Seismological studies at the Hengill geothermal area SW Iceland
Iceland is a sub-aerial part of the mid-Atlantic Ridge which has formed above an E migrating ridge centred hotspot. The Hengill area is a ridge-ridge-transform triple point that contains a central volcano-fissure swarm system and a large geothermal area. A seismological study of this triple point was conducted with the main emphasis on natural earthquake studies. The aims were to study the geothermal prospect and tectonic structure and to evaluate the passive seismic method as a geothermal prospecting tool. The area exhibits continuous small magnitude earthquake activity that correlates positively with surface geothermal displays, and negatively with surface faulting. The log (cumulative frequency) magnitude relationship is linear and indicates a b value of 0.74 ± 0.06. Focal mechanisms for 178 events indicated both shear and tensile crack type movements, the latter being confined to the high temperature geothermal area. Teleseismic and explosion data indicate a low velocity body beneath the central volcano in the depth range 0 - 10 km, flanked by higher velocity bodies to the W and E. Two volcanic systems occupy the Hengill area : the presently active Hengill system and the extinct Grensdalur system. The ongoing seismicity of the area is attributed to contraction cracking due to the action of cool groundwater fluids on hot rook, which, in a tensile stress regime, results in tensile crack formation. The high temperature area is fuelled by two heat sources associated with the two volcanic systems and may be divided into two separate fields that exhibit contrasting reservoir characteristics. Local seismioity studies may be applied to other Icelandic high temperature geothermal areas as a tool to map those volumes of rock that are fueling the geothermal reservoirs. The continuous formation of small tensile cracks on accretionary plate boundaries offers an explanation for the mechanism of dyke injection.