My house is slowly moving to the Northwest, but one would never notice the shift because the speed –less than a centimeter per year– makes a snail seem like a Ferrari, and besides, everything else in sight is also moving in perfect sync.
You see, I live a few miles east of the San Andreas fault, not far from the now-drowned valley that gave the fault it’s name. The San Andreas is a right-lateral strike-slip fault that also is more or less the boundary between the Pacific Plate and the North American Plate. The Pacific Plate is the side that is moving, grinding inexorably northwestwards, but the friction is also tugging the edge of the North American plate along (imagine rubbing the side of one gummi eraser against another). Thus, Half Moon Bay and the Santa Cruz Mountains to the west are galloping northwest at over twice the rate my side is moving and my neighborhood terrane is being dragged along for the ride. Which is some comfort, as Los Angeles sits on the western side of the fault, which means it is moving inexorably towards my neighborhood. Horrors! Sometime beyond 23 million years from now, LA, Burbank, freeways, the whole smoggy mess will be on my doorstep. But for the northern drag exerted on my side of the fault, LA would arrive in half the time.
Now, I don’t really know how fast my home is moving. I am merely relying on the research findings of geophysicists working in the region. And frankly, that has bothered me. So thanks to my friend and surveying wizard, Hans Haselbach, of Haselbach Instruments, I have begun to document the movement for myself. Hans loaned me a Leica Geosystems GPS 1200 surveying systemand we laid in a grid of reference benchmarks on my property surveyed to an accuracy of 5mm horizontal and 2cm vertical. That’s accurate enough that with annual resurveys I’ll be able to establish the speed and azimuth of my northward drift as my home rafts along the edge of the North American Plate. And after the big one rumbles through, I will be able to say with precision just how how far my property was shoved and where it ended up. Watch this space for updates…
(Hans and the 1200, below)