Seismic structure of the Juan de Fuca Ridge: Ocean bottom seismometer results from the Median Valley

Abstract

An axial valley at the northern end of the Juan de Fuca ridge was the site where two new ocean bottom seismometers were tested in a seismic refraction experiment. Detailed underway geophysical data, seismic reflection profiles, and heat flow measurements were already available for area. By combining the refraction results from surface and bottom shots, the axial valley was found to be filled with at least 1.7 km of largely unconsolidated sediments that were deposited in about 0.3 m.y. VP is estimated for the sediments at 1.7 km/s; a P to SV converted wave allows estimation of VS as 0.51 km/s and Poisson's ratio as 0.450. More distant shots show that the crust in the area (all less than 2.5 m.y. in age) is matured and has velocities similar to those of the typical oceanic section given by Peterson et al. (1974). The upper mantle has a normal 8.3-km/s velocity across the ridge, but laterally localized low-velocity mantle must exist beneath the axial valley. This localized low velocity must be even lower than the anisotropically lower velocity commonly observed parallel to the crest in ridge flank studies.