4.08 - Fault Interaction, Earthquake Stress Changes, and the Evolution of Seismicity

Abstract

When an earthquake occurs there is an overall drop in stress which is then re-established by tectonic loading prior to the next event. However in some regions, close to the earthquake, stress increases. It is for this reason that aftershocks occur and one large earthquake can trigger another. Although earthquakes only relieve shear stress, slip is considered to be controlled by friction and as a result a component of compression stress across the fault is thought to be important as well as the shear traction on the fault. This combination of shear and normal stress is known as the Coulomb failure function (CFF) or simply as the Coulomb stress. Whether Coulomb stress changes act by initiating rupture or allowing rupture, once initiated, to be sustained to create a large event is not known. Whichever interpretation is correct the method successfully predicts a large proportion of the events in aftershock distributions and has been very successful in identifying regions where large events occur. For the Greek–Turkish region 41 out of 49 events of M 6.5 or greater have occurred where the Coulomb stress had been increased by preceeding events and none have occurred where the stress had been reduced. Some progress has been made estimating future earthquake probabilities using time-dependent friction laws. While this has social importance, it will be many years before it can be demonstrated to be scientifically valid for large destructive earthquakes. The Earth is not the homogeneous elastic half-space that Coulomb calculations assume and fluids, viscous flow and other effects may play roles in earthqiuake interactions that are not yet understood. Thus while the Coulomb interaction studies have proved to be very successful, many lines of research remain to explore.