Exceptionally preserved neotectonic fault scarps in SW Namibia record large-magnitude structurally-controlled SCR paleoseismicity

Abstract

Namibia is situated within a stable continental region (SCR), far away from plate boundary zones, and is therefore not expected to be at risk of significant earthquakes; the largest events in the instrumental record have a moment magnitude of 5.5.  Despite this, a paleoseismic fault scarp on the Hebron Fault has been interpreted as indicating much larger events have occurred in the past.  In this study, we demonstrate that a relatively small area of SW Namibia contains four more major neotectonic fault scarps.  These 16-80 km long structures have vertical separations between 0.7-10.2 m and could produce earthquakes of Mw 6.4 or greater.  Some of these scarps are interpreted to have formed through repeated failure of the same segment and they highlight the potential for further seismicity that far exceeds the maximum observed magnitude in the national catalogue. We identify strong structural controls on the location and orientation of these fault ruptures which reactivate N-S and NW-SE trending zones of crustal weakness.  These structures may be driven by E-W extension associated with the distribution of gravitational energy caused by the anomalously high elevation of the Namibian Escarpment.  If this explanation of the driving stresses is correct, these and similarly oriented faults represent a previously unrecognised source of continuing seismic hazard.  The discovery of these major fault scarps suggests that fault studies should be incorporated into seismic hazard analyses of stable Southern Africa as has been done in Australia and other SCR regions. Their apparent spatial clustering also merits further study.  At this point it is not clear if this clustering indicates a region of elevated strain rate (relative the surrounding SCR) or alternatively, an area of exceptional preservation due to a semi-arid climate and extensive calcrete-cemented surficial deposits.