Characteristics of Radiation and Propagation of Seismic Waves in Northern Finland, Estimated Based on Records of Local Earthquakes

The characteristics of radiation and propagation of seismic waves required in various seismological applications, especially in seismic hazard assessment, are estimated in northern Finland based on records of local earthquakes (~ 80 records from four stations, M_w ~ 1.5 to 4.1, h ~ 3 to 25 km). Northern Finland possesses relatively low seismic activity; however, rather strong earthquakes occurred there in the past. We applied coda-normalization method to estimate the quality factor Q(f)~ Q₀fⁿ and then verified and corrected the Q(f) estimates at low frequencies by stochastic simulations. We obtained Q₀~ 80 to 100, n ~ 1.1 to 1.2 for hypocentral distances of ~ 20 to 150 km, and Q₀~ 180 to 350, n ~ 0.7 to 0.9 for hypocentral distances of ~ 150 to 320 km. Other parameters were estimated by the empirical methods and stochastic simulation of local earthquakes (M_L > 2.0). We estimated the stress drop (∆σ ~ 200 bars), geometrical spreading (as a two-segment function: 1/R for R ~ 0 to 80 km and 1/80 for R ~ 80 to 300 km), the duration of seismic motion depending on the hypocentral distance, kappa value (κ ~ 0.001 to 0.03 s), and some other parameters. The obtained high estimates of the stress drop, along with rather high Q values, may result in noticeably increased (compared to those that could be expected on average) PGA estimates in seismic hazard assessment in northern Finland.