On a Structure under the Aso Caldera

Abstract

The Aso Volcanological Laboratory is situated within the gigantic Aso Caldera, and many records have been obtained for events occurred around the Kyushu Island. By analyzing those records, a certain systematic azimuthal deflection of P wave has been clearly discerned and seems to reflect some structural feature beneath the caldera. It has been shown that the systematic azimuthal deflection can be interpreted based on the following structural model : a tectonic line or zone runs across the center of the caldera from east to west and splits the basement of caldera into two blocks ; the interface of basement of the northern block inclines nearly eastward with the dip of about 15° and that of the southern block toward NNE with the dip of about 25°. In order to estimate the depth of interface, seismograms of an artificial explosion and earthquakes occurred northward outside of caldera rim were analyzed and then a transversely polarized phase was found in P-coda waves. The phase was interpreted as SV wave converted from P wave incident at the inclined interface of the northern block basement. Based on time difference t_sv-p, the interface of the basement was supposed to be about 2.5km deep in the western part of caldera. It has been suggested, taking into consideration the dip angle of interface assigned by the twin-block model, that the interface extends downward to a depth of 4km or so below the central cone situated rather eastward inside the caldera. It has been also suggested that the magma chamber below the central cone, proposed by one of authors, is located along the tectonic line mentioned above and swells nearly by half above the inclined interface. The tectonic line might have played an important role, such as a feeding channel, in the process of formation of the central cone.