[S02-1] World overview of anthropogenic seismicity I

[S02-1-04] State of the art in 3D reflection seismic interpretation: New insights into a complex structural architecture in the vicinity of Orkney M5.5 event, South Africa

^○Musa Manzi¹, Hiroyuki Ogasawara², Raymond Durrheim¹, Hiroshi Ogasawara², Tullis Onstott³, Artur Cichowicz⁴ (1.University of the Witwatersrand, Johannesburg, South Africa, 2.Ritsumeikan University, Kusatsu, Japan, 3.Princeton University, New Jersey, USA, 4.CGS, Pretoria, South Africa)

In August 2014, the M5.5 earthquake occurred in the Precambrian rocks of the Witwatersrand Basin, South Africa. The M5.5 mechanism was strike-slip faulting, which is different to a typical mine induced earthquake with normal-faulting. It is suggested that M5.5 was caused by dyke several hundreds of meters below the mining horizon. To better understand the structural complexity associated with the M5.5 and its aftershocks, we have processed and interpreted the high-resolution 3D reflection seismic data that were acquired in 2012 for gold exploration. A number of aftershocks fall within the 3D seismic volume providing unique opportunity to directly correlate structures mapped by seismics with the location of the aftershocks. It is envisioned that good seismic mapping of the shale units (4 - 7 km depths) and their associated structures would help to better understand the region's structural framework and to constrain the timing of activity of the faults.

The main outcome of the seismic interpretation has been an improved understanding of the complex structural architecture in the area. In particular, seismics has mapped normal (500 m maximum throw), north-northeast trending major faults and their subordinate structures. Faults are listric in form; dipping relatively steeply (65 - 70 deg.) at 1.5 - 2.5 km below the surface. At greater depths (4 - 5 km), below mining horizons, the dip decreases to about 40 to 50 deg. The seismic sections also exhibit the near-vertical features characterized by high amplitude attenuation associated with fault zones. These features crosscut, but do not displace, the entire stratigraphy of the basin. We interpret these near-vertical features as dykes with thickness from 1 m to 30 m, which may be possible sources of the M5.5 event.

It is, therefore, important to integrate the reflection seismic interpretation with the seismicity data so as to better understand the geometric nature of the structures associated with the M5.5 event.