Converted-wave reverse time migration imaging in subduction zone settings

Abstract

Summary We use a newly developed 2D elastic reverse time migration (RTM) imaging algorithm based on the Helmholtz decomposition to test approaches for imaging the descending slab in subduction zone regions using local earthquake sources. Our elastic RTM method is designed to reconstruct incident and scattered wavefields at depth, isolate constituent P and S wave components via Helmholtz decomposition, and evaluate normalized imaging functions that leverage dominant P and S signals. This method allows us to target particular converted-wave scattering geometries, e.g., incident S to scattered P, which may be expected to have dominant signals in any given data set. The method is intended to be applied to dense seismic array observations that adequately capture both incident and converted wavefields. We draw a direct connection between our imaging functions and the first-order contrasts in shear wave material properties across seismic discontinuities. Through tests on synthetic data using either S → P or P → S conversions, we find that our technique can successfully recover the structure of a subducting slab using data from a dense wide-angle array of surface stations. We also calculate images with a small-aperture array to test the impact of array geometry on image resolution and interpretability. Our results show that our imaging technique is capable of imaging multiple seismic discontinuities at depth, even with a small number of earthquakes, but that limitations arise when a small aperture array is considered. In this case, the presence of artifacts makes it more difficult to determine the location of seismic discontinuities.

Three‐Dimensional Teleseismic Elastic Reverse‐Time Migration With Deconvolution Imaging Condition and Its Application to Southwest Japan

“…This deconvolution imaging condition was first suggested by the exploration seismologists to improve image resolution (e.g., Guitton et al., 2007; Valenciano & Biondi, 2005). Similar implementations in the time domain were presented for reflection and transmission seismic data (Shabelansky et al., 2017) or earthquake data with local sources (Langer et al., 2023). Physically, this imaging condition defines a ratio between the P‐to‐S transmission conversion coefficient and the P‐to‐P transmission coefficient at an interface, thus the STF can be naturally removed.…”