This piece consolidates a number of separate entries in my journal/weblog from December 2004 and January 2005.

« home « Long article index

Learning about tsunamis from the Indian Ocean disaster of December 2004

[Tsunami as its two wavefronts cross southwest of Chile]by Derek K. Miller, January 2005

UPDATE (June 2005): Wondering how far the tsunami went inland? I have a short followup about that.

The day of the Indian Ocean earthquake and tsunami on December 26, 2004, I started posting entries to my journal that drew on the oceanography training from my degree in marine biology, my online research and writing skills, and an interest in tsunamis I'd had since writing a research paper on them a decade and a half earlier. For the next several weeks, I wrote quite a bit on the subject.

Image and animation from NOAA, January 2005.

If you find this article useful, please make a donation to UNICEF (any amount, credit cards accepted) or one of the many other legitimate relief agencies. Don't forget too that, while tsunami victims need assistance desperately, there are also many other people suffering around the world, from Africa to your home town, and donations can help them as well.


[Back to top]

My weblog entries about tsunamis became quite popular in the aftermath of the disaster, attracting hundreds of people a day from all over the world. Some visitors asked questions, and I answered them as best I could in subsequent postings. I've put them all together here and edited them for a better flow in order to make everything I wrote available in one place. They appear in chronological order, with the oldest (from the day of the tsunami) first:

General Info | Tsunami Animation | Donating via PayPal | Before-and-After Satellite Photos | Answering Questions | Why Does the Ocean Retreat First? | How Big a Quake? | Danger in the Pacific Northwest? | Why Not Australia? | How Do They Travel So Far? | Energy, Not Volume | Krakatoa | Dissipation Doesn't Mean No Damage | Death Toll | Animation of Worldwide Effects

General tsunami information

[Back to top]

UPDATE (March 2008): Students from Vancouver Film School have made a summary video infographic based on this page:

Tsunami video still

About 15 years ago, during an oceanography course that was part of my marine biology degree, I wrote a paper on tsunamis—huge surface waves created in the ocean, usually by earthquakes or other seismic events. Tsunamis are often far more dangerous than the earthquakes that generate them, and widespread Indian Ocean disaster of Boxing Day 2004 shows why.

A magnitude 8.9 or 9.0 earthquake is massive on its own: the 2004 event was the fourth largest ever recorded. Tsunamis aside, it killed tens of thousands of people on the northwest coast of Sumatra as structures collapsed. Nothing has shaken the earth's crust as hard since 1964, when a 9.2 quake took place in Prince William Sound in Alaska. Four years earlier than that, the largest quake known, of magnitude 9.5, hit Chile.

But tsunamis resulting from such earthquakes are, in many respects, the worse danger, because while even a huge earthquake is a local event, if it generates a tsunami, the effects can spread more than half way around the world. The 1960 Chile quake caused lethal tsunamis along the coast of South America, in Hawaii, and as far away as Japan; the 1964 Prince William Sound event generated waves that killed people (though not nearly as many) and did extensive damage down the west coast of North America, from Alaska to Port Alberni to the Oregon Coast and California. For most of 2004's victims, there was little or no warning of the waves, either because there was no time or because there is no coordinating tsunami warning system for the Indian Ocean,.

Think of yourself sitting in a bathtub. If you suddenly lift or drop your body, a large wave sloshes along the tub, and might spill over the rim, even if the tub is far from full. A tsunami is essentially the same thing, with an earthquake or other event moving the ocean floor up or down and displacing titanic quanitities of water. In a bathtub, you might move a couple of dozen litres of water with such a slosh, but sea floor movements have in the past displaced more than 100 cubic kilometres of water—billions of bathtubs.

The physics of water waves means that (in general) the longer the wavelength, the faster the wave travels in the open sea. While the length of typical beach waves might be a few dozen or a few hundred metres, a tsunami might be 100 km long. Away from shore, ocean waves don't actually move water very far either, which is why ships, logs, seabirds, and anything else floating can bob up and down in even a large sea swell and not travel anywhere if there isn't a wind or current. Tsunamis are the same: since it doesn't push water ahead of it as it crosses the ocean, the energy of a tsunami wave can travel at frightening speeds—up to 800 km/h, which is as fast as a jet plane.

