17 December 2013

The 500 cubic kilometre landslide that we all missed – triggered by the Tohoku earthquake

Posted by Dave Petley

The Tohoku Earthquake tsunami

The UK environment research council NERC has a press release out today about work that was presented last week at the AGU Fall meeting in relation to the Tohoku earthquake in Japan.  There has been some investigation of the terrestrial landslides triggered by the event, but of course most of the interest has been focused on the tsunami.  There has always been something of a mystery about the Tohoku earthquake tsunami, in part because it was so large and in part because its behaviour is a little strange.  In particular, the tsunami had two distinct peaks some 20 to 30 minutes apart.

Tohoku earthquake


A giant submarine landslide

The press release reports on as yet unpublished research that investigated the tsunami using wave buoy data and modelling, demonstrating that the wave cannot be explained by the earthquake alone.  This suggests a secondary source – most likely a submarine landslide – the potential location of which they tracked down using the wave buoy data.  Dave Tappin of the BGS explains:

‘Using maps of the seabed [I assume this means sonar data], we identified a landslide that was 40 kilometres wide, 20 kilometres long and 2 kilometres thick. That makes it 500 cubic kilometres, so it’s pretty big.’

‘We then used computer models to simulate the tsunami from a dual source; the earthquake and the landslide, and this gave us the high water levels along the north Honshu coast.’

‘An additional check on the landslide source was from an analysis of the wave frequency at the buoys, which showed a high-frequency component that could only be from the landslide.’

This is of course an enormous landslide – the volume is impressive in itself, but a slide with a thickness of two kilometres is genuinely amazing.  I am really looking forward to seeing the submarine imagery of this event.  The Tohoku earthquake and tsunami killed about 16,000 people and left about 2,700 missing.