Don, 40 deg N is getting almost to Mendocino. I don't know the angle that the edge of the subducted slab extends inland, but these might not be so far from it. So, these could be quakes near the edge of the slab. I don't know the area, but could it also be volcanic? I think that Sutter Buttes in the northern central valley is a relatively young volcano (??). Check if the locations are good, and then look for where the edge of the slab should be (I have to look up slab gap vs slab window-I forget which is which....).
Chris