It’s like having some fabled subterranean monster beneath your garden. One minute it’s all butterflies and flowers and skipping in the sun. The next, the earth opens up to devour you.

Countries bordering the Andaman Sea and Indian Ocean have long presented the world with a vast tropical playground of beautiful beaches, rich forests and marine life, exotic cultures and welcoming local people. And the Phuket area enjoys an added attraction — historically, it has appeared all but immune to natural disaster. Even typhoons have been rare and generally weakened by the time they hit this part of Asia. "Disasters" tend to run to such things as a temporary drop in whale-shark sightings among scuba divers.

For untold years, meanwhile, a monster lurked beneath this idyllic garden. Perhaps 40 kilometres beneath the sea bottom not far west of northern Sumatra and running north under the Andaman-Nicobar islands and beyond, colossal pieces of the earth’s crust, subjected to unimaginable pressures countered by friction between these blocks, were trying to slide past one another along a 1,200-kilometre fault line. Given the current state of geophysical sciences, there was no way to anticipate when the slip would come. But come it did. In the event, it set off an earthquake measuring 9.0 on the Richter scale, the world’s biggest in 40 years, and violent enough, according to geologist Kerry Sieh of the California Institute of Technology, to make the Earth wobble on its axis.

The power of the subsequent tsunami was related to the magnitude of the quake and to the fact that this — typically for earthquakes occurring along subduction zones — had involved a vertical displacement of the sea bottom. Basically, two continents are shoving up against each other, with the India tectonic plate pushing under the Burma plate along what scientists refer to as the Andaman Thrust. In this case, the Burma plate was lifted by 10-30 metres, heaving up a massive column of seawater. Boats, even vessels directly above the fault, might not have been much affected at that point. But the upthrust water then proceeded to pour downhill in all directions. The distance between the resulting wave crests was so great that boat passengers probably wouldn’t even have been aware of them. But these waves nevertheless involved enormous volumes of water moving as fast as 800 kph.

In general, with tsunamis, the drama really begins when these behemoths encounter shallow coastal waters. Even there, few tsunamis display the flamboyant breaks so beloved by Hollywood blockbusters. In fact, the sea often first draws away from the shore, exposing the sea bottom for some distance, before flooding back, sometimes repeatedly. (This effect, where, before the first wave strikes, the sea actually recedes for several minutes from the shoreline, may indicate that the tectonic shift that triggered the Aceh earthquake, before the upward thrust, first caused a radical drop in the local sea surface.) This is an especially treacherous feature, since many people are then moved by curiosity or the promise of abundant fish out of water to walk out on the exposed seabed. When the waves finally arrive, there often isn’t time to run to high ground. As the waves slow to perhaps 50-80 kph, they lose some energy due to sea-bottom drag, but the rest is translated into increased height as, within just 10 or 15 minutes, they can soar to 30 metres or even more.

But a deceptively gentle, relatively unforbidding front often conceals the huge volumes of water to follow as they rush toward observers on the beach. The first waves are usually followed by a so-called "run-up". Apparent sea level abruptly rises by several metres, with water surging hundreds of metres in from the shoreline — in extreme cases, 1.5 kilometres or more — where floating debris, rolling vehicles and collapsing buildings all contribute to further casualties. When the surges recede, they can carry still more victims with them out to sea. This explains, in part, why accurate estimates of the dead and missing may never be known.

With the recent tsunami, the scale of the disaster was exacerbated by the very attractions that have drawn so many visitors to the region. Booming tourism had crowded the many coastal resorts and beaches that have proven such an economic boost to the region — among these, most prominently, Phuket and the neighbouring provinces of Krabi and Phang Nga. Environmentally inconsiderate tendencies, especially among smaller tourism operators, to jam right down along the waterline frequently increased the toll. Some environmentalists also argue that destruction of mangrove forests and coral reefs worsened the problem in places, since these natural barriers would otherwise have dampened the force of the waves.

Finally, casualties would have been far fewer if affected areas had had warnings. The tsunamis first struck nearby northwestern Sumatra with devastating force. Despite an earthquake warning issued by the US Geological Survey by email to Bangkok’s seismological authorities, there was no alert given to Phuket and other areas of the impended catastrophe. With warning there would have been up to two hours to try to evacuate coastal regions. (The waves hit India and Sri Lanka within about 4 hours of the quake and, according to a US Geological Survey report, eventually reached Africa, the Pacific Ocean, Hawaii and the west coast of North and South America.) Unlike the Pacific Ocean, however, the Indian Ocean/Andaman Sea hasn’t experienced anything remotely like this, at least since 1833, and authorities simply weren’t prepared for what followed.

The incredible trauma inflicted on the region promises to encourage greater preparedness in future. But we may hope that the monster beneath the garden has itself died in this disaster, and that these seaside resort destinations and local coastal communities, once they recover, will be secure from future ravages of this kind. By Collin Piprell

When the tsunami struck southern Thailand on the morning of 26 December 2004, the effect was devastating. In the first wake of the aftermath, given all the confusion and uncertainty, and given the sometimes overly sensationalist reporting by the international press, it might have seemed that the whole of Phuket's west coast had been utterly destroyed. As the dust began to settle, however, it became clear that some areas had suffered much more than others.

In fact, effects on Phuket and neighbouring areas varied radically from beach to beach. In many cases there was little or no damage or loss of life.

A tsunami, while still out in the ocean, presents a train of waves less than a metre high travelling at tremendous speeds. As it approaches the shore and meets shallow water, the wave slows and compresses; the energy from that wave is then directed upwards, increasing the wave height.

The respective impact of the tsunami was very different from beach to beach. Why was it that Patong and Kamala were hit so hard, when Surin, for example, suffered almost not at all? The most significant factor was the geography of the beaches. The topography of every beach in southern Thailand differs. In any given case, when the recent tsunamis struck, loss of human life and damage to property and the natural environment, on each beach, was determined in part by the shape of the bay, the local seabed, and the surrounding land area.

According to Professor Joe Cann of the University of Leeds Earth Sciences Faculty, in the UK, "The impact of the tsunami depends primarily on the shape of the seabed offshore in relation to the direction the wave is travelling in. Not only on how steep the seabed is, but also