Roger,
I have read articles that the barometric pressure which is manifested by the "earthquake weather" is the possible triggering mechenism. I have looked for the article and have had no luck. I believe it was done by the Cal. Division of Mines and Geology. The study indicated that from November to something like April, "Earthquake weather" happened some 60% of the time for quakes. I assume that these quakes are at least 5.0 or greater otherwise it makes no sense. The big San Fernando quakes (1971, 1994)were all in the first two months of the year and preceeded by warmer than usual weather.
Bob in Woodland Hills