April 25, 2012 | 2
I’m sorry. Very, truly sorry. I know the recent earthquake in Sumatra isn’t precisely breaking news, and I really meant to say something about it earlier. Let me explain: in the immediate aftermath, I was hoping Chris Rowan would do up a Friday Focal Mechanism post about it so I could point you there. He’s brilliant at explaining this stuff. Alas, professional geologists get too busy sometimes to write up breaking news. No worries. The Earth will throw him a nice, big earthquake in a free moment someday. And I found you other tasty links.
My knowledge of earthquakes is the kind that leads earthquake geologists to pat me on the head and say, “Aw, Dana, you’re so cute for trying. Here’s a gold star.” That’s my own assessment, mind: my geologist friends aren’t condescending. What I mean is, I haven’t studied this stuff in enough detail to be comfortable with it, but I can use words like “strike-slip” and “focal mechanism” in some sort of context. But my understanding is pretty elementary.
But this one’s too fascinating to avoid simply because I’m a noob at earthquake geology. For one thing, why didn’t this monster quake (8.6) cause a tsunami worth noticing? I’d planned to demonstrate that in the bathtub, but I haven’t built the proper rig, so we’ll have to try thought experiments.
First, similarities: the December 2004 quake (which was a 9.1 or thereabouts) and the April 2001 quake were enormous. However, a 9.1 is roughly three times bigger and five and a half times stronger than an 8.6. You can calculate the exact numbers. There’s a nice, professional diagram here, but I whipped up a little something for ye myself:
Right. So, in our thoughts, let us splash in our tub. Hold your hands together under water. Now, pretend they are the earth’s crust. Break them apart. Earthquake! Do it three times bigger. Bigger earthquake! Much bigger sploosh, amirite? (I hope I’m right. I haven’t empirically tested this – I’m going off of bathtime fun memories from the days of yore.)
Not that this is going to register on a seismograph, but if your hands really were crust, a bigger “rupture” would show up as a larger magnitude on a seismograph. But the energy released is even larger than that. You can probably measure this by the comparative amounts of water on the bathroom floor if you’ve been experimenting energetically enough. With me so far?
Right. So. This next bit’s harder, because hands really aren’t crust, and you’ll probably have to think of something a little more ingenious. But you can still try this, and still see something of a difference. I know, because I tried it with two rocks in a pan of water. Yes, I’m really that weird.
The 2004 quake was a megathrust earthquake. The key word here is thrust: one side of the rupture is thrust up relative to the other. This is common in subduction zones, like the one between the Indo-Australian and Eurasian plates.
Try it at home: place one hand slightly beneath the other. Then pull the top hand up. Watch the water slosh as it tries to get out of the way. The problem with our rig is that there’s a big empty space under our hands for the water to go. This isn’t the case when the crust goes up – there’s plenty more crust where that came from, whereas we’ve run out of hand. To really get the gist of this, we’d have to push up part of the bathtub and floor, which may upset the owners if you’re renting. Best not. Still, it gives you some idea. When the seafloor suddenly rises, all that water’s gotta go somewhere.
Now, the 2011 quake was a strike-slip. That’s where one side is moving horizontally relative to the other rather than vertically. Hold your hands together, edge-to-edge. Now pull one hand toward you, keeping both hands flat and level. Congratulations! You’ve just done a strike-slip fault. And, if our experiment somewhat replicates natural conditions, you should notice a lot less slosh. The water’s not being pushed around so much.
There you go. Bathtub experiments. And no one can complain about the water on the floor because you are investigating earthquake science. Isn’t science fun?
Now, here’s the reason why this Sumatra quake, despite the lack of a tsunami, was so interesting: it’s one of the largest strike-slip earthquakes recorded. The thing was gargantuan for a strike-slip earthquake.
All of this, of course, is merely an excuse to post some delicious links for you. Without further ado, then:
The Guardian: Why earthquake off Sumatra did not trigger a devastating tsunami. I love it when a paper doesn’t go for the “we’re all gonna die!!!!” schlock and does the science instead.
Harvard Seismology: Preliminary Rupture Modelling of the April 11, 2012 Sumatran Earthquakes. People, if you love awesome gifs and good explanations, this page is for you. Go forthwith.
Geolog: Following today’s earthquake in Sumatra online. Links to some excellent information.
Arizona Geology: Giant Sumatra earthquake recorded in Arizona. Included simply because I find it awesome that my old home state recorded this earthquake.
Hypo-theses: Sumatran Earthquakes 11/04/2012 Recorded at Keele, UK. Least my friends in the UK feel left out of the whole recorded-the-earthquake thing.
Paleoseismicity.org: Strike-Slip Week on Planet Earth. A nice round-up of some very interesting quakes, including our superstar in Sumatra. If you’re looking for your focal mechanism beach balls, there they are. (And if you need a primer on focal mechanisms and have no idea what the beach ball is, see here. Fear not the beach ball! It is our friend.)
Do you know of any other geoblogs that wrote up the quake? Link ‘em in comments.