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

The understanding of the factors influencing the coalescence of intrarift fault segments in actively extending continental lithosphere is limited. The 2009 Mw6.0 Karonga earthquake occurred in the westernmost portion of the Northern Malawi Rift, which hosts clustered intrarift faulting. The 2009 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 St. Mary Fault. To investigate the role of coseismic stress redistribution on fault interactions, we use the slip distribution of the 2009 event and realistic 3D strike-variable fault geometries to compute coseismic Coulomb stress changes around the sSMF and neighboring faults. We provide evidence suggesting that the intersections of the sSMF with the neighboring faults partition strain in a way that directs most of the deformation to >5 km depths. More importantly, the coseismic stress changes from the 2009 Mw6.0 Karonga earthquake promoted a northward interaction between the sSMF and adjoining segments of the fault to the north at shallow depths (<5 km) indicating that hard linkage is underway in the poorly coalesced en-echelon sections of the northern segment. Our results demonstrate how coseismic stresses transfer along evolving intrarift normal fault systems. Over successive slip events, this stress transfer mechanism 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 the progressive maturation of rift basins and transitions from juvenile continental rifting to the development of margins where continental break-up can initialize. • Fault interactions during the 2009 Mw6.0 earthquake in Malawi vertically partition strain. • The coseismic stress changes directed most of the deformation to >5 km depths. • Northward interaction between the source fault and its segments occurs at depths <5 km. • The tress transfer accelerates the linkage of contiguous intrarift fault segments. • This process is relevant for the progressive maturation of rift basins.