Detection of Mantle and Core P- Arrivals, and Analysis of T-waves, Recorded on Ocean Sound-Channel Hydrophones Along the Mid-Atlantic Ridge (10°-35°N)

In February 1999, a consortium of U.S. investigators (NSF and NOAA) began long-term monitoring of Mid-Atlantic Ridge (MAR) seismicity between 15°N and 35°N. The experiment uses six NOAA/PMEL autonomous hydrophones moored within the SOFAR channel on the MAR flanks. The hydrophones record the hydroacoustic tertiary phase or T-wave of oceanic earthquakes from throughout the Atlantic basin. The low attenuation properties of the SOFAR channel allow for a reduction in the detection threshold (cutoff Magnitude) of MAR earthquakes from M_c=4.7 of the land-based seismic networks to M_c=3.0 with the hydrophones (Bohnenstiehl et al., 2002). The improved detection capability of the hydrophones allows for a better view of the overall spatio-temporal patterns in MAR earthquakes (Smith et al., 2002). To assess the waveform analysis capability of the hydrophones we present a preliminary examination of P- and T-wave arrivals recorded from 84 regional MAR and teleseismic earthquakes. The hydrophones (8 bit resolution) detect upper mantle Pn arrivals from regional MAR earthquakes at epicentral distances of 374-1771 km and from events as small as m_b=3.6. The T-waves of regional MAR events are also clearly recorded although the signals are significantly clipped when earthquakes are <400 km distant or m_b>5. A surprising result of the waveform analysis was the identification of P- arrivals from earthquakes outside the Atlantic Ocean basin. The hydrophones detected P-waves from global earthquakes with magnitudes from 5.8 to 8.3 at epicentral distances ranging from 29.6° to 167.2°. Examination of travel times suggests these teleseismic P-waves comprise the entire suite of body-wave arrivals from direct mantle P- to outer and inner core reflected/refracted phases. These global P- amplitudes also exhibit the typical solid-earth wavefield phenomena of a P- shadow zone and caustic at a Δ=142°. There is an apparent 2-second delay in expected {\ P-} arrival times at the hydrophones consistent with acoustic conversion of seismic phases at the seafloor interface and propagation through the water-column. One goal of this study is to use the regional P- arrivals to estimate Pn velocity along the Mid-Atlantic Ridge between 10° and 35°N. The hydrophone arrays provide a unique opportunity to measure Pn velocities in the Atlantic Ocean upper mantle due to the difficulty in placing seismometers on the seafloor for an extended period of time. At this time, a preliminary MAR Pn velocity of 7.9 +/- 0.1 km/sec has been estimated from only 17 Pn arrival times. It is anticipated that with more arrival data we will be able to quantify variations in Pn velocity along the east and west sides of the MAR and southward along the MAR from the Azore hotspot.