Article,
Structure of the Gr\'ımsvötn central volcano under the Vatnajökull icecap, Iceland
Geophysical Journal International, 168 (2): 863--876 (February 2007)
DOI: 10.1111/j.1365-246X.2006.03238.x
Abstract
The subglacial Gr\\'ımsvötn 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 Vatnajökull 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 Gr\\'ımsvötn
caldera rim coincided with inflation of a shallow magma chamber beneath
the caldera, which culminated in a 0.1 km3 eruption in December 1998.
Tomographic inversion of these earthquakes define the extent of a
low-velocity body beneath Gr\\'ımsvötn with a volume of \~20
km3 extending 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 Gr\\'ımsvötn caldera from
regional and teleseismic earthquakes and from two detonations \~150
km east of Gr\\'ımsvötn 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 Gr\\'ımsvötn, 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.
