We studied fracture-controlled geothermal fields in the Hreppar Rift-Jump Block (HRJB). a micro-plate bounded by two NNE rifts and the E-W transform zone of the South Iceland Seismic Zone (SISZ). Distinguishing whether the extensional rift swarm or the transform zone shear fractures host the geothermal activity is challenging. GPS mapping of 208 springs and tectonic analysis indicate that six Riedel shear fracture sets of an older transform zone in the HRJB are permeable. Northerly dextral strike-slip faults are the principal permeable faults, although the highest discharge and temperature are found at their intersections with other fracture sets. Two northerly faults from the HRJB connect to the source faults of the major 1784 and 1896 earthquakes within the active SISZ. The 1784 earthquake caused pressure changes as far north as the studied springs, indicating that earthquakes keep faults permeable over hundreds of years. (C) 2010 Elsevier Ltd. All rights reserved.