Task 2: Deformation rates

Start: August 1996 (month 6)
End: February 1998 (month 24)
Responsible partne: NVI
Cooperative partners: UICE.DG, IMOR.DG

Freysteinn Sigmundsson and Páll Einarsson

The data from the semi-continuous GPS station in the SISZ has been analyzed and the movement of the station is already significant. Relative to a station in Reykjavík (on the North-American plate), the velocity of the Saurbær station is estimated 11.5 mm/year in direction N E. For the first year of observation the relative velocity appears steady in time, about 1 mm/month during the last year. There is no suggestion of temporal variation in the deformation rate in this part of the seismic zone. Also, the inferred velocity is close to expected, about half the 2 cm/year full spreading vector in South Iceland. If the GPS-station were located to the south of the seismic zone (on the Eurasian plate), we would expect to observe the full spreading vector. The semi-continuous data collected sofar will serve as a reference, against which we can compare future measurements.

Based on conventional GPS and levelling, we have information on temporal variation in deformation rates in the Hengill area, at the western end of the seismic zone. There an unusually persistent swarm of earthquakes (M<4.0) has been in progress since 1994. Activity is clustered around the center of the Hrómundartindur volcanic system, in the Hengill area. Geodetic measurements indicate a few centimeters uplift and expansion of the area, consistent with a pressure source at $6.5 \pm 3$ km depth beneath the center of the volcanic system. The system is within the stress field of the South Iceland transform zone, and majority of the recorded earthquakes represent strike-slip faulting on subvertical planes. We show that the secondary effects of a pressure source, modelled as a point source in an elastic halfspace, include horizontal shear that perturbs the regional stress. Near the surface, shear stress is enhanced in quadrants around the direction of maximum regional horizontal stress, and diminished in quadrants around the direction of minimum regional stress. The recorded earthquakes show spatial correlation with areas of enhanced shear. The maximum amount of shear near the surface caused by the expanding pressure source exceeds 1 microstrain, sufficient to trigger earthquakes if the crust in the area was previously close to failure. This study has improved the understanding of triggering of earthquakes in volcanic areas.