In a brief interview segment that I saw broadcast twice on KPIX-TV (San Francisco) news, David Schwartz of USGS said: 'Had this been a magnitude 5, we would have been a little more concerned. At a 5 level, we go to estimating the likelihood that the 5 will be followed by something bigger. But this 3.9 falls below that threshold."

Why is a magnitude 5 earthquake considered more likely to precede a larger event than a magnitude 3.9? Did I misunderstand his statement?

Mary C.