A palaeoseismic trench investigation of early Holocene neotectonic faulting along the Kango Fault, southern Cape Fold Belt, South Africa– part II: earthquake parameters

Abstract

The location and estimated magnitude of a recent surface rupturing earthquake along the eastern part of the Kango Fault traversing the southern Cape Fold Belt, South Africa, is reported here, as well as the minimum recurrence interval and maximum slip rate. These palaeoseismic data are derived from analysis of the mapped logs of an 82 m long, 5 m deep trench excavated across the fault, supported by 12 optically stimulated luminescence (OSL) dates. The tectonic event that formed the observed 2.0 ± 0.06 m free-standing fault scarp at the trench site had a moment magnitude estimated between 6.97 ± 0.01 M and 7.18 ± 0.01 M. According to the recently established Environmental Scale of Intensity, the rupture was devastating (ESI Io = XI). It would have been accompanied by strong ground shaking felt across most of South Africa by the ancestors of the indigenous people groups who experienced the well-known 6.3 M Ceres-Tulbagh event of 29 September 1969 (currently South Africa’s largest and most damaging earthquake). This latter strike-slip event occurred along the western end of the same Ceres-Kango-Baviaanskloof-Coega (CKBC) fault system, but was not accompanied by significant surface deformation. In contrast, the Kango ‘Toorwater’ earthquake reported here displaced the land surface vertically, by over 2 m in places. While the fault scarp extends laterally for at least 84 km, from near the town of De Rust, east of Oudtshoorn, towards the start of the Baviaanskloof in the Eastern Cape, it is unlikely the entire surface rupture length originated during only the most recent event. The palaeoseismic data reported here have contributed to a seismic source characterization model used to update the existing seismotectonic model for South Africa, first established in the mid-1990’s, and revised at various intervals since. The presence of a potentially active, capable, seismogenic structure within a low-seismicity intraplate stable cratonic region suggests that the current 5.3 to 6.3 M m max for the south and south eastern Cape must be reviewed, should these data be included in a revised Probabilistic Sesimic Hazard Assessment (PSHA) for critical structures in the region.