Seismic Hazard Assessment for Bridge Infrastructure Along the East African Rift System

The East African Rift System (EARS) constitutes one of the world's most seismically active intraplate tectonic environments, encompassing the Malawi Rift, Tanganyika Rift, Albertine Rift, Main Ethiopian Rift, and Afar Depression — a 4,000 km chain of diverging continental crust overlain by critical road and bridge infrastructure connecting six EAC member states and two IGAD countries. Despite the well-documented seismic activity of the EARS, systematic probabilistic seismic hazard assessment (PSHA) for bridge infrastructure has not previously been conducted at the multi-country corridor scale. This study presents a PSHA of 45 bridge structures distributed across five seismically exposed road corridors intersecting the EARS, employing a logic-tree framework with multiple ground motion prediction equations (GMPEs) and seismic source models. Uniform hazard spectra (UHS) were computed at 2%, 5%, and 10% probability of exceedance in 50 years. Site-specific hazard was modified for local site conditions using VS30-based amplification factors derived from multi-channel analysis of surface waves (MASW) surveys at each bridge site. Structural fragility curves were developed for four bridge pier configurations representative of the EAC inventory using nonlinear time-history analysis with 40 spectrum-compatible ground motion records. A seismic risk index (SRI) integrating hazard, exposure, and structural vulnerability was computed for each bridge. Results indicate that 17 of 45 bridges (38%) fall in the High or Very High risk category, with the greatest concentration along the Western Rift branch (Lake Tanganyika and Lake Albert corridors). The Arusha–Moshi corridor in Tanzania and the Kampala–Fort Portal corridor in Uganda show mean spectral accelerations at T = 0.5 s of 0.33g and