"The short term probabilities are based on the clustering of earthquakes into foreshock-mainshock-aftershock sequences. One of the primary features of earthquake catalogs is that earthquakes cluster like this. Of course,
aftershocks are much more common than foreshocks, and it is hard to tell foreshocks from the more common background (or unclustered) earthquakes. But we have enough historical experience to be able to estimate the odds that a given earthquake is a foreshock which would mean that it would be followed by something larger. I will resist explaining any of the math because this
is Lucy Jones' specialty. Such forecasts are also made for the odds of large aftershocks occurring, and these have been discussed here recently."

Its interesting to see that the USGS can already give probabilities on a quake being a foreshock for a larger quake, but does nothing with it as far as alerting the public.

Dennis