## 1 Introduction  
Rifts zones with divergent plates host high potential geothermal resources and large hydrothermal systems. The Asal rift is an impelling place to study many features related to the characterization of such hydrothermal systems, as seismicity, ground deformation, high ground temperature, fumaroles activities and geomagnetic anomalies have been observed in the area during the past decades [1; 2; 3; 4]. Earthquake monitoring was consequently carried out for a long period in the Asal rift in order to image the thermal structure of the crust [4]. Thus, the presence of a hydrothermal system was suggested since seismicity can be related to fluid movement in the crust [4; 5; 6]. However, the depth of high linear geothermal gradient for well A5 (absence of convection) is different from the depth of wells A3, A6 and A4 which clearly showed evidence of convection (see Figure 2 in [6]), the permeability reduction from 1 to 2 km observed in deep geothermal wells located at the center of rift (A5, F1 and F3, see Figure 1) and the absence of significant hydrothermal circulation, are fair evidence that the active hydrothermal system can be compartmented.  
Characterization of the subsurface with geophysical methods is an important prior step in geothermal exploration. The magnetotelluric (MT) method is suitable for highlighting conductivity contrasts at depth, and electrical properties are among the most useful parameters for imaging deep geothermal structures, especially in a volcanic context. Electrical conductivity varies with clay content, temperature, rock type, water content. Particularly, the presence of interconnected pore or fracture filled by a fluid or conductive minerals drastically changes the electrical resistivity. Combined with other information, this parameter can help to identify the presence of a magmatic system, hydrothermal reservoir, shallow aquifer, regional aquifer, vertical conduits as faults and the existence of cap rocks [7; 8; 9; 10].  
Dimensionality analysis of magnetotelluric data, which is often used to define what structure (1D, 2D and 3D) of electrical conductivity is more appropriate for the acquired MT data and the determination of geoelectric strike or the direction of the higher electrical conductivity, are useful tools to evaluate the existence of anisotropic layers in the subsurface. In fact, geoelectric strike dependence on the penetration depth of electromagnetic fields can be used to determine the past tectonic activities in a rift zones [11], where tectonic and magmatic activities are dominant processes of rifting, such as in the Asal rift. The aim of this study is to infer the presence of both geoelectric strike and electrical anisotropy direction that may be related to the remanent and current stress fields in an extension context of rift prior to delimit the deep structure and assess the extent of the hydrothermal system in Asal rift. To achieve this objective, MT data acquired in 2008 by ISOR [12] were reinterpreted. Detailed dimensionality analysis of the MT soundings was performed, followed by 2D inversion of resistivity sections across the rift. Joint analysis of dimensionality and 2D inversion modelling were carried out to infer the geoelectric strike direction, existence of electrical anisotropy, presence of potential hydrothermal system, and to identify the main electrical conduction mechanism which accommodates the subsurface resistivity distribution.  
Figure 1: Topographic and main structural elements of **(a)** Dibouti and **(b)** Asal rift. The red rectangle is Djibouti; Greek characters represent the name of major normal faults; MT: Magnetotelluric (modified from [6]).