Multidisciplinary investigations of earthflow processes in the differential erosion furrows morphostructural unit, Northern Rif (Morocco): case study of the Seikha landslide

In the Rif mountain chain, Slow earthflow processes affectthe clayey and flysch formations of the Tangier and Flysch structural units respectively. To understand the underlying geomorphological processes and the trigger-failure relationship between the local mediterranean climate conditions and the earthflow-like morphologies at the Differential Erosion Furrow (DEF) morphostructural unit, a case study is conducted at the Seikha earthflow. The methodology proposed uses a multidisciplinary approach, coupling in situ geophysical and geotechnical tests to study the geometry of the landslide and remote sensing techniques to monitor its activity. Our results indicate that on one hand, the cross-analysis of geological and geophysical results shows that landslide processes at the study area follow a typical terrestrial-style earthflow model, where the geological structures controlling the landscape’s evolution are orientated parallel to the longitudinal stress direction. Vertical and horizontal resistivity variations also allow reconstructing the retrogressive genetic processes responsible for older processes that contributed to the evolution of this hillslope in particular and the DEF morphostructural unit as a whole. On the other hand, the interpretation of multitemporal aerial photographs suggests that the Seikha landslide is in a dormant state and that its acceleration periods follow multiannual cyclic trends related to historic climate and base level fall variations. Seasonal Trends are also emphasized by SBAS (small baseline subset) inSAR (Interferometric Synthetic Aperture Radar) and borehole inclinometer results, which show evidence of slow gravitational deformation that can be accelerated during seasonal rainfall periods.