Influence of Surface Processes on Intrarift Fault Displacement: 1.38 Million Years of Fault Slip Behavior in the Lake Malawi (Nyasa) Rift

Abstract

Surface processes, including hydroclimate variations, affect crustal deformation and fault behavior. These controls are especially important for intrarift faults in continental rift systems, which accommodate substantial strain while experiencing large hydrological load variations beneath rift lakes. The Lake Malawi Rift has undergone large‐amplitude lake‐level fluctuations throughout the Quaternary, with a marked shift in frequency and amplitude across the Mid‐Pleistocene Transition (MPT; 1.2–0.7 Ma). To assess the relationship between intrarift fault slip and lake level loading, we integrated a well‐dated sediment core record with seismic data to measure fault slip histories over 1.38 million years and employed finite element modeling to investigate possible physical mechanisms. Results reveal a 73.0% average reduction in slip rate after the MPT across four intrarift faults. Coulomb stress modeling indicates lake level fluctuations generated stress perturbations of 2–3.6 MPa, more negative values during highstands, indicating that hydrological loading systematically inhibits normal fault slip via clamping. Poroelastic responses varied systematically across fault types, with poorly drained intrarift faults experiencing effective stress changes up to 4.8 MPa during rapid drawdown. The transition from high‐frequency, low‐amplitude pre‐MPT cycles to low‐frequency, high‐amplitude post‐MPT cycles restructured fault loading patterns, reducing the time faults spent near their failure thresholds. These results demonstrate how hydroclimate variations control fault behavior in active rift systems and highlight seismic hazard implications of rapid hydrological load changes under future climate uncertainty.