Imaging the Juan de Fuca subduction plate using 3D Kirchoff Prestack Depth Migration

Abstract:

We propose a new Receiver Function migration method to image the subducting plate in the western US that utilizes the US array and regional network data. While the well-developed CCP (common conversion point) poststack migration is commonly used for such imaging; our method applies a 3D prestack depth migration approach. The traditional CCP and post-stack depth mapping approaches implement the ray tracing and moveout correction for the incoming teleseismic plane wave based on a 1D earth reference model and the assumption of horizontal discontinuities. Although this works well in mapping the reflection position of relatively flat discontinuities (such as the Moho or the LAB), CCP is known to give poor results in the presence of lateral volumetric velocity variations and dipping layers. Instead of making the flat layer assumption and 1D moveout correction, seismic rays are traced in a 3D tomographic model with the Fast Marching Method. With travel time information stored, our Kirchoff migration is done where the amplitude of the receiver function at a given time is distributed over all possible conversion points (i.e. along a semi-elipse) on the output migrated depth section. The migrated reflectors will appear where the semicircles constructively interfere, whereas destructive interference will cancel out noise. Synthetic tests show that in the case of a horizontal discontinuity, the prestack Kirchoff migration gives similar results to CCP, but without spurious multiples as this energy is stacked destructively and cancels out. For 45 degree and 60 degree dipping discontinuities, it also performs better in terms of imaging at the right boundary and dip angle. This is especially useful in the Western US case, beneath which the Juan de Fuca plate subducted to ~450km with a dipping angle that may exceed 50 degree. While the traditional CCP method will underestimate the dipping angle, our proposed imaging method will provide an accurate 3D subducting plate image without heavy computation. This will provide further thoughts for geodynamic research on the evolution of western US.