Continuous deflation of the Askja caldera, Iceland, during the 1983-1998 noneruptive period

The Askja volcano at the spreading plate boundary in north Iceland consists of nested calderas, the latest formed in an eruption in 1875. Several erup tions have occurred since in Askja, the most recent in 1961. Precise leveli ng has been conducted yearly at Askja since 1983. In 1993, a dense GPS netw ork was measured in and around the Askja caldera consisting of more than 20 points, and we remeasured this complete network for the first time in 1998 . Askja subsided during the period from 1983 to 1998. Observed deformation fits broadly with a "Mogi" point source model with best fitting location ne ar the center of the main Askja caldera (65.0448 degreesN, 16.7805 degreesW ) at a depth of 2.8 km. From 1983 to 1991 the yearly subsidence rate betwee n the end points of a leveling profile decayed gradually from similar to 10 mm/yr to an average of -7 mm/yr in the 1991-1998 period. Total subsidence at the Askja center in the 1983-1998 period is at least 75 cm, and the inte grated volume of surface subsidence is similar to0.037 km(3). This period h as: been a "quiet" period at Askja with no eruptions, large earthquakes or known dike injections. Such a high rate of "background" deformation has not been observed at other volcanoes in Iceland. The eruption of a neighboring volcano in 1996 has not resulted in a modified deformation pattern at Askj a, indicating no pressure connection at depth between the two systems. Soli dification may account for part of the observed contraction and subsidence in Askja.