NAF-Tsunami-Phil Cummins

AFTER THE TSUNAMI – HARNESSING AUSTRALIAN EXPERTISE FOR RECOVERY
Canberra, 31 March 2005

The Indian Ocean tsunamis – science and seismics
Dr Phil Cummins
Geoscience Australia

As a scientist involved in monitoring earthquakes, there is a tendency for me to think that my work is done after the tsunami has passed, but as we have seen from the recent earthquake and also as demonstrated by this lithograph from the 1861 Sumatra tsunami, these earthquakes have happened in the past and they are going to happen in the future, so the work is actually never done.

My talk, unlike the previous talks, will be focused on the science behind tsunami. I will just talk briefly about the physics and why I think we perhaps could have better anticipated the Boxing Day tsunami. I will go on to describe the science behind what happened on Boxing Day and just talk briefly about comparing the very recent earthquake with the Boxing Day earthquake, and finally I will talk a bit about suggestions for future research.

Plate tectonics – subduction zones
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This illustrates plate tectonics; I thought I should discuss this briefly. As most of us probably know, the Earth is expelling heat and its interior is convecting, much like a pot of boiling water. Rising material comes up and forms the rigid tectonic plates which cover the Earth’s surface. These move away from these ridges, which are called spreading centres, and eventually cool and densify enough to sink back into the Earth’s interior.

Where this happens you have what is called a subducting plate and an overriding plate. They join in an interplate interface, which is a massive thrust fault called the megathrust. That is where most tsunamis are generated. It is certainly the scene of the largest earthquakes in the world, much larger than any earthquakes that occur on the interior of the plates.

You can see these kinds of features are readily evident when you look at a plot of seismicity, earthquakes on the globe colour coded with depth. All these shallow earthquakes generally occur where these spreading ridges are, and the subduction zones are illustrated by the deep earthquakes. There is actually a progression from shallow to deep along the subducting plate.

Earthquake excitation of tsunami
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So how does an earthquake actually excite a tsunami? That is illustrated in this figure. We have the subducting plate – in this case it is the Indian or Australian plate – sliding beneath Sumatra. If the interplate contact exhibits stick-slip friction, it drags the overriding plate down with it and it deforms the overriding plate. It will come down underneath the ocean, it will be pulled down, but it will also be pushed down more landward, and the entire overriding plate will be pushed landward in a horizontal sense.

Eventually the stress building up on this interplate contact will exceed the strength of the friction, and the plate will simply pop back up into position. It will move a mass of water vertically, and that is what causes a tsunami. You will notice also that on this [right-hand] side of the earthquake there will be a draw-down of water. That is seen as a receding wave, so there is actually a natural warning if you are on this side of the earthquake.

Eventually this tsunami will propagate out, and in some cases it can propagate and still cause damage at great distances.

Comparative tectonics: M>8 Earthquakes: Japan vs Indonesia
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The really big tsunami are caused by magnitude-9 class earthquakes, and these tend to occur where young oceanic lithospheres are subducting. That is illustrated in this figure, where I am comparing Japan with Indonesia. What you see from this colour coding is that this orange and yellow colour indicates a young oceanic lithosphere in the south-west of Japan and off Sumatra, in the case of Indonesia, but if you go to north-east Japan or off Java you have relatively old lithospheres subducting. I have plotted here the magnitude-8-and-above earthquakes, and you will see that where old lithospheres subduct you don’t get many earthquakes above magnitude 8. You do get a few of these normal faulting earthquakes that are in a reverse sense of the thrust earthquakes that I have been talking about. They do occur out here [somewhere on slide!] but generally the earthquakes are smaller than you get where you have young lithospheres subducting.

By and large, the really massive earthquakes – the magnitude-9 earthquakes – occur where young oceanic lithosphere is subducting.

Tsunami generated by the great Sumatran earthquake of 1833 (Mw=9.2)
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So I think, based on that kind of work, comparative tectonics, but also based on the earthquake history in Sumatra and on geodetic measurements of the strain buildup – the way the crust moves and accumulates strain energy – that it really should have been possible to anticipate that Indian-wide tsunami could occur. In fact, we were looking at that last year and did some simulations of the tsunami caused by the great 1833 Sumatra earthquake, and indeed came to the conclusion that it could affect the entire Indian Ocean basin.

I think it is worth reflecting on why this wasn’t appreciated sufficiently, prior to the earthquake, because I think the evidence was there.

Seismic waves generated by the great Sumatra-Andaman earthquake of 2004
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But I will leave that for the discussion groups and go on to discuss what actually happened on Boxing Day. This was a great earthquake that occurred off Sumatra. The first indication of it was the seismic waves, and that is what is illustrated here. These are the ground motion recorded at seismic stations distributed throuthout the globe. Time is going this way [y axis?], so you see the first arriving waves are the largest; distance goes this way [x axis?], so it arrives at close stations first. And these waves are propagating all the way to the other side of the globe – that is 0° and 180°, so this wave has gone all the way to the other side of the globe, it has wrapped back around and these waves, which are roughly 20 seconds period, are still, although you can’t really see the scale, of roughly half a centimetre to a centimetre amplitude as they travel around the Earth over hours. So these are really massive waves.

And yet they don’t really indicate the size of this earthquake. To see that you really have to look at very long-period energy that is not visible on these seismograms. But I just wanted to plot this to illustrate that these are massive waves. You couldn’t actually feel these, because of the long period, but they are of the scale we could actually see.

26 December Sumatra earthquake
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The earthquake appears to have ruptured a very long segment of the Sumatran–Andaman subduction zone. The first indications were that there was a magnitude-9 earthquake that ruptured about a 500-kilometre segment. Some of the analysis of seismic waves and certainly tsunami waves indicated that it was a somewhat larger source, about 1,000 kilometres in length, along the subduction zone. But the aftershocks and crustal deformation indicate that it was 1,300 kilometres. So there has been some confusion about the magnitude of the earthquake, but the length of subductio