Estimating shear velocities in the oceanic crust from compliance measurements by two-dimensional finite difference modeling

The deep seafloor deforms under the pressure loading of linear ocean surface gravity (water) waves at low frequencies (0.003 to 0.04 Hz). The ratio of seafloor displacement to pressure loading as a function of frequency, known as the seafloor compliance function, depends on the shear velocity structure of the oceanic crust and upper mantle. Compliance measurements are used to estimate oceanic crustal structure, particularly within low shear velocity regions such as sediments, fractured rock, and partial melt. Compliance calculated from laterally homogeneous (one-dimensional, I-D) crustal models shows that a buried low-velocity zone (LVZ) causes a peak in the compliance function at wavelengths 4 to 6 times longer than the LVZ depth, and that the compliance amplitude depends primarily on the LVZ shear velocity. A new numerical code allows forward modeling of compliance for two-dimensional oceanic crustal models. The new code demonstrates that the peak in the compliance function directly over a finite width LVZ reaches a maximum value at higher frequency, and is of smaller amplitude, than predicted from 1-D modeling. The compliance maximum persists outside of the region underlain by the LVZ but diminishes in amplitude and shifts to lower frequencies with increasing distance from the LVZ. The numerical models indicate that significant peaks in the compliance function indicate crustal LVZs, but that multiple compliance measurements are necessary to independently constrain the depth, location, and shear velocity of these features.