Age dependence and the effect of cracks on the seismic velocities of the upper oceanic crust

Seismic velocities in young (e.g., <1 Ma) upper oceanic crust increase with depth from ∼2.0 km s−1 at the top of the basaltic crust to ∼6.8 km s−1 at its base. Also, seismic velocities at the top of the upper oceanic crust increase with age, while the velocities at the base remain fairly constant. The increase in seismic velocities with depth and age in oceanic crust can be explained by the stiffening of cracks with increasing overburden pressure and infilling of pore space with alteration products. Both of these mechanisms increase the moduli of the igneous crust and thus raise its seismic velocities.; Using the oblate spheroidal pore-shapes model of Kuster-Toksöz, laboratory measurements of P- and S-wave velocities, densities, and porosities of basaltic mini-cores from Hole 990A on the Southeast Greenland Margin show that pores can be effectively sealed by alteration products, and that the distribution of pore shapes is independent of porosity. Analyses of sonobuoy data collected over 0–7 Ma oceanic crust near the East Pacific Rise using the hidden layer method estimates seismic velocities of the upper oceanic crust. The results of sonobuoy analyses indicate that mean top-of-basement velocities and velocity gradients are 2.8 ± 0.1 km s−1 and 2.7 ± 0.1 s−1 respectively. Results also suggest that top-of-basement velocities increase at a rate of 0.12 ± 0.05 km s−1 Ma−1 . A pressure-dependent asperity-deformation model describes the increase in seismic velocities with depth observed from the sonobuoy data. The asperity-deformation model incorporates a velocity variation of the form V( z) = V₀ (1 + z/ z₀)1/n, where z is depth, V₀ is the velocity at the seafloor, and z ₀ and n are constants. The asperity-deformation model describes how seismic velocities can increase with pressure simply through the stiffening of cracks without a need for a change in mineral moduli. The observed traveltimes are modeled to within an average root-mean-square misfit of 3.5 ms (less than 0.8 percent).