Tuesday, September 27, 2011

Scary geology

I spent the weekend in an area that proclaimed geological drama. The mountains in whose shadow we camped were basaltic, the product of long-ago volcanic events. However, these mountains were elevated and pushed up and out of the way by a granitic intrusion, making them into "roof pendants." We camped upon the granite. It must have been something to see happen, though I'm not sure any sentient witnesses were present 150 million years ago.

There's still geological drama today, though, and lots of sentients to see it and measure it. While in camp, I finally got around to reading some of the scientific reports about what is now called the Tohoku-Oki Earthquake of March this year, and about the Maule Earthquake in Chile in 2010. Both of these megaquakes were extremely well measured, thanks to GPS sensors, and produced reams of data--data that could be distilled into a couple of pictures that can give you nightmares (as if the videos and photos of the quake and tsunami didn't do it). This one just made me gape:

You definitely need to click on that and blow it up. Ideally, you should blow it up so that it's about the size of a football field. See those black arrows on the picture of Japan? You should blow up this picture so that they are 24 meters long. Each arrow represents a report on the horizontal movement of a fixed GPS station during the recent megaquakes. It's a useful exercise to simply pace out 24 meters, just to see how big a distance it is. When you think in terms of a
region moving that far, it's damn big.

That scary picture is from a review article; the primary sources have scary pictures of their own. Here's one from the report on the Chilean megaquake, measured at M8.8.
This figure is, like many figures from the primary literature, so information-dense as to be difficult to grasp. It records many things (movement of the GPS stations, regions of slip from almost a dozen previous earthquakes, etc), but for me the trippiest is the squiggles off to the left. These record not just the final displacement of the GPS stations (the red arrows), but the route they took to get there over the course of three minutes (the trail of dots that connects the beginning and the end of the arrow). I don't know what would be worse--to be in Constitucion and go 15 meters in one direction, or to be in San Javier (SJAV) and three minutes wandering all over. Another figure from the same paper shows vertical displacements; the station at Concepcion (CONZ) actually rose almost two meters.

The figures from the paper about the M9.0 Tohoku-Oki megaquake are even worse, largely because the data is better. All of the sensors used in the Chilean earthquake were on land, and thus pretty far from the actual subduction trench where all the action occurred (it's the black line on that picture). The Japanese had GPS sensors on the ocean floor, right near the subduction zone.

(This last sentence shouldn't make sense--GPS systems require an unobstructed "view" of a satellite to work, so a sensor on the sea floor shouldn't be able to communicate with a GPS satellite. However, by putting a lot of acoustic sensors on the sea floor, a ship or buoy can triangulate to figure out its exact relationship to the sensors, and then communicate with a GPS satellite to figure out its precise location. Here's a figure from the Japanese paper to illustrate:
So, here's the data they recorded, and if doesn't make your hair curl, something is dead in your soul:
So, bits of Japan lurched eastward 24 meters and rose by three meters. I'm trying to picture the place where I'm sitting doing that, and I confess that I am utterly failing in my efforts--let's see, I think it would place me on the roof of my neighbor's neighbor's house.

This is plenty scary on its own--but here I am in Oregon, and the subduction zone off our coast is due for precisely this kind of megaquake. We're always cautioned the geographically separated earthquakes that occur close together in time are
not connected. However, these megaquakes seem to be a different thing, and there's some opinion that they megaquakes around the Pacific rim may cluster--there was a bunch of them back in the early 1960's (Chile, Alaska, Kamchatka/Kuril islands) and now we may be seeing another bunch. I live well inland, but when I do go out to the Oregon coast, I take care to notice all the signs for tsunami evacuation routes. I appreciate geological drama, but its toll on human life overwhelms me. I prefer to view such drama with the remove of a scientific paper, or (as with the Sierras) at the distance of a hundred million years. It's scary enough for me.

Heki, Kosuke (2011). A Tale of Two Earthquakes. Science 322: 1390-1391.

C. Vigny, A. Socquet, S. Peyrat, J.-C. Ruegg, M. Métois, R. Madariaga, S. Morvan, M. Lancieri, R. Lacassin, J. Campos, D. Carrizo, M. Bejar-Pizarro, S. Barrientos, R. Armijo, C. Aranda, M.-C. Valderas-Bermejo, I. Ortega, F. Bondoux, S. Baize, H. Lyon-Caen, A. Pavez, J. P. Vilotte, M. Bevis, B. Brooks, R. Smalley, H. Parra, J.-C. Baez, M. Blanco, S. Cimbaro, E. Kendrick (2011).
The 2010 Mw 8.8 Maule Megathrust Earthquake of Central Chile, Monitored by GPS. Science 322: 1417-1421.

Sato, Mariko, and Tadashi Ishikawa, Naoto Ujihara, Shigeru Yoshida, Masayuki Fujita, Masashi Mochizuki,Akira Asada (2011). Displacement Above the Hypocenter of the 2011 Tohoku-Oki Earthquake. Science 332: 1395.

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