Microseismic observations in the Ceres-Tulbagh aftershock zone, Western Cape, South Africa, and their tectonic implications

Abstract

South Africa is considered a stable continental region where earthquakes from a tectonic source are usually of small to moderate magnitude. In September 1969 a local magnitude (ML) 6.3 sinistral strike-slip earthquake occurred near the towns of Ceres and Tulbagh in the Western Cape, South Africa. This is still the largest earthquake in instrumental history in South Africa. During 2012, a temporary seismic array of 15 surface stations was deployed for a 3 month period in the aftershock zone of the 1969 Ceres earthquake. A total of 168 microseismic events could be located within the boundaries of the array, roughly 30 km x 40 km. Travel times of these events were used for coupled hypocenter-velocity inversion to produce a one dimensional velocity model with station corrections. The hypocenters of recorded events were relocated using the velocity model and local magnitude was empirically derived for all 168 events. P- and S-wave velocity ranges from 4.9 km/s to 6.4 km/s and 2.8 km/s to 4.7 km/s, respectively, from the surface down to 12 km depth. The magnitude of the microseisms ranges from -2.2 < ML < 1.6 with a magnitude of completeness of Mc -1.5, and follow a Gutenberg-Richter distribution with a b -value of 0.9. The microseismic events occurred down to a depth of 15 km within a sub-vertical fault zone roughly 4 km wide, striking SE-NW and passing below the towns of Ceres and Tulbagh. Seismic events appear to occur in two clusters ranging from 0 to 5 km and 8 to 12 km depth, respectively, separated by a 4 km along-strike discontinuity in seismic activity. There is good agreement between the orientations of the strike of the surface trace of the 1969 aftershock plane and the strike of the surface trace of the microseismic plane. Microseismic activity is attributed to the reactivation in basement structures of either the Malmesbury Group, or the Namaqua Natal Metamorphic complex from far field stress transfer from the Southwest Indian Ridge. It is proposed, albeit on speculation, that the presence of microseismic activity along the vertically oriented fault zone could be the manifestation of an incipient plate boundary formation.