Assessment of Regional-Distance Location Calibration Using a Multiple-Event Location Algorithm

Abstract We test the use of a multiple-event seismic location method to improve epicenter accuracy estimates. Regional arrival-time observations of 74 Nevada Test Site explosions with known locations comprise the test data set. We investigate epicenter accuracy as a function of the number of events in the multiple-event system that are constrained at the known hypocenter (calibration), the effect of distance between calibration and unconstrained events, and the use of velocity models with varying travel-time prediction accuracy. Further, we test the utility of using a posteriori travel-time residuals to assess location and travel-time prediction accuracy. We find that constraining one event at the known hypocenter reduces epicenter error for all other events by 58% on average compared to locations produced without constraining events. The incremental improvement in epicenter accuracy rapidly diminishes as more hypocenters are constrained, and incremental location improvement is minimal when the number of constrained hypocenters exceeds 10. Events closest to a constrained event exhibit small location bias. Distinct epicenter bias occurs when the distance between the calibration event and the relocated event is greater than a few tens of kilometers. Last, we confirm that metrics based on a posteriori travel-time residuals are poor indicators of both epicenter accuracy and velocity model-based travel-time prediction accuracy.