Structure of the East Pacific Rise from an Ocean Bottom Seismometer Survey

Abstract

Three seismic refraction profiles have been recorded on the East Pacific Rise near the Siqueiros fracture zone. These profiles have been interpreted using both travel-time analysis and waveform and amplitude studies with the aid of synthetic seismograms. For the profile at the Rise Crest a pronounced low velocity zone has been found at about 2 km below the seabed. The P-wave velocities increase quite rapidly from about 5 km s-1 at the sea floor to 6·7 km s-1 at the lid of the low velocity zone. Mantle arrivals from below this zone indicate a low P-wave velocity of 7·7 km s-1. A second profile, on 2·9 My old crust shows a shallow region of strong velocity gradients which grades into velocities typical of the ' oceanic ' layer without any clear stratification. The mantle velocity is again low at 7·6 km s-1. The third profile situated on 5 My old crust, shows more distinct layering but velocity gradients exist within the stratification. The velocities here are more typical for Pacific refraction profiles and we have a mantle velocity of 8 km s-1. The results show that radical changes occur in the structure of the oceanic crust within a few million years in this region and that stratification becomes more pronounced with age. The most rapid change in seismic velocity with age appears to occur in the top 2 km of the structure and the low velocity zone is certainly absent for crustal ages greater than 1·5 My.