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.