Monday, May 12, 2008

Tectonics of the May 12 Sichuan earthquake

The M 7.9 earthquake that struck Sichuan province in China is already tragic, with at least 10,000 people dead. It's China's worst earthquake since the 1976 Tangshan earthquake - the earthquake that killed at least a quarter of a million people, and maybe many more. MIT's tectonicist Clark Burchfield is quoted in the NY Times as being surprised that an earthquake of that magnitude (which requires a lot of fault to have moved at once) occurred in this particular spot. But although the size might be larger than one would expect, the type of earthquake isn't. This isn't a classic plate boundary earthquake, but it's part of the way that continents respond to continent-continent collisions.

In the big picture, India is moving northward and colliding with Asia. That's what created the immense mountains of the Himalayas, and the highlands of the Tibetan Plateau, in the first place. This earthquake, however, was not on the plate boundary itself. It was off on the eastern edge of the Himalayas, on the boundary between the high country and the Sichuan basin.

Here's the USGS earthquake location map. The pink lines represent the collisional plate boundaries - the boundary between India and geologic Asia runs along the southern end of the Himalayas. This earthquake was within Asia proper.

But continents are... well, continents are kind of wimpy, actually, in terms of deformation. They're made of rock that's easier to break than oceanic crust is. And on top of that, they're thick. And when they run into one another, the continental crust gets even thicker. And that thick, weak crust can break within itself - it doesn't behave like the classic rigid plates that "plate tectonics" is named for.

The crust of Asia breaks in a number of different ways. Parts of it are squeezed out, especially to the east, where it can run over the subducting Pacific plate like a semi over a squirrel. Parts of it actually stretch east-west, creating features like Lake Baikal up in Russia. Parts are pushed over each other, making the height of the Himalayas and Tibet possible. And parts are squeezed over other parts of Asia. And that's what's happening on the northwestern side of the Sichuan basin - the edge of the Tibetan Plateau is running over eastern Asia, in this case, along a classic thrust fault, sloping about 30 degrees down into the ground beneath the edge of the mountains.

You can also see the movement of Tibet in this map of GPS velocities:

(From Gan, Zhang, Sun, and Sun, 2006?))

Each of the arrows represents the movement of a GPS monitoring site compared to the rest of Eurasia. The southern Himalayas are moving north; the Tien Shan mountains, north of the Tarim basin (white, north of the Himalayas) are moving much more slowly, and the eastern Himalayas... are moving east. Or even south.

And at the Sichuan basin, that green blob east of the Himalayas, the GPS velocities drop. There's a change in movement. And that M 7.9 earthquake took place where the change occurs.

The Google earth image (from the USGS Google earth KML) of the earthquake and its aftershocks shows the setting closer-up:

It looks like the earthquake was on the basin-bounding fault. It looks like there are ridges in the basin running parallel to the fault, too - ridges that are cut by rivers. They are probably actively growing folds... which means there are probably active thrust faults running underneath the basin itself, not just under the mountains.

Those faults, at least, don't seem to have moved.


Ron Schott said...

"And at the Sichuan basin, that green blog east of the Himalayas, the GPS velocities drop. There's a change in movement. And that M 8.9 earthquake took place where the change occurs."

Ummm... blog? M 8.9? Oops. Time to fire the copy editor. :-)

Kim said...

Oops! Thanks - it's fixed now.

BrianR said...

Kim, great summary ... I was waiting for blog posts to give me the low-down.

Silver Fox said...

I'm with Brian on this - a very good post. And it seems like I get a lot of news from the Geoblogosphere Feed, these days.