Half of the world's most densely populated cities are located in areas vulnerable to earth-quakes - and there is a very good reason why. James Jackson explains

It has already been a bad century for earthquakes, with well over a quarter of a million people killed in just four catastrophes: at Bhuj in India (2001; 20,000 dead), Bam in Iran (2003; 40,000 dead), Banda Aceh in Indonesia (2004; more than 200,000 dead) and Muzaffrabad in Pakistan (2005; 80,000 dead). As a seismologist, I am frequently asked whether disastrous earthquakes are more common now than they were in the past. It is an easy question to answer. Over the past thousand years, I know of 110 earthquakes that each killed more than 10,000 people - 34 of them happened in the past century. In other words, a disaster of that magnitude happened, on average, about every 11 years from 1000 to 1900AD, about every three years for the next century and now virtually every year.

So, the answer to the question is obviously "yes", and in fact the dramatic increase in the number of earthquakes started in about 1600. But the emphasis must be on "disastrous" - the phenomenon is not caused by any change in the natural behaviour of our planet, as earthquakes themselves are no more or less frequent now than they ever were, but by the way we now live and by our historical response to the natural environment.

In February 1994, the small desert village of Sefidabeh in southeast Iran was destroyed by an earthquake of moderate size (magnitude 6.1). Most of the 300 buildings in Sefidabeh collapsed. They were built from adobe, or sun-dried mud-brick, the traditional indigenous building material from which both walls and heavy roof domes are commonly constructed in the desert. Adobe is a notoriously dangerous material in earthquakes, and the fact that only six people died was because, luckily, the earthquake happened when most people were outdoors.

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But there is more to this story. Sefidabeh is a desperately remote and inhospitable location, sandwiched between the two deserts of the Dasht-e-Margo ("desert of death") in Afghanistan and the Dasht-e-Lut ("barren desert") of southeast Iran; one of the very few stops on a long, lonely trans-desert trade route between northeast Iran and the Indian Ocean. It is the only habitation of any size for nearly 100km in any direction, and yet the earthquake apparently targeted it precisely. Is this a case of extreme bad luck, or is there more to it?

Earthquakes happen when faults move. Faults are giant, planar knife-cuts through rock, in this case extending 20km along the surface and 15km deep into the Earth. The two sides are held together by friction, but occasionally jerk past each other in earthquakes, shaking and vibrating as they do so. As earthquakes happen repeatedly on the same fault, the rocks are progressively ground down to a fine clay. As this clay accumulates underground, it acts as an impermeable barrier to the water table, which can then be much higher on one side of the fault than on the other. Often the water leaks out as springs. The presence of such a spring was the reason the desert settlement at Sefidabeh existed: its name means "white water", inspired by the water that leaks to the surface through white rocks that have been raised to the surface by the fault. The apparent "bull's-eye" targeting of Sefidabeh was no coincidence: because of the fault, Sefidabeh is the only place in the desert where settled agricultural life is possible.

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Other recently "targeted" desert oases in Iran include the famously beautiful Tabas-e-Golshan, visited by Marco Polo (destroyed in 1978, killing 11,000 of its 13,000 inhabitants), and Bam, well known throughout the Middle East for its date plantations (destroyed in 2003, killing 40,000 people out of a population of about 100,000). Again Tabas and Bam were the only settlements of any size for at least 100km in any direction, and the earthquakes were directly beneath them, on faults that in both cases supplied water.

This link between how and where people live and earthquakes is particularly dramatic in Iran, but for many other parts of the great earthquake and mountain belts that run from Italy to China, the situation is similar.

Throughout this region the mountains are largely created by fault movement in earthquakes, pushing one block of rocks on top of another, all ultimately the result of the ongoing collision between the Eurasian plate and the African, Arabian and Indian plates to the south. Large tracts of this area are either low, barren, inhospitable deserts or high, inaccessible and also inhospitable plateaux. Habitations concentrate around the edges of these regions, at the range fronts, because their locations are on trade routes, are of strategic importance in controlling access or are near water supplies.

