The Baltic Shield earthquakes

Abstract

The Baltic Shield area is a low-seismicity area with few seismic events exceeding M L = 4. Since 1979 dense regional seismic networks have been in operation and have allowed accurate location, including focal depths and fault plane solutions, of more than 200 earthquakes in the range M L = 0.6–4.5. The location of the southern Sweden seismicity shows that a long part of the Protogine zone, the border between the younger western and older eastern crustal rocks, marks a change in seismic activity. The focal depths of the earthquakes indicate three seismically different crustal layers: upper crust (0–13 km in northern Sweden, 0–18 km in southern Sweden), middle crust down to 35 km, and the seismically quiet lower crust. The Gutenberg b-value is smaller for the middle crust than for the upper crust. The fault plane solutions show that strike-slip is dominant. Along the Tornquist Line (the southwestern border of the Baltic Shield area) normal faulting occurs. In northern Sweden reverse faulting is more common than normal faulting. The stresses released by the earthquakes show a remarkable consistency considering the small sizes of the earthquakes. The data are consistent with a N60W direction of the regional principal compression, supporting the view that the Baltic Shield seismicity is related to plate tectonic processes rather than to uplift. The spatial distribution of the Baltic Shield seismicity is consistent with a model in which the earthquakes are considered as sudden breakdowns of asperities of normally, stably sliding faults. Aseismic sliding is estimated to be more than 20,000 times more extensive than seismic sliding. The rate at which southern Sweden presently deforms horizontally is estimated to be ⩾ 1 mm/yr.