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P- and T-Wave Detection Thresholds, Pn Velocity Estimate, and Detection of Lower Mantle and Core P-Waves on Ocean Sound-Channel Hydrophones at the Mid-Atlantic Ridge

Oregon State University/NOAA
Hatfield Marine Science Center
Newport, Oregon 97365
(R.P.D., H.M., T-K L., J.H.H., M.J.F.)
Lamont-Doherty Earth Observatory
Columbia University
Palisades, New York 10964
(D.R.B., M.T.)
NOAA/Pacific Marine Environmental Laboratory
Hatfield Marine Science Center
Newport, Oregon 97365
(C.G.F.)
Woods Hole Oceanographic Institution
Woods Hole, Massachusetts 02540
(D.K.S.)

Manuscript received 30 July 2003.

Since 1999 six Sound Fixing and Ranging (SOFAR) hydrophones have been moored along the Mid-Atlantic Ridge (MAR) (15°-35° N). These hydrophones (8-bit data resolution) are designed for long-term monitoring of MAR seismicity using the acoustic T waves of seafloor earthquakes. The completeness level of the MAR T-wave earthquake catalog estimated from size-frequency constraints is m_b 3.0, a significant improvement in detection compared to the m_b 4.6 completeness level estimated from National Earthquake Information Center magnitude-frequency data. The hydrophones also detect the acoustic phase of converted upper mantle P arrivals from regional earthquakes at epicentral distances of 374-1771 km and from events as small as m_b 3.6. These regional P waves are used to estimate a Pn velocity of 8.0 ± 0.1 km sec^-1 along the east and west MAR flanks. An unexpected result was the identification of P arrivals from earthquakes outside the Atlantic Ocean basin. The hydrophones detected P waves from global earthquakes with magnitudes of 5.8-8.3 at epicentral distances ranging from 29.6° to 167.2°. Examination of travel times suggests these teleseismic P waves constitute the suite of body-wave arrivals from direct mantle P to outer- and inner-core reflected/refracted phases. The amplitudes of the teleseismic P waves also exhibit the typical solid-earth wave field phenomena of a P shadow zone and caustic at 144°. These instruments offer a long-term, relatively low-cost alternative to ocean-bottom seismometers that allows for observation of Pn velocities and mantle/core phases arriving at normally inaccessible deep-sea locations.