Seismicity off Fukushima Prefecture is extremely high and many microearthquakes are located in this region by the seismic network of Tohoku University. Since the stations of Tohoku University are located in land area far from the focal region, the accuracy of focal depth is quite low for these shallow earthquakes. The means of improving the accuracy of focal depth is to make seismic stations in this region beneath the sea.
Continuous seismic observation by hydrophones has been carried out at a platform which was constructed for producing natural gas. The platform is located 40km off the coast of Iwaki, Fukushima Prefecture. Output seismic signals from hydrophones are transmitted to the Observation Center in Sendai via the relay station by using radio telemetry. P arrival data at the hydrophone station improve strikingly the accuracy of focal depth of shallow earthquakes occurred off Fukushima Prefecture, which enables us to estimate precisely the location of the upper boundary of the descending Pacific plate. The hydrophone data also made possible to identify the fault plane of the shallow event with magnitude 6.7 which occurred in this region on February 6, 1987.
