DEEP-CRUSTAL EARTHQUAKES IN THE SOUTHERN BALTIC SHIELD

On 1986 July 14, one of the largest earthquakes in the Baltic Shield during this century occurred near Skovde in the province of Vastergotland, Sweden, with a magnitude of M(L)(UPP) = 4.5. It was followed by the so far largest number of recorded aftershocks, more than 20, from any Swedish earthquake. The strongest aftershock, with M(L)(UPP) = 3.4, occurred about one hour after the main shock. A few months later, on November 2, an M(L)(UPP) = 3.6 event took place near Mariestad some 30 km northwest of the Skovde series. All these shocks were located in the lower crust with foci at depths between 20 and 35 km indicating active movements in the shield at depths where mainly ductile deformation is usually assumed. Waveform modelling seems to be an excellent tool to obtain information about the focal depth which is otherwise hard to determine. The calculated seismic moment and stress drop for the Skovde main shock are 5.9 X 10(14) N m and 2.8 MPa, respectively, and for the Mariestad earthquake 2.3 x 10(14) N m and 7.3 MPa, respectively. Focal mechanisms for the Skovde main shock and largest aftershock have been obtained from P-wave polarities and synthetic modelling, and for the Mariestad earthquake from P-wave polarities. The synthetics show good agreement with short-period seismograph records for frequencies up to at least 2 Hz and distances up to at least 200 km. A comparison of the mechanisms indicates that the faulting of the area is complex, with styles ranging from strike-slip to normal, suggesting that other factors than the push from the North Atlantic Ridge, e.g. post-glacial rebound, also contribute to the lithospheric stress pattern. This explanation is corroborated with findings from nearby southern Norway.