The devastating megathrust earthquake of December 26th, 2004 [00:58:53 UTC] occurred on the interface of the India and Burma plates and was caused by the release of stresses that develop as the India plate subducts beneath the overriding Burma plate. The India plate begins its descent into the mantle at the Sunda trench which lies to the west of the earthquake's epicenter. The trench is the surface expression of the plate interface between the Australia and India plates, situated to the southwest of the trench, and the Burma and Sunda plates, situated to the northeast.

Yet, from all the reading I've done in the last three days, it seems clear that predicting tsunamis is a very inexact science indeed. The automated Pacific system takes advantage of many bouys measuring the ocean level, not present in the Indian Ocean, and yet (from what I could tell) even this system often leads to false alarms, leading certain coastal peoples in Hawaii to blithely ignore warnings. Certainly, just from the basic earthquake data (and basic doesn't seem quite the right word reading even a small part of the data) and thus knowledge (?) of the depth and likely movements of the sea bed, it is well beyond the current ability of mankind to predict exactly what waves will be generated and propagated and how they will then inundate various coastlines. The section on "Earthquake size and tsunami size" at www.nerc-bas.ac.uk/tsunami-risks/html/CauEq1Size.htm links to everything you are likely ever to want to know about what we don't know - easily the best general tsunami site I came across in my own travels, which to my surprise was from NERC in the UK. More details to be given in the Shape Of The Tsunami.

For historical perspective it's worth looking at the following passage from the USGS Q&A here

Question: What other significant tsunamis have occurred in the region?

The following destructive tsunamis are listed on a data base maintained by the Tsunami Laboratory, Institute of Computational Mathematics and Mathematical Geophysics (http://tsun.sscc.ru/tsulab/20041226tsun.htm)

1. 1797/02/10 Central part of the western Sumatra. The quake was most felt near Padang and in the area within +/-2 deg of equator. Padang was flooded by powerful waves. More then 300 fatalities.

2. 1833/11/24 South coast of the western Sumatra, estimated rupture from 1 S to 6 S latitude. Huge tidal wave flooded all southern part of the western Sumatra. Numerous victims.

3. 1843/01/05 Strong earthquake west of the central Sumatra. Terrible wave came from the south-east and flooded all the coast of the Nias Island. Many fatalities.

4. 1861/02/16 Exceptionally strong earthquake affected all the western coast of Sumatra. Several thousand fatalities.

5. 1883 Krakatau explosion 36,000 fatalities

A gap of 121 years is quite honestly a reasonable time for complacency to set in. Before that it seems there was something about every twenty years. But nothing of the scale of the current disaster? Even that is difficult to tell of course. The much greater population today on the coasts of Sumatra, Sri Lanka, India and Thailand also comes into play. And the two events that seem to most resemble this one were just ten years apart, in 1833 and 1843.

This gives some, but not all, of the context needed to evaluate the following letter to the London Evening Standard yesterday:

Fingers are being pointed at the stingy response of governments to the Indian Ocean disaster. But the great tragedy remains that tens of thousands of lives could have been saved if the authorities in the first places to be hit had promptly warned the media in neighbouring countries.

The governments concerned are so embarrassed that they are now calling for high-tech tsunami warning systems. But these should not be used to dodge the shameful failure of existing structures. The quake itself was a warning of impending floods but, incredibly, it appears that the Thai meteorological authorities who knew about the quake didn't issue a tidal-wave warning "in case it upset the tourists". The US monitoring authorities reportedly said that they couldn't get through to the right places. Wouldn't a call to CNN have been enough?

It is also extraordinary that only a year ago some authorities in the region claimed that tsunamis were unlikely there. Serious inquiries are needed.

-- Piers Corbyn, Weather Action, London SE1

Based on my own reading I find that last paragraph odious humbug, typically ridiculous and self-righteous media wisdom by hindsight and, in the light of 100-200,000 dead, in the worst possible taste. The rest however is not so easy to discount, from the little I know. Certainly Sri Lanka and India surely could and should have been effectively warned, based on the experience of places nearer the original quake. But even this counsel of perfection depends on early appreciation of the Shape Of The Tsunami. Whatever was understood, the idea of contacting CNN at once seems perfectly reasonable. But, on the other side, surely nothing could have saved the perhaps 100,000 victims in Sumatra? Nothing that it is in the least reasonable to blame others for now. The case of the Thai authorities may be different. How much did they really understand the risk though? It was very much in their own self-interest not to have the disaster that in fact occurred, with many tourists likely to be put off the area in the future.

Here was the take of The Times by yesterday:

Earthquake monitors tried in vain to warn of problems

From Tim Reid in Washington

December 30, 2004

The desperate attempts of three US earthquake monitors to warn Indian Ocean nations of the approaching tsunami emerged last night. The geophysicists at the Pacific Tsunami Warning Centre in Honolulu, part of the US National Oceanic and Atmospheric Administration, were on duty on Christmas Day when the massive subterranean earthquake struck off Sumatra. They knew for hours of the impending catastrophe.

A mere 18 minutes after the earthquake, the officials issued a Pacific-wide e-mailed tsunami warning, but realised that most potential victims were not among the centre’s Pacificnation clients, because the biggest impact would be in the Indian Ocean. The officials in Honolulu knew disaster was approaching, but also knew that the Indian Ocean, where tsunamis are extremely rare, has no oceanwide tsunami detection system, and no warning system.

