Shinjima Island is a small island of ~0.2 square kilometer located northeast of the Sakurajima Volcano, Kagoshima Bay, southwest Japan. According to historical records, Shinjima Island was uplifted and emerged from bottom of the sea at the eruption of the Sakurajima Volcano in 1779. Shinijima Island is formed by horst-and-graben structures composed of numerous E-W striking normal faults due to the vocalnic activity.
On the both sides of the main graben, we found two major graben-bounding faults which vertically offset a key shell bed by 15.9 m, and 23.5 m respectively. In contrast to such narrow and distinctive surface deformation, a N-S trending seismic profile displays a structure that swells up the geologic units broadly at the depths of 200m and 320 m. The profile also suggests no distinctive fault offsets but warped strata are interpreted below the depth of 50 m.
To estimate volume and movement of the magma storage at the 1779 eruption and other historic and prehistoric events, we performed forward modeling to restore the surface deformation in an elastic half-space. A complex model combining a point source inflation and dike intrusions approximately reproduces the current Shinjima landform. The model suggests that the ridge-trough-ridge structure and associated active faults on Shinjima Island have been developed by such volcano-tectonic processes.