There were several foreshocks before the Hector Mines event -- one less than an hour before. They were exactly on the same spot which became the epicenter of the mainshock. I think this is fairly unusual. Of course, a foreshock isn't much help since there are lots of small events the great majority of which are not followed by main shocks. We never know if an event is a foreshock until later.

To see the foreshocks and aftershocks, fetch the Seismic/Eruption program from my Web page (Windows only): www.geol.binghamton.edu/faculty/jones
and click on the Southern California view.

Alan Jones, SUNY Binghamton