In the middle of the ocean, a tsunami might also be very shallow, perhaps a metre or less in height—you'd never notice it among all the other, taller swells unless you had a sea-level gauge to tell you it was passing by. But as it approaches shore and the ocean floor squeezes it (like typical surf waves), a tsunami gets shorter and taller. With its stupendous energy, a tsunami can get very tall indeed, depending on the shape of the bottom. Today's earthquake generated tsunamis 10 m (30 feet) high in some places; some tsunami waves from the 1964 Alaska quake reached 24 m (80 feet).

The stereotype of a tsunami is of a wall of water crashing down, like the giant waves surfers ride off the north coast of Oahu. Some are like that—but they also come ashore and don't retreat right away, driving roiling rivers of swirling water far inland. Other times, they act more like rising tides that just keep rising, or like rivers overspilling their banks. Often, as the wave approaches, the waterline will retreat much farther from shore than usual, and some people who die are exploring the rarely-exposed sea floor there when the big wave comes in. Tsunamis can also oscillate, so that one wave will come in, destroy things, then retreat. As residents return to begin cleaning up, another wave might arrive, perhaps bigger and more violent than the first, and maybe even hours later.

Most disheartening, tsunamis are beyond human control, and sometimes even prediction, more than nearly anything. They are not a consequence of the weather, or climate change, or any kind of human activity—or of any living thing. They're not even affected by the sun or moon. Earthquakes and tsunamis are simply the result of this humungous planet cracking and popping as it goes about its business, as it has for billions of years, and will for billions more.

Indian Ocean tsunami explained visually

[Back to top]

This excellent animation (much larger 3.6 MB QuickTime version and 760 KB extended-time GIF version also available) from the U.S. National Oceanic and Atmospheric Administration (NOAA) shows why Sri Lanka, in particular, was hit so hard by yesterday's tsunami: the quake was not a focused point on the ocean sloor, but a north-south line near Indonesia, and the most intense wave moved directly west, unobstructed, across the Bay of Bengal to Sri Lanka and southeastern India. That also explains why the wave could reach Africa and still kill people thousands of kilometres and several hours away.

Incidentally, the Wikipedia, which is collectively written and edited by anyone with a web connection who wants to contribute (and where I found the animation), has turned into a stupendously thorough news and information resource about this disaster. In addition to the links below, it includes a series of other graphics that help explain the event, and the main news article has been edited more than 500 times, with each change refining and deepening the coverage with material from all over traditional and non-traditional media worldwide. It is the best starting point for finding out about the catastrophe by far. Darren Barefoot also has some good links to first-person accounts.

Tsunami relief donations via PayPal

[Back to top]

PayPal began accepting direct donations for UNICEF a few days after the tsunami. Before that, writer Kevin McDonald set up his own PayPal donation system that ended up raising more than $9000 USD before January 10, 2005.

Initially, I searched around for a way to donate some of the extra money in my PayPal account to tsunami disaster relief in Asia, but none of the major aid agencies, nor PayPal itself, had a way to do that. ("Hint hint!" I wrote. "Amazon's doing it! Get with it, guys.")

PayPal did eventually get with it, and other organizations started to see how PayPal—and the extra money people like me often leave in it—could be an efficient way to receive donations. I found Kevin's page, and while I don't know him, and couldn't vouch directly for the donations, his site looked good, and routed the money through AmeriCares, which is a reputable relief charity.

You can donate to UNICEF directly. I sent $20 USD—like me, I'm sure you can forgo some of those eBay purchases, right?

Before and after tsunami photos: Banda Aceh, Sumatra, Indonesia

[Back to top]

This 1.0 MB PDF file from Digital Globe, a satellite photo service, includes copies and analysis of overhead photos of the city of Banda Aceh, Indonesia, from before and after the Boxing Day tsunami. After looking at them and imagining similar damage where you live, you can understand why tens of thousands are dead in that city alone.

