A Comprehensive Search for Tidal Triggering of Southern California Earthquakes

Tidal forces cause crustal stress changes that might be expected to trigger earthquakes but previous analyses searching for tidal periodicities in earthquake catalogs have yielded generally negative or inconclusive results. This study examines the Caltech/USGS southern California earthquake catalog from 1932 to 2003, which includes 429,886 events. We examine the data for any correlation between event occurrence and lunar phase by plotting histograms of event count versus phase as well as simple plots of event phase versus time. We analyze both the complete catalog and specified subsets of the data, including grouping by magnitude and depth. We also refine our data spatially by binning events in individual 0.5 by 0.5 degree latitude/longitude bins. Finally, we examine over 900 individual similar event clusters containing 50 or more events as defined by cross-correlation analysis. These clusters typically are less than 3 km across and include earthquakes that likely to have similar focal mechanisms and sensitivity to tidal phase. However, none of our results find a significant correlation between tidal forces and seismicity, suggesting that tidal forces do not contribute significantly to earthquake occurrence times anywhere in southern California.