THE 1994-1995 SEISMICITY AND DEFORMATION AT THE HENGILL-TRIPLE-JUNCTION, ICELAND - TRIGGERING OF EARTHQUAKES BY MINOR MAGMA INJECTION IN A ZONE OF HORIZONTAL SHEAR-STRESS
F. Sigmundsson et al., THE 1994-1995 SEISMICITY AND DEFORMATION AT THE HENGILL-TRIPLE-JUNCTION, ICELAND - TRIGGERING OF EARTHQUAKES BY MINOR MAGMA INJECTION IN A ZONE OF HORIZONTAL SHEAR-STRESS, J GEO R-SOL, 102(B7), 1997, pp. 15151-15161
Since July 1994 an unusually persistent swarm of earthquakes (M<4.0) h
as been in progress at the Hengill triple junction, SW Iceland. Activi
ty is clustered around the center of the Hromundartindur volcanic syst
em. Geodetic measurements indicate a few centimeters uplift and expans
ion of the area, consistent with a pressure source at 6.5 +/- 3 km dep
th beneath the center of the volcanic system. The system is within the
stress field of the south Iceland transform zone, and the majority of
the recorded earthquakes represent strike-slip faulting on subvertica
l planes. We show that the secondary effects of a pressure source, mod
eled as a point source in an elastic half-space, include horizontal sh
ear that perturbs the regional stress. Near the surface, shear stress
is enhanced in quadrants around the direction of maximum regional hori
zontal stress and diminished in quadrants around the direction of mini
mum regional stress. The recorded earthquakes show spatial correlation
with areas of enhanced shear. The maximum amount of shear near the su
rface caused by the expanding pressure source exceeds 1 mu strain, suf
ficient to trigger earthquakes if the crust in the area was previously
close to failure.