August 26, 2012
Scientists Suggest Tsunamis Made Much Worse Because Of “Pop-Up” Sediments
Lawrence LeBlond for redOrbit.com - Your Universe Online
Why do some tsunamis, such as the devastating one that struck Japan in March 2011, occur on a much larger scale than scientists expect? The answer, based on a model developed by researchers at the University of Cambridge, lies within the “pop-up” movement of large amounts of sediment.
Undersea earthquakes can release huge amounts of energy as tectonic plates slip across each other´s ridge lines. A study in Earth and Planetary Science Letters suggests that wedges of sediment scraped from these tectonic plates can pop up, giving the resulting tsunami a big boost.
The research also suggests that spotting these wedges of sediment could vastly improve the prediction of large tsunamis.
Professors Dan McKenzie and James Jackson of Cambridge´s Department of Earth Sciences described in the work that what made the Japanese tsunami so severe was the huge collapse of soft material on the seafloor, displacing an equally large amount of water above it, all due to the equally as massive earthquake.
In the case of the Japan earthquake, the fault is a plate boundary, allowing the Pacific sea floor to slide beneath Japan (a subduction zone). The wave formed at the sea surface, as the sea floor moves, can cause untold damage when it hits shore.
McKenzie explained that their pop-up idea arose from studies of the sea floor surrounding the Japan earthquake. “A Japanese submersible went down after the Tohoku earthquake and found not at all what everybody expected - which is that actually the tsunami was generated by the Japanese plate overriding the Pacific plate and moving upwards,” he told BBC News℠ Jason Palmer.
“But what this submersible found was... there was a fault which had moved the other way: instead of Japan moving upwards over the Pacific (plate), actually Japan had moved downward,” he said.
“Data from the Japanese earthquake show a movement of more than 200 feet. Rocks can bend–but they cannot bend to that extent and, anyway, the rocks that moved were sloppy sediments with little strength,” added Jackson. “This suggests that something else was taking place to increase the movement several fold. It was this massive movement that caused the tsunami that swamped the coast of Japan and beyond with such terrible consequences.”
McKenzie explained that when the plates violently slip, a wedge of sediment is squeezed, firing the material upward and outward, turning the energy of the plate into the energy of the ejected matter.
“Let's say you have something wedge-shaped on the floor and you jump on it, the wedge will shoot sideways,“ Prof McKenzie explained. “The weight of the stuff on top shoots a wedge of stuff out. Put some water on the floor and do the same thing, and that will produce a wave.”
Using their new found evidence, the team also studied other unexpectedly large tsunamis from the past, including the 1992 Nicaragua tsunami, the 2004 Sumatra tsunami, and one that occurred in Java in 2006. They found similar characteristics of these fault lines as well.
The findings are promising, the team said, because it potentially provides a means to anticipate where the greatest tsunami risks lie. The team added that their model points to the size of the wedge of sediment rather than the actual size of the earthquake is most important in producing a sizable tsunami. This model could explain why some comparatively small earthquakes have resulted in huge tsunamis.
A better analysis of which faults could result in the biggest tsunami could overcome the human factors that have hindered some warning systems, said McKenzie.
“There's a huge enterprise around the Pacific of predicting them, and the false alarm rate is very high - people evacuate a few times and then nothing happens,” he told BBC News. “That's one of the reasons they're so dangerous - every now and again you have some little earthquake like one that didn't even wake people up in Indonesia but they heard the roar of the wave coming in, but the shaking was so small that they hadn't done anything.”
“We hope that our research represents a step forward in understanding how large tsunamis occur and in what circumstances they are likely to happen,” added Jackson.