Earthquakes begin without any obvious sign of their coming. Two hypotheses have been advanced to describe the underlying physical processes that lead to their nucleation. These are the preslip and cascade models which take opposing views on the role of aseismic deformation in the nucleation process as well as the prospects for prediction. In this paper, foreshocks, the small earthquakes that sometimes precede larger ones, and the initial seismic wave radiation of the earthquake itself are used to examine how earthquakes nucleate and grow. Earthquake are found to begin abruptly and grow irregularly, consistent with laboratory-derived predictions for limited aseismic slip and in agreement with the predictions of the cascade model.
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