10th February 2006 at 00:00
The Earth rumbles for just a few minutes and the result is chaos and misery. Adam Luck investigates the cause of this "act of God"

On a fine November morning in 1755, the Reverend Charles Davy was finishing a letter in his first floor apartment when he noticed that his papers and table were trembling as if brushed by a gentle breeze. Within seconds, however, the house began to shake from the roots of its foundations and the English clergyman heard a distant growling thunder, which initially he attributed to passing coaches.

But then the walls began to rock violently with gaping fissures revealing a darkening sky, large stones began to rain down and the roof's rafters were stripped bare before Davy's terrified eyes.

Fleeing the imploding house, Davy clambered over the instant ruins of Lisbon's once grand St Paul's Church to join the ranks of enrobed bishops, priests and their traumatised congregation close to the supposed sanctuary of the river.

As shock after shock struck, those who were not already on their knees praying were forced onto all fours as the echoes of crashing buildings mixed with the sounds of the dead and dying. But the horror was only just beginning to unfold before their eyes and ears... Davy wrote: "On a sudden I heard a general outcry, 'The sea is coming in, we shall be all lost.' Upon this, turning my eyes towards the river, which in that place is nearly four miles broad, I could perceive it heaving and swelling in the most unaccountable manner, as no wind was stirring. In an instant there appeared, at some small distance, a large body of water, rising as it were like a mountain. It came on foaming and roaring, and rushed towards the shore with such impetuosity, that we all immediately ran for our lives as fast as possible; many were actually swept away, and the rest above their waist in water at a good distance from the banks."

The earthquake of All Saints Day lasted for no more than six minutes but it killed an estimated 100,000, laid waste to one of the world's greatest cities and profoundly shocked Europe.

Davy's ensuing tsunami - which would eventually reach the shores of Cornwall -and a series of fires that raged for five days, sealed the fate of Lisbon, halted Portugal's colonial ambitions in their tracks and ushered in the science of earthquakes or seismology. Just five years later, British engineer John Michell, regarded as one of the founding fathers of seismology, deduced that earthquakes and the waves of energy they release are caused by "shifting masses of rock miles below the surface".

Layer upon layer

To understand earthquakes you have to understand that the Earth is composed of layers. The outer layer, which is 30km thick on average, is the key one.

This layer is composed of about a dozen large, odd-shaped tectonic plates that crash into one another, as well as slide under and over and past one another. All this happens above the semi-molten layer that surrounds the core of the planet. You will invariably find earthquakes at the boundaries of these plates.

The plate boundaries come in all shapes and sizes but conform to four basic types. Constructive, where the plates move apart, are more descriptively called spreading zones. This sees the plates pushed apart by molten rock rising from deep below. These boundaries are usually found on ocean floors with perhaps the best example being the North American and Eurasian plates spreading apart along the mid-Atlantic ridge.

Continental collision boundaries can be found in their purest form in the Himalayas. This mountain range has been created by the Indian plate or sub-continent crashing into the Eurasian plate, or what we call Tibet and China, and pushing ever upwards. Last October this fault line was responsible for the calamity that consumed Pakistan-controlled Kashmir killing 70,000 people and flattening an entire region.

The third is the destructive boundary, which is found in subduction zones where an oceanic plate or slab is pushed down below the adjoining plate into the semi-molten layer deep beneath the surface. This can be best found on the edges of the Pacific Ocean.

Perhaps the most famous, however, and certainly the most keenly studied fault is the San Andreas Fault in California, which represents the final kind of fault: conservative. This is where two plates slide past one another. The boundaries throw up different kind of faults or flaws in the Earth's crust where the rocks have been fractured. Conservative boundaries invariably throw up a strike slip fault, which is a simply sideways movement.

What are the boundaries?

Destructive boundaries usually cause a reverse fault where one layer of rock is pushed over the other, causing a shortening of the land mass.

Constructive boundaries usually cause a normal fault where the rocks are pulled apart and there is a net lengthening of the land mass.

