Four Magnitude-3.4 and larger quakes have struck in two hours, all south of Cape Mendocino, and west of the town of Petrolia, California. Based on the earthquake locations and their focal mechanisms (both by the USGS), the swarm appears to be occurring on a right-lateral reverse fault that connects the northernmost San Andreas-Mendocino Fracture Zone with the shallow portion of the Cascadia Megathrust (as shown by the black half- arrows in the map).
This area has highest quake rate in all of California (http://temblor.net/earthquake-insights/m6-5-earthquake-stresses-offshore-san-andreas-fault-aka-mendocino-fault-zone-1917/). That’s because the two fastest-slipping faults in the western U.S. join, or nearly join, here. It’s also because the bend in both the San Andreas and Cascadia Faults mean that the crust here is being crushed, distorted, and fractured. This distortion cannot be accommodated solely along the great faults, and so there is a plethora of small faults like this one that distribute the stress.
What this event means for the Cascadia and San Andreas Faults is unknown, but it is certainly a swarm to watch.