Studying local earthquakes in the northern Fennoscandian Shield using the data of the POLENET/LAPNET temporary array

Abstract

Abstract. Earthquakes within areas inside continental plates are still not completely understood and the progress in understanding intraplate seismicity is slow due to short history of instrumental seismology and sparse regional seismic networks in seismically non-active areas. However, knowledge about position and depth of seismogenic structures in such areas is necessary, in order to estimate seismic hazard for such critical facilities as nuclear power plants and nuclear waste deposits. In the present paper we address the problem of seismicity in the intraplate area of northern Fennoscandia using the information on local events recorded by the POLENET/LAPNET temporary seismic array during the International Polar Year 2007–2009. We relocate the seismic events by the program HYPOELLIPS and grid search method. We use the first arrivals of P-waves of local events in order to calculate a 3-D tomographic P-wave velocity model of the uppermost crust (down to 20 km) for selected region inside the study area and show that the velocity heterogeneities in the upper crust correlate well with known tectonic units. We compare position of the velocity heterogeneities with the seismogenic structures delineated by epicentres of relocated events and demonstrate that these structures generally do not correlate with the crustal units formed as a result of crustal evolution in Archean and Paleoproterozoic. On the contrary, they correlate well with the post-glacial faults located in the area of the Baltic–Bothnia Megashear (BBMS). Hypocentres of local events have depths down to 30 km. We also obtain focal mechanisms of two selected events with good data quality. Both focal mechanisms are of strike-slip type in which shift prevails over uplift. Our results demonstrate that Baltic–Bothnia Megashear is an important large-scale, reactivated tectonic structure that has to be taken into account in estimating seismic hazard in northern Fennoscandia.