Folarin Kolawole , Estella A. Atekwana, Daniel A Lao-Davila, M. G. Abdelsalam, P. R. Chindandali, J. Salima, L. Kalindekafe

Key Points:
1. Filtered aeromagnetic data elucidates the relationship between earthquake surface ruptures and buried faults along the North Basin hinge-zone, Malawi Rift.
2. Fault segments align with basement shear zone fabric.
3. Strain accommodation in the North Basin hinge zone is modulated by reactivation of the underlying Precambrian shear zone fabric.

Abstract:
We integrated temporal aeromagnetic data and recent earthquake data to address the long-standing question on the role of pre-existing Precambrian structures in modulating strain accommodation and subsequent ruptures leading to seismic events within the East African Rift System (EARS). We used aeromagnetic data to elucidate the relationship between the locations of the 2009 Mw 6.0 Karonga, Malawi earthquake surface ruptures and buried basement faults along the hinge zone of the half-graben comprising the North Basin of the Malawi Rift. Through the application of derivative filters and depth-to-magnetic-source modeling, we identified and constrained the trend of the Precambrian metamorphic fabrics and correlated them to the three-dimensional (3D) structure of buried basement faults. Our results reveal an unprecedented detail of the basement fabric dominated by high frequency WNW- to NW-trending magnetic lineaments associated with the Precambrian Mughese Shear Zone fabric. The high frequency magnetic lineaments are superimposed by lower frequency NNW-trending magnetic lineaments associated with possible Cenozoic faults. Surface ruptures associated with the 2009 Mw 6.0 Karonga earthquake swarm aligned with one of the NNW-trending magnetic lineaments defining a normal fault that is characterized by right-stepping segments along its northern half and coalesced segments on its southern half. Fault geometries, regional kinematics and spatial distribution of seismicity suggest that seismogenic faults reactivated the basement fabric found along the half-graben hinge zone. We suggest that focusing of strain accommodation and seismicity along the half-graben hinge zone is facilitated and modulated by the presence of the basement fabric.