Intrarift fault interactions: Insights from coseismic stress redistribution from large seismogenic segment ruptures, Northern Malawi Rift

The understanding of the factors influencing the active interaction and coalescence of intrarift fault segments in extending continental regions is limited. The 2009 Mw6.0 Karonga earthquake occurred in the westernmost portion of the Northern Malawi Rift, which hosts clustered intrarift faulting. The event ruptured the strongly coalesced southern segment of the St. Mary Fault (sSMF), and subsequently, moderate-magnitude events localized on poorly -coalesced segments that define the northern continuation of the fault. To investigate the role of coseismic stress redistribution on interacting faults, we explore the slip distribution of the 2009 event with realistic 3D strike-variable fault geometries, and compute coseismic Coulomb stress changes around the sSMF and neighboring faults. The results suggest that the down-dip intersections of the sSMF with the neighboring faults partition strain in a way that directs most of the deformation to >5 km depths. Additionally, the coseismic stress changes from the 2009 earthquake promoted interactions between the sSMF and adjoining northern segments of the fault at shallow (<5 km) depths, indicating that upper-crustal hard-linkage is underway in the poorly coalesced en-echelon sections of the northern segment. These results demonstrate how coseismic static stress transfer between evolving intrarift fault systems drive fault linkage over a single seismic cycle. Over successive slip events, such stress transfer processes may accelerate the linkage and coalescence of contiguous intrarift fault segments, amalgamate and deepen sub-basins along-strike, and promote across-rift basin compartmentalization. This process is relevant for fault coalescence over multi-seismic cycles, progressive maturation of rift basins, and transitions from juvenile continental rifting to the development of margins where break-up can initialize.