But the range fronts are often faults, and many of these places have been destroyed in past earthquakes. The problem is that villages grow, and have grown, rapidly, while building quality remains as vulnerable as ever, though it may change from weak adobe houses to poorly built multistorey apartment blocks, and so mortality rates remain appallingly high.

The case of Tehran (daytime population 10 million to 12 million) is instructive. It is situated at the base of the Alborz mountain range front, whose rocks have been elevated by movement on a fault. In former times, the site was occupied by relatively small towns on a major trade route. These predecessors of modern Tehran were damaged or destroyed completely in earthquakes in the 4th century BC, then in 855, 958, 1177 and 1830AD, but the number of people killed was probably quite small by modern standards, perhaps measured in hundreds or thousands. Modern Tehran is a megacity that grew rapidly on the same site in the later 20th century. By contrast, while the Tehran site was occupied by relatively small towns, the ancient city of Tabriz in northwest Iran was always bigger, more prosperous and far more important as a trade route crossroads. Tabriz was devastated by major earthquakes on its nearby faults in 1721 (more than 40,000 killed) and 1780 (more than 50,000 killed), at a time when the population was a fraction of today's.

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The developing world has seen a relentless rise in population and its concentration into large towns, cities and megacities. In the great earthquake belts of Asia, many of these concentrations are adjacent to mountain fronts and faults, in places that provided good locations for agricultural settlements. Yet their development has not been accompanied by a decrease in their vulnerability to earthquakes. The introduction of building codes - often poorly enforced or observed - has had little effect in many of the countries concerned, and mortality rates remain shockingly high. Half the world's megacities of more than 10 million inhabitants are in locations vulnerable to earthquakes, and events that in the past killed a few hundred or thousand people now kill tens or hundreds of thousands, or more. The reason we have not yet had an extreme catastrophe, with more than a million killed in one earthquake, is only because these cities have been exposed for a short time (about 50 years) compared with typical repeat times of earthquakes on faults (usually hundreds or thousands of years).

But a catastrophe of those dimensions seems to me to be inevitable and will probably occur in this century.

Meanwhile, the developed nations have had great success in reducing the earthquake hazard of their urban populations, at least for moderate-sized earthquakes. In California, and increasingly in Japan, earthquakes of magnitude 6 to 7, which can routinely kill tens of thousands in rural areas of the Middle East and Asia, are now principally stories about economic loss. The earthquakes at Loma Prieta in northern California (1989, 64 killed) and Northridge in southern California (1994, 50 killed) occurred in regions that would be considered urban or suburban compared with Bam (2003, 40,000 killed) or Tabas (1978, 11,000 killed) in Iran. Expressed as proportions of the population, the mortality figures of the Californian earthquakes are insignificant compared to those of the two in Iran (roughly 30 per cent and 85 per cent). This is testament to good building design codes, sensibly enforced, though whether those same designs will prove effective in earthquakes much bigger than magnitude 7, in which shaking durations and ground displacements are much larger, is uncertain.

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Nonetheless, the message that good buildings save lives is an issue far more important than demands for earthquake prediction, which has remained scientifically elusive.

What can be done about the appalling earthquake vulnerability of large megacities in the developing world? The problem is immense and urgent, often generating despair in local politicians, who can see no achievable result with the limited resources at their disposal. Certainly, it involves preparation for the inevitable extreme catastrophe, so as at least to attempt to cope with its consequences. But it also needs a sustained determined effort at hazard mitigation and reduction before such a catastrophe occurs, and one that must embrace a large number of cities. The problem of what to do about populations that are already housed in poorly constructed apartment blocks that accompanied the rapid growth of megacities is particularly difficult. One thing at least is clear: we can expect 2 billion people to be added to the cities of developing countries over the next 20 years. This will generate the biggest construction boom in history, and for those people at least, we should try to ensure that this construction conforms to good building practice and good land-use planning.

In this more limited goal, education plays a key, and deliverable, part.

When the public in the developing world start to realise that total destruction is not inevitable, and to demand that their new buildings conform to modern standards in the West, some progress, at least, will have been made.

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James Jackson is professor of active tectonics at Cambridge University. He is giving a Darwin Lecture on Surviving Natural Disasters on February 17.