Thursday, October 01, 2009

Two South Pacific earthquakes unlikely to be connected, say seismologists

The earthquakes in the South Pacific occurred in a region of extraordinary geological activity called the Ring of Fire, which stretches from Indonesia to the coast of Chile. Several tectonic plates converge and create enormous pressure in the Earth's crust. Nine out of 10 earthquakes in the world happen in the region.
It is unlikely the two latest earthquakes are connected, according to seismologists. They were caused by slippages in faults that took place 16 hours and 10,000km apart on two different tectonic plates.
The first earthquake, recorded as magnitude 8.0, happened on Tuesday at 6.48am local time around 200km off the coasts of Samoa and Tonga. Because the earthquake was only 10km beneath the Earth's surface, it caused the seafloor to deform, triggering a tsunami that battered the Samoan capital of Apia.
Most earthquakes in this particular region of the Ring of Fire are caused by the Pacific plate pushing underneath the Australia plate, but Tuesday's quake was different. "This time, as the Pacific plate bent under the Australia plate, it essentially cracked and caused this earthquake," said Stuart Sipkin, a geophysicist at the US Geological Survey in Colorado. The tremors were consistent with slippage along a 100km-long crack.
The Pacific plate moves westwards under the Australia plate at the rate of about 9cm a year.
The second earthquake happened on Wednesday about 30 miles from Padang in Sumatra at 5.16am local time. The magnitude 7.6 quake was much deeper, about 80km beneath the seafloor, which is too deep to cause a tsunami. In Sumatra and elsewhere in Indonesia, all of the damage was caused by tremors.
Experts have warned that a major earthquake is long overdue in the region, but those fears centred on a build-up of stress in another subduction zone, where the Australia plate pushes under the neighbouring Sunda tectonic plate. In the past decade, there have been large earthquakes north and south of the Indonesian islands, leading seismologists to expect more in between. The earthquake on Wednesday happened much deeper than expected.
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WSJ Interactive Map of Earthquakes and Faultlines