Big quakes 'shake the world'

MICHAEL DALY
Last updated 11:48 28/09/2012
Tsunami
Reuters

DEVASTATED: The Natori city in Miyagi prefecture, Japan, is hit by a tsunami on March 11, last year. Scientists now believe large earthquakes can trigger other quakes in far-away locations.

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In an unnerving change of tune, scientists warn that big earthquakes can cause other quakes in faraway locations days later.

The warning follows a study by seismologists from University of California Berkeley and the United States Geological Survey (USGS) after the magnitude-8.7 quake in the east Indian Ocean off Sumatra on April 11.

As well as triggering small quakes during the three hours it took for seismic waves to travel through the Earth's crust, the quake primed some faults to break - up to six days later.

Experts told UC Berkeley's News Centre the findings were a warning to those living in seismically active regions worldwide that the risk from a large quake could persist, even on the opposite side of the workd, for  more than a few hours.

"Until now, we seismologists have always said, 'Don't worry about distant earthquakes triggering local quakes'," said Roland Burgmann, professor of earth and planetary science at Berkeley and co-author of the study.

"This study now says that while it is very rare - it may only happen every few decades - it is a real possibility if the right kind of earthquake happens."

USGS said an "extraordinary" number of quakes of magnitude 4.5 and greater were triggered worldwide in the six days after the April 11 quake.

"These large and potentially damaging quakes, occurring as far away as Mexico and Japan, were triggered within days of the passage of seismic waves from the main shock that generated stresses in the Earth's crust," it said.

UC Berkeley said part of the reason that five times more quakes than expected were recorded during the six days after the April 11 event could be the unusually low number of quakes during the six to 12 days before the event.

That had meant many very-close-to-failure faults were sensitive to a triggering shockwave.

A cascade of smaller, undetectable quakes might have been triggered on those faults that led to larger ruptures.

Alternatively, large quakes could trigger nearly undetectable tremors that were a sign of slow slip underground. That could run away into a larger quake, with some slow-slip events taking days, to a week or more, to evolve.

The east Indian Ocean quake was the possibly the largest strike-slip earthquake - where ground on one side of the fault moves horizontally past ground on the other side - seismically recorded. A similar sized quake in  Tibet in 1950 was of an unknown type.

Felt from India to Australia, including throughout South Asia and Southeast Asia, it is also considered to probably be the largest intraplate quake - meaning it happened within a single tectonic plate - seismically recorded. Most quakes are at plate boundaries.

Previously reported as being magnitude 8.6, the quake was caused by at least four undersea fault ruptures southwest of Sumatra within a two-minute 40-second period. At least two people died directly as a result of the quake, while eight others died from heart attacks.

Scientists from the University of Utah and UC Santa Cruz said the magnitude-8.7 quake and an 8.2 quake two hours later were part of the breakup of the Indian and Australian subplates of the Indo-Australian plate along a boundary that was still unclear underneath the Indian Ocean.

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The process started about 50 million years ago and  would continue for millions more.

"We've never seen an earthquake like this," study co-author Keith Koper, director of the University of Utah Seismograph Stations, said.

"This is part of the messy business of breaking up a plate. This is a geologic process. It will take millions of years to form a new plate boundary and, most likely, it will take thousands of similar large quakes for that to happen."

The 2012 quakes were likely to have been triggered, at least in part, by changes in crustal stresses caused by the magnitude-9.1 Sumatra-Andaman earthquake of December 26, 2004 - the jolt that generated the massive tsunamis responsible for the deaths of 228,000 people in the Indian Ocean region.

In another indication of the impact large quakes can have, USGS scientists also studied an almost complete lack of seismic activity up to 400km away that followed the magnitude-9.1 event - the largest quake in 40 years.

After the massive shake, aftershocks greater than magnitude 4.5 ceased for five years along part of a distant series of linked faults known as the Andaman back arc fault system.

Along a larger segment of the same system, the sideways-slipping transform quakes decreased by two-thirds, while the rate of rift events - earthquakes that happen on a spreading centre - increased by 800 per cent. Those very large but distant seismicity rate changes were unprecedented, a report of the study said.

If a mainshock could promote or inhibit earthquakes far from the fault rupture, it changed not just the local but also the regional risk of subsequent quake damage.

"This is important because damage to buildings can be more strongly influenced by moderate nearby earthquakes than by shaking from the large distant mainshock that triggered those remote shocks," the report said.

Nowhere was that clearer than with the magnitude-6.3 aftershock on February 22, 2011, in Christchurch, which caused 185 deaths.

It also tripled the insurance losses of the original magnitude-7.1 earthquake that struck 45km away in Darfield on September 4, 2010.

- Stuff

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