Abstract
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 (~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 (~0.2-0.5 Hz) of OBSs reach or exceed the high noise model of continental stations. At regional distances M(subscript 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(subscript diff) and PKP waveforms have been recorded per year by OBSs at high corner frequency of 0.1 Hz with a minimum M(subscript w) 5.3-6.0. The number of recording drops to less than 5 per year at 1 Hz with M(subscript w)≥6.4 and distances less than 90°.
Highlights
Understanding the noise spectrum on the sea bed is crucial to the design and installation of ocean-bottom observatories
In 2006, the Institute of Earth Sciences at Academia Sinica started the deployment of broadband ocean-bottom seismometers (OBSs) in the eastern waters of Taiwan
The system was built at the Woods Hole Oceanographic Institute (WHOI) ocean-bottom seismology laboratory, and similar broadband instruments have been employed in various campaigns to study the structure of the upper mantle (Collins et al 2001, 2002, 2008)
Summary
Understanding the noise spectrum on the sea bed is crucial to the design and installation of ocean-bottom observatories. In 2006, the Institute of Earth Sciences at Academia Sinica started the deployment of broadband ocean-bottom seismometers (OBSs) in the eastern waters of Taiwan. The broadband system we used is equipped with a Guralp CMG-3Tc sensor, which has a velocity response flat to 120 s, a gimbal to level the sensor, a Quanterra Q330 datalogger, and a Seascan clock which maintains the time at a drift rate of less than 2 s a year. The. In this study we present the ambient noise levels at these 3 sites and compare them with those at BATS (e.g., Liang et al 2004, 2008; http://bats.earth.sinica.tw) stations installed near the coastline and in the center of the island and two F-net stations (Okada et al 2004). TWKB is located 5 km from the coastline of the Hengchuan peninsula, and TDCB is installed at a quiet site in the middle of the Central Range of Taiwan (Fig. 1). The high and low noise model compiled by Peterson (1993) provides a global reference for continental sites
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