IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS002 Poster presentation 1815
Micropaleontological analysis of 2004 Indian Ocean Tsunami sediments
Prof. Koji Minoura
Geology and Paleontology Non IAPSO
Daisuke Sugawara, Naoki Nemoto, Shinji Tsukawaki, Tetsuya Shinozaki, Fumihiko
Imamura, Kazuhisa Goto
The 2004 Indian Ocean Tsunami, which is one of the largest tsunamis on record, attacked the coastal
areas around the Indian Ocean on 26 December 2004. The tsunami invaded the coast of Thailand with
waves reaching up to 9 meters high (Matsutomi et al., 2005). At Pakarang Cape the tsunami left
thousands of carbonate boulders on the shore. Satellite images exhibit no signs of the boulders before
the tsunami, whereas show distribution of them after the tsunami. The coastal areas are not in the
affect of strong tropical cyclones, and thus the storms were not responsible for the emplacement of the
boulders at the cape. These scientific results, together with the reports of local residents regarding the
absence of such boulders before the tsunami, indicate that the boulders were highly likely to have been
transported as a result of the 2004 Indian Ocean Tsunami. Our numerical calculations indicate that the
reef flat was mostly exposed above the sea surface just before the tsunami attack due to seawater
receding. The first wave reached to the reef edge approximately 130 minutes after the tsunami
generation, and the maximum current velocity of the first tsunami was calculated to be from 8 to 15
m/s between the reef edge and 500 m offshore, and less than 6 m/s on the reef flat. We do not know
the origin of such destructive tsunami currents in the sea, and we cannot make clear the reworking
mechanism of carbonate boulders on the forereef slope. In this context the understanding of current
formation is an urgent necessity of study for evaluating the effect of tsunamis on the environment. It
can be hypothesized that fluid motion by tsunami currents cause bedload transport of grains on the sea
bottom. Rapid grain shearing over the static bed disturbs the ecological adaptation of in- and epi-faunal
organism. Such kind of destructive tsunamis occurred repeatedly in the ocean on a geological time
scale, and probably impacted on the ecology and evolution of benthic living things. In this study we
adopted micropaleontological methods for elucidating the erosion-reworking mechanism of tsunami
currents. We have stocks of surface sediment samples dredged in the offing of Pakarang Cape before
the tsunami (April 2002). By comparing the faunal compositions of benthic foraminifers in the samples
with those in sediments collected after the tsunami (April 2005, Febrauary 2006), we could make clear
the origin of tsunami currents in the offshore bottom. The preliminary results on the benthic
foraminiferal analysis indicate that the currents by the 2004 Indian Ocean Tsunami were formed on the
forereef of around 30 m in depth. The difference in faunal assemblages between the samples of 2005
and 2006 shows the ecological recovery of faunal communities to have been made within a year. We
know well on-land tsunami hazards, however undersea effects by tsunami waves are mysterious. Living
things in the coastal areas receive continually exogenous impacts, and they have ecological adaptability
for environmental catastrophes. By applying this characteristic of shallow sea benthic foraminifers, we
might be able to elucidate the origin of tsunami currents that bring on destructive damages on the coast
Keywords: numericalcalculation, benthicforaminiferalanalysis, coralreef