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Recording teleseismic earthquakes using ocean-bottom seismographs at mid-ocean ridges

Institute of Geophysics and Planetary Physics Scripps Institution of Oceanography, La Jolla, California 92093-0225
Department of Geological Sciences Brown University, Providence, Rhode Island 02912

Abstract

Existing teleseismic data recorded by ocean-bottom seismographs (OBS) are sparse, but they are sufficient for analysis of earthquake detection thresholds under various background noise conditions. Long-period P, S, and surface waves are consistently recorded by OBS's for magnitude 5.7 to 6+ events at ranges greater than 100°. Both Love and Rayleigh waves are recorded for very large events, with high coherence in the 15- to 70-sec period range; high coherence in the 15- to 35-sec range is typical for events of magnitude 5.5 to 6+. Short-period body-wave arrivals (1 Hz), on the other hand, have only been clearly recorded by OBS's in the North Atlantic, during calm-weather periods or, by OBS's in the Pacific, for very large events at ranges less than a few tens of degrees. Seismograms recorded at the East Pacific Rise (EPR) and at the Mid-Atlantic Ridge (MAR) illustrate the high signal coherence between instruments deployed in an array of OBS's. Recordings of the pressure field as well as the vertical and horizontal displacement fields are used to assess the capabilities of OBS sensors and the frequency range of high signal-to-noise arrivals. Contamination of long-period, body-wave arrivals by secondary phases, due to reverberation in the water column, can significantly hinder investigation of relative travel-time anomalies across an OBS array on rough seafloor, particularly at low signal-to-noise ratios. The nature of the reverberations is illustrated in short-period data, and the basic physics behind the differences between the pressure and displacement signals is discussed. A bias of about 0.5 sec can be introduced to relative arrival times, with deeper stations appearing erroneously late, for an array where seafloor depths vary by about 1 km. Reflectivity synthetics provide the basis for designing optimum filters for removing the reverberation bias in long-period, P-wave data from the Mid-Atlantic Ridge, 34° S. The resulting relative travel-time anomaly is 0.4 to 0.6 sec with delays increasing with distance from the axis on the east flank of the spreading center.

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