Microseismic Activity and Basement Controls on an Active Intraplate Strike‐Slip Fault, Ceres–Tulbagh, South Africa

Abstract

Earthquakes far from plate boundaries are poorly understood, because of few well‐studied examples and uncertainty of what controls the location of these events. In 1969 a damaging local magnitude (ML) 6.3 strike‐slip earthquake, with no surface expression, occurred in the Ceres–Tulbagh region in the South African stable continental interior. Here, we present a microseismic study of the Ceres–Tulbagh area, conducted over three months in 2012, in which 172 events recorded on at least three stations follow a Gutenberg–Richter relationship for −1.5<ML<0.5. The events delineate a 5 km wide, subvertical zone that is microseismically active to a depth of 15 km. This fault zone is subparallel to the 1969 aftershock zone and at low angle to the regionally inferred greatest horizontal stress. We argue that the microseismically active zone is guided by inherited structures in the basement geology. This and similar structures may represent significant earthquake hazard in plate interiors.