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

Abstract

Abstract. Since July 1994 an unusually persistent swarm of earthquakes (M< 4.0) has been in progress at the Hengill triple junction, SW Iceland. Activity is clustered around the center of the Hr6mundartindur volcanic system. Geodetic measurements indicate a few centimeters uplift and expansion of the area, consistent with a pressure source at 6.5 + 3 km depth 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 subvertical planes. We show that the secondary effects of a pressure source, modeled as a point source in an elastic half-space, include horizontal shear that perturbs the regional stress. Near the surface, shear stress is enhanced in quadrants around the direction of maximum regional horizontal stress and diminished in quadrants around the direction of minimum regional stress. The recorded earthquakes show spatial correlation with areas of enhanced shear. The maximum amount of shear near the surface caused by the expanding pressure source exceeds 1 •tstrain, sufficient to trigger earthquakes if the crust in the area was previously close to failure.

Evolution of Stress Fields and Faulting in Seismic Zones

“…Large earthquakes, however, may rupture to greater depths, possibly to around 20 km. Historical data, as well as the current seismicity, indicate that earthquakes are commonly most frequent and destructive near the eastern and western ends of the SISZ (BJÖ RNSSON, 1975;STEFANSSON and HALLDORSSON, 1988;STEFANSSON et al, 1993;RÖ GNVALDSSON, 1994;SIGMUNDSSON et al, 1997). This is in agreement with the distinct-element results in Figure 3.…”

“…This applies, for example, to the current seismicity at the junction of the South Iceland Seismic Zone with the western rift zone (Fig. 6); although the frequency of earthquakes in this region is very high, the spatial distribution has to date been essentially limited to the Hromundartindur Central Volcano (SIGMUNDSSON et al, 1997) so that the volume that is homogenised has remained small. Earthquakes are generally common in the central volcanoes of Iceland, but they rarely exceed magnitude M6 and are generally of a magnitude less than M5 (TRYGGVASON, 1973).…”

Temporal stress changes associated with the 2008 May 29MW 6 earthquake doublet in the western South Iceland Seismic Zone

“…Inflation rates of 2 cm yr −1 were observed during that time, with a centre of uplift about 5 km NW of the Kross fault (Sigmundsson et al 1997;Feigl et al 2000). This might have increased NW-SE oriented stress on the N-S orientated faults and potentially had influence on the timing of the 2008Ölfus earthquakes.…”

Hengill geothermal volcanic complex (Iceland) characterized by integrated geophysical observations

“…In mid-1999 the activity returned to the pre-1994 level. Repeated geodetic measurements and InSAR detected uplift (7.5 centimeters) centered south of Hrómundartindur volcano and expansion between 1986 and 1995 in the southeastern area of the Hengill volcano (Sigmundsson et al, 1997;Feigl et al, 2000). This activity is interpreted as a small magma intrusion of about 1 to 3 × 10 7 m 3 and modelled by a Mogi (1958) source at a depth of about 7 km (Feigl et al, 2000).…”