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.
La sistematica de isotopos de helio esta siendo aplicada en la peninsula Iberica con el fin de obtener informacion adiccional sobre las caracteristicas neotectonicas y sismotectonicas del area de estudio. Resultados preliminares sobre la distribucion geografica de las relaciones isotopicas 3He/4He en fluidos terrestres reflejan variaciones de los niveles de emision de helio-3 significativas, y estas estan intrinsecamente ligadas a las caracteristicas sismotectonicas y neotectonicas de la peninsula Iberica.
Neutron activation analysis of U, Th and K has been made on some possible mantle materials, mainly peridotite nodules from continental and oceanic areas. The paper first describes the analytical procedures of U and Th determinations. Results of analysis showed that oceanic nodules have more uniform concentrations of U, Th and K and Th/U, K/U ratios than continental nodules. Average values for 7 oceanic lherzolites are: (1.87±0.27)×10-8 gU/g, (5.48±0.87)×10-8 gTh/g, and (54.2±7.5)×10-6 gK/g, giving a rate of heat production, 26×10-16 cal/cm3·sec, which is about 70% of that for the average chondrite. There is no indication that oceanic lherzolites are more enriched in these elements than the continental ones. These results suggest that, from the thermal point of view, peridotite nodules may not be the representative constituent of the upper mantle, unless the oceanic mantle is capable of transferring heat exceedingly well. The average values of the ratios are Th/U=2.93±0.61, and K/U=(0.29±0.06)×104. The latter is significantly smaller than the crustal value (1×104), which is already smaller than the chondritic value (5×104).
Geodynamic evolution of southern Italy can be understood within the framework of the Mediterranean‐Alpine System. Subduction of a plate along the Sicily‐Calabrian forearc under the Tyrrhenian Sea has been suggested by many geophysicists, although it is not yet confirmed and remains somewhat controversial. Helium isotope ratios provide useful information on the geotectonic structure of the region. We report here the ³He/ 4 He ratios of terrestrial gas samples from southern Italy. The observed ³He/ 4 He ratios are relatively high in the Eolian volcanic arc region and low in the other areas. Dichotomous explanations are presented. Firstly volcanic arc‐forearc hypothesis suggests the subduction along the Sicily‐Calabrian forearc. Secondly horizontal transport hypothesis is described based on the relationship between the ratios and radial distance from the recent spreading basin in Southern Tyrrhenian Sea.
