There have been a wide variety of approaches applied to forecasting earthquakes (Turcotte 1991; Kanamori 2003). These approaches can be divided into two general classes. The first is based on empirical observations of precursory changes. Examples include precursory seismic activity, precursory ground motions, and many others. The second approach is based on statistical patterns of seismicity. Neither approach has been able to provide reliable short-term forecasts (days to months) on a consistent basis.