Hi All. While seeing what I could find for the Lake Tahoe area I also thought I would see what I could dig up in regards to the 08/17/1999 Bolinas quake.

This quake didn’t answer any questions about future quake activity along the northern segment of the San Andreas fault. It did however generate some questions, which to this day no answers have been found.

The moment tensor solution indicates that the quake was a strike slip with thrusting faulting component.

Information obtained from the earthquake waves indicate an up-down direction of slip not the sideways (or lateral) slip expected for the San Andreas fault.

It occurred in a region that has had only one previous earthquake in the last 30 years of detailed seismic recording.

The first observation was somewhat of a surprise. Much of the 300 mile length of the San Andreas fault that ruptured in 1906 (from San Juan Bautista to Cape Mendocino) is currently aseismic (relatively free of earthquakes) and has been since 1906. Scientists consider the San Andreas fault in Northern California to be in a "locked" part of its earthquake cycle.

Some said that this quake may have been an aftershock of the 1906 quake while others say it may be a foreshock to a much larger quake such as those before the 1989 Loma Prieta quake.

The second observation suggests that, in detail, this earthquake probably was not on the San Andreas fault, but rather occurred on a small fault adjacent to the main fault. This may be the cause of the thrusting component observed during the quake.

About 100 books fell from the library shelves, and all the artwork hanging in illustrator Keith Hansen's gallery went askew.

According to Keith Hansen "All the art did a dance," and that the quake hit with a sharp upwards strike then reverberated a few seconds more. "I've felt all sorts of earthquakes but never felt one from below."

The upward motion described by Keith Hansen is an indication that some thrusting was indeed associated with this quake. If the quake did occur on the San Andreas fault the current thinking for the San Andreas fault is going to have to change in regards to it being a right lateral strike slip fault.

The Loma Prieta quake of 1989 also displayed some thrusting. It may be that the fault has always had some thrusting component to it, or the fault itself is starting to change from to a right lateral strike slip to a thrust fault. Now that would be interesting if that were occurring.

It should also be noted that the Yountville quake on 09/03/2000 also had a just a trace of thrusting as indicated by the moment tensor solution.

The 09/22/1999 Rohnert Park quake was a pure thrust quake and did not occur on the Rodgers Creek fault as first thought. As a matter of fact this quake occurred on a previously unknown blind thrust fault.

Of course all of this history. When viewed as separate quakes they indicate one thing, but when viewed collectively they indicate something else. To my way of thinking there could be some compression going on in this area. As to what extent I have no idea.

With each quake I learn a little more. Now if I could only put it all together I may learn that I have learned a lot more then I thought I have. Then again maybe I have a lot more to learn before I can put it all together. I wonder how long that's going to take? Take Care…Don in creepy town