1992年西表島群発地震の特徴について

Abstract

We studied the seismic activity in and around Iriomotejima Island, SW Ryukyu Arc, Japan, during the period from January 1992 through May 1993. In particular, two earthquake swarms, namely, the swarm which occurred off the SW coast of Iriomotejima Island late in August (SW swarm) and that occurred in the NW region of the Island in the middle of September 1992 (NW swarm), were investigated in detail in this paper. The data used in this study were restricted to “felt earthquakes” whose amplitudes were larger than 1gal. As a result, the number of events was not so large, but homogeneous in time and space.The spatial and temporal characteristics of these seismic activities are summarized as follows;1) A high swarm activity in the SW region continued several days. The focal zone extended to the NW-SE direction.2) The focal region of NW swarm migrated from off the NW coast of the Island to the inland area and then expanded to the surrounding areas.3) The focal zones extend in the NW-SE direction. And the vertical distribution of hypocenters shows a sharp seismic boundary plane. This is consistent with the predominant direction of the fault strikes and lineaments in this region.4) Some of the remarkable features observed in the later phases appear to represent reflected waves. These features suggest a possibility of the existence of sharp reflectors such as a magma reservoir beneath the region. However, no signs were seen for any surface volcanic activity.In consequence, it is considered that these earthquake swarms occurred under the stress field acting in the wide area of the SW Ryukyu Arc.