Compressional to shear wave conversion in oceanic crust

Abstract

Summary Mode conversion for compressional to shear waves or vice versa, occurs most efficiently where the seismic wave encounters a large change in seismic velocities over a distance of less than half a wavelength. In the oceanic crust, the interface between water or sediment and the volcanic basement is frequently the site of considerable mode conversion. We examine the effect of changes in the sediment and the basement properties on the efficiency of mode conversion over a range of horizontal phase velocities assuming that the interface is a first order discontinuity. We then show with the aid of synthetic seismograms calculated using the reflectivity method that a small transition zone at the top of the basement over which the velocities increase with depth may have a considerable effect on the amount of conversion. In particular, small changes in the shear wave velocity structure over thicknesses of around half the shear wavelength may markedly change the amplitudes of the variable angle basement reflection and the doubly converted lower crustal shear waves even though we may only consider compressional waves incident on and returned from the crust. Thus, at typical seismic source frequencies, a change of only a few metres to a few tens of metres in the shear wave transition zone thickness, although considerably less than the wavelength of an incident compressional wave may greatly alter the resultant compressional wave crustal seismogram.