HR: 09:30h
AN: T41E-07    [Abstracts]
TI: Radon Along the Dead Sea Transform: a Proxy of Geodynamic and Geophysical Processes
AU: * Steinitz, G
EM: steinitz@gsi.gov.il
AF: Geological Survey of Israel, Malkhei Israel st. 30, Jerusalem, 95501, Israel
AB: Radon (Rn-222) occurring in geogas and water phases in upper crustal levels is often mentioned as a proxy of active geodynamics, with a potential as a precursor of earthquakes. So far the reported variability and complexity of its signals limits establishment of a verifiable scientific basis for this proposition. Most investigations tried to relate the temporal variations to different combinations of atmospheric, hydrologic, geophysical-geodynamic as well as geochemical drivers. An extensive set of stations, measuring radon with a resolution of <1-hour in the unsaturated zone is implemented since 1995 in a 200-km sector along the western margin of the southern DST. Several signal types comprise the measured signal – multi-year variation, a periodic Seasonal Radon (SR) signal, a multi-day (MD; 2-20 days) signal and periodic Diurnal Radon (DR) and sub-diurnal signals (SDR; several hours). Analysis in both the time and frequency domain – per individual sites and per groups of sites situated in identical arid climatic conditions - negates atmospheric influences as the primary forcing of the radon signals in the geogas system, albeit apparent similarity. A mechanical-geodynamic (subsurface) driver is suggested in cases where: a) relation can be shown between temporal and periodic features in the time series of radon and local geologic/structural situations; b) Frequencies typical of Earth tide (M2; O1) occur in the diurnal variation band; c) a statistically significant correlation is established between MD Rn signals as recorded in the Dead Sea and earthquakes that occurred within the Dead Sea rift valley several days after the start times of these anomalies; d) when a bigger amplitude of the DR signal is associated with the rising limb of a MD signal. Characterizing the fundamental statistical properties of radon time series indicates that further and different external (above surface; non-atmospheric) processes determine the periodic (SR, DR, SDR) temporal variation of subsurface radon. The main criteria are: a) cases where the amplitudes of the S1 and S2 diurnal frequencies, dominating the DR signal, vary in a regular seasonal variation pattern; b) the ratios (per 21.3 days) of co-occurring amplitudes of S1 and S2 frequencies define a linear pattern; c) Locations where radon time series are non- stationary in mean and in variance combined with a coupling between variance and mean-level; d) Large SDR signals occurring at depth (85m) in some days with a preferred bi-modal incidence within the 24-hour cycle. The radon signal in subsurface environs is driven by both internal (primarily the MD signal) and external (SR?, DR, SDR signals) driving processes. The first may be linked to mechanical geodynamics and the second may be related to an unidentified extraterrestrial influence - possibly related to solar irradiance. Both mechanisms interact with the local subsurface geological condition to yield the highly significant and complex phenomena observed in radon time series.
DE: 3200 MATHEMATICAL GEOPHYSICS (0500, 4400, 7833)
DE: 5499 General or miscellaneous
DE: 7599 General or miscellaneous
DE: 8099 General or miscellaneous
DE: 8199 General or miscellaneous
SC: Tectonophysics [T]
MN: 2007 Fall Meeting