Scientists identified geological faults in quake-hit region in China a year back

Washington, May 17: A research carried out by earth scientists last summer had determined that geological faults in the Sichuan Basin, China, are sufficiently long to sustain a strong ground-shaking earthquake, an estimate which proved to be right when a massive quake struck the region on May 12.

The international team of scientists that made the estimate included Dr. Alexander Densmore from Durham University, Dr. Mike Ellis from the British Geological Survey (BGS) and colleagues from research institutes in Chengdu, China.

For their research, the team had carefully mapped and analysed a series of geologically young faults that cross the Sichuan Province like recently healed scars.

The team mapped the densely populated Sichuan Basin and adjacent mountains using what is known as 'tectonic geomorphology'.

This technique can demonstrate significant changes in ground movement over time, such as observations of offset river channels, disrupted floodplains, abnormally shaped valleys and uplifted landscape features.

These subtle signals of deformation, when combined with the ability to measure the age of the disfigured landscapes, produced surprising results.

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

This dramatic range, steeper than the Himalayas, is the upturned rim of the eastern edge of Tibet, a plateau that has risen to 5 km in response to the slow but unstoppable collision of India with Asia that began about 55 million years ago and which continues unabated today.

According to the researchers, two long faults in particular, running almost the entire length of the Longmen Shan, showed clear evidence of slip during the last few thousands, and in some cases hundreds, of years.

The rates of slip varied between fractions of mm per year to possibly many mm per year. Millimetre by millimetre, the Longmen Shan are being sliced and displaced.

One of these faults is likely to be the one that gave rise to the 7.9 magnitude earthquake that has now caused 22,069 fatalities.

Geomorphological evidence, gathered by the researchers, 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 BGS suggest both things: more thrust in the SW (South West), nearer the epicenter, and more strike-slip toward its direction of propagation, the NE (North East).


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