Hi All. As most of you know Petra and I have been doing quite a bit of research on the Rodgers Creek fault. I do the research, but if it was not for Petra bringing it to my attention and then being the heartless task master that she is I would not have found the data that has been useful to us in other ways. She deserves the credit. Not me.

Sometimes I come across things that at the time appears not to fit into the area of what were searching for, but I do set it to one side for a later look.

What I found was that the 1957 Daly City earthquake had a very similar mechanism to the Bolinas event, and the Loma Prieta earthquakes. Both were a mixture of strike-slip and reverse-slip motion.

Digging a little further I found that the Rohnert Park and Yountville quakes showed the same type of motion.

The Bolinas earthquake occurred along a section of the San Andreas fault that has almost been devoid of earthquakes since the 1906 earthquake. There is one other section of the San Andreas fault that is showing an increase in activity. This is in the area of Pacifica, which is located across from Bolinas on the other side of the bay entrance.

The October 1969 Santa Rosa earthquakes were on the Rodgers Creek fault and were not anomalous. Rather they reflect the historical record of repeated moderately strong earthquakes and earthquake sequences in the region. The zone of activity just north of town seems to be a region of concentrated moderately deep shocks. The larger earthquakes in this region seem to have minor aftershock sequences a phenomenon probably related to the concentrated source region and to the depth of the shocks. The mechanisms of the earthquakes appear to be right lateral slip on steeply dipping fault planes roughly parallel to the Healdsburg fault to the northwest or the Hayward fault to the southeast. This mechanism seems characteristic of earthquakes in the Coast Ranges east of the San Andreas fault in the Bay Area and northward. (Note northward.)

Just recently I requested a map of the Rodgers Creek fault. What I received was beyond what I had hoped for. On the map it showed where trenches have been dug for the purpose of learning the history of the southern portion of Rodgers Creek fault. What they found was that there have been 3 earthquakes in the last 1000 years for a total offset of 15 feet. There is a remote possibility that the last one was the Mare Island 6.5Md quake on 03/31/1898.

USGS has just recently gave the Rodgers Creek fault a 32% chance of having a major quake in the next 30 years. This puts it at the very top of the list and above all of the other Bay Area faults.

On 02/19/01 there was a 1.7Md quake 5 miles NE of Rohnert Park. This is the location of the 4.2Ml quake on 09/22/99. This is also the first quake to occur on the southern portion of Rodgers Creek fault since the Rohnert Park quake and its aftershocks. This quake is most likely an aftershock, but it may also be an indication that the southern portion of the Rodgers Creek fault is starting to move. Take Care…Don in creepy town.