A Study on Earthquake Triggering: Solution to the Paradox of Coulomb Stress Increase with Frictional Coefficients

Coulomb stress change calculation has been playing an important part in investigating fault interactions and earthquake triggering. However, the results of most workers showed that Coulomb stress changes (or earthquake triggering effect) would become larger and larger with the increase of apparent frictional coefficients. This phenomenon is clearly in contradiction with our common knowledge in which frictional stress always resists fault slips and inhibits earthquakes under any circumstances. By analyzing the formula for calculating Coulomb stress changes (ΔCFS), we found that previous researchers did not take into account the additional ΔCFS which are only resulted from the variations of frictional coefficients. Suppose the depth of typical receiver fault is 15 km, the value of combined ΔCFS will be as large as ∼39.2 MPa when the variation of apparent friction coefficient is 0.1 (e.g., from 0.3 to 0.4), whereas traditional ΔCFS is only 0.8 MPa. If we incorporated the additional ΔCFS in calculation, the above contradiction will disappear completely. Therefore, it is suggested that we should consider changes of combined ΔCFS due to the variation of friction coefficient, especially when we compare different Coulomb stress models with different apparent frictional coefficients.