Rn and CO2 in-depth, as a proxy for pre-seismic activity Hovav Zafrir1,4, Uri Malik1, Elad Levintal2, Noam Weisbrod2, Yochai Ben Horin3, Zeev Zalevsky4, Nimrod Inbar5 1Geological Survey of Israel, 32 Yesha’ayahu Leibowitz, Jerusalem 9371234, Israel, 2The Zuckerberg Institute for Water Research, Ben-Gurion University, 8499000 Sede Boqer, Israel, 3Soreq Nuclear Research Center, Yavne 81800, Israel, 4Faculty of Engineering, Bar Ilan University, Ramat-Gan 52900, Israel, 5Ariel University, Ariel 40700, Israel. (First author e-mail: [email protected]; [email protected]). Abstract The method of long-term monitoring of subsurface gases in shallow to deep boreholes assumes that the climatic influence on geo-physicochemical parameters is limited since its energy decreases with the increase in the thickness of the geological cover. Hence, the monitoring of radon (Rn), CO2 and other constituents above and below the water table in deep boreholes enables to eliminate the climatic-induced periodic contributions, from the residual portion of the signals that are associated with the regional geodynamic processes, as have been proved by us recently for radon(*). Monitoring of radon and CO2 at a depth of several tens of meters along the Dead Sea Fault Zone, between the Dead Sea and the Hula Valley has led to a clear discovery of the phenomenon that both gases are affected by an underground tectonic activity related to the pre-seismic processes of producing earthquakes, even if they are weak. The pre-seismic processes even if not all end with earthquakes, cause the movement of gases in the subsurface geologic media and creating non-periodic signals that are wider than 20 to 24 hours. Hence, monitoring of any other natural gas at depth may show a similar expansion signal and may serve as a precursor for earthquakes. The necessary conditions needed to explore anomalous signals of gases that induced by pre-seismic processes at the depth, as accumulation and relaxation of lithospheric stress and strain, are: a) setup of a monitoring system within boreholes airspace, drilled to active faults, b) verify that there is at least one gas with concentration level few times above the conventional background level of the regional subsurface content, c) utilizing high sensitive detectors to recover changes in the gas content, with detection limit of few percents of the local average (As an example: for radon, the required content is at least 1kBq/m3 and the required sensitivity is better than 5%). (*) Zafrir, H., Ben Horin Y., Malik, U., Chemo, C., and Zalevsky, Z., 2016, Novel determination of radon-222 velocity in deep subsurface rocks and the feasibility to using radon as an earthquake precursor, J. Geophys. Res. Solid Earth, 121, 6346–6364, doi: 10.1002/2016JB013033.