POSTRIFTING ANELASTIC DEFORMATION AROUND THE SPREADING PLATE BOUNDARY, NORTH ICELAND .1. MODELING OF THE 1987-1992 DEFORMATION FIELD USING A VISCOELASTIC EARTH STRUCTURE
Ma. Hofton et Gr. Foulger, POSTRIFTING ANELASTIC DEFORMATION AROUND THE SPREADING PLATE BOUNDARY, NORTH ICELAND .1. MODELING OF THE 1987-1992 DEFORMATION FIELD USING A VISCOELASTIC EARTH STRUCTURE, J GEO R-SOL, 101(B11), 1996, pp. 25403-25421
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.