The 23 october 1904 MS 5.4 Oslofjord earthquake: Reanalysis based on macroseismic and instrumental data

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 newepicenter 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 UppsalaWiechert seismograph. The magnitude of the event has previously been assessed toMS 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.