Volcano-tectonic Earthquakes during the Stage of Magma Accumulation at the Aira Caldera, Southern Kyushu, Japan

Abstract

At Sakurajima volcano, activity of volcano-tectonic (VT) earthquakes has gradually increased since 2002, as the inflation of the Aira caldera progressed since 1993. In particular, VT earthquakes SW off the volcano and in the caldera swarmed during November 2003-February 2004, and then a baseline of GPS across the volcano indicated a significant extension. However, no significant increase in eruptive activity was observed. Location and focal mechanism of VT earthquakes, which occurred during 1998-2005, are determined and the relationship between the seismic activity of VT earthquakes and volcanic activity is discussed. The VT earthquakes originated in an elongated zone extending in NE-SW direction, namely from the NE part of the Aira caldera through Sakurajima volcano to SW off the volcano. This zone coincides with active tectonic zone. The focal zones of VT earthquakes are distinguished into three regions: (1) 0 to 4km deep beneath the summit of Sakurajima, (2) 6 to 9km deep SW off the volcano, and (3) 4 to 14km deep in the Aira caldera. At the summit area, VT earthquakes occurred mostly south of the crater and a few located north, and very few VT earthquakes were found inside the crater. Focal mechanism of VT earthquakes beneath the summit area has variation with depth. Reverse fault type was dominantly obtained at depths from 0 to 2km. Meanwhile at deeper portion of 2-4km, strike-slip fault type is predominant. In contrast, most of VT earthquakes SW off Sakurajima indicated uniquely normal fault type with horizontal T-axes oriented to WNW-ESE direction. Whereas the mechanism of those in the Aira caldera is strike-slip types which one of the nodal lines oriented in NE-SW direction. These are consistent with regional stress field and direction of depression zone inferred from geological study. A hypothetical model adding a tensile fault to dual Mogi's pressure source is proposed to examine the relation of inflation of Aira caldera and the following seismicity SW off the volcano.