Abstract

An ocean bottom seismometer capsule designed specifically for the long-term monitoring of teleseisms has been designed and tested. An efficient triggering algorithm consisting of multiple high-pass filters effectively discriminates between locally generated earthquakes and noise, and teleseisms. During a 1-month deep ocean deployment west of the Santa Barbara Channel, a magnitude 5.9 earthquake at a distance of 76° was recorded, in addition to a number of regional events in the 300- to 450-km range. Noise levels were monitored by automatically recording data at intervals. Vertical noise levels were approximately 20 times greater than those recorded at quiet land sites, and horizontal noise levels were about 5 times greater than that. The instrument consists of a microprocessor-controlled data logging system in three parallel pressure tubes, joined by a common baseplate. Wires between the three tubes are contained within the baseplate. There are no external connectors which are exposed to sea water, allowing the deployment time to extend to 1 yr. Data are recorded on two Braemar cassette recorders with a capacity of 15 Mbits each, for a continuous recording time of approximately 43 hr. This is adequate for the expected data acquisition rate, given the robustness of the triggering algorithm. The sensors are Mark Products L4-C 1-Hz seismometers with useful extended low-frequency response to about 30-sec periods for earthquake signals. The three components are leveled with a mechanical gimbal arrangement. The instrument has been successfully deployed 3 times in the Santa Barbara Channel and deep ocean, and should prove extremely useful for extending teleseismic studies into the ocean.

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