Stress Field in the Unzen Volcanic Area, Kyushu, Japan

Abstract

Reanalyses of shallow earthquakes in the Unzen volcanic area have proved that their focal mechanisms are well explained by mormal faulting, in contrast to the previously supposed strike-slip faulting. Tension axis is in the N-S direction. Predominace of active faults, striking in the E-W direction, and forming a graben-like topography, seems to support the present model. Geodetically, crustal deformation in this area is characterized by marked N-S extension and local subsidence in the graben zone. These evidences suggest the stress system of normal fault type (tension axis is almost horizontal in the N-S direction, and pressure axis is nearly vertical) in this volcanic area, which (1) accumulated the normal fault offsets in the late Quaternary, (2) controlled the crustal deformation in the last some decades of years, and (3) caused earthquakes cccasionally. The above-stated stress system would have been closely related to the volcanism of this area, too.