Seismogenic Fault Reactivation in Western Central Africa: Insights From Regional Stress Analysis

Abstract

AbstractThe onshore continental margins of western Central Africa have been hosting potentially damaging earthquake events for decades; yet, the links between the seismicity, the contemporary stress field, and pre‐existing faults are not well understood. Here, we analyze the regional stress fields offshore and onshore along the coastal margin, and in the interior continental areas using earthquake focal mechanisms, map and characterize the detailed structure of preexisting fault systems in outcrops, and assess their reactivation potential. Our results show a switch from NNE‐SSW transpressive stress regime offshore and near the coastal margins, to NE‐SW compressive and transtensive stress regimes in the cratonic interior (Congo Basin and Kasai Block). We show that regional stresses acting on offshore oceanic fracture zones are compatible with those acting along the onshore areas of the continental margin. Field observations reveal the presence of large fault systems that deform both the Precambrian basement and Phanerozoic sedimentary sequences, with widespread calcite veining, quartz veining, and palygorskite mineralization (with evidence of post‐veining shear reactivation) along the fault zones. Along the margin, the preexisting NNE‐, NNW‐, and N‐S ‐trending strike‐slip faults and normal faults show a high slip tendency (>80%–100%), whereas in the continental interior, the NW‐ and N‐S ‐trending thrust faults are the most likely to reactivate. We argue that favorable orientation of the preexisting faults define the susceptibility of the faults to seismic reactivation. We propose that zones of higher stress magnitudes along distal offshore oceanic fracture zones extend further into the continent and may be driving stress loading on pre‐stressed, favorably oriented fault systems onshore, along the continental margin.