Implications of Geodetic Strain Rate for Future Earthquakes, with a Five-Year Forecast of M5 Earthquakes in Southern California

We construct an earthquake likelihood model based on the hypothesis that earthquake frequency and magnitude distribution are related to geodetic strain rate in two ways: a) seismicity rate is steady and proportional to the average horizontal maximum shear strain rate during interseismic time period between large earthquakes; and b) earthquake magnitude distribution is spatially invariant except for an amplitude constant which is proportional to the maximum horizontal shear strain rate. We use geodetically determined maximum shear strain rate averaged from 1993-2001 to "forecast" earthquakes of M ≥ 5, and compare strain rate with the earthquake occurrence rate. In a preliminary test, there were few earthquakes within the top 10% of the area with highest strain rate, but most of the earthquakes did occur in the next 15% of the region with highest strain rate. About 75% of the earthquakes occurred at ~25% area with highest strain rate. Therefore our preliminary test of the method yields a cautiously promising result. A new forecast for the next 5 years starting January 1, 2006 is presented.