A Tectonic explanation of the May 12, 2008, Sichuan Earthquake

1. A Tectonic explanation of the May 12, 2008, Sichuan Earthquake Professor Emeritus Tanya M. Atwater Member, U. S. National Academy of Sciences Dept. Earth Science, 1006 Webb Hall University of California, Santa Barbara Santa Barbara, CA 93106-9630 atwater@geol.ucsb.edu

2. By the end of the Paleozoic era, about 250 million years ago, most of the continents had collided to form the super-continent Pangea.

3. Pangea lasted about 50 million years, then it began to break apart. India broke away from Antarctica about 120 million years ago and drifted northward across the old Tethys Ocean.

4. India first encountered the southern edge of Asia about 50 million years ago, initiating a continental collision.

5. The India-Asia continental collision has continued ever since, with India ramming ever more deeply into southeast Asia. To view or download a computer movie showing the breakup of Pangea, visit: http://emvc.geol.ucsb.edu/downloads.php

6. The lithosphere of India was old, cold and strong while the lithosphere beneath the rim of Asia was young, warm and weak.

7. Thus, as it collided with Asia, India acted as a rigid indenter. It crumpled and piled up the weak Asian crust in front of it as it entered.

8. As the Tibetan crust became thick and high, it heated up and became unstable. To view or download a computer movie showing the India-Asia continental collision, visit: http://emvc.geol.ucsb.edu/downloads.php

9. The unstable Tibetan crust began to flow sideways, mostly toward the east, in a process called “extrusion tectonics” or “tectonic escape”.

10. The Sichuan basin lies on top of the Sichuan block, an old, rigid block that is embedded within the Asian lithosphere.

11. As the weak Tibetan crust flowed eastward, it encountered this strong block. Some of the flow was diverted southward, moving through a slot between the Sichuan block and the indenting Indian block.

12. The Tibetan edge above the Sichuan basin is laced with large, curved strike-slip faults that guide the crustal flow around the corner. This “flow” is actually accomplished in jerks when earthquakes rupture these faults. Many of China’s largest, most destructive earthquakes occur here.

13. The flow is also pressing eastward against the Sichuan block, forming a steep mountain front and running over the block with folds and thrust faults. During the May 12 earthquake, one of these thrust faults ruptured and moved the mountains as much as 8 meters up and over the basin.

14. The continental collision continues today and into the future, unabated. The obvious conclusion is that large earthquakes in this region are natural and inevitable, so that continual earthquake preparedness is of the utmost importance.

15. Sources and Acknowledgements: Base maps for the S.E. Asia and Sichuan images are from Google Maps, Terrain. http://maps.google.com/maps Fault patterns are after Peter Molnar, American Scientist, v. 74, p. 144, 1986, and v. 77, p. 350, 1989. See also www.tectonics.caltech.edu/2008MayChinaEQ/xichuan.html Thanks to Zhang Peizhen, Xue-ze Wen, Peter Molnar, Douglas Burbank, and many others for discussions, insights, and Sichuan field experiences.