One of the things I did not catch in the web news article is that to synchronize the faults must have very similar slip rates: see a clip from an email I just sent to some people I work with below:

Scholtz in the last issue of BSSA: interesting. The Inner Borderland right-lateral faults may well fit the description of faults that can become synchronized because they may have comparable slip rates, at least along parts of their strikes (N-I-Rose Canyon about 2 mm/yr, probably 1 mm for Carlsbad where it is strike-slip, long-term, 2 mm/yr is reasonable guess for San Diego trough fault, and, if GPS loading is telling us anything (and it may not be), San Clemente fault does not have a lot of present day accumulation left over to match how the islands are moving, so could be in same range even if more important fault?

**added for this post: nothing is known about past earthquakes on the offshore parts of these faults, beyond the quakes on them the last 80 years or so, which generally have been smaller than M6 (some about M6?) except 1933 Newport Inglewood. I may be wrong on that....

Chris