Microseismicity in the middle Okinawa trough

Abstract

We conducted a microearthquake survey with ocean bottom seismometers (OBS) in the middle Okinawa trough in 1984. During about one month of operation, very active microseismicity was observed. The microearthquake activity was characterized by the ceaseless occurrence of numerous small earthquakes in the vicinity of OBS stations. This activity was recorded by almost every OBS station but not by land‐based instruments. Swarm activities also occurred at many places. Average activity levels of earthquakes whose durations are longer than 30 s, recorded at individual OBSs, are from 4 to 20 events per day, which seem higher than those of ordinary mid‐oceanic ridge‐fracture zone systems and other back arc basins. Apparently, the number of smaller size earthquakes is fewer than expected. This might indicate the rather high attenuation of seismic waves or low‐Q of the crust around OBS stations, which makes it difficult to detect small size events at a distant range. Attenuation of seismic waves was large beyond the epicentral distance of about 120 km. This seems to indicate low‐Q in the lower crust. Also, a particularly low‐Q area was found at the northern most part of the research area where high heat flow measurements were obtained. This suggests that igneous activity reaches to the surface in that vicinity from the bottom of the crust where magmatic activity has been implied in other studies. At least two types of swarm activities are indicated for this area. Some have no dominant events, show high “b” value properties, and seem to be volcanic. Others have dominant events and appear to be tectonic in nature. Among the swarm activity, two kinds of signals closely resembling volcanic tremors were observed. From these features, we conclude that the crust of this area is highly fractured and subject to intense tectonic activities, as well as some kind of igneous processes.