Earthquake location accuracies in Norway based on a comparison between local and regional networks

Detailed studies of the low to intermediate seismicity in two coastal regions of Norway have been used in a comparison between earthquake locations from local high-precision networks on the one side and locations using a sparse regional array network on the other side. To this end, a reference set of 32 low-magnitude earthquakes have been located using two local temporary networks in northern and western Norway, with estimated epicenter accuracies better than 5 and 10 km, respectively. Comparisons are made between the local network solutions and the NORSAR Generalized Beamforming (GBF) system, which provides automatic phase association an location estimates using the Fennoscandian regional array network. The median automatic GBF location error is of the order of 20-30 km when four or more arrays detect the event, increasing to about 80-100 Lm when only two arrays are available, and the automatic GBF bulletin is essentially complete down to magnitude M-L = 2.0. Most of the mislocation Vectors of the NORSAR GBF solutions are oriented perpendicular to the Norwegian coast, and with a tendency to pull the location in a southeasternly direction. The GBF performance is clearly better. both in terms of accuracy and completeness, than the performance of the automatic bulletin of;he Prototype International Data Center (PIDC) which uses data from essentially the same network. The analyst reviewed NORSAR and PIDC bulletins show, nut unexpectedly, an improvement in location accuracy compared to the automatic solutions and appear to be of similar quality for the few common events, with an average mislocation of about 20 km. The NORSAR reviewed bulletin is more complete at low magnitudes compared to PIDC, and there appears to be a potential for significant improvements in the PIDC processing of small seismic events in this region.