Sediment dynamic, slope instability and potential tsunami hazard at the outer Thai shelf margin, Mergui Ridge, Andaman Sea
The Thai Andaman Sea coast has been repeatedly struck by tsunamis, such as the 2004 Indian Ocean Tsunami, caused by an earthquake at Sunda Trench. Geological record shows that this was not the first tsunami in the area. Most tsunamis worldwide are of tectonic origin, but submarine landslides can cause destructive tsunamis, too. For the outer Thai shelf margin in the Andaman Sea the state of the slope and whether submarine mass wasting occurs and may add to the tsunami hazard potential was unknown so far. In order to examine sedimentary development, stability and tsunami potential of the slope a new multibeam bathymetry / high resolution 2D multichannel reflection seismic data set was acquired. The new data show that the Mergui Ridge, an area of low sedimentary input, is today shaped by bottom currents producing drift deposits. After uplift and subaerial exposure at the end of Late Miocene, the shelf subsided in Pliocene. Small scale tectonic deformation is ongoing. The development is reflected in older sediments, which are characterized by local erosion and today are partly covered by a drape of undisturbed sediments. Within sediments of the Mergui Ridge 17 mass transport deposits were identified, mainly within drift deposits, showing that the slope was unstable and failed repeatedly. The presence of fluids, instable drift deposits and ongoing tectonic activity may be important preconditioning factors that may lead to future failures. Based on their mostly relative small dimension (ca. 0.3 -14km3), and their occurrence in large water depth, it is unlikely that most failures were tsunamigenic, but it cannot be excluded. Based on thickness of hemipelagic layers between events, recurrence of failures seems to be long, compared to recurrence of tectonic tsunamis. A simple numerical modeling of landslide tsunami propagation and estimation of run up heights (1.0-25.8m) shows, landslides in the area may produce tsunamis but modeled scenarios represent worst case scenarios.