Development of an Ocean Bottom Seismometer to be installed on hard rock by a submarine.

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A digital ocean-bottom seismograph has been developed, which was designed to be installed on ocean-bottom hard rock by using the submarine research ship "SHINKAI 2000".The sensor consists of 1 vertical and 2 horizontal components with a natural pendulum period of 1 second. Ground motions are recorded on PROM memory when their amplitudes exceed over a pre-set level. A triggered event is recorded for a period of 250 second and thus the total of 51 events can be stored on the PROM memory of 4 mega bytes with 12 bits of resolution and at a rate of 50 samples per second.The sensor, amplifier, memories and so on are all put in a 17" glass pressure-vessel. The glass sphere, covered with a hard plastic container, transponder, weight and others are all assembled with a hard leg-frame. All parameters such as starting time of measurement, trigger component (s) , triggering level, and so on are set with commands by an external portable type computer before closing the vessel.The OBS was set in the Okinawa trough for 5 days in September 1988, and recorded 11 seismic events and back-ground noises during the observations.After this observation, some modification were made for this OBS.The aluminium, anti-pressure vessel was used instead of the glass vessel. The amount of memory was increased. And software for triggering was improved.Using a new OBS off the east coast of Ito, Central Japan, where seismic swarm and a submarine eruption occurred in 1989, seismic observation was made before this swarm.