Seismic Source Discrimination at Teleseismic Distances—Can We Do Better?

Abstract

The mb:MS criterion and other routine seismic discriminants can fail, resulting in a number of unidentified disturbances for which alternative methods of discrimination are sought. Direct examination of the P and S source radiation patterns offers one possibility. Such an approach presents difficulties at local and regional distances, where the complexity of body-wave raypaths makes the direct examination of radiation patterns impossible. By contrast, the analysis of radiation patterns at teleseismic distances is straightforward in principle. It has been used for nearly to decades in the relative amplitude method, which was developed by the author and colleagues for source identification and the determination of focal mechanisms. Although the identification threshold for any teleseismic discriminant is likely to be at a higher magnitude than for a successful regional discriminant, previous studies of specific discrimination problems indicate that teleseismic radiation pattern observations can make a significant contribution to the identification of suspicious seismic sources. Here the evidence for this is reviewed, and criteria are suggested for the design of a global seismic network to identify disturbances by means of teleseismic radiation pattern observations. An algorithm to measure the performance of different distributions of stations is used to show that existing and planned networks, including the proposed Primary Seismic Network for monitoring a comprehensive test ban treaty, are inappropriate for global discrimination at teleseismic distances. This inadequacy is shown to arise not from an insufficiency of suitable stations, but from their uneven geographic distribution. Arguments are presented in support of further studies in teleseismic discrimination, particularly to establish additional array stations to improve the global capability of teleseismic discrimination.