5-16. Semi-controlled experiments in South Africa Gold mines

 

　The nucleation process of earthquakes is the most plausible among anomalous processes that can occur just before the occurrence of earthquakes. It has been investigated by friction experiments in laboratories.  However, there is no direct observation of the nucleation process of natural earthquakes. It is pointed out the possibility that the size of the nucleation is not large enough to be recorded by ordinary observations.

  In gold mines in South Africa, earthquakes up to Magnitude 3 (a source size of about 100 m) occur on geological faults resulting from stress concentration by excavations. We can observe earthquakes within a distance of a few to a few tens of meters by sensors installed in boreholes drilled into stress concentration regions. The excavations cause stress buildup, change properties of rocks and microearthquake activities, and at last generate the nucleation process of earthquakes. It is very important to observe these processes by various kinds of sensors, e.g., seismometers, strain meters, and to clarify the nucleation process.

The semi-controlled experiments in South Africa Gold mines have been carried out under cooperation with university researchers. We detected a change in shear stress in the focal region of M2 event that occurred at a distance of about 100 m from the network. The stress change was estimated from changes in b-value and stress drop. Further, an analysis suggested that the ratio of seismic energy to the seismic moment decreased just before the M2 event. If the analysis is valid, it is possible that we detected a stress decrease before the M2 event and that the decrease reflected the nucleation process of the M2 event.

A swarm activity including a M3 event occurred within a distance of about 100 m from a strain meter installed in the Banbanani mine. Unfortunately, no data were transmitted to the surface recording station due to a trouble of data communication when the M3 event occurred. Even about one month a later, however, we detected a remarkable after effect of the event.