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

On the first of January 1992, unusual earthquake activity started in Steigen, northern Norway. By 31 December, 1992, a total of 207 events had been recorded in several pulses. Among these, several events with magnitudes larger than 3.0 (M(c)) were also felt by the people. The epicentral distribution of these events seems to suggest that they all are concentrated in a small region (approximately 10 km in diameter) in the Brennvika Bay (67.8-degrees-N, 14.9-degrees-E), between Leinesfjorden in the north and Nordfolda in the south. The distribution of events in time is characteristic of a swarm in which no clear evidence of a mainshock-aftershock sequence was observed. Macroseismic intensity surveys 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, combined with an observed NE trend in the epicentres and the focal depths, these events correlate with a previously proposed fault zone along Nordfolda. The NE trend has been dominant in the area throughout geological time, starting from the Caledonian Orogeny to the post-Caledonian basin developments, as well as the post-glacial and neotectonic structural features. At present, the deep-seated structural anomaly underneath the Lofoten Ridge seems to be the major factor controlling the structural trends in the area. The activity along the Nordfolda fault zone could be related to this large-scale structure, although the causes of the stresses that would create the seismic activity are not very clear, as there are several possible stress-generating mechanisms involved, such as post-glacial uplift of the land masses, the 'ridge-push' effect from Mohn's Ridge, and vertical stresses generated by the sediment loading in the adjacent basins.