Answers to tsunami questions

[Back to top]

Following my earlier posts about the Indian Ocean tsunami, a couple of people have asked questions about it. Here are my responses. Keep in mind that I am not a tsunami expert, just someone with a marine biology degree who did some research on them 15 years ago, and who has maintained an interest in this fascinating but awful phenomenon.

Tsunami "drawdown": why did the water retreat first?

[Back to top]

The first was from Petula Brown, who was actually vacationing in Phuket when the tsunami hit and witnessed it first-hand. She wrote:

I am still confused about the water retreating as where we were the water disappeared for five or nearly ten minutes allowing people to wander out to investigate more. Why would the water remain out for so long. The first wave was more like a fast rising tide but the waves to follow were crashing monsters destroying everything in its way. Fortunately the sleeping rooms of our resort were 105 steps up from the beach so people were injured but no one from our resort died.

Dr. Stephen Nelson's very technical page from Tulane University provides an explanation of why the water retreated before the tsunami waves arrived:

If the trough of the tsunami wave reaches the coast first, this causes a phenomenon called drawdown, where it appears that sea level has dropped considerably. Drawdown is followed immediately by the crest of the wave which can catch people observing the drawdown off guard. When the crest of the wave hits, sea level rises (called run-up). [...]

Because the wavelengths and velocities of tsunami are so large, the period of such waves is also large, and larger than normal ocean waves. Thus it may take several hours for successive crests to reach the shore. (Fora tsunami with a wavelength of 200 km traveling at 750 km/hr, the wave period is about 16 minutes). Thus people are not safe after the passage of the first large wave, but must wait several hours for all waves to pass. The first wave may not be the largest in the series of waves. For example, in several different recent tsunami the first, third, and fifth waves were the largest."

Essentially, the low water level is just as much a part of the tsunami as the big wave, because tsunamis are waves with very long wavelengths, and waves have both crests (the peak of the tsunami wave) and troughs (the bottom of the same wave).

If the trough reaches shore first, the water level will lower dramatically, far below the normal lowest tide, before the crest comes rushing in. Some areas will encounter the peak first, some the trough, which is why low water levels are not always present before tsunami waves arrive. Following the crest that caused the initial destruction, the following trough is what dragged people and debris back out to sea, followed by the subsequent waves.

How big does an earthquake have to be to create a tsunami?

[Back to top]

It's not so much how big, as what kind and where. Obviously, a huge earthquake far inland can't create a tsunami in the ocean, but Walt Davis had an interesting question:

I read your article on tsunami wave and found the information useful. I read a U.S. Goverment paper that stated that only a 7.5 quake or larger can produce a tsunami. Do you know if this is corrrect?

As far as I understand it, that's not true, because it doesn't have to be an earthquake that causes a tsunami, and it has more to do with the type and location of the earthquake as well as the topography of the sea bottom and shoreline than earthquake magnitude.

A tsunami can arise (at least locally) from something like a landslide in a lake or fjord. The key is that a significant amount of water has to be displaced suddenly—if it is confined to a narrow channel, "significant" may not be all that much, compared to big tsunamis like that in the Indian Ocean. Some examples from my part of the world:

You could, however, argue that many of these events are simply monstrous waves, and not really tsunamis—if your definition of a tsunami is that it must be not only big, but energetic enough to cross significant stretches of ocean. It may be that a large, ocean-wide tsunami created by a seafloor quake alone does require a magnitude of 7.5+ to generate enough energy, but there are also many other circumstances (volcanic eruptions, meteor impacts) that could do so otherwise.

In addition, a big quake that moves laterally (sideways, not up or down) can be way above 7.5 magnitude, but not cause a tsunami—which is why the 1906 San Francisco quake and later smaller ones in southern California, which occur on the lateral-moving San Andreas fault, haven't created tsunamis.

Risky geology off the Pacific Northwest coast?

