Electrical structure of the tectonically active Kalabsha Fault, Aswan, Egypt

In this work, we use the magnetotelluric (MT) method to detect geoelectrical conductivity anomalies in the Earth's crust and link them to local seismic activity. This application affords the unusual opportunity to study the percolation of water from a lake into a fault system and its effect on the induced seismicity. MT measurements were carried out in the period range 0.0046–420 s at nine sites along a 15 km-long North–South profile crossing the Kalabsha Fault, on the western bank of Lake Aswan. Data were analysed by 2D simultaneous inversion of both polarisations. The resulting model is compared with the local seismicity map and reveals the conductive signature of the fault, as well as geological and tectonic stresses prevailing in the Aswan area. Our MT investigations show the following features: The measured MT strike aligns with the seismic epicentre axis corresponding to the Kalabsha Fault. While crossing the Fault, enhanced conductivity is found down to depths of 5 km on a 1–2 km profile segment. At mid-crustal depths (20 km), a very high conductive body is found to coincide with the main seismic cluster in the Aswan area. These observations indicate that seismic activity and high electrical conductivity are related. The link between them is the presence of crustal fluids which are presumably the cause of the high conductivity observed. Their presence is also required to trigger the observed seismicity. In addition, we explain the lower conductivity of the local upper crust in terms of stress-modulated rock porosity. We believe that these results are of general significance, as they could explain the mid-crustal seismicity of tectonically active zones.