These boundaries and faults essentially represent zones of weakness in the Earth's crust. They will bend and they will stretch but eventually the stress will exceed the strength of the rocks that border the fault, which will then break or snap into a new position, just like a rubber band.

This process of breaking will generate vibrations or seismic waves of energy that spread out from the origin or focus of the earthquake and across the surface of the Earth or the epicentre. This is the energy that can destroy all in its path.

The largest earthquakes can be felt across different continents and those that occur beneath the ocean can create giant waves, or tsunamis, because the ocean bed rises or drops suddenly displacing huge quantities of water in the process.

Just why tsunamis happen can be explained by the fact that 75 per cent of the world's seismic energy is found around the so-called Ring of Fire, which stretches for 40,000 km around the Pacific. This centres on the thinner Pacific plate beneath the Pacific Ocean which is forced underneath the surrounding continental plates that mark out the west coasts of South, Central and Northern America, Japan China, the Philippines, Indonesia and Australasia.

This tsunami link also helps explain why in Greek mythology the God of the Sea, Poseidon, or Neptune in Ancient Rome, was ascribed sufficient power to make him an "earth shaker".

By contrast the Eurasian, Indian and African plates which crash into one another, account for 15 per cent of the total global seismic energy. This is released along a band that stretches from the Mediterranean, across the Caucasus, down to the Himalayas and onto Burma.

Given these fault lines are as old as time itself it is no surprise that earthquakes can be found in historical records as long ago as 1831bc in Shandong province in China.

The so-called Middle Kingdom also provides record of the deadliest known earthquake, which took place in 1556 and centred on Hua county, in Shaanxi province. More than 800,000 are believed to have died (in excess of the 60 per cent of the region's population) in an earthquake in the region of 8 on the Richter scale. One of the reasons for the high mortality rate was that millions of people lived in man-made caves, whose crumbling soil was particularly prone to landslides.

Humankind's vulnerability to earthquakes invariably centres on the region's population density and construction of his buildings and whether they are able to withstand seismic waves.

The world's largest recorded earthquake occurred on May 22 1960 in Chile and generated vast tsunamis across the Pacific, but because the stricken areas were sparsely populated fewer than 3,000 died. Charles Darwin on a visit to Chile in 1835 noted that the wood houses and lightweight hovels escaped damage during an earthquake but the brick and mortar buildings of Concepcion were utterly destroyed.

Given the explosion in the world's population over the past few hundred years, however, it is perhaps not surprising that even once remote areas are likely to witness a rising death toll when earthquakes strike. Mix this with the poor design and cheap construction materials that you find in the developing world and you have a lethal combination.

South Asian disaster

Nowhere is this better illustrated than with last year's South Asian earthquake. With a magnitude of 7.6 this quake struck close to the provincial capital of Muzzafarabad and few of the 75,000 victims even had the chance to try to escape from the buildings as they collapsed. Many were pupils crushed beneath their schools, with 120,000 injured and 3.5 million made homeless.

Seismologists believe the energy released was the equivalent of a 30 megaton nuclear explosion.

What made this earthquake particularly lethal, however, was that the fault and focal point was so shallow with a depth of six miles which increases the strength of the seismic waves. But this was not the only reason.

With most developing countries there is no "seismic building code" to enforce the construction of seismic resistant buildings, or if there is it is widely ignored. The violent vibrations generated by earthquakes invariably find out the weak points in any building so it is vital that there must be an equal distribution of strength and stiffness. Crucially, it must also be "ductile" or sufficiently flexible to allow it to move like a rubber band.

In developing countries this means that buildings must be tied with a variety of beams, and engineers must insure that the materials are of sufficient quality. Developed areas such as the US, Europe, and Japan all have strict seismic codes for the construction of new buildings, both small and tall. Skyscrapers are peculiarly adept at surviving quakes because their height tends to dissipate the waves. All too often, once the earthquake has struck in developing countries, western aid flows in but the desperation and chaos of the situation means that cheap and cheerful housing goes up. Sadly these are rarely up to the job and will survive only until the next earthquake.

