PRESSURE CHANGES IN ICELANDIC GEOTHERMAL RESERVOIRS ASSOCIATED WITH TWO LARGE EARTHQUAKES IN JUNE 2000

Abstract

Two large (6.6) earthquakes, which occurred on June 17 and 21 2000 in S-Iceland, caused some major changes in the pressure of several geothermal reservoirs. The pressure changes correlate near perfectly with the focal mechanism of the two quakes, i.e. reservoir pressure increased in areas of rock compression and decreased where dilation took place. Several secondary chances were also observed. The most pronounced are in wells, which happened to tap directly from the two 15-25 km long N-S striking fractures, formed by the quakes. Near instantaneous pressure drop of 1-10 bars were common in those wells. Few reservoirs maintain pressures higher than before the quakes and few appear permanently reduced in pressure. This is suggested to be a consequence of stress driven permeability changes. Some reservoir may have changed from being confined to unconfined as a result of stress changes. Pressure changes were observed up to 75 km away from the seismic epicenters. Many of those wells are now, 6 months after the quakes, still recovering. Also of interest are a few post-quake events of pressure rise/decline, presumed to be a consequence of stress relaxation in the crust. We believe that the currently evolving database of tectonically induced pressure changes in S-Iceland is significant for the general understanding of fractured geothermal reservoirs as well as for seismology.