FAST P-WAVE PROPAGATION IN SUBDUCTED PACIFIC LITHOSPHERE - REFRACTIONFROM THE PLATE

P waves traveling from events in the Tonga-Kermadec seismic zone to st ations in New Zealand are very fast with highly emergent, dispersed wa veforms. Ray tracing has shown the waves to travel close to the subduc ted Pacific plate throughout their length, and synthetic seismogram ca lculations have shown the dispersion requires a very thin (8-12 km) fa st layer. Previous work has been based on data from analogue records a nd one digital, single-component short-period instrument; no polarizat ion analysis was possible, and measurements of dispersion were Limited by the bandwidth. From January 1991 to August 1992 we deployed nine b road band, three-component seismometers in good sites for observing th ese arrivals; the data are augmented by three-component, short-period digital records from new stations of the New Zealand National Network. In this study we analyze 1191 broad-band and 2076 short period Seismo grams from 71 events for polarization of the initial P wave. The polar ization directions are found to be up to 300 off the great circle path and consistently steep (200 from vertical). They are too steep to be explained by standard ray paths or refraction from a fast horizontal l ayer. We invert the polarization directions for a tilted interface ben eath the array and use arrival times to control the depth to the inter face, which is found to lie close to the top pf the subducted plate in ferred from the seismicity. Wet conclude that these precursive, emerge nt P waves have traveled through a fast layer close to the top of the subducted plate and refract upward to the station. A second arrival, w ith lower dominant frequency near 1 Hz and normal travel time, is occa sionally seen on both broad band and Short-period stations, Its polari zation direction is similarly steep but difficult to measure; the evid ence suggests that it also travels within the plate with similar ray p ath to the precursor.