Seismotectonic structure of the Western Pacific arcs

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A new broadband seismic station was installed at Kamensokoye, Northern Kamchatka, Russia. The STS-1 seismographs are installed in an underground seismic vault. The temperature in winter is as low as -50 to -60C,but the temperature in the vault is below but near 0 degree and the three component STS-1 seismographs are in operation.The broadband seismic station at Parapat, Sumatra, Indonesia was upgraded. First a seismic vault was constructed to avoid thermal disturbances caused by sunlight radiation. Long-period large noises found in the previous broadband records at this station cannot be found in the records obtained in a new vault. Also new power units were installed to minimize the interruption of recordings due to electric power stoppage. Secondly digital magnetic tape recording was replaced by optical magnetic disk recording, which proves to be more stable. Further STS-2 instruments were replaced by STS-1 seismographs and the replacement made possible to record seismic motions at longer periods.At Ponpei, Micronesia and Pohang, Korea broadband seismic observation is continuing and good data were collected. At Ponpei, signal cables from the seismic vault to the recording room were replaced and were buried to avoid any meteorological or artificial cutting of cables.Digital waveform data at Ponpei were converted to compressed formatted data and are nearly ready to be open to researchers through the ERIOS system at the Earthquake Research Institute, University of Tokyo. Digital waveform data at Parapat station are available to researchers in Japan and abroad through the ERIOS system. The data were accessed from US and other countries.An analysis of seismic data together with other geophysical data shows that the entire island arc of Indonesia can be divided into three distinct arc regions : West Sunda arc, East Sunda arc and Banda arc. Also large earthquakes along the Kamchatka, Kuril and Japan arcs are analyzed on the basis of pattern of spatial migration.