Geoelectric patterns of active faults in permafrost regions (example of Russian highland and Arctic regions)

Active faults under permafrost conditions in the highland part of Gorny Altai (South Chuya and Kubadru fault zones) and the Lena River delta (Primorsky Fault Zone) were studied using the electrical resistivity tomography technique. The method proved to be effective in identifying active fault zones to depths up to the first hundreds of meters under permafrost conditions. However, the presence of ice-rich rocks with a resistivity greater than 100 kOmega .m limits its application because of the screening effect of the insulating unit. The main criterion for identifying active faults in geoelectric sections is the existence of subvertical zones of reduced electrical resistivity against the background of high-resistivity permafrost rocks. This concerns both the young seismic ruptures formed during the 27 September 2003 Chuya earthquake (M (sub s) =7.3) in the South Chuya Fault Zone and the older Holocene primary seismic deformations in the zones of the Kubadru and Primorsky faults. At the same time, the electrical resistivity values in the zones of active faults and seismic ruptures are too high to assume their saturation with free water. The decrease in electrical resistivity in such zones relative to the host permafrost frame may be due to (1) intensive fracturing of rocks and sediments; (2) occurrence of finely grated material in the core and damage fault zones, on particles of which adsorbed unfrozen water is concentrated; (3) residual thermal anomalies in the case of modern fault activations, so that negative temperatures have already been restored, but the process of frozen strata aggradation is not complete yet; (4) saturation of the geologic section with sand-silty material as a result of liquefaction and fluidization processes during earthquakes. The revealed regularities can be used not only to confirm the zones of morphologically distinctive segments of active faults but also to search for their buried segments in the permafrost areas typical of seismically active highland and Arctic regions in Russia and worldwide.