Earthquake activity in the Southern Vanuatu arc recorded by the Texas digital OBS

Abstract

Abstract.We have reconfigured the Texas digital ocean bottom seismograph (OBS) to operate in a triggered mode and record regional earthquake signals. This paper reports the results of a deployment program designed to test these digital OBS, by monitoring earthquake activity in and near the trench in southern Vanuatu (formerly, the New Hebrides). We successfully recorded hundreds of earthquakes, including 133 located regional earthquakes recorded by three or more stations. We also report J-B residuals for 21 earthquakes reported and located by the ISC. Fourier analysis of seismograms from regional earthquakes suggest that the frequencies of spectral peaks at any station were nearly the same for large, small, nearby, and distant events. However, we obtained very disparate frequencies when we analyzed seismograms for the same earthquake recorded at different stations. The most plausible interpretation is that spectral peaks do not depend on the characteristics of the earthquake source, but instead on site characteristics, or, more specifically, on the coupling of the instrument to the seafloor. To record reliable spectral data, we need to overcome this problem.

Ocean Bottom Seismograph:Instrumentation and Experimental Technique

“…According to this latter report, which was based on a survey conducted through the international marine seismological community, there were a total of 232 OBS's in the world, including three units from NTOU-R.O.C. The Te xas OBS 's have been extensively used in studies of deep crustal structures in the Middle America Trench (Ibrahim et al,1979) and in the Gulf of Mexico (e.g., Ebeniro et al, 1986Ebeniro et al, , 1988, and in studies of micro-earthquakes in the Southwest Pacific (Pontoise et al, 1979;Ibrahim et al, 1980;Chen et al, 1981;Coudert et al, 1981;Frohlich et al, 1990) and in the Aleutian Trench (Frohlich et al, 1982). In this paper, we briefly describe the instrument and the operational procedures for seismic refraction surveys.…”