Intraplate Seismicity and Seismic Hazard: The Gulf of Bothnia Area in Northern Europe Revisited

Abstract

Intraplate seismicity is a challenging subject to study, because earthquake observations accumulate slowly in plate interiors. It is known that great earthquakes can occur in these regions and that shallow earthquakes may break the surface, but examples are limited to a dozen or so. This has obvious implications to seismic hazard and risk. England & Jackson (2011) pointed out that the unanticipated earthquakes in continental interiors have claimed more human lives than earthquakes at plate boundaries in the past 120 years. The present investigation deals with intraplate seismicity of the Fennoscandian (Baltic) shield in northern Europe. The seismicity of the region has been discussed by several authors over the years (e.g., Bungum et al., 1986; Gregersen et al., 1991). The investigation focuses on the Gulf of Bothnia bordered by Finland to the east and Sweden to the west (Fig. 1). The Gulf of Bothnia has long been recognized as one area of enhanced seismicity in the region; even the oldest seismicity maps based on written documentary records show how earthquakes occur on its coasts. It is a seismicity area in miniature: the current seismograph networks register relatively frequent micro-earthquake activity down to magnitude below 0, while the largest observed earthquakes had magnitude above 4. It is not clear whether magnitude 5 has been exceeded during the last three centuries, the time span of the available seismicity record, because the largest earthquakes occurred during the non-instrumental era and their magnitudes are affected by uncertainties. The first short-period seismographs suited for the registration of local earthquakes were installed in the study area in the latter half of the 1950s. All earthquake information prior to that time is defined as historical. Emphasis is laid on the historical data in this study. Many of the largest earthquakes known occurred adjacent to the Gulf of Bothnia, but only those in 1883, 1888 and 1898 have been subjected to a more detailed analysis (Mantyniemi, 2005, 2008). New macroseismic maps are presented for earthquakes of 27 November 1757, 14 July 1765, 13 October 1780, 26 May 1907, 31 December 1908, and 9 March 1909. Many previously unknown reports of these earthquakes were brought to light, when scanning the contemporary press. In addition, attention is paid to the location of the largest historical earthquakes. They are compared with recent instrumental records and reviewed against the seismo-tectonic setting. Evidence for larger earthquakes is analyzed using historical data. The potential for larger earthquakes in the area and implications for seismic hazard are discussed.