Effect of single arrival time on the hypocentral focal depth determination

Accurate determination of focal depth is often an important task in analyzing seismic data. Valuable diagnostic information can be obtained by assessing the effect of individual stations on focal depth determination. Careful analysis of the data may be especially important for a small number of stations that have unusually large influence on focal depth. In this note we present a simple measure of station potential for focal depth, based on the work of Cook (1977). The potential measure is based on the change in estimated focal depth that results from deleting a station and can be calculated easily as part of a standard analysis based on Geiger's method. In a simulated example, the potential measure accurately predicted the effects of station deletion and detected problems related to focal depth determination that were not evident either from residuals or from the diagonal elements of the density information matrix. We think that the focal depth potential measure is a useful tool in diagnosing stations that play a dominant role in determining focal depth and recommend that it be a standard addition to the seismologist's tool kit.