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.
Highlights
The only difference is that the global-scale stacks include more non-contributing cross-terms of seismic waves that propagate outside the great-circle plane in three dimensions
We have shown that coda-correlation consists of discrete and timedependent contributions
Most of these contributions reside in the late coda, 3 hr after the origin time and later, while the early time-interval presents many incoherent constituents with varying energy
Summary
Half a century has passed since pioneering investigations of the properties of coda waves—the late and long-lasting wave trains of earthquakes (Aki 1969; Aki & Chouet 1975). Apart from that, the seismic-event coda on global scale is made of strong reverberations with energy confined in the great-circle plane (SensSchonfelder et al 2015) This contradicts the necessary condition for accurate reconstruction of seismic waves in which the energy distribution should be equal in different directions for the wavefield that is cross-correlated. All features in global correlograms can be explained by means of a mathematical conjecture and simple laws of physics that employs ray theory: they arise due to the similarity between body waves of the same slowness for a given pair of receivers at the Earth’s surface (Pha.m et al 2018). Our analysis enables quantitative interpretation and utilization of coda correlation to constrain Earth structure
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