Rayleigh wave group-velocity maps from ambient noise tomography near Leeu Gamka, Karoo, South Africa

Leeu Gamka is located within a stable continental region, in the hinterland of the Cambrian-Ordovician Cape Fold Belt, which runs parallel to the southern coast of South Africa. Following a swarm of moderate-low magnitude seismicity in the area between 2007 and 2013, documented in the ISC catalogue, researchers from the University of Cape Town deployed an array of 23 geophones between March and June 2015, for the purpose of more precisely locating further events. Although there is no evidence of a fault at the surface, microseismic epicenters aligned along a NW orientation suggest that there may be movement along a blind fault of the same orientation. The anomalous occurrence of earthquakes far removed from an active plate boundary may help improving our understanding of earthquake mechanisms and hazard, while the location of a blind fault may be useful to shale gas exploration in the area.

Potential interest in the Leeu Gamka seismicity and the prospective blind fault motivated further investigation, especially as they occur in a region which has been identified for shale gas exploration. The data used for locating earthquakes was reused to calculate Rayleigh wave group velocity maps. Although the network design was originally optimized for locating earthquakes, with a higher station-density in the centre of the network, a minimum inter-station-distance of 2 km and a maximum inter-station-distance of 60 km, usable Rayleigh wave group velocity maps were obtained.

Our preliminary findings suggest that there is an increase in Rayleigh wave group velocities southeast of a linear feature with a similar orientation and location as the previously located earthquakes. This abrupt lateral change in velocity is interpreted to be a consequence of thick quartzite formations of the Cape Supergroup, with high Rayleigh wave group velocities, having been thrust upwards during the Cape Orogeny juxtaposing them against the lower Rayleigh wave group velocity shale, siltstone, sandstone and diamictite, of the Karoo Supergroup. In this model, the measured earthquakes are most likely a reactivation of an older thrust fault which was active during the Cape Orogeny, but after the deposition of the lowermost Karoo units. This interpretation is consistent with the interpretation of Stankiewicz et al. (2007) who suggested the existence of a blind fault in the area based on the interpretation of a wide-angle seismic refraction line which passes through the study area.

This interpretation highlights the potential risk of the reactivation of blind faults associated with the Cape Orogeny if shale gas extraction and associated wastewater disposal were to proceed. Ambient noise tomography presents a low-cost way both to map the depth of the base of the Karoo Supergroup, and to identify some potentially seismogenic faults in the region, supporting both exploration and associated hazard identification and mitigation.