Seismic observations from an Ocean Subbottom Seismometer in the northwest Pacific Basin

Abstract

One of the primary objectives in placing seismic systems in deep ocean boreholes is the detection of teleseismic earthquakes and nuclear events. If the oceanic basement is substantially quieter than the ocean floor and island sites, then the added data could yield information of considerable value in nuclear test detection. This paper presents and evaluates data relevant to this problem. The smallest teleseismic (Δ > 30°) earthquake observed on the Hawaii Institute of Geophysics Ocean Subbottom Seismometer (OSS ) had a body-wave magnitude of 5.4. Only one nuclear explosion ( mb = 5.6) was clearly observed. With additional filtering and processing, nuclear explosions as small as mb = 5.1 can be marginally observed. OSS records many regional earthquakes of the northwest circum-Pacific area which are not listed in the NEIS catalog. Teleseismic P -wave arrivals for several events contain frequencies as high as 8 Hz above the background noise. No teleseismic short-period S waves were seen, although high-frequency S waves from regional events were abundant. This dichotomy in the observability of S waves at OSS suggests that the lithosphere of the northwest Pacific has a high Q , but that the underlying asthenosphere is more highly attenuative.