The Seismicity of the Nyanza Rift: Implication for its Propagation from the Main Central Kenya Rift Valley

Abstract

A new seismicity map of the Nyanza Rift is presented, using a compiled and unified earthquake catalog of 290 earthquakes spanning 107 years from 1913 to June 2020 and magnitude M_L≤5.2. A magnitude of completeness Mc 4.0 and a b-value of 0.74 confirm that the Nyanza Rift Valley is an active extensional rift. The seismicity patterns confirm that the Nyanza Rift propagates in a WSW direction off the main Central Kenya Rift. The Nyanza Rift lacks, to a large extent, bounding structural controls eastwards of Lake Victoria, implying that Lake Victoria sits in half-graben, except at the Winam Gulf, where the active Kisumu faults, Nyando faults, and Kendu faults bound imply a graben. The seismicity predominantly diffuses and straddles the entire length of the Nyanza Rift, about 250 km, with a breadth of 50 km. Six active faults are identified: The Nandi fault, Kisumu fault, Nyando fault, Kendu fault, Lambwe-Samanga fault, and the Siria-Vitumbara fault. Earthquake depths increase southwestwards, where lower crustal earthquakes 30-38 km confirm crustal thickness increases within the Tanzanian Craton and underneath Lake Victoria. Four seismogenic source zones are delineated as the Tinderet-Timboroa Zone (Zone-1), the Winam Gulf and Homabay-Rangwe Zone (Zone-2), the Southeast Lake Victoria Zone (Zone-3), the Siria-Vitumbara Fault Zone (Zone-4). Within seismogenic source zones, Zone-1 to Zone-3 a north-northeast seismicity trend is observed, implying possible re-activation of buried faults.

Keywords: Seismicity, Seismicity patterns, Seismogenic source zones, West-southwest propagation of the Nyanza Rift Valley

DOI: 10.7176/JEES/14-6-05

Publication date: December 30th 2024