BRITISH GEOLOGICAL
SURVEY News Release issued by The Government News Network on 16 May 2008
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.
NOTES TO EDITORS
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
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
*ENDS*