By Richard A. Lovett
On 27 February, at 3:34 in the morning, Chile was rocked by a magnitude-8.8 earthquake. It was the fifth-largest earthquake since 1900, releasing about 500 times the energy of the magnitude-7.0 quake that hit Haiti on 12 January. Although both countries were devastated, the destruction was worse in Haiti, where an estimated 230,000 people were killed. In Chile the death toll reported on 28 February was 708, although that figure is expected to increase. Natureexamines what happened in Chile and why the outcomes were so different.
Why was the Chilean earthquake so big?
The earthquake occurred along a 500-kilometre segment of the Peru-Chile subduction zone, where part of the Nazca Plate in the Pacific Ocean plunges beneath the South American Plate.
Subduction zones are noted for producing some of the biggest earthquakes -- the 2004 Indian Ocean quake also arose from a subduction zone. But Chile is particularly prone to tremors because the plates there are converging at high speed - about 80 millimetres a year compared to 25-70 millimetres a year for most other plates, says Roger Bilham, a seismologist at the University of Colorado in Boulder.
This means that strain builds up quickly, and the long, straight plate boundary, which follows the coastline, enables big blocks of the fault to slip simultaneously, releasing a lot of energy in a single lurch. The biggest earthquake ever recorded -- magnitude 9.5 -- occurred along the same subduction zone in 1960. The 27 February quake happened on a section of the fault between the site of the magnitude-9.5 quake and a magnitude-7.8 quake that occurred in 1985. Thus, says Emile Okal, a geophysicist at Northwestern University in Evanston, Illinois, it was to be expected that the central section would eventually rupture.
Why didn't the earthquake do 500 times more damage than in Haiti?
Big earthquakes affect larger areas than smaller ones, but don't necessarily knock down more buildings. That is partly because subduction-zone earthquakes occur offshore, and the strength of the shaking diminishes quickly with distance. So, says Seth Stein, a geophysicist at Northwestern University, "a 'large' earthquake close by does more damage than a 'huge' one a little further away".
And the Haiti earthquake was relatively shallow, whereas the Chilean one was deep. Just as damage reduces with horizontal distance from the fault, it also attenuates with vertical distance, says Scott Ashford, head of the School of Civil and Construction Engineering at Oregon State University in Corvallis.
Another factor is the speed of ground shaking. Giant earthquakes, says Okal, tend to put much of their extra energy into slower, long-period vibrations, rather than adding to the strength of the 1-hertz-frequency vibrations most effective at knocking down walls. "You get a kind of saturation," he says. "A larger area is affected, but the intensity does not necessarily grow."
To what extent did better building codes minimize the death toll?
In Haiti, non-existent or badly enforced building codes turned many buildings into "weapons of mass destruction," Bilham wrote last month in an Opinion in Nature (see 'Lessons from the Haiti earthquake').
But although nothing could make Chile invulnerable to earthquakes, its building codes are comparable to California's, says Ashford. "If you look at earthquake risk there are two components," he says. "One is the hazard - what Earth can do to you. The other is how vulnerable your buildings are. In Chile, they have frequent earthquakes, but they've implemented seismic design into their building code, have inspections, and well-engineered structures."
But the South American nation did still take an enormous hit from the earthquake. "As we start looking at the infrastructure, it's going to take a long time to rebuild," Ashford says.
How do the tsunamis from the two earthquakes compare?
Both earthquakes produced deadly tsunamis, with three people killed by waves in Haiti and 16 in Chile. In Haiti, waves up to 3 metres high occurred along the Haitian shore, possibly due to underwater landslides or similar local phenomena (see'Haiti earthquake produced deadly tsunami'). In Chile, waves as high as 2.34 metres were recorded, and at least three people were killed by 3-metre waves on Robinson Crusoe Island, 700 kilometers offshore. The Chilean earthquake also sent waves racing across the Pacific, inducing alerts throughout the Pacific Rim and causing evacuations in Hawaii, where officials remembered the 1960 earthquake, when a tsunami killed 61 people in the coastal town of Hilo. The more recent tsunami was smaller, producing no reported damage in either Hawaii or Japan.
What is the next step?
Although Chile has plenty of seismologists of its own, scientists in many fields will undoubtedly be heading there to see what they can learn from the region's biggest earthquake in 50 yearhs.
In doing so, they will be following in the footsteps of Charles Darwin, who was in Chile during an earthquake in 1835, estimated to have been around magnitude 8.2.
Darwin noted, says Ashford, that earthquake waves are amplified by certain geological features, most notably mountain tops and cliff tops. Something similar was seen in California's 1994 magnitude-6.7 quake in Northridge, Los Angeles, he adds. "Homes along the edge of the cliff in Pacific Palisades suffered significant damage," he says, "where 100 yards inland they barely felt the earthquake."
Ashford hopes to visit Chile by mid-March. "What we're trying to do," he says, "is learn as much as we can and collect the kind of perishable data that help us validate our models, save lives, and protect from future earthquakes."