Wednesday, November 24, 2010

California Has Her Faults; Here's One of Them...

I didn't think it would take long for the geologists among us to figure out that we looking at a fault rupture in yesterday's "minor mystery photo", and I wasn't disappointed. These en echelon cracks are called riedel shears, and are the first cracks that appear as a solid material is being put under (in this case) lateral stress. There was no earthquake; this is a section of the fault that is creeping, meaning that it is moving constantly without building up enough stress to produce large earthquakes.

This isn't just any fault; it is the fault in California, the San Andreas, which forms part of the boundary between the Pacific and North American plates. The section we visited on Saturday is between the town of Hollister and Pinnacles National Monument, south of the 200 mile long rupture from the 1906 quake in San Francisco. How much does the fault move every year?
A clue is provided by a shot I took in 2008, immediately following the road repairs. No damage was yet visible, and none in 2009 either. I was a little disappointed in 2008 to see that the road was fixed, as it took away one of the more visible bits of evidence of fault motion. The picture below is how the road appeared in 2002. The first time I saw the fault crossing in the 1990's, the fracture was a straight break crossing the highway, with a big bump. Thus, it's been repaired at least twice in twenty years

Without human developments, the year-to-year movement would be hard to see because the subtle effects are obscured by weathering and erosion. In the pictures below (about 8 miles south of our damaged highway) the old fencing shows several feet of offset.

Probably the most famous evidence of movement on the creeping segment of the San Andreas is the De Rosa Winery warehouse (formerly the Almaden Winery). The winery was first constructed in the 1800's and has been rebuilt twice after the building foundations failed. The current building is also being ripped apart, along with a culvert that has been photographed many times by visiting geologists (thrilled, no doubt, by the fact that they can combine their field research with wine-tasting!). The winery owners study geologists too; in this article, they are quoted as saying "Their pants always have zippered pockets...The khaki pants and the funny hat, and you know it's a geologist."

I am traveling this Thanksgiving, like so many others, and will have infrequent web access (my phone is only semi-smart and only makes phone calls). I hope you all have a fine holiday, and that you have many things to be thankful for. Below is one dinosaur descendant that is feeling thankful that it won't be eaten this holiday, as it lives in Pinnacles National Monument and enjoys protected status.
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