Precursory Subsurface 222Rn and 220Rn Degassing Signatures of the 2004 Seismic Crisis at Tenerife, Canary Islands

Precursory geochemical signatures of radon degassing in the subsurface of the Tenerife Island were observed several months prior to the recent 2004 seismic-volcanic crisis. These premonitory signatures were detected by means of a continuous monitoring of ²²²Rn and ²²⁰Rn activity from a bubbling CO₂-rich gas spot located at 2.850 m depth inside a horizontal gallery for groundwater exploitation at Tenerife. Multivariate Regression Analysis (MRA) on time series of the radon activity was applied to eliminate the radon activity fluctuation due to external variables such as barometric pressure, temperature and relative humidity as well as power supply. Material Failure Forecast Method (FFM) was successfully applied to forecast the anomalous seismicity registered in Tenerife Island in 2004. The changes in the ²²²Rn/²²⁰Rn ratio observed after the period of anomalous seismicity might suggest a higher gas flow rate and/or changes in the vertical permeability induced by seismic activity.