INCREMENTAL STRESS AND EARTHQUAKES

We use the Harvard catalogue of seismic moment tensor solutions to investigate the statistical properties of incremental static stress caused by earthquakes. Using Okada's (1992) program for a point dislocation in a half-space, we compute normal and shear stress on nodal planes for each earthquake in the catalogue, as well as stress invariants in the focal zone of each event. These accumulated stress values are calculated at the location of any future reference earthquake (pre-stress) and then compared with the stress level measured at the same point after the event (post-stress). Comparison of the statistical distributions for pre- and post-stress indicates that the normal stress level has little influence on an earthquake occurrence. On the other hand, the shear stress in the focal zone of an event is significantly higher before an earthquake than after that earthquake (the stress caused by the reference earthquake is not taken into account in this comparison). Earthquakes are more likely to be induced by incremental shear stress if the stress and the moment tensor of an ensuing event are consistent, i.e. the incremental stress has the same sign as the seismic moment tensor of the reference earthquake. Similar results are obtained if we compare the first and second pre- and post-stress invariants: earthquake triggering is not influenced by the average normal stress (first invariant); contrary to that, the average shear stress (second deviatoric invariant) is significantly larger before an earthquake than after it. The distribution of hypocentres from the PDE catalogue confirms the above conclusions: the incremental stress caused by events in the Harvard list and measured at hypocentre locations of PDE events, is higher for the pre-stress shear component, but shows no significant difference for compressional/dilatational stresses. These findings, if interpreted in a typical framework of the Coulomb failure criterion, would suggest that the effective friction coefficient mu is close to zero. Since this effect is observed in various seismotectonic regions for shallow and intermediate earthquakes (with a depth of as much as 300 km), the conventional explanation for the low value of mu-high pore pressure-is less plausible. We conjecture that these results can be explained by the fractal pattern of earthquake fault geometry which is due to the fault self-organization in conditions of high lithostatic and tectonic shear stress.