Strain changes for the 1987 Vatnafjöll earthquake in south Iceland and possible magmatic triggering

On May 25, 1987, at 11:31, a M = 5.8 earthquake occurred at the southern end of Vatnafjöll volcanic ridge in south Iceland. This is the largest event in the south Iceland lowland since a M = 7.0 earthquake in 1912 which was located approximately 15 km to the west of the Vatnafjöll earthquake. Vatnafjöll is located at the junction of the south Iceland seismic zone, a left lateral transform zone, and the eastern volcanic zone which is a zone of rifting and volcanism. In May and June 1987, several foreshocks and aftershocks were recorded on the local seismic network as well as the mainshock. A clear coseismic step associated with the mainshock was observed at all operating stations of a volumetric strainmeter network in southern Iceland. Steps associated with some foreshocks and aftershocks were also observed at the closest strain stations. Slow strain changes, before and after the mainshock, lasting a few days, were also observed. Forward modeling of the coseismic strainmeter signals of the mainshock suggests a double couple solution where the slip is mostly right lateral strike slip on a subvertical plane with a northerly strike. The solution has a good fit to observations and is in good agreement with interpretation of seismometer data. This solution indicates a stress field similar to that in the south Iceland seismic zone. The slow strain changes, which start about 10 min after the first foreshock, may indicate magma involvement in the process. Changes, associated with an intrusion and pressure release, may affect the strain field and possibly trigger the mainshock. The strainmeter records open up a new view of the seismic strain event as a combination of seismic strain release and a slower process of magma intrusion.