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 Oral Presentation 1756
Deposits of the tsunami of December 26, 2004, In Northern Indonesia:
Simelue Island and the medan coast of Sumatra Island
Dr. Razzhigaeva Nadya
paleotsunami paleotsunami
Larissa Ganzey, Tatiana Grebennikova, Elena Ivanova
Sediments deposited during the tsunami of December 26, 2004, in coastal areas that differ in their
structure and orientations relative to the tsunami front are studied with defining of the factors
controlling particular features of the sedimentation under different wave intensities. The data obtained
by the international expedition that studied the aftereffects of the tsunami in question from January-
February, 2005. The sediments were studied along profiles orthogonal to the shoreline with available
estimates of the tsunami run-up heights and distances. The lithology of tsunami-related deposits and
data on various fossils (diatoms, foraminifers, and mollusks) are analyzed. The data obtained
demonstrate that the sedimentation patterns during the tsunami were substantially variable in different
coastal areas, which is explained by some other reasons in addition to the different wave transformation
on the coasts with different configuration. A significant role in this process belongs to the
geomorphologic structure and composition of the flooded zone and underwater coastal slope. Variably
oriented coseismic motions is one of the factors influencing the sedimentation patterns. The most
intense erosion occurred in the Northern part of Simelue Island , where the tsunami run ups exceeded
10 m. The thickness of the eroded layer amounts at least to 1.5 m. Coral reefs suffered the most
intense erosion, particularly in the areas of coseismic uplifting and on the low shores subjected to a
direct tsunami impact. The lithology of tsunami deposits depends on both the particular features of the
tsunami and, largely, on the material sources. Under tsunami run ups up to 3-5 m high, when the
erosion zone is limited by the upper part of the coastal slope, the beach, and adjacent old ramparts, the
tsunami deposited well-sorted sands texturally similar to their counterparts from the beach and draining
area. The textural characteristics of the sands are well sustained both vertically and horizontally. When
the material from destroyed reefs is the main source for the tsunami deposits, they become more
heterogeneous. Their most intense destruction occurred in the areas of coseismic uplifting. Here near
the shoreline, the tsunami deposited moderately and poorly sorted sands, in the extended flooded zone,
high tsunami run ups result in the sorting of this material, its grain size decrease, the disappearance of
the coarse-grained fractions, and a unmodal grain-size distribution. The taxonomic composition of the
microflora and fauna in the tsunami sediments shows that they mainly originate from the sediments of
the erosion zone, coral reefs, and the upper part of the coastal slope. The paleotsunami deposits
discovered are compared with their recent counterparts. Grants RFBR 05-05-64063, FEB RAS I № 06-I-
ОНЗ-106.
Keywords: tsunami, sediments