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 a peridodite composition for the uppermost mantle, a lower limit of 8% for the volume percentage of partial melt and a minimum temperature of about 1450-degrees-C in the asthenosphere is consistent with the observations.