First results from the Hawaiian SWELL pilot experiment

In a year-long pilot experiment to the southwest of the Hawaiian Islands, w e recorded teleseismic intermediate-period Rayleigh waves on ocean-bottom " L-CHEAPO" instruments using differential pressure gauges as sensors. We ana lyzed over 70 events and obtained accurate phase velocity estimates at peri ods between 15 and 70s. The average seismic structure beneath the pilot arr ay does not deviate significantly from a standard seismic model of 100 Myr old oceanic lithosphere. However, we find strong lateral velocity variation s across the array with large negative anomalies appearing within roughly 3 00km of the island chain. We are able to image the edge of the Hawaiian Swe ll and hence demonstrate the importance of making phase velocity measuremen ts on the ocean floor.