SEISMIC EVIDENCE FOR AN EARTHQUAKE NUCLEATION PHASE

Near-source observations show that earthquakes initiate with a distinc tive seismic nucleation phase that is characterized by a low rate of m oment release relative to the rest of the event. This phase was observ ed for the 30 earthquakes having moment magnitudes 2.6 to 8.1, and the size and duration of this phase scale with the eventual size of the e arthquake. During the nucleation phase, moment release was irregular a nd appears to have been confined to a limited region of the fault. It was characteristically followed by quadratic growth in the moment rate as rupture began to propagate away from the nucleation zone. These ob servations suggest that the nucleation process exerts a strong influen ce on the size of the eventual earthquake.