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Abstract

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 "SUB 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 "SUB L" (UPP) = 3.4, occurred about one hour after the main shock. A few months later, on November 2, an M "SUB 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. The calculated seismic moment and stress drop for the Skovde main shock and for the Mariestad earthquake are given. Faulting of the area is shown to be 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. -from Authors