The western Pacific is a region of high tectonic activity where large earthquakes occur caused by subduction of the Pacific plate or other interplate interaction. The task of this group is to continue operation of the network of the former POSEIDON ( Pacific Ocean Seismic Data Observation Network ) project, and to complete a high-performance broad-band seismic network by installing instruments at a few new sites in the western Pacific. The network operated by Group I is composed of stations in the western Pacific region, including several oceanic islands in the Japanese territory, Korea, Russia, Philippine, Indonesia, Belau, Papua New Guinea, and Micronesia. At each station, a broadband seismometer system ( Fig. 2 ) is installed that is the standard system for the OHP project. The station spacing of this network is about 1,000 km. This spacing is too coarse to elucidate the fine structure of local heterogeneity or to study seismicity after a large earthquake, and therefore studies using temporary arrays of portable instruments will be also necessary. A mobile observation system that consists of self contained portable broadband seismometers will be developed for this purpose.

This group will also install networks of GPS and geomagnetic observations. Surface displacement will be monitored by a network of GPS observatories. The primary purpose of this network is to detect plate motions and intraplate deformations. Fig. 3 shows some preliminary results: the velocity vectors of block motion derived from GPS records for the eight months from July 1996 to February 1997. It is surprising that the velocity pattern is consistent with plate motion models such as NUVEL-1, although the observation period was only eight months. This result strongly suggests that further accumulation of data will provide detailed information on tectonic movements in the western Pacific region.