Three-dimensional Kirchhoff-approximate generalized radon transform imaging using teleseismic P-to-S scattered waves

Abstract

Teleseismic imaging techniques utilizing mode converted/scattered waves are gaining importance due to the deployment of increasingly dense broad-band seismograph arrays. Although common-conversion point (CCP) stacking is widely used to determine structure from Ps or Sp scatteredwavefields isolated by receiver function (RF) processing, this method is limited due to its assumption of a layered medium: Dipping events and diffractions are not treated correctly. As an extension of previous 2-D generalized Radon transform (GRT) imaging methods, we present a 3-D Kirchhoff-approximate imaging technique to migrate scattered waves in 3-D. We first derive the 3-D migration formula for P-to-S conversions using the GRT solution to the linear inverse elastic wave scattering problem. Then we illustrate the Kirchhoff method using finite-difference synthetic seismograms from several 3-D models. Next, we apply the method to two portable broad-band array data sets in the western United States to image the Mendocino Triple Junction and the High Lava Plains (HLP) crust and uppermost mantle structures. From the HLP data, we construct the Ps transmission coefficient images with three-component Green's functions. The 1.0 and 0.5 Hz images show a continuous undulating Moho, as well as three negative upper-mantle events at 50-80 km depth. Compared to the CCP images, the Moho is more clearly imaged, particularly near 117.5°W-117.8°W at the western edge of the Owyhee Plateau. The three negative events in the upper mantle correlate well with the top of three low-Vs zones (-3 per cent contour) in the Rayleigh wave tomography model. The migrated Ps RF data from Mendocino clearly image the rapid decrease in depth of the lithosphere-asthenosphere boundary from ~65 km beneath the subducting Gorda Plate to 30-50 km beneath the Coast Ranges slab window. The final image is consistent with, but has higher resolution than the Vs structure determined from joint receiver function/Rayleigh wave inversions and images from Ps CCP stacks. Although restricted by array aperture and spatial aliasing criteria to dense, large aperture arrays, the 3-D Kirchhoff technique provides a promising tool for imaging complicated lithospheric structures that are poorly mapped or unresolved by more traditional imaging methods. © The Authors 2012. Published by Oxford University Press on behalf of The Royal Astronomical Society.