A probabilistic seismic hazard analysis has been conducted for a potential nuclear power plant site on the coast of South Africa, a country of low-to-moderate seismicity. The hazard study was conducted as a SSHAC Level 3 process, the first application of this approach outside North America. Extensive geological investigations identified five fault sources with a non-zero probability of being seismogenic. Five area sources were defined for distributed seismicity, the least active being the host zone for which the low recurrence rates for earthquakes were substantiated through investigations of historical seismicity. Empirical ground-motion prediction equations were adjusted to a horizon within the bedrock at the site using kappa values inferred from weak-motion analyses. These adjusted models were then scaled to create new equations capturing the range of epistemic uncertainty in this region with no strong motion recordings. Surface motions were obtained by convolving the bedrock motions with site amplification functions calculated using measured shear-wave velocity profiles.
Efficient detection and high-fidelity quantification of surface changes resulting from underground activities are important national and global security efforts. In this investigation, a team performed field-based topographic characterization by gathering high-quality photographs at very low altitudes from an unmanned aerial system (UAS)-borne camera platform. The data collection occurred shortly before and after a controlled underground chemical explosion as part of the United States Department of Energy's Source Physics Experiments (SPE-5) series. The high-resolution overlapping photographs were used to create 3D photogrammetric models of the site, which then served to map changes in the landscape down to 1-cm-scale. Separate models were created for two areas, herein referred to as the test table grid region and the nearfield grid region. The test table grid includes the region within ~40 m from surface ground zero, with photographs collected at a flight altitude of 8.5 m above ground level (AGL). The near-field grid area covered a broader area, 90–130 m from surface ground zero, and collected at a flight altitude of 22 m AGL. The photographs, processed using Agisoft Photoscan® in conjunction with 125 surveyed ground control point targets, yielded a 6-mm pixel-size digital elevation model (DEM) for the test table grid region. This provided the ≤3 cm resolution in the topographic data to map in fine detail a suite of features related to the underground explosion: uplift, subsidence, surface fractures, and morphological change detection. The near-field grid region data collection resulted in a 2-cm pixel-size DEM, enabling mapping of a broader range of features related to the explosion, including: uplift and subsidence, rock fall, and slope sloughing. This study represents one of the first works to constrain, both temporally and spatially, explosion-related surface damage using a UAS photogrammetric platform; these data will help to advance the science of underground explosion detection.
ABSTRACT Field and desktop mapping studies were conducted for the stable continental region in the Western Cape Province of South Africa to characterize fault activity of four fault systems, including the Worcester, Groenhof, Piketberg-Wellington, and Colenso faults. The geologic studies presented here were in support of a Probabilistic Seismic Hazard Analysis (PSHA) for a nearby nuclear power facility site. Previous studies performed by the South African Council for Geoscience in the region suggested evidence of near-surface co-seismic deformation (De Beer, 2004; De Beer et al., 2008). The goal of this study is to re-assess the prior interpretations of these four faults and gather the required data for including them in a seismic source model for use in a PSHA. The primary aspects to include in the characterization are the recency of movement, slip rate, kinematics, and geometry. To improve the interpretation and target sites, the study used a satellite-derived digital elevation model and aerial imagery for six areas, totaling over 900 km2 of data. Limited Quaternary cover, or other late Cenozoic deposits that overlie the Precambrian and Paleozoic bedrock structures, resulted in difficulty constraining the recency of faulting. The new observations presented in this study suggest that reactivation and surface rupture along pre-Cenozoic faults of the four fault systems have not occurred in at least the last 10 ka. Further, the lack of youthful tectonic geomorphology and deformation of Quaternary stratigraphy indicate that surface faulting has not occurred in the late to middle Quaternary along any of these four structures.
The understanding of subsurface events that cannot be directly observed is dependent on the ability to relate surface-based observations to subsurface processes. This is particularly important for nuclear explosion monitoring, as any future clandestine tests will likely be underground. We collected ground-based lidar and optical imagery using remote, very-low-altitude unmanned aerial system platforms, before and after several underground high explosive experiments. For the lidar collections, we used a terrestrial lidar scanner to obtain high-resolution point clouds and create digital elevation models (DEMs). For the imagery collections, we used structure-from-motion photogrammetry techniques and a dense grid of surveyed ground control points to create high-resolution DEMs. Comparisons between the pre- and post-experiment DEMs indicate changes in surface topography that vary between explosive experiments with varying yield and depth parameters. Our work shows that the relationship between explosive yield and the extent of observable surface change differs from the standard scaled-depth-of-burial model. This suggests that the surface morphological change from underground high explosive experiments can help constrain the experiments' yield and depth, and may impact how such activities are monitored and verified.
careful literature research, LANL identified an alternative methodology to achieve our technical objectives and fully support critical model parameterization. Very-low-altitude unmanned aerial systems (UAS) photogrammetry appeared to satisfy our objectives in lieu of GB LIDAR. The SPE Phase 2 baseline collection was used as a test of this UAS photogrammetric methodology.
Southernmost Africa, with extensive upland geomorphic surfaces, deep canyons, and numerous faults, has long interested geoscientists. A paucity of dates and low rates of background seismicity make it challenging to quantify the pace of landscape change and determine the likelihood and timing of fault movement that could raise and lower parts of the landscape and create associated geohazards.
To infer regional rates of denudation, we measured 10Be in river sediment samples and found that south-central South Africa is eroding ~5 m m.y.−1, a slow erosion rate consistent with those measured in other non-tectonically active areas, including much of southern Africa. To estimate the rate at which extensive, fossil, upland, silcrete-mantled pediment surfaces erode, we measured 10Be and 26Al in exposed quartzite samples. Undeformed upland surfaces are little changed since the Pliocene; some have minimum exposure ages exceeding 2.5 m.y. (median, 1.3 m.y.) and maximum erosion rates of
We directly dated a recent displacement event on the only recognized Quaternary-active fault in South Africa, a fault that displaces both silcrete and the underlying quartzite. The concentrations of 10Be in exposed fault scarp samples are consistent with a 1.5 m displacement occurring ca. 25 ka. Samples from this offset upland surface have lower minimum limiting exposure ages and higher maximum erosion rates than those from undeformed pediment surfaces, consistent with Pleistocene earthquakes and deformation reducing overall landscape stability proximal to the fault zone.
Rates of landscape change on the extensive, stable, silcretized, upland pediment surfaces are an order of magnitude lower than basin-average erosion rates. As isostatic response to regional denudation uplifts the entire landscape at several meters per million years, valleys deepen, isolating stable upland surfaces and creating the spectacular relief for which the region is known.
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