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

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.