Seismicity and Surface Deformation in Kamanjab Inlier, Northern Namibia

The last two decades have seen the onset of felt earthquakes, including occasionally damaging events, in the Kamanjab Inlier, a block of Paleoproterozoic crystalline basement in northern Namibia. The Geological Survey of Namibia (GSN) and the Council for Geoscience, South Africa (CGS) deployed a temporary network of 10 seismic stations within the Kamanjab Inlier from June to September 2018 and cataloged ~1500 events. We used a neural network-based earthquake phase detector, EQTransformer, to enhance the published GSN catalog to >9000 detections. The double-difference earthquake relocation of ~4500 events reveals two distinct major and three minor spatial clusters that we interpret as local discrete faults that intersect the NE-dipping seismogenic fault of the 4 April 2021 Mw 5.4 earthquake, which is the largest instrumentally recorded earthquake in Namibia to date. We name the Mw 5.4 host fault "Anker Fault" and constrain its orientation using Sentinel 1 Interferometric Satellite Aperture Radar (InSAR) to image surface uplift and subsidence patterns. Given the sudden onset of the 2018 seismic activity and the absence of dams, mineral or energy exploration projects nearby, we eliminated the possibility of anthropogenic triggering. We suggest that the proximal cause for 2018 seismicity is shallow groundwater migration, possibly associated with nearby hot springs and modulated by tidal forces. The Kamanjab Inlier area has shown an increase in the number and magnitude of earthquakes from 2018 to 2021, which could pose a seismic hazard in the future. Our study introduces an earthquake detection and relocation workflow that can be adopted for regions with limited instrumentation.