Hi All. The most recent quakes near Laytonville, California are kind of interesting. At least to me anyway. They occurred near the known end of the Maacama Fault and it is this that makes them interesting. Petra and I cross over this fault every time we go to the place she calls Mecca. Come to think of it there is another fault we cross near Squaw Rock. This is the Calloyomi Fault and is a east/west trending fault. You can see the trace of this fault from the highway. The whole area is laced with slides looking for a place to occur.

According to the USGS “A single-fold seismic reflection section constructed from the large-aperture data shows a discontinuous layer of lower-crustal reflectivity that deepens to the east and extends from the coast to the western edge of the Great Valley. The high-reflectivity layer appears to be offset beneath the Maacama (MF) and Bartlett Springs (BSF) fault zones, which suggests that these faults extend through the entire crust and into the upper mantle.”

It has been suggested that in the case of the Maacama Fault this could be the source of the heat that powers the geysers. However there may be another source, which a study shows.

The Bartlett Springs Fault may be the source for the magma chamber under Lake Pillsbury that could erupt as a volcano in 400,000 years. However research by Rice University and others suggest another possible source.

“Research led by Rice University geologists estimates that within 400,000 years a new volcano could erupt in northern California, relatively soon in geologic terms.
The findings suggest that magma located in chambers about 20 kilometers, or 12 miles, into the earth's crust could rise to the surface in the Lake Pillsbury area, about 120 miles north of San Francisco.”

“The research is published in the February issue of Geology in an article titled, "Fluids in the lower crust following Mendocino triple junction migration: Active basaltic intrusion?" Authors include Alan Levander and Timothy Henstock of Rice University, Anne Meltzer and Bruce Beaudoin of Lehigh University, Anne Trehu of Oregon State University (OSU) and Simon Klemperer of Stanford University. The research is part of the Mendocino Triple Junction Seismic Experiment (MTJSE), a National Science Foundation Continental Dynamics project involving Rice, Lehigh, OSU, Stanford and the United States Geological Society, and focusing on the Mendocino Triple Junction, a region where the Pacific, Gorda and North American plates meet and slide past each other in northwestern California.”

"These are the brightest features I have ever seen in the Earth's crust," said Levander, professor of geology and geophysics at Rice and leader of this study, looking at the data from the study. "After the '93 experiment when we had preliminary reports, the first thing we thought was that we found a magma chamber. We knew we had something interesting, but it took the second experiment to actually verify it."

“Levander and the team suggest that the seismic reflections result from a complex system of basalt melt created from upwelling layers of rock after the triple junction moved northward.”

“As the triple junction system moves north, it makes a hole in the mantle, which fills with a partially molten mantle rock including basaltic elements. One of the consequences of the plate geometry in the area is that south of this triple junction region, the base of North America is exposed to the deeper levels of the mantle.”

“There is no intervening shield of partially solidified mantle so this hot rock comes straight up, reaches the base of the crust, and in the process undergoes a phenomenon called decompression melting, producing some volume of basalt, an intermediate rock. This process goes on as this region moves upward and it has left behind a string of volcanoes, all but one of which are extinct. The Clear Lake area, just 50 miles south of Lake Pillsbury, is the last active site, with modern geothermal activity--now used as hot springs and a spa. Moving south, the sites get older and older.”

Two of those older sites may be the Gilroy Hot Springs located to the east of Gilroy, California and the Mercy Hot Springs located in the Little Panoche Valley south of Hollister, California. In the case of the Gilroy Hot Springs it is located just to the east of the Calaveras Fault. In the case of the Mercy Hot Springs they are located just to the north and a little to the east of the Griswold Fault. This by the way is the entrance to the Griswold Canyon, which has the most bizarre landscape and rock formations I have ever seen. There are ridges of serpentine that appear to have been pushed up out of the earth. A little to the south there are rock ridges that remind me of the mud balls I use to make when I was kid. I have no idea what they are comprised of. I don’t think they are lava as there is no indication of flow layering and I have never seen any volcanic bombs that look like these do.

I think I may have gotten carried away a little bit here. Okay, so I more then gotten carried away. Anyway hope you enjoy. Take Care…Don in creepy town

• Lake Pillsbury