The leading source of error in seismic event locations is travel-time perturbations caused by three-dimensional Earth structure. The reciprocity of travel times between sources and receivers provides a method for testing the effectiveness of empirical methods for improving event locations that rely on nearby calibration events of known location. We apply this approach to travel-time residuals obtained by Engdahl et al. (1998) for almost 100,000 teleseismic events. By analyzing the residual patterns at thousands of seismic stations of known location, we characterize the spatial coherence of station/event mislocation vectors. We find that, on average, calibration events are likely to improve locations only if they are located within 100–150 km of the target events. For 84 events of known location, we find that applying source-receiver reciprocity can significantly reduce location errors by correcting for the teleseismic residual pattern observed at stations close to the target events. These results have implications for efforts to improve event locations for nuclear explosion monitoring purposes.

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