Long‐period S to P converted waves and the onset of partial melting beneath Oahu, Hawaii

Strong S ‐ P converted waves are observed at the long‐period Global Digital Seismograph Network (GDSN) station Honolulu (HON), Oahu, Hawaii, preceding mantle S by 11±1 s. The lead times and polarities of precursors relative to S are consistent with a seismic velocity decrease at a depth between 70 and 80 km probably marking the asthenosphere‐lithosphere boundary (ALB) beneath HON. Comparison of the data with synthetic seismograms calculated for models containing a first order seismic discontinuity at 75 km depth suggests that the S wave velocity decreases by at least 15% across the ALB. The sharpness of the ALB cannot be resolved with long‐period data; the same results are obtained if the velocity decrease extends over a depth interval up to 40 km wide. Assuming aperidodite composition for the uppermost mantle, a lower limit of 8% for the volume percentage of partial melt and a minimum temperature of about 1450°C in the asthenosphere is consistent with the observations.