Questions: What percentage of major earthquakes are preceded by at least one foreshock? What size of a foreshock? In what preceding time period can it be considered a foreshock? Within how large an an area can it be considered a foreshock? Is it true, the larger the earthquake, the larger the foreshock(s), if they are to occur at all?

I remember reading something on this board about some small earthquakes that occurred in Santa Monica Bay (?) in the day or days preceding the Northridge quake. How would one determine that they were related (that's not exactly closeby) or was that just coincidence?

I know that for the obverse, it can be said that for any earthquake there is a 5% chance, or 1 in 20, that any earthquake will be followed within a week or so by an aftershock. So I would like to know if any studies have been done in the other direction concerning foreshocks, not aftershocks.

Barbara