Spatial and Temporal Distribution of Stresses Associated with the Aftershock Sequence of a Moderate Earthquake and Volcanic Eruption near the Izu Peninsula, Japan.

Abstract

In July, 1989, a seismic swarm, including an ML 5.5 mainshock on July 9, occurred east of Izu Peninsula, central Japan. Temporary seismic stations were deployed from July 13 to July 16; during this period, a submarine eruption and the largest aftershock (ML 4.8) occurred. Seismic activity during the period was analyzed using hypocenters and focal mechanism solutions obtained from both temporary and routine earthquake observations. The spatial and temporal distribution of solutions indicate that strike-slip and normal events are segregated in space and time and that there was a local stress field within limited spatial and temporal windows.The spatial and temporal distribution of solutions obtained suggests that the magma related to the submarine eruption penetrated the mainshock source area and filled the deeper part of the eastern half of the study area. This magma penetration induced a local tensile stress which reached to within 1.5 km of the surface after the largest aftershock, and then decreased rapidly and diminished rapidly after the end of the study period. The northwestern end of the mainshock source area was temporarily controlled by redistributed local stresses induced by the mainshock.