The Hengill ridge-ridge-transform triple junction is a complex tectonic unit containing two volcanic systems, one active and one extinct, and widespread geothermal resources. The area exhibits continuous, small-magnitude earthquake activity. Good constraint was obtained for the radiation patterns of 178 small earthquakes within the area, of which 50% are non-double couple and are interpreted as resulting from tensile crack formation. These events occurred in the depth range 2--7 km and indicate that at these depths in the area extensional stresses must be prevalent, that outweigh the compressive effect of the overburden. The tensile crack events were mingled with double couple events, and were confined to the high-temperature geothermal area. Outside of this, seismic faulting was almost entirely shear.

During a 3-month recording period, tensile crack type events were of low magnitude and accounted for only 2% of the seismic stress release. The fault plane of the shear events was deduced from the orientation of the tensile crack strikes. The continuous small-magnitude seismicity is attributed to the action of circulation groundwater fluids on hot rock at depth which remove heat and cause contraction of the rock, thereby reducing the effective confining stress. This process induces cooling contraction cracks that release thermal and regional stress in a mode that is consistent with the regional stress regime. In close proximity to the accretionary plate boundary, this stress regime is extensional, and some of the fractures formed are tensile cracks. The spatial distribution of the continuous earthquake activity represents a map of the heat source of the geothermal area. In the Hengill area the earthquake locations indicate that the high-temperature geothermal area is fueled by two main distinct heat sources. These are associated with the two volcanoes, and separate geothermal fields that exhibit contrasting reservoir characteristics. Continuous small-magnitude earthquake activity is commonly associated with geothermal areas worldwide, and the results from the Hengill area indicate that they may be used elsewhere as a prospecting tool to gain unique information about the heat source.