The disastrous earthquake and tsunami that hit Indonesia and several neighbouring countries on 25 December 2004 was a reminder to all about the great forces of nature and it was also a reminder to earth scientists that low-probability events also occur, albeit very rarely. It was known of course that the subduction zone along the Sunda Arc was capable of producing mega-earthquakes, but no similar event was know to have occurred for several hundred years.
Following the 2004 tsunami, the Norwegian Geotechnical Institute (NGI) has in cooperation with NORSAR conducted a major study for the region, with particular emphasis on Thailand. NORSARs work here was mostly on the source, where one of the conclusions was that it most likely will take at least 3-400 years before a similar event can be expected to happen again in this region. This has been supported by subsequent paleoseismological investigations. Until then the risk will gradually increase in this part of the Sunda Arc. Further south and east, however, the risk may have become greater, which is the subject of an ongoing study in cooperation first of all with Indonesia and the Philippines, also in cooperation with NGI.
 A seabed dislocation model (left) for the main M 9.3 Sumatra 2004 earthquake, with movements up to 5 meters. The model was used to produce the tsunami snapshot to the right, 1 hour and 20 minutes after the earthquake and just before the tsunami reached the tip of Phuket. The modelled waves are about 3 meters above mean level (and 3 m below also) which matches quite well the actual observations. The work included also a number of less dramatic but more likely tsunami scenarios, as a basis for specific recommendations on mitigation measures.Seabed dislocation model.
Up to now, initial seafloor displacements resulting from tsunami-triggering earthquakes are calculated using a simple, analytical model, allowing only for very simple model geometries and homogeneous material properties. The amount of slip for a given seismic moment depends not only on fault plane properties, but also on the shear modulus (rigidity) of the surrounding material, which may vary with depth in subduction. Curvature of fault plane and seafloor topography may additionally influence the resulting displacement pattern. Such inhomogeneities can only be captured by a numerical model.
To this end NORSAR is now developing a finite-element (FEM) numerical method to deal with surface displacement computations resulting from slip on a fault. Our work has been based on GeoFEST software (Jet Propulsion Laboratory) where one can account for material inhomogeneities, heterogeneous slip distributions along the fault plane and irregular fault geometries in 2-D or 3-D. It is also possible to include multiple faults, seafloor topography and friction on the fault.
The figure below shows an inhomogeneous FEM mesh used to calculate seafloor displacement resulting from slip on earthquake faults in a subduction zone, subdivided to represent accretionary wedge, continental crust, oceanic plate and asthenosphere. The last figure shows the influence of a change of subduction angle from 0° to 90°. The results are very similar to the results from analytical solutions, but allowing for more complex and realistic situations.
FEM mesh used to calculate seafloor displacement
The influence of a change of subduction angle from 0° to 90°.
Grey and black lines for steep and shallow subduction angles, respectively.
Left: vertical displacement, right: horizontal displacement.
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