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

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
[email protected]
By the end of
the Paleozoic
era, about 250
million years
ago, most of
the continents
had collided to
form the
supercontinent
Pangea.
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.
India first encountered the southern edge of Asia about
50 million years ago, initiating a continental collision.
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
The lithosphere of India was old, cold and strong while
the lithosphere beneath the rim of Asia was young, warm
and weak.
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.
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
The unstable Tibetan crust began to flow sideways, mostly
toward the east, in a process called “extrusion tectonics”
or “tectonic escape”.
The Sichuan basin lies on top of the Sichuan block, an old,
rigid block that is embedded within the Asian lithosphere.
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.
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.
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.
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.
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.