Sunday, November 19, 2017

An Iconic Bit of the Calaveras Fault in Hollister is Gone (But it will be back)

Corner of Locust and Central Avenues in 2016
I go on field studies trips year after year, and my camera is always around my neck, to the amusement of my students. They sometimes wonder why I would take pictures of things I already have dozens of pictures already. I sometimes wonder the same thing when I am trying to track down a specific shot from my archives of tens of thousands of pictures. But for a teaching geologist, there is a very good reason:

Things change.

Sometimes it is sudden, like a flood in Yosemite or on the Tuolumne River that actually alters the look of a landscape. It's nice to be able to catalog before-and-after views of a place. But in others, it is because of the incremental geologic changes. That's especially true with a couple of faults in Central California, the San Andreas and the Calaveras.
Corner of Locust and Central Avenues in 2013
Yesterday I updated the spot on Highway 25 near Pinnacles National Park where the San Andreas fault crosses the road (and I deeply appreciate the widespread response, especially on Twitter). The spot visibly changes every year. For decades, geologists have also been tracking the creeping of the Calaveras Fault in downtown Hollister. Generations of geology class field trips have walked several city blocks, tracking the fault as it offsets streets, sidewalks, curbs...and houses! I hope that all who do so remember to stay on sidewalks and not become nuisances to the residents. They have enough to contend with when you think about it.
The corner of Locust and Central Avenues in 2001

One curb has been iconic; it's been an illustration in any number of textbooks and PowerPoint presentations. The corner of Locust and Central Avenues is offset by the Calaveras Fault adjacent to the crosswalk, so it can be observed easily without bothering residents. It's one of the most vivid examples of right lateral offset imaginable, and if one knows the age of the sidewalk, the changes can be used to calculate the yearly rate of movement on the fault. The break is not a perfect measure because the deformation is spread out for several yards on both sides of the break (note how the sidewalk is curved in the pictures above).

So you can imagine my surprise on Saturday to find that the iconic curb disappeared sometime last year. It was for a good reason, as the city put in a wheelchair ramp, but it was still a shock. I was disappointed for a moment for my students until I realized that for the first time in 15 years, we have a brand new baseline of fault movement. Because as surely as the curb was offset before, it will continue into the future. We'll be watching for the first of the tensional cracks in the concrete, eventually to be followed by total rupture and offset curbs.
Corner of Locust and Central in 2017
Geology never stops.

1 comment:

Anonymous said...

Garry, based on your Twitter link I ended up reading a bit about the Pallet Creek excavations and paleoseismology, if I got that right. I understand the average of 137 years or so between major earthquakes on the SA fault in the vicinity. What I don't see is a measure of the variability in that average. I use CVs or RSDs to describe averages at work. Others use confidence intervals. I'd like to know how to interpret a 20 year or so delay in a major earthquake on the SA. Within expectations? A 100 year delay? Thanks in advance.