Three-dimensional magnetotelluric imaging of the crust and uppermost mantle beneath the Sambe volcano, Southwest Japan

The Sambe volcano is a large lava dome of dacite to andesite composition, situated in the Sanin District, Southwest Japan, where the Philippine Sea plate has been subducting along the Nankai Trough. The Sambe volcano is emplaced at the center of an elliptical caldera with a diameter of about 6 km, with eruption products characterized by lava domes, lava flows and pyroclastic materials covering an area of at least 30 km2 [Committee for Catalog of Quaternary Volcanoes in Japan, 1999]. The latest eruption took place before 3,600 years. Magnetotelluric (MT) methods can provide electrical resistivity images of the crust and upper mantle, detecting variations related to melt and/or fluids flow. A great deal of effort has been made on obtaining information about the two-dimensional resistivity structure of active volcanoes and surrounding geothermal fields by MT soundings. However, since the Sambe volcano is surrounded by electrically conductive sea and has a topographic variations, measured MT data are expected to be seriously biased by distortion of electric fields. Recently, the inversion algorithm of Sasaki [2004] solves simultaneously for static shift and 3-D subsurface conductivity distribution parameters. In this study, we have tried to determine a three-dimensional (3-D) electrical resistivity structure beneath the Sambe volcano by MT soundings MT stations were deployed around the Sambe volcano which is located about 15 km away from the coastline of Japan sea. All of 21 MT sites were set up in the land area. The data were collected using five component wide-band MT instruments (Phoenix MTU-5 system) in November, 2008. The data were acquired in the frequency range from 320 to 0.00055 Hz. The recording duration for the sites ranged from 2 to 9 days. A simultaneous remote reference measurement was carried out at the Sawauchi site (900 km northeast of the study area). The observed apparent resistivity and phase data were inverted simultaneously using the 3-D inversion code of Sasaki [2004]. In this inversion, the 3-D blocks were set up in the crust and upper mantle. These block size in the horizontal and vertical directions were 5-10 km and 1.2-20.14 km, respectively. The obtained 3-D resistivity model through the inversion shows as follows. (1) Although the initial model of this inversion was set as a uniform earth of 100 ohm-m, the existence of sea water was well imaged as conductive layer near the surface of the sea area. (2) Beneath the Sambe volcano, prominent conductive anomaly (< several tens ohm-m) is visible at depths greater than 10 km in the crust. (3) The cut-off depth of the crustal earthquakes becomes shallow toward the summit of the Sambe volcano, and it is coincident with the upper boundary of the conductive zone. In addition, low-frequency events occurred at a depth of around 30 km at the northern edge of the conductive zone [JMA, 2006]. (4) There is a clear correlation between the low seismic velocity anomaly [Nakajima and Hasegawa, 2007] and the conductor. (5) These results indicate that the conductive anomaly is due to the existence of partial-melting zone and/or hydrothermal fluids related in the Sambe active volcanic system.