An Integrated Approach to Seismic Event Location: II. Sources of Location Uncertainty for Teleseismic and Local Network Data, (Period Covered - 1 July 1991 through 28 February 1994)
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The locations of seismic events determined from travel times by independent agencies differ for at least three reasons, including differences in picking of or availability of the phases, effects produced by individual structural peculiarities of each station, and systematic mislocations caused by the choice of the travel time model used for relocation. The present study evaluates earthquake locations for events in three geographically diverse regions chosen to represent a broad spectrum of location problems. These are: Macquarie Ridge; Bucaramanga, Colombia; and Efate, Vanuatu. The evaluation methods include a comparison of locations reported by different agencies, an analysis of the sources of the variance of travel-time residuals, and a determination of the volume of groups of events thought to originate from a common source region. For the data analyzed it appears that station-dependent effects are a more significant source of location differences than are effects related to picking phase arrivals or choice of velocity model. Also, this study finds many situations where it seems inappropriate to utilize the strategy of minimizing travel time variance to improve location quality.