Petra...I sometimes qualify my posts by saying that I am not an expert...but in fact I am an expert on certain parts of the Borderland, and know something about all of the U.S part of it. The Borderland (not the singular...it is one vast region) extends from half way down Baja north to just south of the northern Channel Islands..(some people include those islands and Santa Barbara Channel, but I don't). It extends from the coast to the deep Pacific Sea floor at the base of Patton escarpment (get out the fault map of California...or check on line). I don't know which earthquake you are referring to...you have to give a Lat Long (but you can figure it out yourself from the info above and a map....but my guess is it was in the middle of the Borderland
Hmm...I'm not aware of a historical quake larger than about M6 within what I define as the Borderland, but there must have been some (Mark Legg would know better). The 1812 Santa Barbara region quake was well over a M7 (was 13 days after a San Andreas quake). There is a lot of controversy where that was....it is known from a trench (Dolan, Rockwell, others) that there was a very large quake on the San Cayetano fault in the last 350 years or so (I visited the trench...the slip was something like 5 meters in the trench). The authors, or some of them, think it was 1812....but I suspect it was near Santa Rosa Island.....near the edge of the borderland. I posted a few weeks ago that perhaps there could be a megaquake...a M8 or so...that would be beneath a large part of the northern Borderland.

Earthquakes in the Borderland have nothing to do with current seafloor spreading directly. They have everything to so with northwest movement of the Pacific plate, as do most of the earthquakes in California.
Chris