Don,

I don't know, offhand, about this particular pattern. I would guess it relates to the current subduction north of Cape Mendicino, which angles a bit south as it comes inland. There has also been hypothesized to be peices of slab broken off and attached to the North America (remnant slabs) and hotter places between the remnant slabs (slabless windows).

For this quake:
1988/09/30 00:39:38.09 39.8060 -121.3545 239.93 3.60 Mc 38 336 478 0.91 UW

you should email Steve Malone at UW:
steve@geophys.washington.edu

240 km sounds unlikely as a depth in California, and why would UWash report a quake here?

John