Small Stress Change Triggering a Big Earthquake: A Test of theCritical Point Hypothesis for Earthquakes

Whether or not a small stress change can trigger a big earthquake is one of the most important problems related to the critical point hypothesis for earthquakes. We investigate global earthquakes with different focal mechanisms which have different levels of ambient shear stress. This ambient stress level is the stress level required by the earthquakes for their occurrence. Earthquake pairs are studied to see whether the occurrence of the preceding event encourages the occurrence of the succeeding one in terms of the Coulomb stress triggering. It is observed that the stress triggering effect produced by the change of Coulomb failure stress in the same order of magnitudes, about 10^-2MPa, is distinctly different for different focal mechanisms, and thus for different ambient stress levels. For non-strike-slip earthquakes with a relatively low ambient stress level, the triggering effect is more evident, while for strike-slip earthquakes with a relatively high ambient stress level, there is no evident triggering effect. This water level test provides an observational support to the critical point hypothesis for earthquakes.