On the estimation of seismic detection thresholds

Abstract

The incremental detection capability of a seismic station, i.e., the probability of detection as a function of event magnitude, is usually estimated either by using the exponential magnitude-frequency relationship of earthquakes (the recurrence method) or by comparing the station detections to an independent reference system (the direct method). The recurrence method usually yields significantly lower 90 per cent thresholds than the direct method, while a hypothetical “true” threshold would lie between these values. This is explained theoretically as resulting from the global variance of observed signal amplitudes for a given seismic event. A maximum likelihood estimation procedure is developed for the direct method, and approximate confidence limits on the estimated detection thresholds are derived. The two estimation techniques are applied to determine the event detection capability of the NORSAR array for the Japan-Kuriles-Kamchatka region, and the results are related to previously published estimates.