Space-Time Clustering and Correlations of Major Earthquakes
  Earthquake occurrence in nature is thought to result from correlated elastic stresses, leading to clustering in space and time. We show that occurrence of major earthquakes in California correlates with time intervals when fluctuations in small earthquakes are suppressed relative to the long term average. We estimate a probability of less than 1% that this coincidence is due to random clustering. 
James R. Holliday, John B. Rundle, Donald L. Turcotte, William Klein,   and Kristy F. Tiampo