Seismic hazards in Uganda
Abstract
A probability seismic hazard map of Uganda is presented, based on instrumentally-derived data. The Uganda catalogue, compiled by Twesigomwe for the period 1900 to 1991, has been used. The magnitudes in the catalogue have been homogenised to surface wave magnitude (Ms); the conversion of body wave magnitude (m b) and local magnitude (M L to Ms were carried out where necessary using formulae derived for the Ugandan earthquakes. Based on seismicity and tectonics, the region was divided into 13 seismic source areas, each of which contributes to the seismic hazard throughout Uganda according to its specific seismicity. The uncertainties in the input parameters were accounted for by using a logic tree approach. On the logic tree, each uncertain parameter is represented by a node and branches emanating from that node represent alternative values on that parameter value and their assigned likelihood of being correct. Due to a lack of strong motion data, a semi-theoretical approach was used to develop an attenuation relation for the region. The mean peak ground acceleration (PGA), which is exceeded on average once every 50 years, was calculated. The resulting hazard map suggests that the whole of Uganda except in or close to the rifts, can expect to experience a PGA of between 0.5 and 0.6 m s -2, equivalent to an earthquake of intensity V-VI (slight damage) on average once every 50 years. In or close to the Western Rift, the expected PGA is between 1.0 and 2.2 m s -2, equivalent to an earthquake of intensity VII-VIII (moderate to heavy damage) on average once every 50 years. The frequency of occurrence of a PGA=2.0 m s -2 (heavy damage earthquake) for various parts of Uganda was also calculated. The results indicate that northeast Uganda can expect a destructive earthquake on average once in more than 3000 years, while south of latitude 0.5°N, except in and close to the Western Rift, can expect a destructive earthquake on average once every ∼1000-1500 years. In or close to the Western Rift the return period for destructive earthquake is on average less than about 50 years.
- Publication:
-
Journal of African Earth Sciences
- Pub Date:
- February 1997
- DOI:
- 10.1016/S0899-5362(97)00035-3
- Bibcode:
- 1997JAfES..24..183T