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

On May 25, 1987, at 11:31, a M = 5.8 earthquake occurred at the southern en d of Vatnafjoll volcanic ridge in south Iceland. This is the largest event in the south Iceland lowland since a hi = 7.0 earthquake in 1912 which was located approximately 15 km to the west of the Vatnafjoll earthquake. Vatna fjoll 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 ri fting and volcanism. In May and June 1987, several foreshocks and aftershoc ks were recorded on the local seismic network as well as the mainshock. A c lear coseismic step associated with the mainshock was observed at all opera ting stations of a volumetric strainmeter network in southern Iceland. Step s associated with some foreshocks and aftershocks were also observed at the closest strain stations. Slow strain changes, before and after the mainsho ck, lasting a few days, were also observed. Forward modeling of the coseism ic strainmeter signals of the mainshock suggests a double couple solution w here the slip is mostly right lateral strike slip on a subvertical plane wi th a northerly strike. The solution has a good fit to observations and is i n good agreement with interpretation of seismometer data. This solution ind icates a stress field similar to that in the south Iceland seismic zone. Th e 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 held and possibly tri gger the mainshock. The strainmeter records open up a new view of the seism ic strain event as a combination of seismic strain release and a slower pro cess of magma intrusion.