Wavelet analysis of geochemical time series: continuous CO2 flux measurements at the summit cone of Teide volcano, Tenerife, Canary Islands

Tenerife Island (2034 km²) is the largest of Canarian archipelago and is characterized by three main volcano-tectonic axis: the NS, NE and NW dorsals and a central caldera, Las Cañadas, hosting the twin stratovolcanoes Pico Viejo and Teide. Although Teide volcano shows a weak fumarolic system, volcanic gas emissions observed in the summit cone consist mostly of diffuse CO₂ degassing. The first continuous automatic geochemical station in Canary Islands was installed at the south-eastern foot of summit cone of Teide volcano in 1999, with the aim of improving the volcanic monitoring system and providing a multidisciplinary approach to the surveillance program of Teide volcano. The 1999-2011 time series show anomalous changes of the diffuse CO₂ emission with values ranging between 0 and 62.8 kg m^-2d^-1, with a mean value of 4.7 kg m^-2d^-1. The CO₂ efflux increases remained after filtering the time series with multiple regression analysis (MRA), were soil temperature, soil water content, wind speed and barometric pressure explained 16.7% of variability.

We analysed the CO₂ efflux time series by using the Continuous Wavelet Transform, with the Ricker wavelet, to detect relevant time-frequency patterns in the signal. The wavelet analysis showed, at low frequencies, quasi-periodical oscillations with periods of 3-4 years. Moreover, during the intervals of highest levels of CO₂ efflux the analysis evidenced also oscillations with a period of about 6 months.

Our data show in 2002 a marked peak of the filtered CO₂ signal. The beginning of this increase is nearly coincident with a similar signal on the data of CO₂ emission, coming from periodic surveys performed yearly on the area of Teide summit cone since 1997. We interpret these signals as an “early warning” associated to the 2004 seismo-volcanic unrest in Tenerife. A similar coincidence was observed also for the interval 2006-2009, which was followed by an increase in the local seismicity of Tenerife as well, characterized both by an increasing number of small earthquakes occurring, respectively, mostly along the NW dorsal and in the southern part of the NE dorsal of Tenerife.

Our study reveals that wavelet analysis on the continuous CO₂ efflux measurement could help to detect anomalous degassing periods, possibly indicating impending seismo-volcanic unrest episodes and/or eruptions. Finally, it is important to remark that the data presented in this work, constitute one of the longest time series of continuous CO₂ efflux measurements in an active volcanic area, hence providing an important benchmark for similar measurements worldwide.