We've been having little earthquakes here in the Bay Area recently. That is, we have them every day but we can't usually feel them, so what I really mean is we've been having earthquakes which rattle the china a little. Last Sunday we had a good one, a 4.4 centered in San Jose, and Tuesday we had a little 1.6-er centered just a couple of miles from my house. Because the San Andreas fault runs right through this part of California we get a lot of shallow earthquakes, the kind that occur 1-10 miles underground. They're momentarily exciting, but they don't typically last very long. I enjoy looking at the USGS-UCB website to see what's been going on in Quakeland USA from time to time.
The quake that hit Seattle yesterday was a deep earthquake, taking place at least 30 miles underground. It lasted a long time, 45 seconds or so, and it was a 6.8 magnitude. Buildings collapsed, highways crumbled, damage was extensive. It was a subductional faulting, meaning the colliding Juan de Fuca plate which is slowly sliding under the North American plate just got a little lower at the plate boundary. Here in the Bay Area we experience a different kind of quake. Ours are the result of shearing force, not compression or tension, which is called sometimes called strike-slip faulting. The plates are in the process of sliding past one another instead of bonking into one another or being pushed away from one another, and that friction release is why we get so many little quakes all the time. Most plate boundaries are either normal faulting, a result of subduction, or thrust faulting due to colliding land masses. The San Andreas fault is unique on the west coast of the Americas. Another area that is at the mercy of the same strike-slip faulting is Turkey. Unfortunately, their building codes are less restrictive than California's and they experience far greater loss of property and life every time the plates move strongly.
Having grown up in earthquake territory they don't hold non-stop terror for me. I remember being fascinated by the jiggling water stream coming from the faucet when I was brushing my teeth during the 1964 Alaska quake. I tend to quietly get up from whatever I'm doing and find a support beam to stand under if a quake goes on for more than 5 seconds, but I don't panic. Even in the 1989 quake, watching the windows bow in and out of the building I was in, I was deeply interested in everything as it happened even as I gradually became frightened by the intensity of the quake. It doesn't seem as terrible when you know it's just the earth recycling its crust. "Come on, make way, new seafloor coming, don't hold things up, it's time to make some magma!"
In general, I am impressed that someone worked out just how the convection movement of the earth's lithosphere and athenosphere operate, thus creating a unified theory explaining the drifting of the continents and the recurrence of earthquakes and volcanos in certain areas of the world.
Plate tectonics are really neat.
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