Three-dimensional seismic velocity structure beneath Aso Volcano, Kyushu, Japan

Abstract

Tomographic results for P-and S-wave velocity structure beneath the active Aso Volcano, Kyushu, Japan, using 800 well-recorded earthquakes and ten shots recorded by an eight-station seismic network, are presented. A 68% variance reduction was achieved upon simultaneous inversion for hypocenter and velocity structure. Well-resolved velocity anomalies associated with the active crater reveal heterogeneity up to 26% slower and 18% faster in P velocity, and up to 31% slower and 22% faster in S velocity, than the one-dimensional model. The largest anomaly is seen over the upper 11 km in the central and northern parts beneath the central cones. Two low-velocity regions are imaged. The first region, a 10×15-km region encompassing the upper 3 km centered near the caldera wall at Tateno Valley, is characterized by P velocities up to 19% slower (20% for S). The second low-velocity region is associated with the central cones and active magma conduit system at 6 km depth. Velocities as low as 4.3 km/s (up to 26%) in P and 2 km/s (31% slower) in S characterize the 7-km-wide volume. The magma chamber is roughly spherical in shape, centered at 6 km depth, flattens at 10 km depth, and is located between Mt. Kishima, Mt. Eboshi, and Mt. Naka, the present focus of magmatism. A sharp velocity contrast at the depth of 3 km, with high velocities to the southwest and lower velocities to the northeast, characterizes different abutting structures associated with the Oita-Kumamoto Tectonic Line.

Velocity structure in the crust beneath the Kyushu area

“…Below the Aso volcano, Sudo and Kong (2001) showed a large amount of low-velocity anomaly, over 20% both Pand S-waves, in the upper crust below the central cones. Our results do not show such an anomaly.…”

“…In terms of the upper crust, some studies have obtained details on velocity structures beneath the active volcanoes using the local seismic network (e.g. Ohmi and Lees, 1995;Sudo and Kong, 2001;Yoshikawa et al, 2005). Therefore, here we compare their results and discuss a linkage of shallow structures and structures in the lower crust beneath the active volcanoes.…”

Effects of the 2016 Kumamoto earthquakes on the Aso volcanic edifice

“…A dip of about 85° southward maintained underground caused the Aso-N·C rupture bands could have extend toward the south side of −30 to −20% S velocity perturbation named LD at a depth of about 6 km below mean sea level (msl) beneath Kusasenrigahama, as reported by Sudo and Kong (2001). The LD low-velocity region is considered to be a magma chamber in the partially molten magma system of Aso volcano (Sudo and Kong 2001). Therefore, we assume that the Aso-N·C rupture bands could not have reached a depth of more than −2 or −3 km msl beneath Kusasenrigahama because rocks at this depth experience plastic deformation as a shallow magma storage region, which is not conducive for volcanic earthquakes (e.g., Japan Meteorological Agency 2013).…”

“…For example, we observed a dip of 84°S in the Aso-C rupture band at Site 12. A dip of about 85° southward maintained underground caused the Aso-N·C rupture bands could have extend toward the south side of −30 to −20% S velocity perturbation named LD at a depth of about 6 km below mean sea level (msl) beneath Kusasenrigahama, as reported by Sudo and Kong (2001). The LD low-velocity region is considered to be a magma chamber in the partially molten magma system of Aso volcano (Sudo and Kong 2001).…”

Genetic relationship between post-caldera and caldera-forming magmas from Aso volcano, SW Japan: Constraints from Sr isotope and trace element compositions

“…Miyoshi et al (2005) supported this multiple -magma -chamber hypothesis by performing simple model calculations using petrographic and trace element data pertaining to the post -caldera volcanic products. Geophysical studies revealed low -velocity anomalies in the middle -to -shallow crust beneath the Aso caldera, indicating the possible presence of two magma chambers (Sudo and Kong, 2001;Abe et al, 2010).…”