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The earthquake that caused the Samoan tsunami and a major devastating tremor an hour later in Indonesia were only linked by timing, experts said yesterday.
Bill Fry, a seismologist at GNS Science in New Zealand, said that while the quakes on Wednesday were on the same fault line, they were 10,000km apart and could not be considered connected.
"You can get quakes that are close temporally and spatially as one transfers stress to another place against the fault but that's not possible this far apart," he said.
Earthquake frequency needs to be studied over a period of thousands of years to establish patterns, he said, and it was impossible to draw conclusions about a cluster of seismic events occurring over a few days.
"The frequency at which we have large quakes over a period of days, or even weeks and months, is quite chaotic."
More than 100 people are feared dead in Samoa, American Samoa and Tonga, while more than 1000 are feared killed in Sumatra.
Samoa and Indonesia lie on the active seismic zone around the Pacific plate known as the Pacific Ring of Fire.
But according to the The Iris Wilber seismic monitor, which is supported by America's National Science Foundation, up to a dozen earthquakes registering a magnitude of 4 and above are recorded around the world each day. A series of aftershocks occurred around the boundaries of the Pacific area yesterday.
Professor Gary Gibson, from the Royal Melbourne Institute of Technology's Seismology Research Centre in Australia, agreed that it was impossible to draw any connection between the two. "There is no known mechanism to connect them," he said.
"What's happening is perfectly normal ... Earthquakes often seem to cluster more than they actually do, but that is mainly just a psychological perception because you get a big one and then become more alert to others."
Professor Tim Stern of Victoria University's geology department said that the quakes in Indonesia and Samoa were caused by different types of techtonic activity.
A normal fault occurs when plates move away from one another or extend. When plates come together, or compress, it is called a reverse fault. Professor Stern said reverse faults were responsible for some of the most destructive earthquakes in the world.
Strike slip faults, which often occur in NZ, happen when plates move alongside one another generating friction. Subduction is the term used when plates move beneath one another.
The Samoan quake, with a magnitude of 8, was caused by a crack in the outer side of the Pacific plate as it fell into a trench beneath the Australian plate. Because the plate was pulled apart when it cracked it was called an extension fault.
"It was the bending of the down-going quake that caused the earthquake rather than a collision of the plates themselves," he said.
The Sumatran quake was caused by the plates being thrust together.
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Pacific Cooperation Foundation
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