Is the subject an expression, or did I make it up? There was a M4.1 south of Santa Rosa Island. This on the page that Petra linked to makes a rather odd string of roughly evenly-spaced quakes. I linked to the historical seismicity of the USGS-NEIS page. It is farthest west quake at that latitude. It is south of the fault system responsible for the islands (Channel Islands thrust and west continuations), and since that fault system dips north, it is not on it.
If located at all accurately, it is west of the Ferello fault (also called the Santa Rosa Ridge-Cortes Bank fault). Since that fault dips east where I have seen it imaged, and it must dip east to be responsible for the giant anticline, it is not on that fault either. Instead, it is located near the Patton Ridge fault zone. We have reason to think this is a rather active right-lateral fault. Wife in suburbia and I have proposed several times to study it...to NOAA and then to NSF. We got 3 excellents on the first round of reviews with NSF, but the panel was less impressed...and each time we resubmitted, oddly, the reviews were worse. So, we gave up for now. And, this is for a survey of a region with 10% of the plate motion about which we know next to nothing.

Just by its length, this fault system is probably capable of a M7.5. But, the brittle crust may not be thick enough out there for such a quake...wait, I'm guessing...as I said, we know nothing (see Crouch 1981 for paper on this area).
Chris

• Outer Borderland quake