EARTHQUAKE SWARM IN STEIGEN, NORTHERN NORWAY - AN UNUSUAL EXAMPLE OF INTRAPLATE SEISMICITY

On the first of January 1992, unusual earthquake activity started in S teigen, northern Norway. By 31 December, 1992, a total of 207 events h ad been recorded in several pulses. Among these, several events with m agnitudes larger than 3.0 (M(c)) were also felt by the people. The epi central distribution of these events seems to suggest that they all ar e concentrated in a small region (approximately 10 km in diameter) in the Brennvika Bay (67.8-degrees-N, 14.9-degrees-E), between Leinesfjor den in the north and Nordfolda in the south. The distribution of event s in time is characteristic of a swarm in which no clear evidence of a mainshock-aftershock sequence was observed. Macroseismic intensity su rveys as well as synthetic modelling indicate shallow focal depths in the range 5-8 km. Based on a composite focal mechanism, indicating an oblique-slip fault striking NE with a normal component dipping NW, com bined with an observed NE trend in the epicentres and the focal depths , these events correlate with a previously proposed fault zone along N ordfolda. The NE trend has been dominant in the area throughout geolog ical time, starting from the Caledonian Orogeny to the post-Caledonian basin developments, as well as the post-glacial and neotectonic struc tural features. At present, the deep-seated structural anomaly underne ath the Lofoten Ridge seems to be the major factor controlling the str uctural trends in the area. The activity along the Nordfolda fault zon e could be related to this large-scale structure, although the causes of the stresses that would create the seismic activity are not very cl ear, as there are several possible stress-generating mechanisms involv ed, such as post-glacial uplift of the land masses, the 'ridge-push' e ffect from Mohn's Ridge, and vertical stresses generated by the sedime nt loading in the adjacent basins.