AHEAD European Archive of Historical EArthquake DataEuropean Archive of Historical EArthquake Data

Archives and libraries were visited to find previously unknown documents testifying to the Lurøy, Norway, earthquake of 31 August 1819 in northernmost continental Europe. The focus here is on Sweden, Finland, and Russia, which are important for determining the area of perceptibility east of Norway. The new written sources include 12 notes or entries in original archived documents, six contemporary newspaper reports, and two recollections written down years later. The original documentation uncovered is contributory to establishing the authenticity of the observations in Finland and Sweden. The dates of the original documentation allow tracing of the dissemination of eyewitness accounts in writing from the inner area of perceptibility southward to the larger documentation and population centers. New sources of information include weather reports of the Royal Swedish Academy of Sciences, minutes of its meetings, and correspondence sent to the Senate in Finland. The minutes of meetings of the Academy indicate that ample data were collected in the Swedish province of Västerbotten. We found no original Russian documentation but uncovered national newspapers that are more reliable than the previously used Parisian newspaper. To increase transparency, we provide the first list of macroseismic data points (MDPs) including the respective documentation that testify to the Lurøy earthquake. A macroseismic intensity was assigned to a locality, using the European Macroseismic Scale of 1998, when adequate information was available. Accounting for the uncertainty of intensity assessment, the magnitude was estimated as moment magnitude M=5.9±0.2⁠, reconfirming the ranking as the largest onshore or nearshore earthquake in the historical seismicity record of Fennoscandia. In addition to the reappraisal of the 31 August 1819 earthquake, a macroseismic map is provided for the earthquake of 17 February 1819, which was felt in northern Finland and Sweden. Some of its MDPs were previously associated with the Lurøy earthquake.

The two largest known pre-1850 Scandinavian earthquakes are the 22 December 1759 Kattegat event (located at 57.7°N-11.1°E) and the 31 August 1819 Nordland event (located at 66.4°N-14.4°E), the latter being the largest north European near-shore earthquake of the past few centuries. The 22 December 1759 event caused minor damage to buildings (MMI VII) on either side of the Kattegat in Northern Jutland and in the Swedish province of Bohuslän, and was felt up to 600 km away. The 31 August 1819 earthquake caused widespread damage (MMI VIII) to stone components of wooden buildings in the sparsely populated region of Nordland Norway, as well as very extensive rockfalls, liquefaction phenomena and a remarkable variety of disturbances in fiords and in the sea. The earthquake was felt throughout northern Scandinavia over distances of up to 800 km.

The purpose of this paper is to present the results of an evaluation of the seismicity of western Scandinavia. Intensities, with reference to the MSK scale, have been assessed for the larger earthquakes, in most cases from primary sources, and isoseismal maps have been constructed for the most important events, while for smaller shocks one or more isoseismal radii were estimated. In all, over 3,200 earthquakes have been retrieved for Northwest Europe including about 300 artificial events such as chemical explosions, rock-bursts and mine explosions. Of these, 500 events occurred in western Scandinavia. Surface-wave magnitudes were reassessed for 205 events, using surface-wave amplitude-period data, and all earthquakes recorded by more than 6 stations were relocated for the period prior to 1955. A calibration formula, obtained from the combination of macroseismic and instrumental data of the 20th century, has been used to assign magnitudes to all events for which there is macroseismic information available, thus deriving a homogeneous body of data covering a period of 180 years. It is shown that the largest earthquake in the region since 1800 occurred on land and that it had a magnitude in excess of 6.0. This is contrary to the current belief, based on short-term data, that the larger events in western Fennoscandia occur offshore in the continental shelf areas.