Chile earthquakeChile quake occurred in zone of increased geological stress

The massive, 8.8-magnitude earthquake that struck Chile on 27 February occurred in an offshore zone that was under increased stress caused by a 1960 quake of magnitude 9.5, according to geologist Jian Lin of the Woods Hole Oceanographic Institution (WHOI).

The earthquake, some 300-500 times more powerful than the magnitude 7.0 quake in Haiti on 12 January, 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 Lin, “which is one of the fastest rates on Earth.”

Lin and colleague Ross S. Stein of the U.S. Geological Survey in Menlo Park, Caalifornia, have studied the region extensively, and alerted the scientific community to a build up of stress along the interface of the two plates in a 2004 paper in the Journal of Geophysical Research. “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. We do not yet have good enough science to say why one place took only 3 months and another took 50 years. But even 50 years is short enough [to fall within] in a person’s lifetime. Thus, we should consider the earthquake interaction possibilities seriously.”

In Haiti, Lin point out that and others have calculated that the 12 January rupture has heightened stress further east along the Enriquillo Fault, thereby increasing chances of a quake in that