Proceedings of ICGG7 (2003): 6–7 © Copernicus GmbH 2003 ICGG7 2003 Radon soil increases before volcano-tectonic earthquakes in Colombia G. Garz´ on 1 , D. Serna 1 , J. Diago 2 , and C. Mor ´ an 3 1 Instituto de Investigaci´ on e Informaci ´ on Geocient´ ıfica, Mineroambiental y Nuclear – INGEOMINAS, Volcanological and Seismological Observatory at Manizales, Colombia 2 INGEOMINAS, Volcanological and Seismological Observatory at Popay´ an, Colombia 3 INGEOMINAS, Volcanological and Seismological Observatory at Pasto, Colombia Abstract. Continuous studies of radon concentration changes in soils for the purpose of earthquake monitoring have been carried out in three colombian districts and in the edifices of Galeras and nevado del Ruiz volcanoes since 1995. In zones of active faulting have been measured radon soil emissions between 1000 and 2500 pCi/L. In an inter- section of two active geological faults have been measured levels of 25 000 pCi/L. In the present work appears a compilation of examples of the registered anomalous radon emissions in several stations before earthquakes of tectonic character. Examples of reg- istered radon increases before: (1) events of magnitudes be- tween 2 and 4; (2) the occurrence of seismic swarms; and (3) the Quind´ ıo (Colombia) earthquake (Mw = 6, 2) of January 1999, are described. A model of transport mechanism for the studied isotopes is presented. 1 Introduction In studies of geological fault traces and researches of tectonic activity state, isotopic studies have been gotten up since few decades. Radon-222 is the isotope that more had been inves- tigating. Simultaneously are measured some gases like he- lium, hydrogen, carbon dioxide (Garzon, 1996); and isotopes like thoron (Rn-220), radium-226 and carbon-13 (Heilig- mann et al., 1997). Gas concentrations from the Earth’s in- terior are controlled by lithology and structure of the stud- ied zone (Williams-Jones et al., 2000). But in the time their emissions can change by geodynamic and/or meteorological influences. Radon-222 is in constant emission from the Earth’s inte- rior towards the atmosphere. For several decades it had been proposed: (1) the existence of anomalous emissions of this gas on active geological faults; (2) temporary variations re- lated to changes of atmospheric conditions; and, (3) its possi- Correspondence to: G. Garz´ on (garzon42@hotmail.com) ble forerunner manifestation before an earthquake (Ulomov and Mavashev, 1971; Moran et al., 2001). 2 Methodology At the present time in Colombia, its installed a network of 22 isotopic stations near Pasto city at Nari˜ no district; four stations near Purace town at Cauca district; and six stations between Manizales city and nevado del Ruiz volcano at Cal- das district. With the purpose to measure radon emissions on active ge- ological faults, in all three Volcanological and Seismological Observatories at INGEOMINAS is used the E-perm system (Electret Passive Environmental Radon Monitor) proposed by Kotrappa et al. (1988). A Pylon instrument with a Lu- cas cell is used for radon measurements in a gas telemetrical system, which is tested from 1997 at some fumaroles in the center of active crater at Galeras volcano (Faber et al., 2002). 3 Results and discussion The most relevants radon soil increases measured for our working group, before seismic events are: 1. For a time period of ten months Garzon (1996) reported some radon soil increases in Barranco station at iso- topic network installed by the Pasto’s Volcanological and Seismological Observatory. Radon soil increase was of 350 pCi/L, which apparently preceded the occur- rence of microearthquakes of magnitudes until of 2,1 in the Richter scale in a radius from the isotopic station <6,0 Km. 2. Comparative studies between radon soil emissions and number of volcano-tectonic earthquakes (M > 1, 5) registered by the Pasto’s seismological network were carried out. In the same Barranco station and for a time window of five years, radon soil increases were