Constraints on coda wave interferometry estimates of source separation: the acoustic case*
David Robinson 1 2 4 Malcolm Sambridge 1 Roel Snieder 31 Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia.
2 Risk Research Group, Geoscience Australia, GPO Box 378, Canberra 2601, Australia.
3 Center for Wave Phenomena and Department of Geophysics, Colorado School of Mines, Golden CO 80401, USA.
4 Corresponding author. Email: david.robinson@anu.edu.au
Exploration Geophysics 38(3) 189-199 https://doi.org/10.1071/EG07019
Submitted: 4 August 2006 Accepted: 31 July 2007 Published: 19 September 2007
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.
Key words: earthquake location, coda waves, coda wave interferometry, relative location.
Acknowledgments
This work has received the support of the Research School of Earth Sciences at The Australian National University; Geoscience Australia, an Australian Federal Government Agency; and the Center for Wave Phenomena at the Colorado School of Mines. Heiner Igel is thanked for supplying the original acoustic finite difference software. The manuscript has been improved courtesy of reviews by Marthijn de Kool (GA), Jürg Hauser (RSES), Matthew Purrs (GA), an anonymous reviewer, David Denham and the editorial team of Exploration Geophysics (Leonie Jones and Lindsay Thomas).
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* *Presented at the Australian Earth Sciences Convention, June 2006, Melbourne.