The concept of 'critical earthquakes' applied to mine rockbursts with time-to-failure analysis

We report new tests, performed on rockbursts in deep South African mines, of the concept that large earthquakes are 'critical points'. We extend the concept of an optimal time and space correlation region and test it on the eight main shocks of our catalogue, provided by the South African company ISS International. In a first test, we use the simplest signature of criticality in terms of a power-law time-to-failure formula. In spite of the fact that the search for the optimal correlation size is performed with this simple power law, we find evidence both for accelerated seismicity and for the presence of log-periodic behaviour in the cumulative Benioff strain with a preferred scaling factor close to 2. We then propose a new algorithm based on a space and time smoothing procedure, which is also intended to account for the finite range and finite duration of mechanical interactions between events. This new algorithm provides a much more robust and efficient construction of the optimal correlation region, which allows us to use the log-periodic formula directly in the search process. In this preliminary work, we have only tested the new algorithm on the largest event in the catalogue. The result is of remarkably good quality, with a dramatic improvement in accuracy and robustness. This confirms the potential importance of log-periodic signals. Our study opens the road for an efficient implementation of a systematic testing procedure of real-time predictions.