Scientists Identified Earthquake Faults in Sichuan Last Year

The State Council today ordered all flags to be kept at half-mast and all public amusements suspended for three days from Monday as China begins an official mourning period for victims of the May 12 earthquake. The Beijing Olympic torch relay in the eastern city of Ningbo will be put off for three days until May 22, the Olympic organizing committee said.today.

The earthquake in Sichuan occurred under some of the steepest and most rugged mountains in the world, the Longmen Shan: the Dragon's Gate Mountains.

Steeper than the Himalayas, this range is the upturned rim of the eastern edge of Tibet, a plateau that has risen up to 7500 meters above sea level and is characterized by topographic relief of more than five kilometers over distances of less than 50 km in response to the slow but unstoppable collision of India with Asia that began about 55 million years ago and continues today.

Aerial view of the quake-striken town of Yingxiu in the Longmen Shan mountains. May 14, 2008 (Photo courtesy Xinhua)

Research published by earth scientists last summer in the international journal "Tectonics" found that geological faults in China's Sichuan Basin "are sufficiently long to sustain a strong ground-shaking earthquake, making them potentially serious sources of regional seismic hazard."

The international team of Chinese, British, Swiss and American scientists mapped and analyzed a series of geologically young faults that cross Sichuan province like recently healed scars.

The team included Dr. Alexander Densmore of the Institute of Hazard and Risk Research at England's Durham University and Dr. Mike Ellis, head of science for climate change at the British Geological Survey, as well as colleagues Y. Li and R. Zhou from research institutes in Sichuan's capital city Chengdu.

The team mapped the densely populated Sichuan Basin and adjacent mountains using a technique known as tectonic geomorphology. With this technique, scientsts can demonstrate changes in ground movement over time, such as offset river channels, disrupted floodplains, abnormally shaped valleys and uplifted landscape features.

The scientists calculated the average rate at which the faults must be slipping using these subtle signals of deformation combined with the ability to measure the age of the disfigured landscapes using cosmogenic radionuclides - cosmic rays - that bombard the Earth from all corners of the universe.

Cosmic rays form new short-lived isotopes of trace elements that are found usually in common quartz, or sand grains. But, the scientists explain, cosmic rays only generate these special isotopes on or near the ground surface.

This means that scientists can use them to measure how long the surface has been exposed to the sky, or the age of a certain landscape feature such as a river terrace or alluvial fan. "If these features are disrupted by a fault, it is a simple matter to calculate the average rate at which the fault must be slipping," the team reports in its "Tectonics" paper.

Two long faults in particular, running almost the entire length of the Longmen Shan range, showed clear evidence of slip during the last few thousands, and in some cases, the last few hundreds, of years, they found.

The road from Dujiangyan city across the steep Longmen Shen mountains is destroyed by a landslide caused by Monday's earthquake, hampering the transport of rescue workers and relief supplies. May 14, 2008 (Photo courtesy Xinhua)

One of these faults is "likely" to be the one that gave rise to the 7.9 magnitude May 12 earthquake, the scientists said today in a statement.

"Exactly why the Longmen Shan are here is a mystery. Unlike the Himalaya, which form the southern boundary of Tibet and whose faults chatter continuously with small earthquakes, faults in the Longmen Shan, remnants perhaps of geological events hundreds of millions of years ago, have historically only produced earthquakes up to magnitude 6," the authors write.

Geomorphological evidence, described in the "Tectonics" paper, suggests that the mapped faults are very steep with dominantly lateral or strike-slip displacements taking place over time scales of thousands to hundreds of thousands of years.

This contrasts with shorter-term measurements using Global Positioning Systems which suggest a greater proportion of thrust or shortening displacement than lateral displacement.

The observations of seismologists at the British Geological Survey suggest both things - more thrust in the southwest, nearer the epicenter of the quake, and more strike-slip toward its direction of propagation, the northeast.

To read the paper, "Active tectonics of the Beichuan and Pengguan faults at the eastern margin of the Tibetan Plateau," click here.

Copyright Environment News Service (ENS) 2008. All rights reserved.