Swedish NDC evaluation of selected events in connection with SPT1

The Swedish NDC evaluation in connection with the First System-Wide Performance Test (SPT1) has focused on the IDC event location accuracy and a comparison between the Reviewed Event Bulletin (REB) and a list of events produced by the Swedish National Seismic Network (SNSN). In particular we were interested in getting absolute location errors for events observed in Sweden and whether the corresponding IDC error ellipses represented real mislocations. For this purpose a limited number of events in the REB were selected and true locations were retrieved. These events include explosions from the Aitik open pit mine in the northern part of Sweden and underwater explosions from an area in the Stockholm archipelago. Evaluation of the IDC location errors based on the selected explosions showed that both the location accuracy and the reported error ellipses need to be improved due to the requirements of a potential On-Site Inspection. Implementation of new travel time tables in 2004 seemed to cause a slight increase of the absolute location errors of the Aitik explosions. In order to obtain accurate locations of the Aitik explosions several readings of the Sn-phase (in addition to Pn) seems to be required. This conclusion was drawn from an investigation of the defining phases used in the IDC event solutions. Combining seismic data from the IMS and the national network from relatively small explosions at two nearby sites in western Sweden substantially improved the location accuracy. Finally, a comparison between the REB bulletin and a list of earthquakes mainly in Sweden defined by the Swedish national network showed that the REB was complete down to a magnitude of about 3.2.