Using coda wave interferometry for estimating the variation in source mechanism between double couple events

Abstract

It is shown how a change in orientation between the source mechanism of two identically located double couple sources can be estimated from the correlation of the coda waves excited by their sources. The change in orientation is given by the root mean square of the change in strike, Δφs, dip, Δδ and rake, Δλ of the double couple. It is not possible to determine Δφ s, Δδ or Δλ individually from the cross correlation. Applicability of the theory is tested using synthetic waveforms generated from a 3D finite difference solver for the elastic wave equation. Changes in strike, dip and rake are tested independently and simultaneously. In each case a crossover point is identified such that the actual change in orientation is within one standard deviation of the coda wave interferometry (CWI) estimates for all rotations below the crossover. After the crossover, the CWI estimates give a lower bound on the change in orientation. Crossover points of 30°, 62°, and 56°, respectively are observed when the strike, dip and rake are varied independently. When all angles are varied simultaneously by the same quantity the crossover point is 17°. The new theory can be applied in combination with existing coda wave interferometry techniques for estimating source separation. It creates the potential for joint relative location and focal mechanism determination using information from seismic coda recorded at a single station.