Thursday, October 8, 2009
The 'Perfect' Quake This Way Comes
It's early evening thousands of kilometers away at the offices of the United States Geological Survey in Golden, Colorado, and Sumatran tectonics expert Richard Briggs
is studying a series of seven-plus magnitude earthquakes that had just struck off the island of Vanuatu in the South Pacific.
"What's most interesting about the Vanuatu events were that they were a doublet - a couple magnitude 7.8ish - and a large aftershock of 7.3. These three were obviously related," Briggs, 37, told Asia Times Online on October 7.
"Doublets are rare and so we'll all be interested to see how it went down in detail. I really hope there's no bad local tsunami damage." Briggs says things like that all the time. He studied under world-leading paleoseismologist Kerry Sieh at the California Institute of Technology (Caltech), and conducted three years of post-doctoral field work in Sumatra from 2005 to 2007. To Briggs, the massive fault line that parallels the west coast of the island of Sumatra is the "Sunda megathrust". He describes the "shaking" and "earth rolling" of a major earthquake with a deep understanding of the appalling power unleashed.
In 2005, Briggs contributed to a white paper signed by Padang officials and international agencies that claimed, "No one is able to predict scientifically to the nearest day, week or even year when a great West Sumatran earthquake and tsunami will strike next. But the scientific evidence strongly suggests it will occur within the lifetimes of most young people living along the coast today - such earthquakes occur about every two centuries and the last occurred 172 and 208 years ago. It is very unlikely that any valid prediction will be more specific than this ..."
Why earthquakes - and why Sumatra?
RB: I proposed to study the Great Sumatran fault [an inland fault comparable to the San Andreas fault in California]. Then the 2004 and 2005 Sunda megathrust ruptures happened and I was drawn instead to this amazing story of massive shaking, waves
and re-arrangement of the Earth's surface. There are few geologic events so dramatic and demanding of our awe, fear and understanding as great subduction earthquakes.
RB: Our understanding of earthquakes has advanced mightily over the last century, but we're still trying to decipher an incredibly complex system with relatively little data. Much of the time we're groping in the dark until the way forward is lit by another large fault rupture. Plate tectonics proceeds slowly compared to the human lifespan and to make meaningful progress on understanding earthquakes we need to join the limited data we have with clever analyses, a global perspective, and often pure intuition. It's a very stimulating field.
Considering the recent seismic history of west Sumatra - the 2004 tsunami, triggered by a 9.2 earthquake, and powerful earthquakes in 2005, 2007 and last week - is it some kind of "perfect storm" earthquake area?
RB: Since 2000, the Sunda megathrust has been in the spotlight, popping off in a series of large and great earthquakes, and the historical and paleoseismic records show that these bursts have occurred before. Other subduction zones - for example, the Aleutian Islands and the coast of South America - have gone through similar phases of energy release. Subduction zones are where we observe the largest and most frequent ruptures, because the fault areas are large due to shallow-dipping megathrusts and ruptures are frequent where subduction zones are locked and convergence rates are high.
What's your assessment of Padang as an earthquake or tsunami site?
RB: Padang sits in a tectonic vise. To the west, the Sunda subduction zone is locked and loaded just offshore, and when it last ruptured in 1797 and 1833, large tsunamis caused extensive damage along the coast. To the east, the Sumatran strike-slip fault - analogous to the San Andreas in the US, or the North Anatolian fault in Turkey - has ruptured in over a dozen large earthquakes since the late 1800s. Beneath Padang, the Australian plate creaks and groans as it descends, and this internal failure of the descending slab is what caused the devastating earthquake last week.
That said, the hazard situation in Padang is not too different from the rest of West Sumatra. But what is different is the risk, which is the collision of hazard with the human-built environment. We've just seen the terrible results of how sustained ground-shaking toppled poor construction there. More alarming is the fact that Padang has hundreds of thousands of people living only a few meters above sea level and within a kilometer or two of the coast. Careful geologic work over the last decade has shown that megathrust rupture offshore Padang is extremely likely sometime in the next few decades.
Is it fair to say this quake is nothing compared to what experts expect to happen?
RB: No expert would minimize the obviously terrible destruction in Padang now. The earthquake [September 30] event was strong and the damage severe in places, and it was clearly a terrible disaster. But the event that we still worry about is a larger rupture of the megathrust offshore that might generate not only shaking, but a large tsunami as well that could inundate coastal cities, including Padang.
Asia Times Extract from story filed by Charles McDermid
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