Mapping faults around Adama Town:Implication for future earthquaketriggering potential

Abstract

The northern Main Ethiopian Rift (MER) manifests a complete finger print evolutioncharacteristics of continental rifting from rift initiation to incipient sea floor spreading.Adama is located in the southern extreme of the active volcano-tectonic segment of thenorthern Main Ethiopian rift. We did a structural and geomorphological mapping offaults to investigate their future earthquake triggering potential. We have collectedabout 129 structural (faults and fractures) measurements. All measurement data werecollected from inside Adama town and its surroundings. We calculated palestressdirection of the area which generated all the faults using kinematic analysis techniques.Our data shows the exerted force which is responsible for deformation in the area isoriented about N 1120E to N 680W. The results of our kinematic analysis is statisticallysimilar with the current regional stress direction which is inferred from continuesgeophysical monitoring. Our data shows minor deflection stretching direction withrespect to the regional to the south. We suggest that the minor deflection could beresulted due to crustal strength heterogeneity. We have evaluated faults also throughmorphological investigations using stream profile index. We identified a sharp jump ofelevation (Knick point) along stream profile across fault planes. We checked each ofknick points in the field and suggested that it is an indication of past earthquakeoccurrence evidence. From the integration of structural data and geomorphologicalevidences, we recognized that faults around the town mainly the western side faults areactive. Like historical earthquake of the 1996 could reoccur in this area due to slip alongone of the nearby faults. Pilot study on ground cracks around the town shows asignificant relationship with faults of the area. We suggest that the weathered rockfragments accumulation along the slope of the fault planes especially in the east of thetown can result landslide even by a smaller magnitude of earthquake occurrence. using atomic absorption spectrophotometer, whilst Na and K was analyzed using flame photometer.Turbidity for Awash River water sample was measured using standard method. Derived parameters(SAR and RSC) were estimated from the measured constituents using appropriate equation. Finally,the water under investigation was classified to the different suitability classes as per the criteriaestablished by the USSLS (1954) and Greenberg et al. (1992). Soils of the area exhibited high rangeof variation with respect to ECe values (Appendix Table 4A). ECe value varied from 0.33 dS/m to82.1 dS/m and 0.4 dS/m to 37.5 dS/m, respectively for soil samples taken from Melka Sedi and MelkaWerer farms. Regarding the magnitude of the problem, sever salinity problem was observed at MelkaSedi and former Banana farm areas. Even though the extent was less in terms of area affected,salinity problem in Melka Werer area was also observed. It was also observed that several hectaresof cotton cultivated farms were at saline phase as witnessed from manifestation of frequent salinitypatches elsewhere in the command area indicating the possibility of salinity expansion to take placein near future at more faster rate than ever seen beforeThe sodium adsorption ratio ranges from 0.7 to 27.2 in Melka sedi and 0.6 to 9.4 in Melka Wererfarm areas (Appendix Table 4B). Soil sodicity problem were observed in Melka Sedi farm areasespecially in some fields of former Banana farms, which had a very strongly sodic character. Soils ofMelka Werer were not very much affected by sodicity problems so far. The analytical result obtained(Appendix Table 4) indicates that the electrical conductivity of the saturated extract at soil depth of 0-30 cm varies from 0.33 to 3.93 dS/m for non-saline, 23.00 to 45.90 dS/m for saline sodic soils and4.02 to 82.10 dS/m for saline soils. Whereas the sodium adsorption ratio of the saturated extractvaries from 0.6 to 4.05 for non-saline, 12.75 to 27.19 for saline sodic and 0.79 to 11.80 for salinesoils. Hence considering the distribution of salt affected soils in Amibara Irrigation Schemes, about34 % of the soils are salt affected soils.From the results, considering the soil reaction (pH) of the study area almost all of the soil sampleshave a pH greater than 7 indicating general alkalinity reactions. Regarding soil salinity and sodicityassessment, detail soil salinity and sodicity map of the study area was conducted due to extremeheterogeneity of soils with respect to salinity and sodicity and occurrence of wide range of theproblem. From the results around 34% has been mapped as saline soil and out of which around 20%as strongly to severe saline. Sodicity problem, only 1% has been mapped as slightly sodic to verystrongly sodic soil. Combining both parameters (ECe and SARe values) were resulted 34% as salinesoil and 0.05% as saline sodic soils. From the maps soil salinity/sodicity problems were alsoobserved in Melka Werer areas but the extent was less compared to Melka Sedi area.