This is
remarkable and lucky. We have a
creditable and clear stratigraphy to act as a benchmark. The important take home is the outright of
meaningful periodicity. I am actually
glad of that. Far too often we chase
such patterns because we inclined to see them.
This informs us very clearly otherwise and it is a timely caution. Three thousand years is almost enough.
This is good
science and we will now systematically track down tsunami events
worldwide. It will be difficult work but
I do believe an accurate history of coastal inundations is needed. Tens of thousands died in a moment in 2004
and this is not unique. What is unique
is our lack of decent historical data and our willingness to ignore what little
we have.
Let us make it
simple. Geology can find a way to strike
any given exposed coast. It is certainly
not common but it is not impossible. For
that reason, broad set backs are needed as a matter of prudence. The same holds true for hurricane damage.
These setbacks
need to include riverine estuaries, most often choked with commerce.
This simply
needs to become global practice. We
are giving up nothing to do so. Vancouver and Seattle are coastal cities
wonderfully barriered by a mountain range that easily quells dangerous offshore
quakes as well as blocking tsunamis and moderating storms.
Sumatra coastal cave
records stunning tsunami history
By Jonathan Amos Science correspondent, BBC News, San Francisco
11 December 2013 Last updated at 21:06 ET
The cave retains the deposits washed ashore by huge waves over
thousands of years
A cave on the northwestern coast of Sumatra
holds a remarkable record of big tsunamis in the Indian Ocean.
The limestone opening, close to Banda Aceh,
retains the sandy deposits washed ashore by huge, earthquake-induced waves over
thousands of years.
Scientists are using the site to help
determine the frequency of catastrophes like the event of 26 December 2004.
This is being done by dating the cave's
tsunami-borne sediments, which are easy to see between layers of bat droppings.
###
Because people thought they had no history of
such things, they thought it was impossible”
Prof Kerry SiehDirector, Earth Observatory of Singapore
"The tsunami sands just jump right out at
you because they're separated by guano layers. There's no confusing the
stratigraphy (layering)," explains Dr Jessica Pilarczyk.
"It makes for interesting field work; I'm
not going to lie to you. The bats get very excited when people are disrupting
their space. But from a geologist's point of view, this cave has the most
amazing stratigraphy," she told BBC News.
Dr Pilarczyk was speaking here in San
Francisco at the American
Geophysical Union (AGU) Fall Meeting, the world's largest
annual gathering of Earth scientists.
She is part of a team of researchers - led by
Prof Charles Rubin - from the Earth Observatory of Singapore,
an institute of Nanyang Technological University that is investigating the
coastal history of Indonesia's largest island.
###
The tsunami record from about 7,500 to 3,000 years ago is
impeccable, say the scientists
Sumatra's proximity to the Indo-Australia and
Sunda tectonic plate boundary, and the giant earthquakes that occur there,
means its shores are at risk of major inundations.
Understanding how often these occur is
important for policy and planning in the region.
The Acehnese cave lies about 100m back from
the swash zone at current high-tide. Its entrance is also raised somewhat, and
this prevents all waters from getting into the opening - apart from tsunamis
and severe storm surges.
Dr Pilarczyk and colleagues have dug trenches
through the alternating bands of bat guano and sand to piece together the
cave's history.
The scientists know they are looking at
tsunami deposits because they can find debris in the sediments of seafloor
organisms such as microscopic foraminifera. Only the most energetic waves could
have lifted and carried this material into the cave.
The investigations are ongoing but the team
thinks it can see deposition from perhaps 7-10 tsunamis. The geometry of the
cave means these events would likely have been generated by earthquakes of
Magnitude 8, or more. By way of comparison, the devastation wrought by 26
December 2004 stemmed from a M9.2 tremor.
Dating the old deposits is obtained by
radiocarbon analysis of organic debris caught up in the bands, such as molluscs
and pieces of charcoal from old human-lit fires.
Work is under way to date even the insect
remains eaten by the bats and now immersed in the guano layers.
Today, the cave is so full of sand and bat
droppings that any new event will essentially overwash and erode the most
recent deposits. "The 2004 tsunami completely inundated the cave,"
comments Prof Rubin.
Nonetheless, the stratigraphy from
about 7,500 to 3,000 years ago is impeccable.
"What we think we have is actually a
near-complete sequence of late-Holocene deposits. This is amazing because
usually the records we have are fragmentary at best. This coastal cave is a
unique 'depot centre', and it's giving us a remarkable snapshot of several
thousands of years, allowing us to figure out every single tsunami that would
have taken place during that time," said Dr Pilarczyk, who is affiliated
also to Rutgers University, US.
The team's other investigations along the
Acehnese coast are filling in the period from 3,000 years ago to the present.
And the
take-home message from all this research is that the biggest tsunamis are not
evenly spaced through time. Yes, there can be long periods of quiescence, but
you can also get major events that are separated by just a few decades.
Co-investigator Prof Kerry Sieh says this is a
cautionary story.
"2004 caught everybody by surprise. And
why was that? Because nobody had been looking back to see how often they
happen, if they'd ever happened," he told BBC News.
"In fact, because people thought they had
no history of such things, they thought it was impossible. Nobody was prepared,
nobody had even given it a second thought. So the reason we look back in time
is so we can learn how the Earth works and how it might work during our
watch."
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