Large temporal variations in seismic and volcanic activity and in strain rates, coincident over large areas, have been observed both on long-term and short-term basis in Iceland. Such strain waves or strain pulses increase the probability of seismic and volcanic activity in the crust over a large area.
Historical and geological evidence describe a large temporal variability in seismic and volcanic activity [#!Stefansson/etal:1993!#,#!Larsen/etal:1998!#]. An example of a period of high activity was the creation of a new island in an eruption on the Reykjanes ridge in early 1783, followed a few weeks later by the huge eruption of Laki, in the central highland of Iceland. In 1784 followed the largest earthquake in the history of Iceland (M=7.1), which occurred in the South Iceland seismic zone.
Short-term variations in stress and deformation fields, attributed to effects of plume activity, have been observed instrumentally over large distances. The intensified instrumental observations in Iceland related to seismic risk mitigation projects like the SIL project of the Nordic countries and the PRENLAB and PRENLAB-2 projects of EU have indicated that strain waves, probably related to intrusive episodes can be observed over large distances, 100-200 km. However, it has been very difficult to confirm such connections [#!Tryggvason:1989!#,#!Foulger/etal:1992!#,#!Stefansson/etal:1996!#,#!Stefansson/etal:1998!#].
The temporal variations in activity are still poorly understood, although it is often assumed that they are triggered by a common large source, i.e. a large intrusion of basaltic fluids into the crust from below. It is of an enormous significance to be able to understand the underlying mechanism of them and to monitor them intensively both near their origin as at distance.