Probabilistic seismic hazard assessment in Namibia

Namibia is located in Southern Africa on a Stable Continental Region (SRC). Despite the fact that a number of earthquakes have been observed or recorded in Namibia since 1910, with the July 31, 2009 and the April 4, 2021 earthquakes both of moment magnitude (Mw) of 5.6, no up-to-date country-wide seismic hazard study has been conducted. As a result, the purpose of this research is to conduct a comprehensive Probabilistic Seismic Hazard Assessment (PSHA) for the entire country in order to aid in planning and seismic risk mitigation. The earthquake catalogue developed for this study spans the years 1910-2021 and includes both historical and instrumental events. Data from the International Seismological Centre (ISC), the Advanced Network of Seismological System (ANSS), and Namibia's National Seismological Network (NSN) were used to create the catalogue. The catalogue was then declustered to remove fore- and aftershocks as well as similar events; the resulting catalogue was used in conjunction with available geological evidence to identify, delineate, and characterise 11 seismic source zones as well as one fault source, the Hebron/Dreylingen fault. Four Ground Motion Prediction Equations (GMPEs) were carefully selected and used in the seismic hazard computations. The GMPEs were implemented in the calculations using a logic tree formalism with equal weights, which assisted in addressing the uncertainties associated with both the seismic sources and the ground motion models. Peak Ground Accelerations (PGA) obtained for a 10% chance of exceeding in 50 years ranged from 0.017 g to 0.149 g. The highest levels of hazard were observed in Namibia's north-western, north-central, central and southern regions. This study also produced seismic hazard maps that show the distribution of acceleration at different response periods (0s (PGA), 0.1s, 0.15s, 0.2. s, 0.3s, 0.5s and 1.0s) computed for a 10% probability of exceedance in 50 years. The findings from this Namibia's first large-scale seismic hazard study are expected to make a significant contribution for future land use planning in facility and infrastructural development in the country.