HR: 0800h
AN: G51B-0835    [Abstracts]
TI: Monitoring Radon Along the Dead Sea Transform (Israel) - an Indicator of Seismic and Aseismic Deformation Transients in the Upper Crust
AU: * Steinitz, G
EM: steinitz@mail.gsi.gov.il
AF: Geological Survey, Malkhey Israel 30, Jerusalem, 95501 Israel
AU: Begin, Z B
EM:
AF: Geological Survey, Malkhey Israel 30, Jerusalem, 95501 Israel
AU: Zafrir, H
EM:
AF: Geological Survey, Malkhey Israel 30, Jerusalem, 95501 Israel
AU: Malik, U
EM:
AF: Geological Survey, Malkhey Israel 30, Jerusalem, 95501 Israel
AU: Balogh, B
EM:
AF: Geological Survey, Malkhey Israel 30, Jerusalem, 95501 Israel
AU: Gazit-Yaari, N
EM:
AF: Soreq NRC, Soreq, Yavne, 81800 Israel
AU: Piatobratov, O
EM:
AF: Geological Survey, Malkhey Israel 30, Jerusalem, 95501 Israel
AB: Radon is being monitored at three arrays of stations located in a 200 km sector along the western boundary fault of the Dead Sea Transform (DST). Measurements are conducted in the geogas of the unsaturated zone of different rock units - Precambrian igneous and metamorphic rocks, Cretaceous syenite and sub-recent unconsolidated gravel. Alpha and gamma detectors, placed in the unsaturated zone at depths of 1.5 to tens of meters, gather radon and ancillary information at a high time resolution (< 1 hour). Long time series (up to +10 years) display systematic and recurring signals, enabling to discern several variation patterns: 1) Multi-year (MY); 2) Seasonal; 3) Multi-day (MD); 4) Diurnal cyclic and non-cyclic Radon Signals (DRS). The recurrence of similar signals in different geographic and geological settings indicates that similar driving processes are acting in these settings. Establishing the geodynamic nature of the signatures and signals in the time series of radon is based on three approaches: (1) Negation of atmospheric influence as the primary forcing - by demonstrating that the variation of the radon in geogas is basically unrelated to the local ambient atmospheric conditions. This is shown by searching for correlation and fit in both the time and frequency domains. (2) Analyzing radon signals in the geological, spatial, time and frequency domains. Systematic relations are observed among signals from different stations within an array at spatial scales of 0.1 to 25 km. Time series of radon exhibit temporal correlation among stations in the array. Several modes of correlation occur, based on the time scale of the radon signal - MY, MD, DRS. Systematic time-offsets of the signals, confirmed by cross-correlation analysis, are observed among stations within an array. In the case of DRS different patterns of daily cyclic signals, derived by FFT analysis, are associated with geologic and tectonic elements. Particular correlations between the overall MD radon level and the amplitude of cyclic DRS indicate a mechanical coupling among the driving processes. (3) Establishing a statistically significant correlation between MD Rn anomalies as recorded in the Dead Sea (DS) and earthquakes that occurred within the DS rift valley several days after the start times of these anomalies. To explain our results we assume that plate movement along the DST episodically deforms the tectonic segments within the DS rift valley. Upon reaching a certain strain (probably in the order of 10-9-10-8) enhanced emanation of radon from a source rock occurs at depth, and is transported upwards by a carrier-gas, to be detected as a MD Rn signal. As the overall deformation continues, further strain along the DST may either result in aseismic slip or else it may exceed the failure threshold somewhere within the DS rift valley, so that an earthquake occurs there within a few days after the generation of a Rn anomaly in the DS area. This may explain how weak earthquakes are correlated with Rn anomalies that are recorded tens of kilometers away from their epicenters, but within the same tectonic context. We assume that most of the Rn anomalies that are not correlated with earthquakes (~60%) signify strain transients that are related to aseismic slip. The observations and results determine radon as a sensitive proxy of subtle deformation transients in the upper crust. This sets radon as an important tool for geodynamic precursor research. Establishing radon, within a specific tectonic scenario, as a proxy of subtle geodynamic activity enables its use there as a reference for testing of further proxies of deformation transients.
DE: 1207 Transient deformation (6924, 7230, 7240)
DE: 1209 Tectonic deformation (6924)
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
SC: Geodesy [G]
MN: Fall Meeting 2005