The officials told the Honolulu Advertiser that they realised there was no way to alert potential victims because they did not know who to contact. Charles Mc Creery, director of the National Oceanic and Atmospheric Administration’s Honolulu centre, said: “We tried to do what we could. We don’t have contacts in our address book for anybody in that part of the world.”

His colleague, Barry Hirshorn, said: “We talked to the State Department Operations Centre and to the military. We called embassies. We talked to the navy in Sri Lanka, any local government official we could get hold of.” The officials worked through the night sounding the warning as the tsunami continued to sweep across the ocean. But their efforts were in vain.

Thailand’s meteorological department knew of the earthquake an hour before the tsunami struck and issued warnings, but may have been overcautious for fear of damaging tourism. The Thai Government has begun an inquiry.

The questions raised here are worth writing about at the end of the first week. But we must also do something about the repugnant Culture Of Blame that so often applies in the UK in such situations, in the media and to some degree in the general populace. Our generosity in the face of such disaster is the other side of both coins. The very lack of ability to predict this tragedy is surely part of what moves so many to give - that and the fact that many of the more affluent have spent holidays on these very same beaches and feel at least some guilt for the horrendous and unforeseeable loss of and to those local people who normally serve them so cheerfully.

I still doubt the story that "they knew for hours of the impending catastrophe" and did everything they could yet no thought was given of a call to CNN. It was still Christmas Day in the USA when the above detection of the earthquake was made. This has to have been a factor in the lack of alacrity and inventiveness in the response.

There are other shorter-term ways of predicting a tsunami. One that may be investigated in future is that no animals seem to have perished alongside a number of human beings in a wildlife park in Sri Lanka that was inundated. The rule that all human beings should move away from the coast when all animals do may I guess lead to a few false alarms! But in conjunction with other factors, such as detection of the original quake, this would appear a fruitful line of inquiry.

Even more surprisingly, up to five minutes warning is often given by the sea itself, as outlined in www.noaa.gov/question.html

Ironically, in deep, open-ocean water, tsunami waves are often less than a meter high and can travel at speeds up to 1,000 kilometers per hour (621 mph). However, as a tsunami wave approaches shallower waters along the coast, the leading edge of the wave begins to slow down while the rest of the wave begins to “pile up” behind it—causing it to grow in height while maintaining its strength. The crest of this wave can be several meters high by the time it reaches the shoreline. Sometimes, however, the crest of the wave isn’t the first to arrive—the trough is. This is often the case when the tsunami originates from an oceanic earthquake associated with land subsidence or sinking, which causes the water column to drop down at the earthquake site. In this case, instead of extremely high water levels, the first sign of a tsunami is what appears to be an unusually low-tide. Although onlookers might be intrigued by this unusual site, this major withdrawal of the sea should be taken as a warning that a tsunami wave will soon follow.

As the coastal ocean waters recede from the shore, it often leaves large portions of the sea floor exposed. Individuals who do not recognize this as a common precursor to tsunami waves often find themselves gravitating toward the exposed shore. Unfortunately, they often perished as they rush to gather fish left high and dry on the exposed beach or to view never before seen rock and reef formations—only to be hit moments later by the incoming wall of water. Experts believe that a receding ocean may give individuals more familiar with “nature’s tsunami warning signal” as much as a five-minute warning to evacuate the area. This cycle may be repeated several times as successive wave crests arrive five minutes to an hour apart. Seek higher ground and stay out of danger areas until an "all-clear" is issued by a competent authority.

Unlike so many other missed opportunities to warn the unwary, a ten-year-old English girl apparently saved one hundred people on a Phuket beach through remembering this aspect of her recent geography lessons. This was the Lead Story of The Sun in the UK on 1st January but published on the Web in full at another News International site, nypost.com/news/worldnews/37579.htm:

The adults were curious, but Tilly froze in horror. "Mummy, we must get off the beach now!" she told her mother. "I think there's going to be a tsunami." The adults didn't understand until Tilly added the magic words: "A tidal wave."

Her warning spread like wildfire. Within seconds, the beach was deserted — and it turned out to be one of the only places along the shores of Phuket where no one was killed or seriously injured. Last night, Tilly was being hailed as a savior. "I think it's phenomenal that Tilly's parents and the others on the beach are alive because she studied hard at school," said Craig Smith, the American manager of the JW Marriott Hotel where Tilly's family was staying.

The story was also summarised in The Times (here)

Girl’s sea warning saved a hundred

A girl aged ten saved a hundred fellow tourists from the tsunami because of a geography lesson about the giant waves. Tilly Smith urged her family to get off Maikhao beach in Thailand after seeing the tide rush out and boats on the horizon begin to bob violently.

The youngster, recalling a recent school project on quakes, turned to her mother Penny and said: “Mummy, we must get off the beach now. I think there is going to be a tsunami.” Penny and her husband Colin alerted others and they cleared the Phuket beach just in time. It was one of the few beaches where no one has been reported killed or seriously injured.

Last night Tilly, from Oxshott, Surrey, told The Sun that credit for her quick-thinking should go to Andrew Kearney, her geography teacher at Danes Hill Preparatory School.