Detection of a crack-like conduit beneath the active crater at Aso volcano, Japan

To constrain the source of long period tremors (LPTs), we deployed a very d ense broadband seismic network consisting of totally twenty-four stations a round the active crater of Aso volcano in Kyushu, Japan. The spatial variat ion of the observed signal amplitudes reveals that the source of LPTs consi sts of an isotropic expansion (contraction) and an inflation (deflation) of an inclined tensile crack with a strike almost parallel to the chain of cr aters. The detected crack has a dimension of 1 km and its center is located a few hundred meters southwest of the active crater, at a depth of about 1 .8 km. The extension of the crack plane meets the crater chain including th e active fumarole at the surface, suggesting that the crack has played an i mportant role in transporting gasses and/or lava to the craters from below. This work also demonstrates a powerful usage of broadband seismometers as geodetic instruments to constrain subsurface structures at active volcanoes .