Structure of the Grimsvotn central volcano under the Vatnajokull icecap, Iceland

The subglacial Grimsvotn central volcano, lying within a volcanic zone directly above the core of the Iceland mantle plume, is one of the most active in Iceland. Local, regional and teleseismic earthquake data recorded on a temporary seismometer array across western Vatnajokull icecap during the summer of 1998 have provided a three-dimensional image of the shallow crustal structure of the volcano. Microearthquake activity at depths of 1-4 km along the Grimsvotn caldera rim coincided with inflation of a shallow magma chamber beneath the caldera, which culminated in a 0.1 km(3) eruption in December 1998. Tomographic inversion of these earthquakes define the extent of a low-velocity body beneath Grimsvotn with a volume of similar to 20 km(3) extending to similar to 3 km below the surface. This low-velocity body is flanked by high velocities under the caldera rim. Delays in the P-wave arrival times through the Grimsvotn caldera from regional and teleseismic earthquakes and from two detonations similar to 150 km east of Grimsvotn are 0.10-0.15 s greater than the delays through the uppermost 3-4 km of crust shown by local earthquake arrivals. This suggests the presence of a further low-velocity body at depths greater than 3-4 km beneath Grimsvotn, presumed to be due to the presence of melt. Using the distribution of local seismicity and shear wave attenuation we estimate the maximum lateral extent of the region containing partial melt to be 7-8 km E-W and 4-5 km N-S. P-wave delays require a thickness of less than 1 km of pure/high percentage partial melt, assuming a sill-like magma chamber.