Hi Chris. The Hayward fault is sort of like two faults in one. There is the northern segment and the southern segment.

The segment of the Hayward fault from San Pablo Bay south to the border between Berkeley and Oakland is referred to as the northern Hayward fault, which may connect under the bay with the Rogers Creek fault that runs through Napa County. The segment of the fault at the same border south to Fremont is the southern segment. It is this segment that is felt could have a large quake.

Roland Bürgmann assistant professor of geology and geophysics set out several years ago to clarify the confusing history of earthquake activity along the northern Hayward fault. If, as trenching evidence suggests, the northern segment was the site of a major quake sometime between the mid-1600s and the arrival of Spanish colonists in 1776, why hasn't another quake occurred since then? With the help of radar interferometry and data from global positioning satellites, plus analysis of repeating microquakes six miles below the surface it was determined that the deep portions of the northern segment steadily slip at about the same rate as the surface does. Putting all this information together, he estimated that the northern Hayward fault slips underground at a rate of about 5 to 7 millimeters per year, essentially the same rate as at the surface. The similar rates indicate that the fault is slipping freely without locking. This means the rocks deep below the surface aren't locked and building up strain that could be released in a catastrophic quake.

Assuming (I hate that word) that the estimated creep of 5 to 7mm a year is correct then the likelihood of the Rodgers Creek fault being connected to the Hayward fault is false, or the southern segment of the Rodgers Creek fault is locked tight as very little to no creep is being recorded there. In December of 1981 there was a small swarm of quakes about 1 to 2 miles west of the fault near Petaluma. The largest quake in that swarm was about M>3.4. From there south to the San Pablo Bay there have been almost no quakes at all. Measurements at the surface done by the University of Stanford show no movement at the surface. Their measurements are taken from a point near Sonoma Mountain across the valley to a point on the west side of the fault. I have no idea as to where that point is located in regards to some thrust faults on the west side of the valley. Very few quakes have been recorded in the area of the thrust faults. It isn’t until you cross the foothills into the area of the San Andreas fault that you see quakes. However when I plotted the quakes on a topo map I found that they followed a east/west line as opposed to a north/south line you would expect to see if the quakes were associated with the San Andreas fault. No east/west fault is shown on the 1994 fault map in that area.

When Petra took me to her study area one the first things I saw were a lot of sag ponds. The second thing I saw was that the whole area was a slide looking for a place to happen. The other interesting thing I saw was that the area had volcanic material all over the place. This ranged from rocks formed by flow layering to volcanic bombs to volcanic glass, which by the way is almost like sand. Down the hill from her study area is a small creek. I found some cinnabar but have no idea if they had mercury in them or not.

The Rodgers Creek fault is my first choice for the next large to major quake in the Bay Area. Of course like everything else the other faults in the area may not agree with that. Take Care…Don in creepy town