ABSTRACT: The concept of Coulomb Stress Triggering has been used widely in crustal seismology to gain insight into the probability to experience earthquakes, following the occurrence of historical earthquakes. The key idea is to estimate changes in shear- and normal- stress on faults. Such estimates plus assumptions regarding internal strengths along fault contacts are used to calculate so-called Coulomb Failure Stress (CFS). Published results indicate that the majority of aftershocks fall within areas of increased CFS (typically up to 4 bar), while earthquakes are delayed in areas of decreased CFS. The same concept is in use in the South African underground tabular mining environment, but referred to as Excess Shear Stress (ESS). Average ESS levels as well as increase in ESS due to mining reflect the probability of triggering large events, and hence may be used for mine seismic hazard assessment. One aim of this paper is to relate the concept of ESS to Coulomb Stress triggering, and to consider the relative scales, i.e. magnitude of stress changes, area of influence along geological structures and loading rates. A case study of stress triggering from the tabular underground mining environment is discussed, where input information is known to a fair degree of accuracy, and the main features of the seismic source mechanism is understood.
1 INTRODUCTION
Coulomb stress triggering in earthquake mechanics is concerned with quantifying stress changes along crustal faults, due to stress changes caused by historical earthquakes. This should lead to an advance or delay in time of future earthquakes, either on the same fault or other faults in the area. Although there are indications that dynamic ground motion is an important potential trigger, the starting point is to consider static stress changes.
