Seismic reflectors beneath the central cones of Aso Volcano, Kyushu, Japan

Abstract

The subsurface structure of Aso Volcano, central Kyushu, Japan, has been imaged down to 10 km below sea level and notable features of subsurface structure in the central cones were revealed by a 3-D seismic reflection analysis. The 3-D reflection analysis included reflection enhancements and 3-D migration. The reflection enhancements and the NMO correction were applied on waveform data from the seismic experiment ASO98. The 3-D migration with a Kirchhoff integral was processed for 1397 NMO corrected traces to calculate the reflectivity at about 60 000 image points in an approximately 7 km by 7.5 km square region. The results of the 3-D migration show that the subsurface structure beneath the central cones can be divided into three parts, the western part, the eastern part, and the northwestern part. Bright reflectors appear in most of them and a significant discontinuity is inferred. The reflector horizon at 2 km below sea level spreads beneath the western and the eastern part and is probably the top surface of the Pre-Aso volcanic rocks or the basement rocks. Reflection voids appear in the western part and just beneath the active crater. The reflector void in the western part implies a hot region and is most probably the known seismic anomalies, such as low velocity region or attenuation region. Another reflector void beneath the active crater can also be associated with a hot region, apparently surrounding the conduit system of the current volcanic craters. The major discontinuity is located beneath the northwest flank of the central cones. This discontinuity divides the northwest region with weak reflectors from other parts. The discontinuity may be the trace of the Oita–Kumamoto Tectonic Line, which is the major tectonic line in central Kyushu, crossing the Aso caldera.