Multichannel Alignment of S Waves

Abstract

\n High-resolution earthquake locations and structural inversions using body waves rely on precise delay-time measurements. Subsample accuracy can be realized for P waves using multichannel cross correlation (MCCC), as developed by VanDecar and Crosson (1990), which exploits redundancy in pairwise cross correlations to determine delays between similar waveforms in studies of mantle structure using teleseismic sources (common source and multiple stations) and regional studies of structure and seismicity (multiple sources and common station). For regional S waves, alignment is complicated by the additional degree of freedom in waveform polarity that is expressed for sources with different moment tensors. Here, we recast MCCC within a principal component framework and demonstrate the equivalence between maximizing waveform correlation and minimization of various singular value–based objective functions for P waves. The singular-value framework is more general and leads naturally to an MCCC linear system for S waves that possesses an order of magnitude greater redundancy than that for P waves. Robust L1 solution of the system provides an effective means of mitigating outliers at the expense of subsample precision. Residual time shifts associated with higher-order singular vectors are employed in an iterative adaptive alignment that achieves subsample resolution. We demonstrate application of the approach on a seismicity cluster within the northern Cascadia crustal fore-arc.