## 2 Geological Background  
The junction of three diverging plates--Arabia, Somalia and Nubia in East Africa--is a well-known depression called the Afar depression, where the expression of tectonic and magmatic activity is manifested as rift segments. Asal is the first inland rift segment of the Afar depression resulting from the westward propagation of the Aden oceanic ridge (Figure 1). The emerged part of the Asal-Ghoubbet rift is bounded in the east by Ghoubbet Bay and in the west by Lake Asal (Figure 1). The opening date of this rift is close to 1 My [13]. Normal faults with principal direction of northwest-southeast that can have subvertical scrap over 100 m and a deep rift valley of about 300 m, are the main observed structural elements at the surface. The central emerged part of the rift is narrower than the marginal parts. Such structural and topographic settings are common features of slow spreading oceanic ridges. Deformed paleo shoreline from a preexisted freshwater lake high stand dated to 9 ky and the recent basaltic lava flow that cover mainly the rift axis zone highlight that this area has been subject to recent volcanic and tectonic activity [14; 15]. The topography of the emerged rift resulted from successive steps of magmatic-tectonic activity. At the early time, almost 100 ky ago, the Fieale volcano was built by magmatic activity, then following tectonic activity faulted the crater that went under a last period of weak magmatic activity. Volcanic edifices of this later period can be observable at the surface but actives normal faults have dismantled most of them [13; 16]. Micro-seismic activity, fumaroles and faults creep activity concentrated at the surface of Fieale Caldera, low seismic velocity zone under Fieale and surrounding areas is evidence supporting the idea that the central volcanic system cited above persisted up to now under the Fieale Caldera [3; 4]. Intense seismic swarm with an earthquake of magnitude greater than five that happened on November 1978 in the Asal-Ghoubbet rift was followed by a fissured basaltic eruption lasting one week and which ended by the extrusion of an approximate volume of 2 \(\times\) 10\({}^{7}\) m\({}^{3}\) yr\({}^{-1}\) of lava that gave rise to the Ardoukoba volcano [15]. Observed deformation and geodetic data between 1972 and 1979 allowed to deduce an absolute uplift of the rift shoulders of about 0.2 m, 2 m extension with direction perpendicular to the rift axis (north-east) and 0.7 m absolute subsidence of the inner floor controlled by normal faults activity [13; 15; 17]. Observed deformation at the surface was interpreted as the result of injection of two magma filled dykes, one under the Ghoubbet Bay and another under the emerged Asal rift [15; 18]. In addition, co-seismic period in 1978 of earthquake epicenters record during 5 days at the same episode of deformation in the Ghoubbet Bay had a trending direction of N120\({}^{\circ}\)E, which corroborates with the direction of dyke injection described above [13]. Two months of continuous seismicity recorded after that event indicated extension of the seismic activity in the Ghoubbet pass and the transfer zone between the Asal-Ghoubbet rift and Tadjoura submerged rift segment [13; 15]. In their study, [19] hypothesized that rift extension at a shallow depth is mainly controlled by opening fissures and normal faulting in both the Ghoubbet seafloor and the emerged Asal rift. Unfortunately, detailed structural and kinematic features that could explain how fissures, faults and dyke are related are still missing. Seismicity recorded between 1979 and 1984 with moderate magnitude was concentrated in the northeast of the rift axis, in the emerged part of the rift [4]. Local geodetic measurements performed at different times suggest that extension in the Asal rift has stabilized since 1990 to an approximate velocity extension of 16 mm yr\({}^{-1}\)[17].