Use of Back Azimuth Measurements in Seismic Event Location with Regional Data

Conventional methods for seismic event location, which are based on measurements of signal arrival times, may be inadequate for events detected at only regional distances, because there may be an insufficient number of observations. This shortcoming can be alleviated by supplementing the measurements of P-wave arrival times with measurements of the events back azimuth made by applying Smarts 1978 three-component processor to the regional phase Lg. Back azimuth measurements can also be used as an analysts tool to calculate an approximate epicenter for use in the association of mixed events with the appropriate signal arrivals or for use as the initial value in an iterative location scheme. An algorithm is herein presented which performs event location by treating arrival time measurements and back azimuth measurements in an analogous fashion. Tests of this algorithm which were run in the depth-restrained mode for two events show that it can be effective for sparse data sets but also that it can lead to large errors if there is a poor geometrical distribution of detecting stations. Computations of the confidence region about the epicenter demonstrates an insufficiency in the theory for calculating F-statistic confidence ellipses for azimuths and travel-time data if the measurement variances are not known a priori. The theory is satisfactory for the standard chi-squared ellipses.