Seismic event coda-correlation's formation: implications for global seismology

Abstract

SUMMARY The seismic-event-coda correlograms are characterized by many prominent features, which, if understood thoroughly, could supply valuable information on the internal structure of the Earth. To further refine our knowledge and be able to utilize that information, all-embracing comprehension of coda-correlation's formation apart from a conjecture, is a pre-requisite. Here, we conduct a comprehensive analysis that aims at a quantitative ‘dissection’ of the formation mechanism of coda correlation. Our analysis presents relevant implications for global seismology. We demonstrate that coda correlation is dominated by a few contributions, most of which arise from the late-coda time window, 3 hr after the earthquake origin time. Our identification analysis confirms that the contributions are cross-terms between body waves. That represents an observational proof of the conjecture that coda-correlation features are formed due to body waves arriving at a pair of receivers with the same slowness. We further quantify the relationship between body-wave cross-terms and event-receiver geometries and Earth structure, which has significant practical implications. Our analysis demonstrates that body-wave cross-terms that contribute to the same coda-correlation feature sample the Earth along fundamentally different paths. They are significantly different depending on event locations, although the resulting time variation is quite small if the late coda (e.g. 3–9 hr after event origin time) is used. That explains why the late coda is more effective than an earlier time window in producing relatively stable features, as empirically suggested by previous studies. Our study enables quantitative and practical understanding of coda-correlation features in terms of their formation progress, and this opens a way to distill valuable information about Earth structure from coda correlation.