Hi Chris. Being as how the quake is almost a pure strike-slip it most likely occurred on the Green Valley fault also known as the Concord/Green Valley fault. There is another fault in the area, which is the Soda Creek fault, but it is almost due north of Napa. If the quake has been located correctly it occurred to far to the east to be associated with that fault. Take Care…Don in creepy town

From USGS hazard program.
“Although the overall zone is highly complex and includes some significant active secondary traces such as the Cordelia fault [Harlan Tait Associates, 1994], most of the working group felt that the Concord-Green Valley fault zone is likely to fail in one major event, but a few were concerned that a stepover capable of stopping ruptures might exist under Suisun Bay. A single event model (C34, Table A-1) involves a Mw6.9 every 180 yr, but the opposite alternative of independent Concord (C3) and Green Valley (C4) fault events would produce a Mw6.5 every 110 yr and a Mw6.7 every 150 yr respectively. In view of the absence of events on the two faults in the ~150+ yr historic record, the latter alternative seems somewhat suspect. The model given in our database is an average of these two opposing assumptions. Applying a Gutenberg-Richter distribution using a single segment model (C34) for maximum magnitude Mw6.9 every 180 yr achieves a similar result to our model [Frankel and others, 1996; Petersen and others, 1996a, b].”