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!

Location of J-FAST drilling program

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).

Tectonic Setting of New Zealand

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.

3 thoughts on “The risk of megathrust generated tsunamis”

  1. This is certainly attention-grabbing, You’re extremely qualified blogger. I have registered a person’s rss and appear onward to trying to find further within your excellent posting. Furthermore, We have contributed your internet site around my web sites

  2. Interesting detail. Drilling the start zones. Just like doing profiles in avalanche start zones. Same problem. Weak slick layers.

    As to your other question. We had a quake here off shore of Vancouver Island last night located part way to the Cascadia Subduction line. Lots of people felt it shaking and posted about it on Twitter and FaceBook. That’s how most of us found out about it. No one I know knows of any Canadian government sites for information. I immediately went to the USGS earthquake site to figure what all the postings were about. The tsunami notice was issued 31 min after the earthquake from the Alaska Tsunami Centre. Canada? Not so much.

    6.6 – 94km S of Port Hardy, Canada
    in Kyuquot Sound off of Vancouver Island
    2014-04-23 20:10:12 UTC-07:00

    The reason I wound up at your site was I was googling the distance from shore and the time of arrival of the last 2 great mega thrust earthquakes to try get a handle on how much time we would have to evacuate if it took 31 minuits for the warning to be issued and the Subduction fault was 60 some miles off shore.

    Strange small world we live in. Nice website.

    gragor

    1. As I understand it you have a similar tectonic setting up towards Vancouver as off New Zealand. It always surprises me how long it takes for tsunami waves to propagate across the oceans, but then again there are usually vast distances involved. That being said, it is usually the near shore events that are the most deadly. You really need good warning systems in those situations, but even then they might not help. Another worry are earthquakes that generate submarine landslides that in turn generate huge localised tsunamis – see http://en.wikipedia.org/wiki/1998_Papua_New_Guinea_earthquake. While the location and/or size of the earthquake might not cause a tsunami, it might trigger an undetectable submarine landslide elsewhere that does cause a tsunami.

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