Magma plumbing system inferred from absolute hypocenter determination in Iwate volcano, Japan, using 3D velocity structure derived from active seismic survey

Absolute locations of volcanic earthquakes are indispensable for understanding magma plumbing system and forecasting the volcanic eruption. Based on the 3D velocity structure from the active seismic survey at the Iwate volcano, Japan in October 2000 [Hamaguchi et al., 2001, in the poster session of this symposium], we obtain precise hypocenters of volcanic earthquakes and discuss the 1998 volcanic unrest with relations to the structure. The data set is the P- and S-wave travel times of volcanic earthquake around the Iwate volcano in 1998. Since the 3D structure of the volcano is obtained for only P-wave, S-wave velocity is assumed to keep the relation of Vp/Vs as 1.66. The weights of P- and S-times are 1.0 and 0.05, respectively. The hypocenters are searched with the downhill simplex method [Press et al., 1992] until the variation of the weighted residuals at each iteration becomes less than 1 ppm which is the criteria of convergence. Travel times are calculated with the pseudo-bending method [Um and Thurber, 1989; Zhao et al., 1992]. The average residual of the initial hypocenters calculated by the 3D structure was 0.2 s. That of the final became 0.08 s. Seismic activity started in the central part of the high P-wave velocity column beneath the western caldera of the volcano in February 1998. Low-frequency earthquakes (LF) with predominant frequency of 2 - 5 Hz occurred at the western tip of the high-velocity body around 0 km depth in March 19 and terminated after 8 days. This activity suggests that magmatic fluid accumulates in the western tip of the high-velocity region. Another LF occurred just beneath the crater lake in the western caldera locating the top of the high-velocity body from March to August. These LF earthquakes are located just beneath the ground surface suggesting that the activity of fluid reaches the surface. In April to July, the activity of volcano-tectonic earthquakes (VT) with predominant frequency more than 10 Hz reached its climax in the western tip of the high-velocity region. The VT activities may be triggered by the fluid accumulated at the western tip. As mentioned above, the absolute hypocenters and detailed 3D velocity structure reveal clearly the magma plumbing system in the Iwate volcano.