The use of this aid has long been a cause of friction, not least over who will control its distribution in what are often politically sensitive areas. India and Pakistan's rumbling dispute over Kashmir has never been far from the surface in the aftermath of October's catastrophe.

The two governments were unable to agree even over the use of helicopters to reach cut-off areas in the Himalayan foothills. Pakistan's sensitiveness was further exacerbated by claims that it had been short changed in aid from the west.

The Indian Ocean tsunami, which killed more than 200,000 people in December 2004, fanned out across Indonesia, Sri Lanka, Thailand and even Somalia.

But while aid poured into these countries, Pakistan has received just a fraction of the required amount. Pakistani politicians believe this is because the tsunami killed western tourists in well-known resorts while Pakistan is little known in Europe or America.

Seismology's holy grail

What is certain is that the people of the region will have little better protection when the next earthquake strikes. Some experts believe this is going to be sooner than many would wish.

Roger Bilham, one of the world's leading seismologists, points to records that have helped identify four faults in the area and a pattern of earthquakes coming in clusters. The last major series started in 1501 and culminated in the largest quake in 1555. He said: "I would think that the present earthquake would be equivalent to the one in 1501 which means that there exists a possibility of another major slip in Kashmir's future. Of course 50 years may sound an awfully long time to a politician but it is the perfect length of time to get the structure of the buildings right."

What marks Bilham out is an uncanny ability to predict earthquakes. Not only did he forewarn of the Kashmiri quake but he also predicted the Sumatran tsunami. This, however, does not mean humankind is close to the holy grail of seismology: the ability to predict accurately when and where quakes will take place and with what magnitude.

Countries most vulnerable to seismic activity have spent countless millions on research to identify possible earthquake markers. This has focused on how rocks crack and expand under stress, as well as analysing foreshocks, levels of radon gas, and electrical and magnetic factors. Like Bilham, these scientists also pore over records to deduce from history the likelihood of future earthquakes. Unfortunately, simply because a strain is released along one section of the fault does not a lessening of tension: it might increase the strain on another part. This is why earthquakes so often come in groups.

Those tempted to question what relevance earthquakes have to Britain, however, should think again.

They do happen. Between 200 and 300 occur each year although with a record of 11 deaths in nearly 500 years, including one listed as "22 April 1884, Colchester, Wivenhoe, shock, (uncertain)", we should not, perhaps, be unduly concerned. For those poring over the records in seismically active countries there is, however, consolation to be found that there is no proof that earthquakes are on the increase.

No doubt the Reverend Davy would have sighed 'Amen' to that.

Seismic sizes

In the 1930s the US seismologist Charles F Richter devised a simple numerical scale that has become synonymous with measuring the magnitude of earthquakes. The Richter scale typically uses values of between 1 and 9 to express the total energy released by an earthquake, with each increase of 1 representing a 30-fold increase in released energy.

A good comparison can be found by equating a Richter magnitude of 5 with that of an explosion of 1,000 tons of TNT, while a magnitude of 6 is the equivalent 30,000 tons of TNT or a 30 kilotonne nuclear explosion. To get an idea of the scale of this the atom bomb dropped on Hiroshima in 1945 was a 15-kilotonne device.

The Richter measurement or seismogram is obtained using a seismometer typically at no greater a distance than 600km from the earthquake, with adjustments for the figure made depending on the distance.

What separates seismologists, however, is what form of energy they use to measure the magnitude of an earthquake, how and when. The energy or vibrations that radiate out from the fault rupture are primarily surface waves, which travel along the surface of the earth, and body waves, that travel up through the earth.

The original Richter scale used local magnitude, which was simply the largest trace or mark on the seismogram.

There is also surface wave magnitude, which solely measures surface waves; and body wave magnitude, which measures the first few waves to arrive on the seismogram.

None of these, however, is ideal because they only measure one part of the seismogram and can suffer from saturation when there is a larger earthquake.

Most earthquakes are now measured using what's called moment magnitude, which integrates all the waves and, in effect, measures the total energy released over time.

Moment magnitude is usually adjusted over time because it is the average figure taken from a large batch of seismometers, whose readings are continually complementing the initial reading.

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