A set of Ocean Bottom Seismographs (OBS) has been constructed re cently at the National Taiwan Ocean University (NTOU) for offshore seismic surveys and earthquake observations. The instrument is identical in design to the recently upgraded Texas OBS. It is controlled by an 80C88 micropro cessor. Signals from up to four channels of sensors are recorded digitally on either a cartridge tape or a disk. It was successfully field tested during R/V Ocean Researcher I cruise No. 277 in 1991, and has been in use in subsequent cruises.

A series of ocean-bottom seismograph experiments were conducted to establish velocitv models in the southwestern off shore area of Taiwan from .. 1992 to 1993. Although the newly established system is still in its early stages, t. he data collected make it possible to define a crustal structure in the vicinity of the northern Manila Trench. The arrivals of deep refraction with apparent velocity at about 8.1 km/sec help to define the depth of the Moho discontinuity 'vhich is about 12 km below sea level. A thin crust of merely 5 to 6 km in thickness underlying the sediments to the west of the trench in the study area suggests an oceanic origin of the crust there. The thickness of the sediments is maintained over 3 km throughout the area, but a thicker crust to the east of the trench than that to the west is demon strated.