|Corner of Locust and Central Avenues in 2016|
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|
|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|