Seismological studies of the Western Rift Valley of Africa

Intensity, strain release, and first‐motion studies of earthquakes along the Western Rift Valley of Africa, using data from the seismograph network of the Institut pour la Recherche Scientifique en Afrique Centrale in the eastern Belgian Congo, during the period from May 1953 to April 1956 are presented. Regions of activity shifted during the period of observation, an increase in activity west of Lake Albert in July 1956 being especially noticeable. Intensity maps for three earthquakes show an elongation of isointensity regions parallel to the Rift. The rate of strain release in the area increased with the commencement of the Lake Albert shocks, changing from 35 × 10 8 ergs ½ to 155 × 10 8 ergs ½ per 100 days. Strain released during several aftershock sequences was proportional to the logarithm of the time after the initial shock. First‐motions from Rift Valley earthquakes to the stations of the Institut pour la Recherche Scientifique en Afrique Centrale (IRSAC) do not give a unique determination of the predominant type of faulting. The data are consistent with dip slip faulting along steeply clipping faults parallel to known faults in the region with motion such that the elevated areas are being further elevated and the depressed areas further depressed. They are also consistent with strike slip faulting along near‐vertical faults with the eastern sides moving north. In agreement with the near station results, first‐motion data from distant stations for two shocks west of Lake Albert indicate some combination of reverse dip‐slip and sinistral strike‐slip motion along a fault plane dipping toward the Rift Valley.