## 5 Discussion
### Subsurface Conductivity Structure and Associated Interpretations
#### 5.3.3 Potential Magma Reservoir Inferred from Joint Interpretation  
The presence of magma reservoir at depth will be inferred with joint interpretation of the 2D conductivity model proposed in this study and previous studies. The passive seismic study of [4] revealed that the main volcanic activity with the most seismicity recorded under the rift was concentrated beneath Fieale caldera zone (Figure 1). Those authors interpreted it as volume of hot rocks where the main seismogenic crust lie between 3-5 km depth overlying above a deeper magma chamber. They argued that the deformation due to the volume of hot molten rocks is mainly aseismic and that the brittle to ductile transition is shallower at the center of the rift than at its shoulders. Thus, in this case, the deformed partly hot molten rock may contain open fissures/fractures that can potentially induce seismicity determined between 3-5 km depth. Seismic reflection profiles suggested low velocity mantle beneath the Asal rift where the presence of the dyke under the rift axis was inferred [13]. Geochemical and petrological constraints revealed evidence of shallow melting path beneath the rift axis and deeper melting path at the rift shoulders [47]. Moreover, molten rock or magma reservoir are expected to be highly conductive medium due to the high temperature [71] but neither beneath Fieale Caldera nor beneath the center or close to the rift axis can be found a high conductivity anomaly for the invariant and TMmodels at great depth (Figure 9). Consequently, magma reservoir and/or molten rocks does not exist at great depth surveyed by the 2D conductivity of the invariant and TM models or those models are not able to detect magma that would result in a high conductive medium. Moreover, the TE and TE-TM models (Figure 9 and Appendix A) are able to resolve a high conductivity anomaly at the center of rift between 2 and 5 km depth that may correspond to the magma reservoir. Since the last 30 kyr, the rifting process reached a steady state and a cooling magma is expected under the rift [13; 15; 17]. In this case, a cooling magma intruding the shallow crust during the past magmatic-tectonic events will likely be imaged as relative resistive medium having approximate electrical properties of the embedding geological layer [71]. Thus, the author argue that R2 relative resistive layer (Figure 9) may contain potential partly molten rock based on the following elements. Sub-vertical dipping of R2 can be observed at the center of Asal rift with significant conductance, whereas the conductance decreases at the shoulders of the rift suggesting lateral migration of the injected hot material into dykes. Moreover, the conductivity contrast between R2 and R3 can indicate a considerable petrophysical properties contrast, notably the temperature which in turn affect the density where the low conductive layer may correspond to the layer of high-density. In this scenario, buoyancy variation generated by the density difference facilitate the migration of the molten rocks from greater depth level to a shallow depth. Furthermore, when the buoyancy force is not enough to maintain the upwelling magma reservoir to the shallow strata losing its buoyancy, the hot magmatic materials intruded forms a magmatic reservoir or intrusion at the corresponding depth. It is surprising that most models of ground deformations [15; 18] proposed a maximum depth of 4 km from the surface reached by the dyke injection close to the center of the rift where lie the source of deformation which is consistent with the top of the R2 layer and the bottom of C4 layer (Figure 9). Vertical resistive structures at the center of Unzen graben in Japan was interpreted as cooled dyke that may have acted previously as volcanic conduit [69]. However, the synclinal form of C4 and R2 can reveal the symmetrical lateral propagation of molten rocks to the shoulders of the rift.