RECENT KATTEGAT EARTHQUAKES - EVIDENCE OF ACTIVE INTRAPLATE TECTONICS IN SOUTHERN SCANDINAVIA

On June 15, 1985, an earthquake with a local magnitude M(L)(UPP) value of 4.6 occurred in the Kattegat area close to the Swedish-Danish border. It was one of the largest earthquakes in Sweden and Denmark during this century. Two more events occurred in the same area: on April 1, 1986 (M(L)(UPP) = 4.2), and May 24, 1990 (M(L)(UPP) = 3.3). The derived focal mechanisms have north-south trending P-axes which deviate by 45-degrees from the NW-trending compressive stress field postulated by the ridge-push theory. The mechanisms can, however, be explained by local neotectonism. Both the locations and focal mechanisms, strike-slip faulting on NW striking planes, correlate well with the dominant neotectonic feature of the region, the Skalderviken depression. Seismic moments of the 1985, 1986 and 1990 events were 3.6 x 10(14) Nm, 1.4 x 10(14) Nm and 6.0 x 10(12) Nm, respectively. The 1985 earthquake had an estimated maximum intensity of VII (modified Mercalli scale) and was felt over an area with a mean radius of 180 km. The 1986 earthquake had a maximum estimated intensity of VI and a radius of perceptibility of 100 km. Despite the recent low seismicity of the area, the earthquakes studied here indicate the potential for the occurrence of major events. This is supported by the historical seismicity.