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On 23 October 1904, the area of the Oslofjord was struck by an Ms 5.4 earthquake that was felt over much of northern Europe. We collected information available from seismological bulletins of the time and also seismogram copies from a number of seismic stations in Europe. This allowed, for the first time for this earthquake, an instrumental epicentral location, even if the observation capabilities at that time were poor due to low seismograph amplification and timing problems. After a careful selection and weighting of published onset times, the reported observations from the seismic stations in Uppsala, Hamburg, Potsdam, Göttingen, Leipzig, and Tartu could be used for this instrumental location of the event. We also performed an inversion of the available macroseismic observations, based on the kinematic function (KF) for the radiation of body waves from a line source. Because the problem is nonlinear and also bimodal for pure dip-slip mechanisms, we used a sharing niching genetic algorithm to perform the inversion. The new epicenter obtained from the KF intensity inversion is consistent with the new instrumental solution, giving a location in the lower crust (25-30 km) and close to the eastern coast of the Oslofjord, near the junction of two major fault zones. The KF inversion was moreover able to constrain the fault-plane mechanism with an almost vertical rupture plane striking north-northeast-south-southwest (206°-212°) with a mixed mechanism (approximately 64° rake angle), the ambiguity of which is resolved by the polarities of the Pn onset observed on the horizontal components of the Uppsala Wiechert seismograph. The magnitude of the event has previously been assessed to Ms 5.4, based on the size of the felt area observations, and a value of Ms ~5.4 has now been independently confirmed both through the new intensity inversion and through the instrumental data. [ABSTRACT FROM AUTHOR]

Logic trees have become a popular tool in seismic hazard studies. Commonly, the models corresponding to the end branches of the complete logic tree in a probabalistic seismic hazard analysis (PSHA) are treated separately until the final calculation of the set of hazard curves. This comes at the price that information regarding sensitivities and uncertainties in the ground-motion sections of the logic tree are only obtainable after disaggregation. Furthermore, from this end-branch model perspective even the designers of the logic tree cannot directly tell what ground-motion scenarios most likely would result from their logic trees for a given earthquake at a particular distance, nor how uncertain these scenarios might be or how they would be affected by the choices of the hazard analyst. On the other hand, all this information is already implicitly present in the logic tree. Therefore, with the ground-motion perspective that we propose in the present article, we treat the ground-motion sections of a complete logic tree for seismic hazard as a single composite model representing the complete state-of-knowledge-and-belief of a particular analyst on ground motion in a particular target region. We implement this view by resampling the ground-motion models represented in the ground-motion sections of the logic tree by Monte Carlo simulation (separately for the median values and the sigma values) and then recombining the sets of simulated values in proportion to their logic-tree branch weights. The quantiles of this resampled composite model provide the hazard analyst and the decision maker with a simple, cleat', and quantitative representation of the overall physical meaning of the ground-motion section of a logic tree and the accompanying epistemic uncertainty. Quantiles of the composite model also provide an easy way to analyze the sensitivities and uncertainties related to a given logic-tree model. We illustrate this for a composite ground-motion model for central Europe. Further potential fields of applications are seen wherever individual best estimates of ground motion have to be derived from a set of candidate models, for example, for hazard maps, sensitivity studies, or for modeling scenario earthquakes. [ABSTRACT FROM AUTHOR]

The 31st of August 1819 Lurøy earthquake has, for a long time, been considered as the largest known historical earthquake in Fennoscandia, possibly even on the mainland of northern Europe. Triggered by a recent claim that the magnitude (MS) of this earthquake should be 5.1 rather than 5.8, which implies a reduction in radiated energy by about a factor of 10, we have reassessed the original macroseismic observations together with the published literature. Our conclusion is that a MS magnitude of 5.8 for the 1819 Rana region earthquake still remains a reasonable, justifiable and defendable estimate. Such a magnitude is moreover consistent with present hazard models for the region, which include the possibility that magnitude 6+ earthquakes can occur today in the most seismically active areas in Norway, such as the coastal parts of western Norway, Nordland and the Oslo rift zone. Rock avalanches and landslides, potentially triggered by earthquakes, could moreover generate tsunamis in fjords and lakes and constitute the greatest seismic hazard to society in Norway. [ABSTRACT FROM AUTHOR]

To investigate source-scaling relations for small earthquakes (Mw - 1.8 to 1.2) we have determined source parameters for numerous events (∼ 1500) from the 1400-m-deep Pyhäsalmi ore mine in Finland. In addition to a spectral integration approach, we have fitted Brune, Boatwright, and Haskell spectral-shape models to the observed spectra and investigated attenuation influences. Of three different constant Q models (200, 350, and 800), a Q of 350 in combination with the Brune spectral model satisfied the data best. We have also investigated the frequency dependence of Q using the spectral decay method and found that Q increases with frequency. For selected events from two distinct clusters, we compared source parameters derived from constant Q models with source parameters using the multiple empirical Green's function (MEGF) approach. By using constant Q models, the apparent stress seems to increase with magnitude, whereas results based on the MEGF approach indicate constant apparent stress with magnitude. In comparison with results from other studies that cover a larger-magnitude scale, we find apparent stresses that are about 1 to 2 orders of magnitude smaller than most of those. A modified M0 ∼ fc-(3 + ε) scaling relation allows for increasing apparent stress with magnitude and can hence combine this study's results with apparent stresses found for large earthquakes. However, within the limited-magnitude range of our data, apparent stresses seem constant. [ABSTRACT FROM AUTHOR]

Logic trees are widely used in probabilistic seismic hazard analysis as a tool to capture the epistemic uncertainty associated with the seismogenic sources and the ground-motion prediction models used in estimating the hazard. Combining two or more ground-motion relations within a logic tree will generally require several conversions to be made, because there are several definitions available for both the predicted ground-motion parameters and the explanatory parameters within the predictive ground-motion relations. Procedures for making conversions for each of these factors are presented, using a suite of predictive equations in current use for illustration. The sensitivity of the resulting ground-motion models to these conversions is shown to be pronounced for some of the parameters, especially the measure of source-to-site distance, highlighting the need to take into account any incompatibilities among the selected equations. Procedures are also presented for assigning weights to the branches in the ground-motion section of the logic tree in a transparent fashion, considering both intrinsic merits of the individual equations and their degree of applicability to the particular application. [ABSTRACT FROM AUTHOR]