ABSTRACT

The northwestern region of the Iberian Peninsula was traditionally considered a seismically stable area. However, the 1995 and 1997 Galicia earthquakes of magnitude 4.6 and 5.3 produced significant damage in the region (intensity EMS: V), and these events changed the seismic hazard map used for the building code in Spain. Searching for earthquake precursors of these seismic events was performed, and precursory geochemical signatures of the 1995 and 1997 Galicia earthquakes were detected. These precursory signatures were observed in CO₂-rich groundwaters from a 120 m deep well at Cabreiroá (Ourense, Galicia), ∼90 km distance from the epicentres. Secular variations of the dissolved Cl^- in these CO₂-rich groundwaters showed clearly an increase trend three months before the occurrence of two earthquakes of magnitude 4.6 (November 29 and December 24, 1995) in Lugo (Galicia). After the precursory variations of Cl^- prior to the earthquakes in 1995, a continuous radon monitoring system was installed at Cabreiorá's well. A significant increase of radon activity in the CO₂-rich ground waters was detected four weeks prior to the magnitude 5.3 Galicia earthquake of May 21, 1997. Precursory changes in the chemical and isotopic composition of terrestrial fluids have been mainly attributed to the development of microcracks caused by the regional tectonic strain. The occurrence of these precursory geochemical signatures of CO₂-rich groundwaters of magmatic origin in the NW Iberian peninsula probably suggests that high fluid pressure at depth play a major role in triggering earthquakes in the region.