Estimation of Ground Motion Levels Due To Earthquakes of the Albertine Graben In Uganda.

Abstract

Present probabilistic seismic hazard maps for the Albertine Graben, for peak ground acceleration and the spectral ordinate at periods of 0.2seconds and 1.0seconds;with 10 % probability of being exceeded on average once in 50 years were determined using data derived from different sources.The catalogue was prepared by collecting datafrom International agencies, review of a catalogue by Twesigomwe,(1997). The magnitudes on the catalogue were homogenized to moment magnitude (𝑀𝑤). This was done by converting both the body wave magnitude (𝑚𝑏) and surface wave magnitudes (𝑀𝑠) into moment magnitude using the regression equations derived for the Albertine Graben. Based on seismicity and the tectonics of the Albertine Graben, the region was divided into Four (04)seismic source areas, each of which contributes to the seismic hazard in the Albertine Graben.The maximum peak ground acceleration(PGA)values 26% g to 30% g for return periods of 50 and 100years on rock site conditions and 28%to 32% g on soil site conditions of the Albertine Graben. These correspond to earthquakes of intensity (I -VIII) which are perceived to cause from no damage to moderate or heavy damage. The ground shaking will vary from not being felt to severe (Table 2.1). These results are in agreement with Twesigomwe,(1997), whose result for the East African Rift System are in the range (10 -22) % g, Midzi V. et al.,(1999) for EARS, for 50years return period which are in the range of (10 -26.0)% g.The spectral accelerations observed on rock and soil site conditions when T=1.0seconds is 27%g to 32% g while for T=0.2seconds, 29%g and 32%g on rock and soil site conditions the Albetine Graben respectively. This shows that at shorter periods of vibration, rock sites register higher spectral acceleration values while at longer periods they register lower spectral acceleration values compared to soil site conditions of the Albertine Graben.The suggests that at shorter periods(higher frequencies) of vibration on rock site conditions, the seismic energy moves with high amplitudes but dies away when the period of vibration is prolonged (lower frequencies) while for soil site conditions, the amplitude of the seismic wave is reduced at shorter periods and is amplified as the time of vibration is prolonged as shown in figures (5. 5 to 5. 12).