Browsing by Author "Vuorinen, Tommi"

The ground motion induced by an earthquake and its attenuation as a function of magnitude and distance can be estimated by using various ground motion prediction equations (GMPE). From a historical perspective, the Fennoscandian shield has been a seismically quiet area with a scarcity of strong earthquakes. This has made the area nominally an area with a low seismic hazard. The lack of quantative strong motion data in an area with low seismic hazard and the subsequent lack of advanced theoretical models of seismic response have hindered the developement of GMPEs for the region. Using more direct empirical methods, which do not depend on pre-existing models and simulations of Fennoscandian seismicity, and by taking advantage of the comparatively large Fennoscandian shield ground motion database two GMPEs targeted for the Fennoscandian shield area were created. The created GMPEs are based on an existing attenuation relationship that targets Eastern North America, which is a stable continental area similar to the Fennoscandian shield. The current and historical stage of relative seismic quietness was preceded by a considerably more active phase during and after the deglaciation of the Weichselian continental ice shelf ca. 9000 - 15000 years ago. There is ample geological evidence that this postglacial faulting caused some of the largest known earthquakes that have ever occurred in a stable continental area. The moment magnitudes of the largest postglacial earthquakes have been estimated to have been in excess of 8.0. The magnitude contrast between the geologically recent postglacial seismicity and the available Fennoscandian ground motion database, which consists mostly of low magnitude events, implies that an empirical method can not provide a GMPE capable of reliably estimating the ground motion induced by a potential large postglacial faulting event. It is unlikely that such an event could occur today, but not completely impossible. The two GMPEs presented here are based on a ground motion database consisting of 6465 recordings from 1701 events with magnitudes between -1.0 – 5.2. The events were observed at 84 seismic stations around the Fennoscandian shield between 2003 and 2014. The first GMPE is an empirical model which takes an existing GMPE and uses a non-linear least squares regression method to refit the constant coefficients of the model to our regional ground motion database. The second GMPE is a referenced empirical model which works by multiplying the ground motion prediction of an existing GMPE with a function of certain seismological parameters. The multiplying function's coefficients are then fitted to the ground motion database. The resulting equations provide a reasonably good model of the peak ground acceleration (PGA) and spectral accelerations (SA) at 9 different frequencies: 0.5 Hz, 1.0 Hz, 2.5 Hz, 5 Hz, 10 Hz, 20 Hz, 25 Hz, 33.3 Hz, and 40 Hz. The GMPEs were further assessed by applying them to an independent regional earthquake and various higher magnitude external events that have originated in a presumably similar stable continental area. Based on these comparisons, the upper magnitude limits of applicability were independently estimated for PGA and each SA frequency. A tendency of the ground motion estimate to improve with increasing frequency at higher magnitudes can be seen. The distance ranges of the GMPEs were determined to be identical with the original base model.