News in Science

Island's rise and fall points to next mega-earthquake

Earthquake island A long-term study of an island off the coast of Chile has provided insight into how mega-earthquakes and tectonic strain can affect land height.

The study reported in the journal Nature Geoscience, concludes that a drop in the height of an island in the lead up to a mega-earthquake was caused by the underlying tectonic plate dragging the island down.

The research examined changes in the height of Isla Santa Maria off the central Chilean coast, between two mega-earthquakes in 1835 and 2010, providing the best overview yet obtained of a complete seismic cycle -- the build up of stress between major earthquakes.

"This is the first time we've really been able to make this type detailed measurement," says the study's lead author Dr Robert Wesson of the United States Geological Survey in Denver Colorado.

Wesson and colleagues analysed nautical survey records of Isla Santa Maria taken in 1804, 1835 and 1886, together with modern surveys and GPS data from 2008 and 2010 to measure the vertical movement of the island during the complete cycle between the two mega-earthquakes.

Darwin and the Beagle

"We were able to use old navigational charts made by Captain Robert FitzRoy of the HMS Beagle during Charles Darwin's journey, when he visited this island just after the big earthquake in 1835," says Wesson.

FitzRoy's nautical survey following the magnitude 8.5 earthquake showed the island had been uplifted by between 2.4 and 3 metres.

"He saw dead mussels and other shell fish all over the rocks which were uplifted by the earthquake a few weeks earlier," says Wesson.

Follow-up surveys taken in 1886 showed little change from the 1835 elevation.

"However, when I went to the island in 2008, I looked for the evidence that Fitzroy had described with the wave cut platforms infringing the island, and I couldn't see them, it looked as if they were underwater," says Wesson.

Wesson and colleagues returned to Isla Santa Maria in 2010, undertaking modern echo sounder surveys and GPS measurements, which show the island had subsided by about 1.4 metres since the 1886 readings.

The big one hits

On February 27, 2010, just six weeks after Wesson's 2010 measurements, Isla Santa Maria was struck by the magnitude 8.8 Maule earthquake, causing an uplift of 1.8 metres.

"We were able to show that the island had been at a very low level in 1804, had popped up about 2 or 3 metres in 1835, probably had not changed much through about 1886, then subsided by 2010 when it popped up again," says Wesson.

Isla Santa Maria is located directly above a mega-thrust fault where the Nazca plate under the Pacific Ocean slides under South America.

Between earthquakes, the mega thrust remains locked, and as the Nazca plate subducts under the edge of South America, it drags Isla Santa Maria down with it.

When the earthquake occurs, the fault instantly unlocks, allowing the island increase in height again.

"If the fault was unlocked, then subduction continues and no strain builds up, but once the fault locks, then strain builds up and you start dragging the island down, so the subsidence of the island as it goes down is an indication that strain is building up," says Wesson.

"If we're really going to understand how the probability of future earthquakes changes over time, we need to have ways of measuring the amount of elastic train that is stored up.

"With a very long view, if we could measure this amount of deformation over a very long time, then we could -- in principle -- make some estimate of how much strain is storing up and consequently how likely a great earthquake is."