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Chile quake tops Haiti, but less deadly

Chile quake

There is no direct link between the magnitude of an earthquake and its deadliness, say experts (Source: Reuters/Ivan Alvarado )


While rescuers are still searching through the rubble, experts says many factors explain the difference between the approximately 700 dead so far in Chile while the grim tally in Haiti has topped 220,000.

An earthquake's magnitude reflects the amount of seismic energy released at its epicentre, usually the meeting point of tectonic plates pushing up against or pulling away from each other.

The Haiti quake, which struck on January 12, measured 7.0 in magnitude, while the one in Chile, at 8.8, was nearly two orders of magnitude greater.

Each notch on the scale represents a 10-fold increase in amplitude, or the degree of shaking of the ground, and a more than 30-fold jump in the amount of energy released.

"But there is no direct link between magnitude and its deadliness or level of destruction of human habitat," says Robin Lacassin, a seismologist at the Institut Physique de Globe in Paris.

Quakes in unpopulated areas

Some 15 to 20 quakes every year top magnitude 7.0, but most of them go unnoticed except by scientists because they occur in the ocean or unpopulated areas.

Both recent quakes occurred near large cities, so other factors account for the difference.

"The earthquake near Port au Prince was very shallow, only about 15 kilometres below the surface," compared to about 35 kilometres below the ocean floor for Chile, Lacassin says.

But proximity alone did not account for the massive destruction of huge loss of life in January, he and other experts point out.

"The quality of construction and building codes in Haiti were obviously not as strong as those in Chile," says Dr David Galloway, a seismologist at the British Geological Survey.

The region along the western coast of South America has been hit several times before by major quakes, including the largest on record - a monster in 1960 measuring 9.5 on the Richter Scale - very nearby.

"Chile knows earthquakes. Their codes are more stringent," says Galloway.

It does not help that Haiti is beset by deep poverty and a history of dysfunctional governance.

Construction critical

The critical role of construction quality is highlighted by comparing the Haiti quake with a similar one in Kobe, Japan in 1995.

"The Kobe earthquake was 6.9 on the Richter Scale, was almost as superficial -- less than 20 kilometres under the surface -- and very near the epicentre," says Lacassin.

It was also the same kind of so-called slip-strike fault in which two plates rub past each other.

The death toll in Kobe was just over 5,000, considered shockingly high at the time because the city had been built to withstand intense ground shaking, but still only a fraction of the Haiti toll.

There is also an element of luck, depending on what time of day the pressure that has built up within Earth's surface layer, over decades or centuries, finally gives way.

"In Chile, a lot of car parks collapsed, but there was nobody in them because it was the middle of the night," notes Galloway.

Had the shaking started as people arrived at work or headed home, the tolls would be far higher, he says.

Tags: earthquake, earth-sciences