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One good shake leads to another


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POSTED: Monday, November 02, 2009

The recent “;clustering”; of earthquakes magnitude 7 and larger in the western Pacific and Indonesia could be the result of a “;triggering”; effect, Hawaii geophysicists say.

“;Earthquake triggering is a well-known phenomenon,”; said Cecily Wolfe, a geophysicst/seismologist at the University of Hawaii Institute of Geophysics and Planetology.

Whether the current sequence of earthquakes “;is simply a coincidence”; or due to triggering is a question for research and debate, she added.

The first in the latest series was a magnitude-8 earthquake Sept. 29 in the Samoa region. It spawned a tsunami that killed about 178 people and devastated coastal villages in Samoa, American Samoa and northern Tonga.

Twelve hours later a magnitude-7.6 earthquake occurred in southern Sumatra, about 6,000 miles from Samoa, killing at least 1,000 people. And three large earthquakes and a small tsunami occurred Oct. 7 in Vanuatu, more than 1,500 miles from Samoa.

               

     

 

SAMOA SHAKES AGAIN

        A moderate, 5.9-magnitude quake occurred yesterday about 180 miles southwest of Apia, Samoa, the U.S. Geological Survey said.
       

The quake, at a depth of 8.9 miles, was on the flanks of the Tonga Trench, the same general area of a swarm that affected Samoa and Tonga in late September and early October.

       

 

       

Gerard Fryer, a geophysicist at the Pacific Tsunami Warning Center at Ewa Beach, said he suspects the Samoa earthquake tipped the earthquake fault “;over the edge”; for the Vanuatu earthquakes.

He said the Samoa earthquake was somewhat unusual in that it resulted from a rupture entirely within the downward plate rather than a classic subduction earthquake between two plates in the earth's crust. Moving west, the Pacific plate is subducting or slipping under the Indo-Australian plate along a zone that extends north from New Zealand, creating the Kermadec and Tonga trenches.

“;Such earthquakes tend to be more efficient in radiating seismic energy, so the earthquake could well have had a greater than normal triggering effect elsewhere,”; he said. But the energy radiation was fortunate for the Samoas, he said, because the shaking was so strong people heeded it as a warning, and this saved lives.

Numbers vary, but the U.S. Geological Survey says about 17 earthquakes of magnitude 7 and greater occur annually, Wolfe said, “;so they're not uncommon.”;

The recent earthquakes also happened as part of the earth's subduction zone systems “;where earthquakes of those sizes are known and expected to happen,”; she said. “;So it is not surprising that these systems had fault zones that were ready to rupture in large earthquakes.”;

Many large earthquakes trigger a series of local aftershocks, and “;dynamic waves from large enough earthquakes can sometimes trigger earthquakes at more remote distances,”; she said.

But it is not typical for a sequence of large earthquakes to happen so closely spaced together or to “;cluster,”; as some seismologists describe it, she said.

A high number of magnitude-8 earthquakes occurred worldwide in 2005 and 2006 after the 2004 magnitude-9.3 Sumatra quake, which some scientists suggested may have resulted from triggering, said Wolfe.

A 7.9-magnitude Hawaii earthquake April 2, 1868, caused several magnitude-6 earthquakes—probably aftershocks—for 20 years on the Big Island, she said.

“;One idea is that some fault systems are critically stressed and ready to rumble,”; Wolfe explained, “;so that only a very tiny stress change from another earthquake is all it takes to cause a domino effect. ... We know that the bigger the earthquake is, the bigger the trigger it has.”;

Although earthquakes cannot be predicted, she said, some scientists have devoted their careers to studying earthquake clustering and triggering, which can help to assess seismic hazards and “;understand where the next domino in the sequence may occur.”;

 

Giant quakes cluster, too, as history shows

There is “;good historical reason”; to believe giant earthquakes—with magnitudes 9 and up—cluster as well as smaller ones such as those recently in the Samoas and Vanuatu, says Gerard Fryer, Pacific Tsunami Warning Center geophysicist.

The latest ones were big, with magnitudes greater than 7, but they were not gigantic, he said.

In the 20th century, he said, “;all three giant earthquakes were squeezed into a dozen years: 1952, Kamchatka (magnitude 9.0); 1960, Chile, magnitude 9.5; and 1964, Alaska, 9.2.”;

A large number of “;almost-giant”; earthquakes occurred midcentury, he said: 1946, eastern Aleutians, magnitude 8.5; 1950, Assam, Tibet, magnitude 8.6; 1957, central Aleutians. 8.8; and 1965, western Aleutians, 8.7.

“;Smaller clusters of great earthquakes (magnitude 8 and greater) occurred around 1906 and 1923,”; he said. “;The historical record seems to be saying that big earthquakes do trigger each other.”;

Fryer said all big earthquakes were jammed together in the last century, but after 1964 there were no magnitude-9 temblors for about 40 years. “;Then wham, in 2004 we were all surprised. It was like the earth waking up again.”;

The 9.2-magnitude Sumatra-Andama earthquake of 2004 was followed by an 8.6-magnitude earthquake in 2005 in northern Sumatra and an 8.5 earthquake in 2007 in southern Sumatra, he pointed out.

“;Is another magnitude-9 earthquake coming soon? The record of the last century certainly makes another magnitude-9 event seem very probable,”; Fryer said, adding, “;Statistically, we would have to expect it in the Pacific, since this is where most earthquakes occur.

“;We have to make sure our emergency procedures are honed, our personal 'go' bags are ready and that we all know what to do when the inevitable happens,”; he said.