Revisiting Svenskby, Southeastern Finland: communications regarding low-magnitude earthquakes in 1751–1752

This investigation examines the contemporary documentation of a sequence of low-magnitude earthquakes at the fringes of the Kingdom of Sweden, today Southeastern Finland, in 1751–1752. A total of 11 pages of original correspondence sent from the target village of Svenskby to the Swedish capital Stockholm are reviewed. Newspaper accounts from Sweden and Russia are included in the analysis, and a timeline of the reporting is constructed. A newly created catalog shows over 30 distinct events between the end of October and December 1751 (Julian calendar). The assignment of macroseismic intensity to the earthquakes is hampered by loud acoustic effects that accompany and/or constitute the observations. Maximum intensities are assessed at IV–V (European Macroseismic Scale 1998), and maximum macroseismic magnitudes in the range of MM1.9–2.4, and were probably observed at short epicentral distances close to the ground surface. Comparisons to macroseismic data related to instrumentally recorded earthquakes in the region support the notion of low magnitudes. The data from 1751 provide an analog to modern macroseismic observations from geothermal stimulation experiments. Such experiments have acted as a spur for considering seismic risk from low-magnitude earthquakes whose consequences have seldom previously been a matter for concern.