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Finding fault

Geoscientist 17.3 March 2007

Amarendra Swarup reports on how Leciester University geologists are stripping away Alpine forests – in the interests of science and public safety.

The sudden Kashmir earthquake of 2005 is a powerful reminder of the potentially disastrous seismic hazards posed by faults in mountainous regions. At magnitude 7.6 it was similar in intensity to the infamous 1906 San Francisco earthquake, leaving over 80,000 people dead and another 3.3 million homeless. At the root of this devastating impact lay the fact that the earthquake faults were hidden by forests and so went unnoticed by scientists.

Now, geologists Dickson Cunningham and Kevin Tansey at the University of Leicester may have taken an important step forward in mitigating the consequences of another such disaster. They have developed an innovative airborne system that is capable of mapping the distribution of recently active faults, including fault lines never before seen because they are hidden by forest cover in mountainous terrain. Using powerful aircraft-mounted lasers, the new technique allows researchers to virtually "deforest" the landscape, revealing details of the underlying topography such as any active fault traces.

“Locating earthquake-prone faults in forested mountainous regions and understanding the potential seismic hazard they pose to local population centres has always been a problem to geoscientists” explains Cunningham. “Many regions of the world have undiscovered active faults hidden by dense forests, including Indonesia, India, Northwest North America, the Andean nations and the alpine countries of Europe. Unfortunately for people living in these regions, these faults can be ticking time bombs.”

Preliminary studies by the Leicester team in the southeastern Alps in Slovenia have already demonstrated that their airborne laser system – code-named LiDAR (Light Detection and Ranging) – could prove vital for earthquake monitoring in many areas across the world. Working in collaboration with Slovenian geoscientists, the duo spent four years examining two major plate boundary faults – the Idrija and Ravne strike-slip faults in northwest Slovenia. The area is ideal as it is a very seismically active part of the Alps, having experienced moderate earthquakes in 1998 and 2004 as well as a devastating one in 1511 that killed 12,000 people. Furthermore, little is known about the fault architecture in this high alpine landscape as it is heavily forested.

“We wanted to understand the specific faults that have produced these earthquakes including their exact locations, segmentation, rupture history and to better assess the earthquake hazard they pose for the future” Cunningham says.

The answers, as it turns out, all lie within LiDAR – a new low-altitude technique pioneered in 2002 for geological applications in Seattle. Cunningham and Tansey simply adapted the method to the high-relief alpine landscapes of Europe to map seismically active faults for the first time.

Airborne LiDAR uses a powerful laser to map out the ground surface in huge swathes. The system provides very accurate height measurements to within a few centimeters at high resolution, allowing a detailed topographic model of the ground surface to be obtained. This can reveal distinctive surface features produced by active faults such as scarps – lines of cliffs formed by the fracturing of the Earth's crust – and offset streams, displaced laterally or vertically by faulting, even if such features are extremely subtle. LiDAR can be used in forested regions because the trees can be stripped away. This is because some LiDAR laser pulses penetrate the tree canopy and are reflected back off the forest floor to the airborne sensors. These can then be separated. 

The results have been outstanding. In 2004, LiDAR surveys over the two major Slovenian fault systems believed responsible for the historic earthquakes produced strikingly clear images of both faults unlike any previously obtained, revealing detailed information about their structural architecture, movement history and landscape geometry. In one survey, the researchers were even able to discriminate fault breccia from intact country rock simply by the textural characteristics of the ground surface.

“This was the first attempt at using airborne LiDAR in high-relief alpine landscapes” says Cunningham. “We could not have obtained the same perspective on the ground, and aerial photos would not have revealed the fault traces.”

Mapping these traces is vital for assessing whether the faults are likely to produce large or small earthquakes in the future. Further, thanks to these unique images, geologists can quickly and efficiently identify sites suitable for a more detailed historical study and current nature of the faults, giving them insights into both the recurrence interval of major earthquakes as well as making estimates of the timing and magnitude of the next major event.

Field excursions in August 2006 verified the remote LiDAR observations, and provided overwhelming new evidence for previous fault activity. Cunningham and Tansey are now following up their initial results with further research and have established the UK’s first inter-disciplinary LiDAR research unit at Leicester with support from both the Ordnance Survey and the British Geological Survey.

Further reading

Application of airborne LiDAR to mapping seismogenic faults in forested mountainous terrain, southeastern Alps, Slovenia, Cunningham, D., Grebby, S., Tansey, K., Gosar, A. and Kastelic, V., Geophysical Research Letters 33, L20308 (2006).