It is estimated that a total of one million earthquakes occur on our globe annually. Over 90 per cent of these are below three on the Richter scale. Approximately one hundred earthquakes annually measure six or more on the Richter scale. The destructiveness of an earthquake is also related to its depth. Earthquakes whose hypocentre is located within 70 kilometres of the Earth's outer surface are the most devastating.
Intraplate stresses accumulate slowly and are guided to old weakness zones.
Of the Nordic countries, only Iceland is located in a seismically sensitive area, the Mid-Atlantic Ridge. In the Scandinavian countries, the most vulnerable area to earthquakes is the south-western coast of Norway. Finland, Sweden and the Kola Peninsula, whose solid bedrock is estimated to be three billion years old in places, are located in the Fennoscandian shield. The horizontal compressional stress owing to the expansion of the North Atlantic Ocean causes intraplate earthquakes in these areas. Other causes are believed to result from old remnant stresses and postglacial land uplift following the last Ice Age, which is approximately one centimetre per year at its highest in the Quark in the Bothnian Sea.