Astute Californians will have noticed (and in many cases felt first-hand) this pattern:30 December
- 5.8 earthquake under Mexicali (about a hundred miles inland from San Diego)7 and 8 January
- 4.1 and 3.8 in San Francisco Bay Area9 January
- 6.5 off the coast of Ferndale, CA (Humboldt County, way up there in Northern California). Apparently this one was right at the Mendocino Triple Junction where the North American, Pacific, and the subducting Juan de Fuca plates come together. It's also deeper than the average strike-slip quake.
- 4.1, again under Mexicali (6 hours after Ferndale quake)
[Added later] 12 January
- Here's the big one on the other side of the North American Plate, a 7.0 in Haiti
See the trend? The trend is that property values in Portland and Seattle will be depressed for the next few weeks. You can get real-time earthquake information here
, though they drop off after a week.
Years ago I'd noticed, like many people, that when there's a destructive quake in one part of the world, there's often another one (or more) within a few days. Interestingly, they're frequently at opposite ends of the same tectonic plate. It turns out that earthquake triggering
is known to geologists but we don't yet know enough to predict where the next one might happen. But usually these triggerings are far from each other; and usually they're triggered by bigger quakes than these. My question is whether mid-size quakes are associated with quakes closer to them, rather than across the ocean?
In my last post about redwood habitat I mentioned how nature's real modus operandi, when you look closely, is to move with jerky, inelegant jumps
instead of the inexorable beauty that we seem to read in the distant fossil and mineral record. When you live through something, you necessarily look closely.
For example: the Pinnacles
are the best National Monument you've never been to, near the northern Central Coast, and they're the other half of an extinct volcano that died about 20 million years ago when the Pacific Plate started slipping northward as it is today, instead of subducting under North America. (The other half is near Lancaster, CA but it's not much to look at
.) Today the two halves have drifted 300 miles apart. So you can If you work out the rate at which they move, does that mean that every year, the fault moves exactly an inch? No - but if you take measurements once every century, you're more likely to get an answer closer to that rate (8.25 feet). And what do you know! In the twentieth century, the San Andreas in the Bay Area moved 9 feet. Except it was almost all at one time, in 1906:
Talk about inelegant. This weekend the people in Humboldt County sure are. I should add that in some places the fault does creep noticeably, but this doesn't stop the jerks from occurring. Go here
to see street curbs that have been separated over the years by fault motion.
Because I'm a nice guy I once tried to stop all this plate motion by standing on the fault at the Carrizo Plain
and pushing south on the Pacific Plate but I guess this means it didn't work. My attempts to make Mount Shasta erupt by jumping up and down on the summit met with similar results.
For fellow wanna-be geo-nerds: I've been interested in the Mendocino Triple Junction and West Coast plate-boundary geology for a while; here's a post
I did in May about that.