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

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 mb ~3.0, a significant improvement in detection compared to the mb 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 mb 3.6. These regional P waves are used to estimate a Pn velocity of 8.0 (plus-minus) 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 (delta) ~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.