[Back to top]

Finally, Walt also asked:

Another questain is about a underwater fault line that runs off the Northwest coast. Do you happen to know the name of it.

There are five separate tectonic plates in this region: in order of size, the Pacific Plate, the North America Plate, the Juan de Fuca Plate, the Explorer Plate, and the South Gorda Plate.

The two major north-south lateral faults are the San Andreas, running through California and reaching the sea at San Francisco Bay, and the Queen Charlotte, running just west of the Queen Charlotte Islands. There are ocean spreading ridges between some of the plates, and the Cascadia Subduction Zone forms a trench just offshore from Vancouver Island and the Washington, Oregon, and northern California coasts.

I guess the best name for what you're looking for is the Cascadia Trench or Cascadia Subduction Zone—that's the region most prone to a strong vertical earthquake, and to potentially generating a tsunami.

More tsunami answers: why not Australia?

[Back to top]

Following up on my earlier answers to tsunami questions, Garrison Moore of Virginia in the U.S.A. asked:

I am wondering why the Tsunami did not hit Australia which on the map seems to be at least as much on the path as Africa.

Flat maps can be deceiving. If you look at the animation here, which better represents a spherical view of the earth, or this wider view, you can see two things:

  1. As I mentioned in an earlier post on my site, tsunamis are not necessarily always circles of waves emanating from a point source; they can be directional. In this case, the main paths of the tsunamis were directly to the west and east of the earthquake, not so much north and south. So the hardest-hit areas were directly west and east, regardless of distance. Northwestern Sumatra and Thailand, Sri Lanka and India, and even Somalia (thousands of kilometres and hours away) got big waves. Bangladesh (which is not only nearby, but very low-lying, and so would seem particularly vulnerable), southern Sumatra, much of Burma, Antarctica, and other areas had much smaller waves and less destruction.
  2. Australia is not only southeast of the earthquake, it is somewhat shielded from it by most of the islands of Indonesia, including Sumatra itself.

The combination of the directional tsunamis and the significant shielding meant that—unlike other partially shielded areas (southwestern Sri Lanka and the southwest coast of India, for instance), where the tsunamis were powerful enough to diffract around the coastlines and still cause damage—even the exposed northwest coast of Australia saw only relatively small effects: larger swells and surf, but no tsunami-style run-up and destruction.

How can a tsunami go so far?

[Back to top]

Bill Copeland of Binnington Copeland & Associates in South Africa wrote to me to ask about why tsunamis travel so far. I feel a bit out of my league trying to explain wave behaviour to an engineer, but hey, being out of my league never stopped me before, so here we go...

Dear Mr Miller,

I am a Professional Engineer and am mystified by the fact that the Engineering principal of the inverse law does not appear to apply to Tsunami waves. Whilst I can understand that a big bang under the water will create a large vertical movement of water above it, I fail to understand why the volume of displaced water does not remain constant, and the amplitude of the wave diminish in proportion to the distance it travels (due to the larger circumference or increasing wave front). From reading the reports of the 26th December 2004 tsunami, one can only gain the impression that the wave front hitting the various countries was many thousands of kilometers long, and the volumes of water huge, which appear to have been much greater than the volume which could have been generated by the original earthquake.

Can you or your advisors help me please?

Energy, not volume

[Back to top]

I don't have any advisors (other than Google!), but as far as I understand it (and I'm neither an engineer nor a geophysicist), it's not as though tsunami waves don't dissipate like others, it's just that they contain so much energy that even as they do, they remain powerful enough to destroy and kill. And it does not have to be amplitude that changes; so can period and wavelength, or the number of waves in the train.

Tsunamis are also not so much about volume displacement as about energy transfer. The water shifted above a quake does not move across the ocean, i.e. a log floating at the surface above the epicentre would not have been carried to Thailand or Somalia. So the water that drowned people in Somalia was African water, and that in Thailand was Thai water—just the energy that pushed it came from the Andaman sea floor several hours away.

