Geologists at the Woods Hole Oceanographic Institution (WHOI) say the massive, 8.8-magnitude earthquake that struck Chile last week occurred in an offshore zone that was under increased stress caused by a 1960 magnitude 9.5 earthquake.

 Some 300-500 times more powerful than the magnitude 7.0 quake in Haiti Jan. 12,  the earthquake ruptured at the boundary between the Nazca and South American tectonic plates. The temblor was triggered when the "subducting" Nazca plate was thrust under the South American plate, uplifting a large patch of the seafloor and prompting tsunami warnings throughout the Pacific Ocean. The two plates are converging at a rate of 80 mm per year, says WHOI geologist Jian Lin, "which is one of the fastest rates on Earth."

"In 2004, we calculated that the 1960 magnitude 9.5 earthquake has caused large stress increase on both the northern and southern ends of its rupture," said Lin. That quake, centered a few hundred kilometers south of Saturday's earthquake, was the largest instrumentally recorded earthquake in the world. It killed 1,655 people in southern Chile and unleashed a tsunami that crossed the Pacific, killing 61 people in Hawaii and 185 in Japan. Saturday's "quake picked up where the 1960 rupture ended in the north," Lin said.

"This story is quite similar to the Dec. 26, 2004 magnitude-9.0 Sumatra earthquake, which was followed by a magnitude 8.7 quake on its southern end on 28 March 2005," he said. "The only difference is that it took 50 years for the northern neighboring section of the 1960 [Chile] earthquake to rupture, while it took only 3 months for the southern adjacent segment to rupture in Sumatra.

In Haiti, the Jan. 12 rupture has heightened stress further east along the Enriquillo Fault, thereby increasing chances of a quake in that region, which "comes within five kilometers of Port-au-Prince," Lin said.

The latest Chile quake, which had killed more than 700 people as of Mar. 1, was centered some 65 miles west-southwest of Talca, Chile, about 21.7 miles below the ocean's surface, "relatively shallow for a subduction quake," said Lin. It struck about 200 miles southwest of Santiago, the country's capitol. Saturday's earthquake had a "much longer" rupture zone—500-600 km—than that of the Haiti quake—35-50 km.

So what made the Haiti earthquake so much more devastating?

"First, as a nation, Chile is much better prepared for earthquakes than Haiti. People in Chile today still remember the pain of the 1960 quake," Lin said. In addition, coastal Chile has a history of other very large earthquakes.

Since 1973, there have been 13 events of magnitude 7.0 or greater. Approximately 870 km to the north of the Feb. 27 earthquake is the source region of the magnitude 8.5 earthquake of November 1922. That great quake significantly killed several hundred people and caused severe property damage. The 1922 quake generated a 9-meter local tsunami that inundated the Chile coast near the town of Coquimbo; the tsunami also crossed the Pacific, washing away boats in Hilo harbor, Hawaii.

"In contrast, the last catastrophic earthquake in Haiti was 240 years ago," Lin said, "and thus few people were aware of a string of 'earthquake bombs' lying next to Port-au-Prince until Jan. 12.

"Second, the economy of Chile is much better than that of Haiti. Thus, building codes are better developed and enforced in Chile. The contrasts between the aftermaths of the Chile and Haiti quakes reminded us, once again, that 'earthquakes do not kill people, buildings do.'"