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

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.