Constraints on coda wave interferometry estimates of source separation: the acoustic case

Abstract

Synthetic experiments are used to test the applicability of coda wave interferometry (CWI) as a means for estimating distance between sources of nearby earthquakes. Acoustic waves for 45 sources are propagated through a Gaussian random medium. A pair-wise analysis of resulting waveforms illustrates the applicability of CWI as a tool for estimating source separation. Results suggest that, when the waveforms are filtered between 1 and 5 Hz, CWI provides accurate estimates of the separation for source-pairs separated by δ < 250 m. The technique provides a lower bound on the actual separation when δ > 250 m. The CWI breakdown distance of 250 m is likely to vary with frequency content in the waveforms.The interpretation of CWI source separation estimates is aided by the construction of a conditional probability density function (PDF) P(δt ∣ δCWI), which describes the probability of actual separation δt for given CWI estimates δCWI. The conditional PDF provides a constraint on event separation that is asymmetric. It can be used independently of, or combined with, standard travel-time techniques to improve earthquake location.