Revising the 1759 Kattegat earthquake questionnaires using synthetic wavefield analysis

In aseismic regions the lack of large earthquakes in modern times makes the study of historical earthquakes essential as such events often serve as the yardstick for estimating seismic hazards. Since the largest earthquakes with few exceptions took place hundreds of years ago we may claim that their recurrence times are very large or alternatively their magnitudes are biased upwards. Scandinavia is no exception in this regard and one interesting event in particular is the Kattegat earthquake of 22 December 1759. It is unique in the sense that a macroseismic questionnaire was circulated in Zealand within days of its occurrence. The puzzling feature here is that intensity ranges from VII to I across Zealand but then given I=III for Hamburg and other North German cities. We have chosen two avenues in reassessing the magnitude of the Kattegat earthquake, namely (1) 2D finite-difference (FD) waveform modelling to evaluate possible amplitude amplifications due to sub-surface basin structures and (2) re-assessing the intensity reports of the 1759 Kattegat earthquake in light of the new European Macroseismic Scale (EMS-98). FD results show that the presence of the Danish and the North German basins led to shear wave amplitude amplification by factors of 10 and 5, respectively. This is equivalent to a significant increase or overestimation of the area of perception (tied to I=III). The re-assessment of the macroseismic observations in light of the above results and the EMS-98 downgrade the Kattegat earthquake MS magnitude from 5.6 to 5.1.