## 4 Discussion and Conclusions  
The GPR profiles made it possible to image the underground structures at the two ends of the fault which are associated with opposite vertical deformations (compressive in NW, extensive in SE). The overall fault dip to the north-northeast, probably more vertical in its central section, inducing an extensive counter-slope scarp in the SE section with a recent accumulation of deposits. The 250 MHZ antenna appears to be the best adapted to the local context, with depth penetration close to the target for most of the paleo-seismological trenches of the order of 3 m.  
The 250 MHz GPR images of the Songino fault show the evolution of the sub-surface deformation mode induced by the arched geometry of the Songino fault. We observe a clear compressive structure at its NW section, strike slip at its central section and extensive structure in its SE part with an apparent vertical displacement of around 1.2-1.3 m. This shows that it is necessary to make GPR measurement at various places along the fault to follow its behavior. As the fault is also associated with a strike-slip shift, a major component  
Figure 5: (**a**) GPR image of the profile at the southeastern part of the Songino fault (see red arrow of Figure 2 for the location) obtained with a 250 MHz antenna. The topographic corrections and depth conversion are performed using a velocity of 0.1 m/ns. (**b**) A zoom between 20 and 70 m horizontally is presented; black arrow indicates the fault zone. A change in GPR reflectivity is observed on both sides of the fault scarp at 52 m horizontally at the top of the smoothed fault scarp. at the central part decreasing toward the NW and SE, the observed vertical displacement in GPR profiles does not correspond to real offset. These non-destructive observations allowed us to observe recent geological deposits, potentially datable, as well as sub-surface deformations induced by past earthquakes. This helps us to optimize the location of a paleo-seismological trench which will be presented in detail by [PERSON] (2021 [15]), in preparation). This study confirms that, after morphotectonic analysis, the GPR method contributes to the characterization of active faults, in particular near UB in Mongolia, in a context low slip rate where the geomorphologic features have been strongly smoothed due to erosion processes combined with a very long return period. The main result of this study is that it is possible to distinguish between active fault compression and tensile effects by combining the GPR imagery with the geomorphology of active faults. Despite the very shallow penetration depth (up to 2-4.0 m with the 500 and 250 MHz antennae), the information obtained by the GPR profiles was very useful for better choosing the location of paleoeismological trench sites. The next step of this study will be the paleoseismological investigations close to GPR profiles. These will be compared with our observations and GPR results before giving the final conclusions on this active fault and its relations with regional structures around UB city. Data curation, M.B., N.T.-A., A.S. and U.M. ; investigation, N.T.-A. ; methodology, M.B. and A.S.; validation, M.B. and A.S.; data acquisitions, N.T.-A., M.B., A.S.; field work, A.S., U.M. and M.B. ; data processing, N.T.-A. and M.B. ; writing--original draft preparation, M.B. ; writing--review and editing, A.S. and M.B. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. Data Availability StatementData are available by the authors upon request. The authors would like to thank [PERSON] for making the topographic measurements. Many thanks to [PERSON] and the two students, [PERSON] and [PERSON], who participated in the acquisition of the GPR data. We thank our colleague [PERSON] for valuable discussions during the preliminary morphotectonic field survey.