Geoscience Reference
In-Depth Information
3.2
Depositional Signatures of Tsunami
Wave 5
3.2.1
Buried Sand or Anomalous Sediment
Layers
Wave 1
Wave 2
Wave 3
The commonest signature of tsunami is the deposition of
landward tapering sandy units up to 50 cm thick sand-
wiched between finer material and peats on flat coastal
plains. Tsunami sand units form part of a coherent landward
thinning splay of fining sediment extending up to 10 km or
more inland. The number of units is often characteristic of
the causative mechanism of the wave. For example, tsunami
generated by submarine landslides generally produce wave
trains with two to five large waves. Each wave is capable of
moving sediment inland. Multiple sand layers cannot be
produced by storm surge because it occurs as a single wave
event. In contrast, earthquake-generated wave trains, while
consisting of tens of waves, tend to produce only a single
wave that is large enough to transport sediment inland.
Without additional evidence, it may be difficult to separate
an earthquake-generated tsunami deposit from that pro-
duced by a storm surge.
Figure 3.4 shows the relative grain size and sorting in a
52 cm thick sand unit at Ardmore, Scotland, deposited by
large tsunami from the Storegga slide off the west coast of
Norway 7950 years ago (Dawson et al. 1988 ; Dawson
1994 ). This tsunami deposited fine sand and silt as much as
80 km inland across numerous estuarine flats—termed
carseland—that are now raised along the east coast of
Scotland (Long et al. 1989 ; Dawson et al. 1998; Smith et al.
2004 ). The tsunami and its wider impact will be described
in more detail in Chap. 7 . Each wave in Fig. 3.4 is num-
bered sequentially. The first wave, as expected, had the
greatest wave energy and moved a greater range of grain
sizes. Subsequent waves moved finer sediment, probably
because of decreasing wave height or the unavailability of
coarser sediment. While the last wave was smaller, it
transported a wider range of grain sizes than the previous
three waves. This may reflect the recycling of coarse sedi-
ment seaward by channelised backwash.
Considerable attempts have been made to date anoma-
lous sand layers and relate them to historical tsunami
events. Along the Sanriku coast of northeast Honshu Island,
Japan—renowned for deadly tsunami—up to thirteen well-
sorted sand layers can be found intercalated within black
organic muds in swamps and ponds (Minoura et al. 1994 ).
The sand layers are spatially extensive, but do not form
erosional contacts with the underlying muds. This suggests
that sediment settled from suspension in quiescent waters
stranded in depressions after rapid drowning by tsunami that
exceeded 1 m in height. The layers have been correlated to
known tsunami events originating off the coast and from
Wave 4
0.35
0.30
0.25
0.20
0.15
0.10
sandy
Mean grain size (mm)
silty
Fig. 3.4
Grain size and sorting relationships throughout a 52 cm
thick
sand
unit
at
Ardmore,
Scotland,
deposited
by
a
tsunami
associated with the Storegga submarine slide
distant sources in the Pacific Ocean (Table 3.1 ). Seven of
the tsunami formed locally; however, four of the events
originated from either Chile or the Kuril Islands to the
north. Surprisingly, the severe Chilean Tsunami of May 22,
1960 did not flood any of the marshes. In addition, no event
prior to 1710 is preserved in the sediments despite this
coastline having historical records of earlier events. The
stratigraphic evidence designates this coastline as the most
frequently threatened inhabited coastline in the world. The
November 1, 1755 Lisbon Tsunami, generated by one of the
largest earthquakes ever, deposited sand layers along the
Portuguese coast (Dawson et al. 1995 ) and as far as the Isles
of Scilly off the west coast of England (Foster et al. 1991 ).
In the 20th century, the Grand Banks Tsunami of November
18, 1929, which was produced by a submarine landslide,
laid down sand layers on the Burin Peninsula in New-
foundland (Tuttle et al. 2004 ). Following the Chilean Tsu-
nami of May 22, 1960, sand layers were deposited in the
Río Lingue estuary of south-central Chile (Wright and
Mella 1963 ), while the Alaskan Tsunami of March 27, 1964
deposited sand units along the coasts of British Columbia
(Huntley and Clague 1996 ). More recently, this signature
was observed on the island of Flores, Indonesia, following
the tsunami of December 12, 1992 (Minoura et al. 1997 )
and along the Aitape coast, Papua New Guinea, as the result
of the tsunami of July 17, 1998 (Gelfenbaum and Jaffe
1998 ). In both of the latter cases, landward-tapering wedges
of sand were deposited over 500 m inland from the shore-
line. These examples will be described in more detail in the
chapter on earthquake-generated tsunami. By far the
thickest sand layers were deposited by the recent Indian
Ocean Tsunami of December 26, 2004. In Banda Aceh,
Indonesia, the sand layer was a massive 0.7 m thick and
contained angular pebbles and soil rip-up clasts (United
States Geological Survey 2005a ). In Sri Lanka, the sand
layer was 0.37 m thick (United States Geological Survey
2005b ). This event will be described in detail in Chap. 6 .
 
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