Basal reflector under the Philippine Sea plate

In order to place constraints on the observability and properties of the lithosphere-asthenosphere boundary (LAB), we sought SH precursors to sS from earthquakes beneath the Philippine Sea Plate in the frequency band of 0.01-0.3 Hz. By using array methods to detect precursors, we located discontinuities between 50-70 km depth along a 600 km transect across the Philippine Sea. As a result, discontinuities could be detected at depths of 50-70 km across the Philippine Sea by using both an array method and seismic imaging technique. The lateral coherence and amplitude ratio relative to sS provide constraints on possible mechanisms by which the discontinuity arises. The amplitude ratios, though scattered, suggest minimum shear-wave contrasts of -6%. The change in depth of the discontinuity along the profile does not suggest a thermal origin of the signal. We find that the depths and discontinuity properties agree with previously reported properties of the LAB in the Philippine Sea Plate attributed to melt presence.