Compressional and Shear Wave Velocities in the Upper Crust

Abstract

New reflectivity vs. grazing angle data at a number of thinly sedimented sites in the Pacific Ocean spanning 0.5 to 64 m.y. reveal that compressional and shear wave velocities, Vpand Vsof the uppermost crust increase with geologic age. Measurements between 40 and 64 m.y. reveal compressional and shear critical angles of 60 ± 3° and 9° ± 3° respectively. The corresponding compressional and shear speeds are 3090 ± 300 m/s and 1564 ± 15 m/s respectively. The relatively high estimated shear speeds are in sharp contrast to inferred shear speeds in 0.5 m.y. old crust, viz., 800 m/s, whereas the estimated compressional speeds in the older crust only slightly exceed Vpin young crust, which is about 2400 m/s.