Post-caldera boundary fault reactivation by regional tectonic stress at Akaigawa Caldera (Hokkaido, Japan)

Understanding the post-caldera reactivation of caldera boundary (ring) faults is necessary to both reconstruct the geomorphological histories of calderas and assess volcanic and seismic hazards. Although magmatically induced reactivation events have received much attention, those caused by regional stresses have not. Therefore, we here conducted a tectonic and geomorphological reconstruction of recent activity on the Akaigawa Fault, which extends along the northwestern rim of the floor of the early Pleistocene Akaigawa Caldera (western Hokkaido, Japan), to clarify how reactivations induced by regional stresses contribute to the geomorphological evolution of calderas. Two Middle Pleistocene lacustrine terraces and two Late Pleistocene fluvial terraces are developed in the Akaigawa Caldera, and the Akaigawa Fault deforms them with a vertical downthrow to the southeast, increasing the height of the northwestern somma relative to the caldera floor. The distribution of the Akaigawa Fault and the post-caldera vent strongly implies that the fault is the caldera boundary fault, and thus that activity on the Akaigawa Fault reflects the reactivation of the caldera boundary fault, which is inferred to be an outwarddipping reverse fault system. The & sigma;1 axis of the regional stress field is orthogonal to the fault, indicating that its reactivation is induced by the regional compressive stress. Our findings suggest that the reactivation of caldera boundary faults by regional stresses should be considered in reconstructions of the geomorphological evolution of calderas and associated hazard assessments.