Iceland is one of the most favorable places in the world for geothermal energy production, thanks to favorable geological conditions. The location of Iceland on an intersection between the Mid-Atlantic Ridge and the Iceland hot spot create the unique geological condition for geothermal activity in the country. It is known that the hydrothermal activity extends at least down to 3 km depth but the lower limits for utilization is still not known. The background temperature gradient in Iceland in the uppermost 1,5 km is ranging from 40 to 150?C and shows no sign of a general decrease with depth. Linear extrapolation of these gradients leads to partial melt at 8 -30 km depth.
In order to get information on the temperature depth relationship below the depth of the presently utilized geothermal systems, we have analyzed the relationship between the maximum focal depth of earthquakes and the near surface temperature gradient. This is done by analysis of the SIL earthquake catalogue, but the Icelandic Meteorological Office runs the SIL seismic network. The seismic data cover the period from 1991 to June 2003. Of these around 108000 events met our requirement regarding maximum error estimate in the depth determination. By comparison of the temperature gradient, the maximum focal depth of earthquakes and available estimates of the temperature at the bottom of the seismogenic crust worldwide, we conclude that the temperature gradient in Iceland must decrease considerably with depth below the depth penetrated by boreholes. We suggest two possible explanations, increased thermal conductivity with depth or heat production by repeated intrusions into the lower part of the brittle crust. The proposed increase in thermal conductivity could partly be due to reduction in porosity with depth in the upper crust and partly due to increased share of mantle material in the lower crust. For possible deep geothermal systems in Iceland this means that temperature at 3-5 km depth are lower than previously was expected but still very high and exploitable by deep holes.

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