PLATE BOUNDARY DEFORMATION AND CONTINUING DEFLATION OF THE ASKJA VOLCANO, NORTH ICELAND, DETERMINED WITH GPS, 1987-1993

GPS geodetic measurements were conducted around the Askja central volcano located at the divergent plate boundary in north Iceland in 1987, 1990, 1992 and 1993. The accuracy of the 1987 and 1990 measurements is in the range of 10 mm for horizontal components; the accuracy of the 1992 and 1993 measurements is about 4 mm in the horizontal plane. Regional deformation in the Askja region is dominated by extension. Points located outside a 30-45 km wide plate boundary deformation zone indicate a displacement of 2.4 +/- 0.5 cm/a in the direction N99 degrees E +/- 12 degrees of the Eurasian plate relative to the North American plate in the period 1987-1990. Within the plate boundary deformation zone extensional strain accumulates at a rate of similar to 0.8 mu strain/a. Displacement of control points next to Askja (<7 km from the caldera center) in the periods 1990-1993 and 1992-1993 show deflation and contraction towards the caldera. These results are in accordance with the results obtained by other geodetic methods in the area, which indicate that the deflation at Askja occurs in response to a pressure decrease at about 2.8 km depth, located close to the center of the main Askja caldera. A Mogi point source was fixed at this location and the GPS data used to solve for the source strength. A central subsidence of 11 +/- 2.5 cm in the period 1990-1993 is indicated, and 5.5 +/- 1.5 cm in the period 1992-1993. The maximum tensional strain rate, according to the point source model, occurs at a horizontal distance of 2.5-6 km from the source, at the same location as the main caldera boundary. Discrepancies between the observed displacements and predicted displacements from the Mogi model near the Askja caldera can be attributed to the regional east-west extension that occurs at Askja.