A S-wave Reflector in Middle Crust beneath Western Aira Caldera, Inferred from Controlled-source Seismograms.

Abstract

A seismic reflector at 13.6 km beneath western Aira Caldera, northmost part of Kagoshima Bay, Kyushu, Japan is discussed through later phase analysis of controlled source seismograms obtained in the experiment in 2008. A prominent seismic later phase appears at 8.3 s in travel time on the seismograms in northeastern Sakurajima Volcano for ray paths across western Aira Caldera in the seismic experiment in 2008. Back-azimuth of the later phase orientates center of western Aira Caldera and apparent velocity of 3.7 km/s is measured in the seismic array at northern Sakurajima Volcano. The later phase is inferred as P-SV converted reflection at 13.6 km depth from its travel time and a corresponding reflector located in the east off Osakigahana point, western part of Aira Caldera. Negative impedance contrast at the reflector is inferred from negative polarity of the onset of the later phase. Some clear later phases after S arrival, which appears in seismograms of natural earthquakes through trans-caldera ray paths, can be explained as SS reflection at the reflector. The reflector inferred as top surface of a possible magma reservoir of Aira Caldera, from its negative polarity and its location at depth of the west of known pressure source from preceding geodetic surveys.