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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 NE.
Peer-reviewed paper
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. Tectonics. 2007;26.
Web link to peer-reviewed paper
http://www.geography.dur.ac.uk/documents/densmore/densmore_etal07.pdf
Cosmogenic radionuclides
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 (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 www.bgs.ac.uk
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