Abstract
Long-term monitoring of Rn-222 and CO2 at a depth of several tens of meters at the Sde-Eliezer site, located within one of the Dead Sea Fault Zone (DSFZ) segments in northern Israel, has led to the discovery of the clear phenomenon that both gases are affected by underground tectonic activity along the DSFZ. It may relate to pre-seismic processes associated with the accumulation and relaxation of lithospheric stress and strain producing earthquakes. This approach assumes that the climatic influence on Physico-chemical parameters is limited at depth since its strength decreases with the increase in the thickness of the geological cover. Hence, the monitoring of natural gases in deep boreholes above the water table enables to reduce the climatic-induced periodic contributions, and uncover the residual portion of the signals that seem to be associated, as indicated recently with regional geodynamic processes. The plausible pre-seismic local movement of the two gases at depth, is identified by the appearance of discrete, random, non-cyclical signals, wider in time duration than 20 hours and clearly wider than the sum of the width of the periodic diurnal and semidiurnal signals driven by ambient climatic parameters. These non-cyclical signals may precede, by one day or more, a forthcoming seismic event. Hence, it is plausible to conclude that monitoring of any other natural gas that is present at depth, may show a similar broadening signal and may serve as a precursor too. The necessary technical conditions enabling to distinguish between anomalous signals of gases that may be induced locally by pre-seismic processes at depth, and the relatively low periodic signals that are still established at depth related to external climatic conditions, are presented in detail.
Highlights
4) The amplitudes of the radon periodical signals are controlled by the intensity of the climatic driving force, in linear dependency with the pressure gradient according to the existing physical model, and with largest non-linear variations induced by the ambient temperature gradient, that according to the ratio between the radon level in winter to summer varies by a factor of 3–10 while the temperature varies within 10% span (28 change vs. 285 K)
5) There can be a number of ways for analyzing, identifying the nature of the anomalies in the measured time series, dismount to various components, and isolating the parameters operating as driving forces for the radon movements beneath the surface
Choosing a frequency cutoff less the diurnal, excludes all the unwanted diurnal, semidiurnal, and higher periodical signals that are induced by the atmospheric parameters and proved that we measured radon anomaly that its emergence lasts for more than 20 h and that these non-cyclical signals may precede by several hours or more a forthcoming seismic events even if they are weak
Summary
OverviewPrevious research on radon has been carried out in the last 45 years in various geological environments such as soil (e.g., Akerblom et al, 1984; Schubert and Schultz, 2002; Fujiyoshi et al, 2006; Muga, 2017; Cannelli et al, 2018; Siino et al, 2019), solid rocks (e.g., Vulkan et al, 1992; Steinitz et al, 2007; Barbosa et al, 2010), fans of erosion streams (e.g., Steinitz et al, 1992), subterranean structures such as caves, tunnels and mines (e.g., Hakl et al, 1996; Przylibski, 2001; Muramatsu et al, 2002; Cigna, 2005; Richon et al, 2005; Mentes and Eper-Pápai, 2015; Sahu et al, 2016) and groundwater (e.g., Igarashi et al, 1995; Atkins et al, 2016; Barberio et al, 2018). 3) Most of the radon daily signals that are recognized as induced by climatic parameters, appear once or twice a day at specific times: early morning and afternoon (e.g.,, 4–6 AM and 5–7 PM, in 60 m depth at Sde-Eliezer site, Figure 9C).
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