Don,
Thank you for your research! From that it looks as if the probability of any earthquake being a foreshock to a larger one depends on its fault and location. That probably was too much for David Schwartz to explain in a brief interview but will help me understand what seismologists say in future interviews following earthquakes. I'll leave the calculating (and speculation) for them and their computers. Meanwhile, my emergency supplies are updated, with help from Petra's preparedness site.

Mary C.