Basal reflector under the Philippine Sea Plate

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. As a result, a discontinuity with a velocity reduction downwards could be detected at depths of 50–70 km across the Philippine Sea by using both an array method and seismic imaging technique, in the frequency band of 0.01–0.3 Hz. 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 −5.8 per cent. The change in depth of the discontinuity along the profile does not suggest a thermal origin of the signal. The lateral variation of the discontinuity depth indicates that it is neither a solid–solid phase transition nor a thermally mediated change in rheological fabric of the plate. 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, between 0.01 and 7 vol. per cent.