Ambient Noise and Teleseismic Signals Recorded by Ocean-Bottom Seismometers Offshore Eastern Taiwan

Broadband records from ocean-bottom seismometers deployed in the Okinawa trough and the Huatung basin were analyzed to provide seafloor noise characteristics and the detection thresholds for teleseismic body and surface waves. Ambient noise levels on the horizontal components are 10 - 40 dB higher than on the vertical component, with the sensor seated on the surface of the sediment. On the vertical components, infragravity waves are 10 - 30 dB more energetic at the shallower Okinawa trough sites 2000 m depth) than at the deeper Huatung basin site (similar to 4700 m). From 0.03 to 0.2 Hz, the Huatung basin noise levels are comparable to that of the broadband stations in Taiwan on a quiet day. The microseism peaks (similar to 0.2 - 0.5 Hz) of OBSs reach or exceed the high noise model of continental stations. At regional distances M-w 6.5 is required for recording prominent Rayleigh waves if the source radiation is unfavorable, but 6.2 is sufficient for a favorable focal mechanism. Several tens to over one hundred high-fidelity P, P-diff and PKP waveforms have been recorded per year by OBSs at high corner frequency of 0.1 Hz with a minimum M-w 5.3 - 6.0. The number of recording drops to less than 5 per year at 1 Hz with M-w >= 6.4 and distances less than 90 degrees.