British Geological Survey (UK)
Scientists identified earthquake faults in Sichuan, China
Only last summer research published by earth scientists in the international journal Tectonics concluded that geological
faults in the Sichuan Basin, China "are sufficiently long to sustain a strong ground-shaking earthquake, making them
potentially serious sources of regional seismic hazard."
An international team of scientists including Dr. Alexander Densmore (Institute of Hazard and Risk Research, Durham
University), Dr. Mike Ellis (Head of Science for Climate Change at the British Geological Survey) and colleagues from
research institutes in Chengdu, carefully mapped and analysed a series of geologically young faults that cross 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 (using cosmogenic
nuclides that bombard the Earth from all corners of the universe), 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.
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 much
like salami. 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. 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.
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 BGS suggest
both things: more thrust in the SW, nearer the epicentre, and more strike-slip toward its direction of propagation, the
NOTES TO EDITORS
Densmore, A.L., Ellis, M.A., Li, Y., Zhou, R., Hancock, G.S. & Richardson, N. Active tectonics of the Beichuan and Pengguan faults at the eastern margin of the Tibetan Plateau.
Web link to peer-reviewed paper
The ability to constrain the estimates of fault slip rates comes from the analysis of cosmogenic radionuclides or CRNs.
Cosmic rays bombard the Earth's surface constantly and from all parts of the universe. They form new short-lived
isotopes of trace elements that are found usually in common quartz, or sand grains. However cosmic rays only generate
these special isotopes on or near the ground surface, which means that scientists have the ability to measure for how
long the surface has been exposed to the sky, or the age of a certain landscape feature (e.g. 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 British Geological Survey
The British Geological Survey (BGS), a component body of the Natural Environment Research Council (NERC), is the
nation's principal supplier of objective, impartial and up-to-date geological expertise and information for decision
making for governmental, commercial and individual users. The BGS maintains and develops the nation's understanding of
its geology to improve policy making, enhance national wealth and reduce risk. It also collaborates with the national
and international scientific community in carrying out research in strategic areas, including energy and natural
resources, our vulnerability to environmental change and hazards, and our general knowledge of the Earth system. More
about the BGS can be found at http://www.bgs.ac.uk
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