Tectonic conditioning revealed by seismic refraction facilitates deep-seated landslides in the western escarpment of the Main Ethiopian Rift

Abstract

Landslide is a geo-hazard phenomenon that has been taking lives and causing severe property damages all over the world mostly in mountainous areas. The Main Ethiopian Rift has a unique tectonic setting with complex geological and geomorphological features, coupled with continuously deteriorating environmental conditions, which made its escarpments vulnerable for landslides. The study area is located near the Debre Sina town, within the Yizaba Wein locality, which has been severely affected by frequent landslide problems. This work was carried out using a multidisciplinary approach based on geological, geomorphological, kinematic analysis and geophysical survey. Seismic refraction investigations were carried out along the Yizaba Wein landslide main scarp to determine the depth to the bedrock and to the failure plane, to assess the stability of the slope, to locate possible structural features and to identify the extent of recent landslide activity, and to study the subsurface situation. The seismic measurements were made along three nearly orthogonal survey lines in the recently affected area. A high-resolution 2D P-wave survey was conducted using a 24-channel seismic unit. The seismic refraction results revealed four layers of geomaterials with distinct physical characteristics that contained a subsurface landslide anomaly within the layers. The layers were interpreted according to the major lithological units, from top to bottom: (i) clay, loosely cemented colluvial sediments and highly weathered agglomeratic basalt; (ii) highly to moderately fractured porphyritic basalt, ignimbrite-volcanic ash and rhyolite/trachyte; (iii) moderately to slightly fractured ignimbrite, rhyolite/trachyte and basalt and (iv) very strong, massive, fresh rock/bedrock. Faults and weak zones have also been identified in the bedrock based on the abundance of fractures and subsidiary faults resulting from damage of rocks and change of lithology due to variable fault rock formation strongly influencing the wavefield distribution which usually causes a local decrease of the velocity value. The main findings show that the landslide in the Yizaba Wein locality was caused by its complex geological-structural setting and downslope movement of the underlying pyroclastic sediment facilitated by heavy rainfall. Considering the similar geological and tectonic settings, similar mechanisms can be assumed for other landslides along the rift margins and associated highlands of Ethiopia.