INTERVIEW
The 'perfect' quake this way comes
By Charles McDermid
HUA HIN, Thailand - 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 ..." [1]
As the Padang disaster developed, Briggs spoke to ATol's Charles McDermid in a
series of phone calls and e-mails from September 30 to October 4.
Asia Times Online: What, in your opinion, is the power of an
earthquake on the human psyche?
Richard Briggs: Earthquake survivors often report a severe sense
of disorientation during severe groundshaking, a whole-body confusion and loss
of moorings - both figurative and literal. Our frame of reference is the ground
we stand on, and when that reference frame shifts without warning, something
deep in us is affected. The earth simply isn't supposed to move, we assume, and
when it does we are profoundly unsettled. The unexpected widescale destruction
of our built environment, dramatic land-level changes along coastal areas, and
accompanying tsunamis - and the loss of human life that often accompany these
events - make earthquakes singularly terrifying and memorable.
ATol: 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.
ATol: And why were you in Padang? [An ancient trading city of
900,000, built by the Chinese in the 1600s and later colonized by the Dutch and
British.]
RB: Padang is the staging area for most of our fieldwork in West
Sumatra, and I have many fond memories of friends, and the anticipation of
field missions or of seeing the city lights and Masakan Padang after weeks or
months at sea. It's terrible to see such good people hurting so much right now.
ATol: It seems the study of earthquakes is akin to the T-Rex for
paleontologists or, say, Ulysses for lit majors ... is there any truth
to that? A Holy Grail of some sort?
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.
ATol: 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.
ATol: 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.
ATol: When the first 7.9 magnitude earthquake hit Padang on
September 30 at 5:16pm, many locals ran for high ground fearing a tsunami - was
the area at risk of this, and is it still?
RB: Padang is still at risk from tsunamis that might accompany
sudden movement of the sea floor during a large megathrust earthquake. Tsunamis
slammed into the Sumatran coast following large megathrust ruptures in 1833 and
1797. [The latest] earthquake did little to decrease the amount of strain
stored in patches that ruptured in these earlier events.
ATol: 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.
ATol: Did the local government in Padang do enough ahead of time
to reduce the damage and loss of lives?
RB: It's the next one they need to prepare for. It's awful to
say, but in tectonic terms, this recent event was just noise in the system.
Hopefully, this helps focus preparation for the offshore megathrust event. Now
the threat is no longer abstract. RB: Tsunami inundation models
have shown that thousands of people will be in a precarious situation when the
offshore megathrust ruptures next. They may have trouble making their way to
higher ground. For example, much of the city center is hemmed in by a wide
canal with only a few narrow bridge crossings, and this is simply a big
infrastructure problem that needs to be addressed. There has been some work on
this, mostly by dedicated community groups, but much infrastructure improvement
is needed.
ATol: What role should - or can - science play in the rebuilding
process?
RB: Careful geologic observation has provided evidence of large
megathrust earthquakes that recur every few hundred years offshore Padang - and
the last large event was about 175 years ago - so science has already provided
a characterization of the threat.
Tsunami modelers have also explicitly outlined how far inland, and how deep, a
tsunami might be. So, a worst-case scenario [a devastating tsunami] and rough
time frame [sometime in the next few decades] has been established.
The next steps are education and infrastructure improvements, and these are
largely local decisions. Let's hope that a culture of education and
infrastructure improvement takes root and flourishes in Padang.
In fact, this approach would be valuable worldwide, because as a species, we
mostly have our heads in the sand when it comes to preparing for natural
hazards.
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