Stress transfer and seismicity changes before large earthquakes

In recent years, observational and theoretical descriptions of spatio-temporal patterns of seismicity have focused on two fundamental (and controversial) observations: static stress (Coulomb) interactions between earthquakes and accelerating seismic moment release before large earthquakes. While there have been several documented examples of static stress changes influencing the space-time pattern of seismicity following great earthquakes (main shocks and aftershocks), there have been few attempts to link this method to the evolution of seismicity before great earthquakes (precursory seismicity and foreshocks). In this paper, we describe a simple physical model that links static stress modeling to accelerating moment release before a large event. For practical reasons, it is not straightforward to apply this technique as a method of forecasting future large earthquakes. However, after the large event has occurred, the region of stress accumulation can be calculated with precision based on the known source parameters of the earthquake. This region can then be examined for seismic moment rate changes prior to the event. As examples, we have examined all M⩾6.5 earthquakes in California since 1950 in regions defined by their pre-event stress fields, and find a period of accelerating moment release before all of these events. While we illustrate the model using seismicity in California, the technique is general and can be applied to any tectonically active region. Where sufficient knowledge of the regional tectonics exists, this method can be used to augment current techniques for seismic hazard estimation.