Earthquakes and seismic hazard in Finland

Abstract

In this paper the seismicity of Finland has been re-evaluated on the basis of a new regional magnitude scale, which gives somewhat lower magnitude values than the previously used macroseismic scale. Macroseismic magnitudes for Finnish earthquakes have been calculated by using the value 4.0 for the coefficient of attenuation, and the value 2.0 for the lower limit of intensity at the boundary of the felt area. Using these parameters, we obtained macroseismic magnitudes comparable to those of the instrumental M L-scale. The distribution of seismicity in Finland is presented in an epicentral map and maximum-intensity maps (1880–1980). These maps indicate some regional variation of seismic activity. The seismicity was also considered as a function of time using strain release curves. The b-value 0.59 was evaluated from the log N(M) relationship for the last hundred years. The time variation of b-values was also studied. Estimation of magnitude and intensity extremes by Gumbel's method yielded the value 5.0 for the upper limit of magnitude and VI-VII for the upper limit of intensity. The return periods for different magnitudes and intensities were calculated as well as the probabilities that they are exceeded.