15:30 〜 16:30
[S02-P-05] Geological and velocity structures of the Orkney M5.5 fault, South Africa
In August 2014, the largest seismic event (M5.5) in a South African gold mining district took place near Orkney. It has been a question whether the M5.5 event was natural or mineinduced, and what geological structure was re-activated to cause the event.
It is also known that this earthquake occurred below the mining horizons in anarea characterized by complexgeological structures. However due to no economic interests at these depths, very little geological information is known. To better understand the structural complexity of the area, we have interpred the 2D and 3D reflection seismic data, which were acquired for gold exploration. The 2D/3D reflection seismic data are characterized by strong seismic reflectors, for example, at a dolomite-lava boundary, a lava-quartzite boundary, and quartzite-shale boundary. On the mining horizons from 2 to 3 km depth, the data show high optimum mapping of major fault zones, which correlate well with structures mapped underground. Interestingly, there is less disturbance at the depth of the M5.5 rupture (at least 5 km long horizontally and 3 km wide in the dip direction). It is suggested that a dyke hosted the M5.5 event. The near vertical dyke has been identified by underground mapping, however it is not visible on the seismic data probably due to its thickness being below the seismic detection limit.
It is also very important to calibrate the velocity field below the mining horizons in order to have more accurate absolute locations of targets of scientific interest of the M5.5 fault. To achieve this, we will convert our 3D seismic volume from two-way-travel times to depth using the stacking velocities obtained from the data processing. Subsequently, we will constrain our P-wave seismic velocity calibration using the stacking velocities and exploration borehole data. Following the calibration, the geological structure will be compared with the aftershocks and the mainshock fault slip distribution.
It is also known that this earthquake occurred below the mining horizons in anarea characterized by complexgeological structures. However due to no economic interests at these depths, very little geological information is known. To better understand the structural complexity of the area, we have interpred the 2D and 3D reflection seismic data, which were acquired for gold exploration. The 2D/3D reflection seismic data are characterized by strong seismic reflectors, for example, at a dolomite-lava boundary, a lava-quartzite boundary, and quartzite-shale boundary. On the mining horizons from 2 to 3 km depth, the data show high optimum mapping of major fault zones, which correlate well with structures mapped underground. Interestingly, there is less disturbance at the depth of the M5.5 rupture (at least 5 km long horizontally and 3 km wide in the dip direction). It is suggested that a dyke hosted the M5.5 event. The near vertical dyke has been identified by underground mapping, however it is not visible on the seismic data probably due to its thickness being below the seismic detection limit.
It is also very important to calibrate the velocity field below the mining horizons in order to have more accurate absolute locations of targets of scientific interest of the M5.5 fault. To achieve this, we will convert our 3D seismic volume from two-way-travel times to depth using the stacking velocities obtained from the data processing. Subsequently, we will constrain our P-wave seismic velocity calibration using the stacking velocities and exploration borehole data. Following the calibration, the geological structure will be compared with the aftershocks and the mainshock fault slip distribution.