Thermal and crustal structure of the Aso volcano and surrounding regions constrained by gravity and magnetic data, Japan

Abstract

The Aso volcano forms a large caldera that is one of the most active volcanoes in Japan. Much evidence suggests the existence of a high-temperature zone beneath the active central cones of the Aso volcano and a recent study using seismic observations revealed an attenuation in amplitude of seismic waves that pass through the central region of the Aso caldera. The SWW-NEE-trending Futagawa active fault, a part of the Oita-Kumamoto Tectonic Line, which dips north, lies to the west of the caldera and joins the caldera rim but there are no scars within the caldera. An elongate magnetic positive anomaly from the central cones to the west exists close to the Futagawa fault, this anomaly connects the central cones and the two lateral volcanoes west to the caldera. Magnetic analyses through spectral method and forward modeling delineate a highly magnetized two-dimensional buried prism source dipping north beneath the fault. The lower bounds are about 10 km depth outside the caldera and quite shallow depths of about 3 km inside the caldera. The area to the north of the Oita-Kumamoto Tectonic Line forms a gravity depression whereas the area to the south is a regional gravity high. A two-layered two-dimensional gravity model yields a normal fault dipping north with a throw of 1.3 km, whereas a multi-layered two-dimensional gravity model gives a depression of high-density basement within the caldera. The greatest depth is about 2 km below the surface. Available temperature-depth data suggest the possibility of a sudden change between the inside (high-temperature zone) and the outside (low-temperature zone) of the caldera. Therefore, the difference in the lower boundary of magnetic sources between the area inside and outside the caldera is possibly attributed to the difference of depth of the Curie point. Magnetic sources along the Futagawa fault are likely to be related to materials produced by the volcanic activity. If so, the passage for magma for Aso volcanism could be along the Futagawa fault.