Haiti sits on two seismic fault lines and a major disaster was expected
Seismologists have known for years that a devastating earthquake was likely to hit Haiti. They just did not know when. Although Tuesday’s devastation was wreaked by the most severe quake to hit Port-au-Prince since 1751, the island has been the victim of major seismic activity several times since.
In 1946, a severe tremor in the region triggered a tsunami in the neighbouring Dominican Republic that killed almost 2,000 people. But what made the consequences of this week’s earthquake so much worse than previous ones was that it happened only 8km below the surface and only 15km from the Haitian capital.
The problem in geological terms is that the Haitian half of the island of Hispaniola sits sandwiched between two fault lines on the divide between the North American tectonic plate and the Caribbean plate.
Seismologists estimate that the Caribbean plate is heading eastward at a rate estimated at between seven and 20mm a year – a seemingly miniscule amount but enough to produce tremendous pressures building up beneath the earth’s crust as these vast slabs grind against each other.
This movement has produced two fault lines, called strike-slip faults, to the north and south of Haiti: the Septentrional fault in the north and the Enriquillo-Plaintain Garden fault, which caused Tuesday’s disaster.
“It’s been locked solid for about the last 250 years,” said Roger Musson, a seismologist from the British Geological Survey.
“It’s been gathering stress all that time as the plates move past each other and it was really just a matter of time before it released all that energy. The question was going to be whether it would release it all at once or in a series of smaller earthquakes.”
That question was answered at 21.53 GMT on Tuesday when the quake measuring 7.0 on the Richter scale sent its deadly shock waves – equivalent to the force of 35 Hiroshima nuclear bombs, according to scientists – through the streets and shantytowns of Port-au-Prince.
Although there have been more powerful quakes in Haiti’s history, the most recent measuring 7.2 in 1887, the fact that this one was comparatively shallow in the ground meant that its power was scarcely dissipated by the time it reached the surface.
Its closeness to Port-au-Prince also meant that extensive loss of life was inevitable in a city of more than a million souls, the vast bulk of them crowded together, often on hillsides, in a country where there is neither the money nor expertise to construct buildings capable of withstanding severe tremors.
Haiti’s plight has been made worse by the fact that only 3% of the island remains as forest. When Europeans first arrived there 500 years ago, they marvelled at the density of the trees, but subsequent deforestation, mainly to make charcoal, has left the terrain bare and vulnerable to landslides.
David Rothery, a planetary scientist at the UK’s Open University, said: “From the pictures I have seen, and from what I know of Haiti’s impoverished economy, I doubt if buildings there have been constructed with earthquake resistance in mind.
“They are at risk of further collapse caused by aftershocks, of which there have been several strong ones.
“The debris in the streets suggests that people would have been killed or injured by falling masonry if they tried to flee buildings while the ground was shaking, rather than sheltering under a table until motion had ceased.”
“It is many decades since a comparably strong quake has hit Haiti, and I wonder if the population was adequately aware of what they could do to protect themselves.”
It proved to be this combination of natural disaster allied to man-made failings that has led to such a dreadful loss of life in Haiti. Last year, the US Geological Survey recorded 17 earthquakes of magnitude 7.0 or greater, but none wrought the devastation now being experienced in Port-au-Prince.
Professor Roger Searle from the Earth Sciences Department at Durham University said the quake was equivalent to the energy release of about half a megaton of TNT. Earthquakes of this size can cause relatively slight damage in well-designed and constructed buildings, but considerable destruction in poor ones.
”The last major earthquake in this part of Haiti was 1860; in 1692, a major earthquake in Jamaica caused 2,000 deaths. To date the United States Geological Survey has recorded 33 aftershocks greater than magnitude 4.5 (large enough to cause at least minor damage)”, said Professor Searle.