CHARACTERISTICS OF GROUND FISSURES AND THEIR POSSIBLE MODE OF DEVELOPMENT IN LAKE ZIWAY AREA, ETHIOPIA

The central segment of the East African Rift system in Ethiopia is dissected by extensive ground fissures whose mode of development is poorly understood. Ground fissures are generally linear with some being curvilinear extending for 1 -2 kilometers and having a 1 – 3m wide open cracks. Ground fissures especially affecting sediments can be classified as geohazards that cause damage to farm lands and transportation infrastructure such as road and rail ways. Field studies were performed around Lake Ziway of Ethiopia, where several fissures are mapped within a small area (~ 5 square kilometers). Past research have attributed their formation to extensional movement due to active rift tectonics, hydro compaction, and piping. Field investigations on ground fissures in the Ziway area do not show extension tectonics, earthquake events or hydro compaction as plausible causes. Studying the timings of past earthquake events, there appear to be no direct or indirect relationship with ground fissure events. However, ground fissure events, as verified from historical Google Earth imagery appear to be related to heavy rainfall events. Based on field observations, ground fissures commonly cutting through pyroclastic sediments are surface manifestation of internal erosion of sediments. Internal erosion create subsurface conduits that grow over time and collapse forming fissures. Therefore, ground fissures are actually collapse structures and not extensional features. Other geomorphologic features indicative of internal erosion including disappearing streams, sinkholes, blind gullies, and pipe mounds are also identified in the study area. These features are consistent with topography described as pseudokarst that typically develop in areas underlain by soils/sediments susceptible to internal erosion. Internal erosion of soils/sediments is promoted by gap graded grain size distribution, where the fine grain size fraction erodes through voids between larger grains. In the case of the study area, pumice layers are found to be the most affected by internal erosion. This is explained by their bimodal grain size distribution and ultra-low density (specific gravity = 0.6) that make them unstable under low seepage stress.