Monitoring of radon anomalies in the Rio Casanay and a thermal spring near the El Pilar fault shortly after the July 9, 1997 earthquake (Mw=6.9) in the state of Sucre (Venezuela)

Immediately, after the July 9, 1997 earthquake (Ms = 6.8) in the state of Sucre, Venezuela, we began measuring radon in water to investigate the possible correlations with the seismic activity and to study the meteorological affects. Sampling periods were for 3-5 days with 4-8 weeks in between each. During the first two sampling periods, the seismic activity was high with several minor events (Mb greater than or equal to4.0) and anomalous radon concentrations were measured and considered as possible precursors for the events. We have also shown that the radon activity was stable except for daily meteorological effects during periods of low seismic activity. The radon activity during the first year after the major event on July 9, 1997, at the beginning of the rainy season increased from about 50 pCi/l to about 350 pCi/l for all sampling points. Then abruptly dropped to 100 pci/l at the beginning of the next rainy season for points #1 and #2 in the Casanay river and stayed relatively stable during the second year. At the start of the third year, the values began to increase again, similarly to the first year. We concluded that this was caused by the heavy rainfall at the beginning of the rainy season. While at the sampling points at Hotel Cristal, thermal spring pool, it decreased slowly during the second year but increased again at the onset of the third year. Finally, we have concluded as other investigators, that monitoring radon activity alone is insufficient to predict minor earthquakes (Mb greater than or equal to4.0), but in some cases it can be considered as a precursor.