AN EXERCISE IN AUTOMATING SEISMIC RECORD ANALYSIS AND NETWORK BULLETIN PRODUCTION

A long-standing problem in observational seismology is that of automat ing network operation. In this study we report an experiment that uses the detector described in Ruud and Husebye (1992) for automatically p icking P and S arrivals in local event records from the Norwegian Seis mic Network (NSN). The analysis was performed on prescreened waveform segments known to contain local events. For automatic epicenter determ ination a novel, robust grid-search method well-suited for estimation problems with nonGaussian observational errors is introduced in order to handle outliers. In our experiment, 49 local events from the August 1991 NSN bulletin were located. The number of detecting stations vari ed from 3 to 10, with an average of 5 of a total of 15 stations. P- an d S-picking errors were small, mostly within 0.5 sec for both P and S. Mis-identification of the P- and S-phase was more frequent; this affe cted about 15% of the picks. Even for events with several large arriva l time outliers (up to 40 to 50% of total) satisfactory epicenter dete rminations were obtained. The median location difference of our ''auto matic'' solutions compared with those in the NSN bulletin based on ana lyst picked and identified phases was 15 km. For 90% of the events the difference was less than 50 km.