169-2 Seismic Source Characterization in the Western Cape Province, South Africa.

Session: Advances and Challenges in Seismotectonic Studies in Slow-Deforming Regions

Presenting Author:

John Stamatakos

Authors:

Stamatakos, John¹, Cawthra, Hayley², Claassen, Debbie³, Coppersmith, Ryan⁴, Johnson, Courtney⁵, Midzi, Vunganai⁶, Montaldo Falero, Valentina⁷, Mulabisana, Thifhelimbilu⁸, Smart, Kevin⁹, Neveling, Johann¹⁰

(1) Restired, Southwest Research Institute, San Antonio, TX, USA, (2) Council for Geoscience, Pretoria, South Africa, (3) Council for Geoscience, Pretoria, South Africa, (4) Coppersmith Consutling, Walnut Creek, CA, USA, (5) Slate Geotechnical Consultants, Berkeley, CA, USA, (6) Council for Geoscience, Pretoria, Tunisia, (7) WSP Geotechnical Consulting, San Francisco, CA, USA, (8) Council for Geoscience, Pretoria, South Africa, (9) Southwest Research Institute, San Antonio, TX, USA, (10) Council for Geoscience, Pretoria, South Africa,

Abstract:

We recently completed a probabilistic seismic hazard analysis (PSHA) for the Duynefontyn nuclear site near Cape Town, South Africa. This site lies within a stable continental region and is thus characterized by slow tectonic deformation and low rates of seismicity.

The geological landscape is largely the product of several periods of Precambrian and Paleozoic contractional and transpressional tectonism that resulted in a complex terrane of faulted and folded sedimentary and metasedimentary strata.  This complexity is most evident in the Cape Syntaxis, where the past periods of deformation produced an orthogonal patchwork of faults and folds.  To characterize the seismic hazard, the PSHA was supported by a series of regional seismological, geological, and geophysical studies that included a reexamination of the historical earthquake record, field reconnaissance investigations that sought evidence of recent surface faulting, a regional marine terrace investigation that evaluated vertical deformation, and offshore bathymetric surveys looking for fault displacement of the now-seafloor.

Although the Western Cape has experienced large earthquakes in the past, most notably the 1809 Cape Town Earthquake, our analysis identified only one fault with sufficient evidence of activity to warrant inclusion in the PSHA seismic source model. Otherwise, we modeled future earthquakes using a source zone configuration, with a magnitude-frequency distribution based on the composite project earthquake catalog (including both historical and instrumental events). We treated future earthquakes in the PSHA analysis as virtual ruptures rather than simple point sources. The geometry (strike, dip, length, depth) and faulting style (normal, reverse, strike-slip) of the virtual ruptures were based on the existing faulting architecture. We favored those fault geometries and faulting styles identified as most likely to recur under present-day stress conditions based on a 3DStress® analysis of the Western Cape Province. The spatial distribution of future earthquakes included two equally weighted branches on the logic tree, one assuming that future earthquakes will be located near the locations of past earthquakes and one assuming future earthquake locations are equally likely throughout the source zone. This approach allowed us to model future seismicity in a manner that reasonably represents past seismicity and fault activity while accounting for the current stress regime. Our approach also allowed us to capture the epistemic uncertainty and aleatory variability of the PSHA inputs.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10038

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