change, which means that the inflation of deflation of the magma chambers did not make any stress change in the focal area in spite of high volcanic activity accompanied with eruption. Comparing the stress using the optimal solution inferred from geodetic data, stress tensors were calculated by FEM, and there is stress disturbance induced from increasing or decreasing pressure of magma chamber only in the region which close to the chamber. From these facts we can say the body of magma chamber did not reach at the focal area, thus earthquake swarm was generated by volatile substance which can make weaken the bed rock when they soak into the rocks. By combining geodetic data and stress field from seismic data, we obtained more reliable shape for magma bodies. It allows us to evaluate quantitatively process of volcanic activity.