I was enjoying the cultural splendours of St Petersburg when the devastating tsunami of December 2004 hit. Since the only source of information that CNN news could present to us was their weather-person, I became, as the token geologist in our tour group, responsible for passing on what I knew and correcting media inaccuracies. The anniversary of that appalling event is now approaching, and there are several reports currently in the geo-news of the ongoing forensic work on the earthquake and the tsunami. Workers at Caltech have established that the 2004 and subsequent events only released stress along particular segments of the subduction zone (for example, the build-up since the major earthquake of 1861), and that other areas remain stressed and liable to rupture in the not-too-distant future. "From what we saw," says geologist Jean-Philippe Avouac, director of the Tectonics Observatory and one of the paper's lead authors, "we can say with some confidence that we're probably not done with large earthquakes in Sumatra." (http://www.sciencedaily.com/releases/2008/12/081203131042.htm). And, just to emphasize the continuing surprises that our planet has for us (i.e. the things that make geology exciting), research at the University of Washington has shown that the 2004 Sumatra earthquake triggered low-level seismic events along the San Andreas fault: "We found that an earthquake that happened halfway around the world could trigger a seismic signal in the San Andreas fault. It is a low-stress event and a new kind of seismic phenomenon," said Abhijit Ghosh, a University of Washington doctoral student in Earth and space sciences. (http://www.eurekalert.org/pub_releases/2008-12/uow-gio121008.php and read more at Andrew Alden's great portal to all things geological at http://geology.about.com/b/2008/12/11/triggered-tremor-from-half-a-world-away.htm).
But this is a blog about sand - where is all this seismology stuff leading?
In order to have any hope of avoiding the tragedy of the 2004 event in the
likely future earthquakes, a combination of approaches is needed. Prediction
with sufficient accuracy remains out of the question, but the development of the
Indian Ocean Tsunami Warning System (IOTWS) is vital, and the design and
effectiveness of this
system depends on our understanding of the character and
timing of past tsunamis - hence the forensics. On an island just off the coast
of Aceh most people safely fled to higher ground in 2004 because the island's
oral history includes information about a devastating tsunami in 1907.
When you look at the "before and after" images of the Aceh coast, and if you
can think beyond the scale of the human tragedy, one obvious question is "where
have the beaches gone?" The upper image shows the answer - strewn for long
distances inland. (Images credit Ikonos images copyright Centre for Remote
Imaging, Sensing and Processing, National University of Singapore and Space
Imaging, see http://www.nasa.gov/vision/earth/lookingatearth/indonesia_quake.html).
These deposits give us vast amounts of information about tsunamis past and
present. The photo below shows a cross-sectional
view of 2004 tsunami
deposits at the village of Lampuuk in Aceh. A 73-cm thickness of sand was
deposited over
soil (dark material in bottom center), in beds indicating
multiple pulses of sand deposition. The lowermost bed, 45 cm of sand grading
upward from coarse to fine, settled from suspension in a single wave and can be
used to characterize the wave's height and velocity (http://soundwaves.usgs.gov/2005/03/,
photo by Guy Gelfenbaum).
The layers of sand swept inland by historic tsunamis provide a crucial
historic and geologic record. Although destructive tsunamis are rather rare
events at a given coastal location, to estimate the existing tsunami hazard
properly, long-term observations are required. Geologic records of
pre-historical tsunamis can extend the historical and instrumental records to
the recent past and fill in gaps in the historic and geographic records. An
international team, supported by the US IOTWS program, and led by Kruawun
Jankaew of Chulalongkorn University in Thailand, have been digging hundreds of
pits at locations in Thailand and Sumatra to reveal this record, and have
recently reported their results (http://www.sciencedaily.com/releases/2008/10/081029141037.htm).
The photo shows
Dr. Kruawun in one of these excavations. The pit reveals a
regular sequence of sand layers interrupting the normal dark soils - the topmost
layer is the sand deposited by the 2004 tsunami, the lower ones successively
older events over the last few thousand years. Radiocarbon dating of bark
fragments in soil below the second sand layer led the scientists to estimate
that the most recent predecessor to the 2004 tsunami probably occurred between
A.D. 1300 and 1450 and there is clear evidence of two earlier tsunamis during
the last 2,500 to 2,800 years.
The knowledge gained from this work contributes to an effective warning system and tsunami education. In the words of Dr. Kruawun, "Many people in Southeast Asia, especially in Thailand, believe, or would like to believe, that it will never happen again. This will be a big step towards mitigating the losses from future tsunami events."
So, I guess that the message here is that sand can help save lives. I shall end this post with what I think is a poignant and provocative image. this time from the sands left by a tsunami that devastated the Japanese island of Okushiri in 1993. A clock, half-buried in the sand, provided one small piece of timing data for the event (http://www.ngdc.noaa.gov/nndc/struts/results?eq_1=27&t=101634&s=0&d=4&d=44).
The image of Dr. Kruawun, what do the altering colors of the stick measure - time scale? - how much time?
Fantastic pictures, I recall seeing something similar on the discovery channel (or network of channels) where they were doing the same in the coastal areas of New England/New York (though I think it was more related to hurricane surge).
Posted by: Jesse | February 06, 2009 at 04:24 PM
Jesse - thanks for the comment and for looking at my blog!
I believe that the stick is a meter measure and that each segment is 10 cms. It's amazing how regular the sand occurrences are and how similar they are in thickness. As for time, the dark soil layers represent a long period of time, the sands an essentially instantaneous event; it looks as if the recent soil layer is around 20 cms thick and that represents the time since the last tsunami around seven or eight hundred years ago.
And yes, there's a whole science of "paleotempestology" which looks at the sedimentary records of ancient storms and hurricanes - as well as tsunamis - and a lot of this kind of work has been done around all the coastal regions of the US.
Posted by: Sandglass | February 06, 2009 at 04:51 PM
tsunamis are very danger because many of them die of tsunami.
Posted by: anupargavi | December 20, 2013 at 02:12 PM