Three-dimensional upper crustal heterogeneity and anisotropy around Hole 504B from seismic tomography

We have used over 16,000 compressional wave travel times recorded on 11 oce an bottom seismometers to determine the magnitude of P-wave heterogeneity a nd anisotropy in the upper 2 km of the oceanic crust in an similar to 150 k m(2) area around Hole 504B, the deepest hole drilled into the oceanic crust . Our best fitting one-dimensional, isotropic upper crustal velocity model for the 150 km(2) area around the drill site is similar to velocity models at Hole 504B determined using downhole sonic logs, vertical seismic profile s, oblique seismic experiments and conventional refraction profiling. There is no evidence seismic layer 2 is anomalously thin at Hole 504B, suggestin g that the occurrence of the seismic layer 2/3 boundary within the upper sh eeted dike section, which has been documented in Hole 504B, is a general fe ature of this 5.9 Ma a crust. On the scale of a few kilometers and averaged over seismic wavelengths of a few hundred meters the heterogeneity in uppe r crustal P wave velocity around Hole 504B is comparatively small. Within t he 571-m thick extrusive section maximum P wave velocity differences are si milar to 300-400 m s(-1) (a 6%-8% velocity anomaly). These velocity differe nces decrease to <100-200 m s(-1) (<2%-3% of the average velocity) within t he sheeted dike section >1 km into the crust. The highest upper crustal vel ocities are found beneath basement ridges west and south of Hole 504B, whil e the lowest upper crustal velocities occur beneath the flanking basement t roughs. There is an azimuthal dependence to the travel time residuals (slow direction N-S) which is consistent with 2%-4% crack-induced anisotropy in the upper crust. The magnitude of upper crustal heterogeneity observed near Hole 504B is significantly less than that found in "zero-age" crust at the Mid-Atlantic Ridge but comparable to that observed off-axis in crust forme d at faster spreading ridges.