POSTRIFTING ANELASTIC DEFORMATION AROUND THE SPREADING PLATE BOUNDARY, NORTH ICELAND .1. MODELING OF THE 1987-1992 DEFORMATION FIELD USING A VISCOELASTIC EARTH STRUCTURE

A third Global Positioning System (GPS) survey of a regional network s urrounding the Krafla volcanic system, north Iceland, was conducted in 1992 following a major crustal spreading episode which began in this system in 1975. Differencing the 1992 results with those from 1987 and 1990 reveals a regional deformation field with a maximum, rift-normal expansion rate of 4.5 cm/year near the rift, decreasing to 3 cm/year at large distances. The time-averaged spreading rate in north Iceland, 1.8 cm/year, cannot account for this deformation. The vertical deform ation field reveals regional uplift throughout the network area at its maximum closest to the rift and decreasing with distance. Three diffe rent models are applied to study the postdike injection ground deforma tion: (1) stress redistribution in an elastic layer over a viscoelasti c half-space, (2) stress redistribution in an elastic-viscous layered medium, and (3) continued opening at depth on the dike plane in an ela stic half-space. Using model 1, the effects of historical episodes in the region are subtracted from the observed displacement fields, and t he remaining motion is modeled as relaxation following the recent Kraf la rifting episode. The best fit model involves a half-space viscosity of 1.1 x 10(18) Pa s, a relaxation time of 1.7 years, and an elastic layer thickness for northeast Iceland of 10 km. The vertical field ind icates that the Krafla dike complex rifted the entire elastic layer. U sing model 2, the motion 1987-1990 and 1990-1992 can be simulated adeq uately given the survey errors, but the 1987-1992 deformation is poorl y fitted, suggesting that a more realistic geophysical model is requir ed. Using model 3, a range of dikes will fit the deformation field.