Back in the late 80s and/or early 90s, there was a lot of talk about the Anza seismic gap: the one part of the San Jacinto fault that had not failed in a long time, while most of the rest of the San Jacinto failed in pieces during the 20th centuray. As far as I know, it still has nor failed, and if today's quake holds as less than 6, it will not fill the gap. I forget how big a quake would be needed to fill the gap (or, in other words, how many km of fault is unbroken), but I would guess it would be a high M6 or about a 7...
There must be more known about this in the last 15 years.....
Todays quake had a foreshock a few days ago...would not amaze me if this was a foreshock....but I guess this is historically the case only 5 or 10% of the time...
Chris