Hi Steve. I thank you for asking the question about the “first motion mechanism” for the Lake Elsman quakes. Your question compelled me to find the answer. In doing so I may have found the answer to a question I have been asking myself. As to whether or not it’s the correct answer remains to be seen.

Some time ago I was told to watch the area east of the Rodgers Creek fault for signs as to when the next major quake could occur. I wondered about that. The segment of the Rodgers Creek fault between Santa Rosa and San Pablo Bay is locked tight. There is little to no indication of movement anywhere along that turkey. In the past however there have been some small quakes to the east of the fault. One such quake was the Bennett Valley quake. This quake had a magnitude of M>4.2. The focal depth was placed at a depth of 7 km and the fault plane solution indicated reverse-slip motion along a generally east-west trending plane. The quake was centered about 4 miles southeast of Santa Rosa in Bennett Valley east of Taylor Mountain and near the intersection of Bennett Valley and Grange Roads. Looking at the “moment tensor solution” it indicates that the quake was almost a pure thrust quake. That isn’t the motion one would expect to see if the quake was associated with the Rodgers Creek fault.

The Loma Prieta quake occurred because the two quakes at Lake Elsman allowed the San Andreas fault in that area to loosen up just a bit. In other words it was given enough room to move. Up until those two quakes occurred it was being compressed so tightly that it couldn’t move. It just might be possible that the same thing is occurring with the Rodgers Creek fault. Take Care…Don in creepy town