Seismic Anisotropy in the Upper Crust at DSDP Site 504B

Abstract

Seismic anisotropy in the upper oceanic crust is observed in borehole data obtained at Deep Sea Drilling Project (DSDP) Site 504B on DSDP Leg 92. Particle motion analysis of converted shear wave arrivals from explosive sources at various azimuths reveals a set of patterns which is indicative of hexagonally isotropic structure with a horizontal symmetry axis. There are four diagnostic patterns: 1) Along symmetry axes, where vertically polarized shear waves (SV) are generated, but horizontally polarized shear waves (SH) are not generated, the particle motions are purely vertical; 2) For azimuths at which both SV and SH are generated and the SH velocity is significantly faster than SV, a cruciform pattern with horizontal first motion is observed; 3) For azimuths at which both are generated and the SV velocity is significantly faster than SH, a cruciform pattern with vertical first motion is observed; and 4) for azimuths at which both are generated and SV and SH velocities are similar elliptical particle motions are observed. The shear wave particle motions and compressional wave travel times (from a 2 km radius circle) are consistent with an anisotropic model with hexagonal symmetry. The compressional wave velocity has a two theta variation between 4.0 and 4.8 km/sec. The symmetry axis is horizontal with an azimuth of N20°W ± 10°. The spreading direction at the site (6 my age) is north-south. The observed seismic anisotropy is most probably caused by the preferred orientation of large scale fractures and fissures in upper layer 2 which were created in the early stages of crustal development by near axis extensional processes and normal block faulting.