By Sam Bain
Natural disasters are often staggering in their scale, but few can compare to the shocking extent of damage and life loss resulting from the 2004 Indian Ocean and the 2011 Tohoku-Oki earthquakes and accompanying tsunamis.
We have all seen the horrifying images and videos from these events. The spread of mobile phones with reasonable quality cameras means that within hours of a disaster images begin to appear on the net. For me, there is one video in particular which captures the terrifying raw power unleashed by these disasters. It was captured from on top of a solid building and shows the rapid speed and destructiveness of the tsunami. It also clearly demonstrates the huge amount of displaced water that is involved.
There is an ongoing effort by the academic community to understand what exactly occurred during the Tohoku earthquake. As part of this work scientists from the International Ocean Discovery Program (IODP) have been working to drill the subduction interface where the Tohuku earthquake occurred. The J-FAST drilling program involves drilling through the overlying Eurasian Plate, through the plate boundary interface, and then into the subducting Pacific plate in water depths of 7 km. This water depth combined with drilling through the fault interface resulted in a drill string of nearly 8 kilometres!
One of my lecturers at the University of Otago, Dr Virginia Toy, was part of the J-FAST project and has summed up the current results nicely here. The findings are fascinating. Basically, the drilling revealed a thin (~5m thick) layer of sheared clay rich rock at the fault plane. This rock had almost no frictional strength when earthquake conditions were applied to samples in the lab. This low strength rock allowed massive movement along the fault plane and resulted in the seabed being displaced by more than 50 meters. This movement resulted in the surge of water that devastated eastern Japan. Of real concern to New Zealand, and to other countries near subduction interfaces, is whether this type of rock can be found on their local interface. If it exists then the risk of large displacements and associated tsunamis may be higher.
The 2004 Indian Ocean and 2011 Tohoku-Oki tsunamis have definitely raised the public profile of tsunamis generated from subduction interface earthquakes. This seems to be translating into funding for scientific research into these events, which will hopefully help us predict and minimise the impact of future events. As a geologist living in New Zealand, I am acutely aware we live astride a plate boundary with active subduction occurring off our east and south-west coasts. There have been efforts to improve tsunami monitoring systems and provide a tsunami warning network (through sirens, public broadcasting, the internet, and text messages).
I am sure many of you are based near subduction zones also. It would be interesting to hear what, if any, tsunami preparations have been implemented in other areas. I do not believe that any Leapfrog products have been used for plate boundary scale models of geology as yet, but if so it would be great to hear about that too.