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

• Published in: Geophysical Journal of the Royal Astronomical Society Vol. 45; no. 2; pp. 305 - 320
• Main Authors: Orcutt, John A., Kennett, Brian L. N., Dorman, LeRoy M.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.05.1976
• ISSN: 0956-540X; 0016-8009; 1365-246X
• DOI: 10.1111/j.1365-246X.1976.tb00328.x

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.