A tsunami wave train moves as ocean swells do, by raising and lowering the water level as it passes by. In mid-ocean, a tsunami is unnoticeable without a sea-level gauge, since it might take an hour to raise and lower sea level by one metre—and you'd never notice that with all the bigger swells out there. But the energy contained in that slow, slow peak and trough is (as we have all seen) stupendous.

Sound around the world

[Back to top]

Think of another, similar event in the same part of the world: the eruption of Krakatoa in 1883. Ignore the tsunamis and earthquake damage and focus only on the sound. At the site of the eruption the bang surely would have been deadly on its own, purely from sound pressure, like the shock wave of an atomic bomb. It dissipated rapidly as it spread outward, but even in South Australia it sounded like nearby dynamite being blasted.

More recently, on a Sunday morning in May, 1980, my father rushed downstairs because he thought our water heater had exploded. It turned out that Mount St. Helens, hundreds of kilometres southwest of us in the United States, had erupted that morning, and when the sound reached Vancouver it was still loud enough to seem like an explosion within the house.

Of course, like sound, energy does dissipate as a tsunami moves. The energy expended at and near the site of the quake is much larger than across the ocean, which is why buildings in northwestern Sumatra collapsed, totally aside from the tsunami, but in India and Africa the quake itself only registered on instruments.

Less powerful, but still powerful enough

[Back to top]

Tsunamis are remarkably efficient ways of moving energy, but the waves that reached the coast of Africa were not by a long shot as energetic as those that pounded Sumatra and Thailand. They might have been just as high by the time they ran up beaches—that is determined more by the slope of the sea floor than by the strength of the original source of the waves—but they did not travel as far inland in Tanzania as they would have on some theoretically identical beach in Sri Lanka, or in Indonesia. I suspect that the series of waves in the wave train was smaller at farther shores too, and that there were fewer noticeable waves on African shores than Southeast Asian ones.

The mathematics of quake magnitude are beyond me, but this may help:

Down at the bottom, the chart indicates that this quake released the energy of the explosion of several billion tons of TNT. You could probably strew that several billion tons very thin, along all the coastlines affected by the tsunami, and if you detonated it, you'd still kill a lot of people. You could throw most of it away, and strew what was left along those shores, and maybe kill the same number of people.

By the same logic, even a tsunami that is significantly less energetic than when it started can still be damaging, or deadly. If you live 100 metres from the shoreline on a nearly flat piece of land, a wave that reaches 200 metres inland will drown you just as well as one that reaches a full kilometre. If you're trapped in a hut completely consumed by brown swirling water, whether it's two metres or ten metres to the surface doesn't matter. If a tsunami engulfs the tree you're clinging to for seven minutes, you're just as dead as if it lasted fifteen—even if only half the volume of water passed through.

By the horrible numbers

[Back to top]

And, in the end, the numbers tell the story: nearly 100,000 dead in Indonesia, 50,000 in Sri Lanka, 10,000 in India, 5,000 in Thailand—and 300 in Somalia. It's an imperfect measure, and gruesome, but (taking into account the directionality of the tsunami, differing coastal population densities, and energy absorbed by intervening land masses and sea floor features) that looks like a clear inverse distance relationship to me.

Worldwide tsunami animation

[Back to top]

This has been all over the news already, but the animation from the U.S. National Oceanic and Atmospheric Administration (NOAA) showing how the Indian Ocean tsunami spread around the whole world (2.8 MB QuickTime) is still worth a link.

Notice how two fronts from the tsunami actually cross and interfere with one another in the southeast Pacific, between Chile and Antarctica.

If you find this article useful, please " title="[Donate any amount to UNICEF]">make a donation to UNICEF (any amount, credit cards accepted) or one of the many other legitimate relief agencies. Don't forget too that, while tsunami victims need assistance desperately, there are also many other people suffering around the world, from Africa to your home town, and donations can help them as well.

« home « Long article index

Page BBEdited on 6-Mar-08

© 2004–2008 Derek K. Miller. Some rights reserved. You may use content from this site non-commercially if you give me credit, under the terms of my Creative Commons license.
Valid XHTML 1.0!