I totally agree with Don's theory.

In response to Michael, I can tell you that what Don described above seemed to take place before Loma Prieta. I was living in the Bay Area at the time and remember it like yesterday. Starting in 1986-87 the bay area saw a definete increase in seismicity. It started with that 5+ in around the Fremont area in March or April of 86. To this day I've never heard a good explanation for what fault that one was on, we just know it didn't involve to Calaveras or Hayward, this one could be compared to the Yountville quake of September. Then in 87 through 88 there were 4 pointers every 2 or 3 months and one or two other 5+, mostly on the Calaveras (but all in the S. bay region.) then in 88 we had that 5.6 foreshock in the Santa Cruz mountains which had no aftershocks, looking back now that was a total tell tale sign, then in August 89 another 5.2 foreshock and then boom, Loma Prieta and then quiet for about 10 years, and now I agree with Don and Petra, this pattern appears to be begining up in the N. Bay. There were about four or five 5+ and four or five 4+ quakes leading up to Loma Prieta, ALL in the S. Bay area. So far the N. bay has seen two 5+ and one four and a couple of threes.