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International Tsunami Expedition in Southern Madagascar, August 29-September 13, 2006.
SUMMARY
The International Tsunami Expedition has finished a two week survey of the
southern coast of Madagascar. The research team, consisted of Dr. D.Abbott,
Lamont Doherty Earth Observatory of Columbia University, USA, Dr. E.Bryant,
University of Wollongong, Australia, Dr. V.Gusiakov, Novosibirsk Tsunami
Laboratory, Russia, Dr. W.Masse, Los Alamos National Lab, USA. The team was
accompanied by two representatives of the University of Antananarivo,
Andriamiranta Raveloson and Hoby Raza. The team obtained field data from large
chevrons detected earlier this year using Google Earth imagery. The main
purpose of this field trip was to measure the vertical and horizontal extension
of the chevrons, to study their genesis and morphological features, and to
obtain marine shell in order to determine their age. Another important goal
of the expedition was to verify whether or not the chevrons discovered on
satellite images represent mega-tsunami deposits and if so, to confirm the
inferred run-ups derived from the imagery.
The expedition, using two rented 4-weel drive vehicles, studied over 150 km
of the southernmost Madagascar coast. Participants made several traverses of
the four largest chevron formations in this area - near Faux Cape, Cape Saint
Marie and along the coast of Fenambosy and Ampalaza Bays. All the chevrons
consist of marine sand transported by water, in some cases over the edge of
a coastal escarpment over 150 m high. In contrast to wind-blown dunes, which
consist of a well sorted, unimodal size distribution, the chevrons are unsorted
with a broad range of particle sizes, from small boulders down to clay particles.
They also include marine shells. Dump deposits, consisting of a mixture of rock
fragments and marine shell, that are typical of mega-tsunami processes were
found eastward from Lavanono to Cape Saint Marie. Many of the rock fragments
were not locally derived. The team documented maximum runups of 86 m above
present day sea level at Ampalaza, 186 m at Fenambosy, 205 m at Faux Cap and
192 m at Cape St Marie. Each of the chevrons represents lateral transport of
sediment onto the coast over many kilometers: 20 km at Faux Cap, 30 km at
Fenambosy, and 45 km at Ampalaza.
The measured run-up heights and in land penetration over this extended part
of the coast are far beyond the range produced by the largest historically known
tectonic tsunamis (seismic and volcanic). Such a great run-up can be produced
only by a large-volume submarine landslide somewhere along the nearby continental
slope or by an oceanic impact within the Indian Ocean. In the later case, the 29
km Burckle crater found by D.Abbott in 2005 (Abbott et al,2005) at 30S, 61E on
a fracture zone of the Southwest Indian Ridge is a good candidate for the source
of the mega-tsunami responsible for formation of these chevrons. The Burckle
crater is geologically very young, most probably about 4500 to 5000 years old.
If C-14 dating of shell collected from the chevrons matches the probable age of
the Burckle crater, this will be an important result to prove the reality of
the threat of cosmogenic tsunamis in the recent past in the world's oceans.
The expedition was sponsored by the WAPMERR (World Agency for Planetary Monitoring
and Earthquake Risk Reduction), Geneva, Switzerland
as a part of their cooperative project with the Novosibirsk Tsunami Laboratory
for the development of the World-Wide Tsunami Database and methods for
long-term tsunami risk estimation.
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