Active tectonics, crustal deformation and seismotectonic background for a realistic seismic hazard assessment in southern Africa

Abstract

The aim of this thesis is to determine the impact of improved seismotectonic model on seismic hazard assessment in southern Africa. This is achieved through a combination of several approaches in order to adopt a multidisciplinary strategy so as to enhance our understanding of earthquake occurrence in intraplate southern Africa. The characterisation of strong earthquakes and faults within intraplate regions through geological, geophysical and tectonic geomorphology studies, is beneficial to the improvement of seismic hazard assessment.The first chapter describes the research methodology. The second chapter focuses on the 3 April 2017 MW 6.5, Moiyabana (Botswana) earthquake which occurred in the continental interior of the Nubian plate and in a seismogenic region previously considered as stable. We analyse the mainshock and aftershocks sequence based on a local seismic network and local seismotectonic characteristics so as to constrain the rupture geometry and correlate it to that from the InSAR analysis of Sentinel-1 images (ascending orbit). Focal mechanism solutions of the mainshock and aftershocks display predominance of NW-SE trending and NE dipping normal faulting. Stress inversion of the focal mechanisms produced results that are compatible with a NE-SW extension under normal faulting regime. Although the seismic strain rate is of low level, the occurrence of the 2017 Moiyabana earthquake in the central Limpopo Mobile Belt classifies the intraplate region as an active plate interior. The third chapter presents the Tugela and Greytown fault in Kwazulu-Natal in Southeast Africa, as well as the reappraisal of the 1932 St Lucia MW 6.9 earthquake. We perform detailed investigations in seismotectonics and tectonic geomorphology of the 1932 St Lucia earthquake area with the objective of characterising physical parameters of the seismogenic fault necessary for the seismic hazard assessment in the KwaZulu-Natal (KZN) region. First, we delineate the seismi