Velocity anisotropy and delay-time maps of the mantle near Hawaii

Abstract

Standard and special seismic refraction profiles resulting in 1147 critically refracted mantle travel times were conducted across the Hawaiian arch to investigate the velocity characteristics of the upper mantle and the depth of the M discontinuity. The data are analyzed by a modified three-dimensional delay-time or time-term method in which the refractor velocity is assumed to possess anisotropy and lateral variations in velocity. The results show that the upper mantle immediately beneath the M discontinuity is anisotropic with respect to compressional waves. The maximum velocity occurs in an east-west direction and is 0.6 km/sec greater than the minimum, which occurs in a north-south direction. Lateral variations in the mean mantle velocity, which averages 8.16 km/sec for the entire area, are also indicated, although the evidence is not as definitive as for the anisotropy. The crustal thicknesses reflect the topographic arch, being relatively thin along the crest of the Hawaiian arch and thick in the Hawaiian deep. In the area on the arch directly north of the island of Hawaii, the crust is about 6.0 km thick, but it thins to about 5.3 km in both the southeast and northwest directions along the crest. The crust thickens beneath the southwestern flank of the arch and reaches thicknesses of 7.5–8.0 km in the Hawaiian deep.