Geoscience Reference
In-Depth Information
northeast coasts of Iceland and the northeast tip of Scotland
in just over 2 h (Ward
2001
). Figure
7.10
shows that the
tsunami then took another 8 h to propagate through the
North Sea (Henry and Murty
1992
). Most of the tsunami's
energy was focused towards Greenland and Iceland. Based
on shallow-water long-wave equations, the tsunami was
3 m high approaching the coast of Greenland and Iceland,
and 1 m high approaching Scotland. Maximum run-up
heights were 10-15 m for the first slide and 5-8 m for the
second. Along the east coast of Scotland, modeled run-ups
ranged from 3.0 to 5.5 m. These values may be conserva-
tive. If the slide moved at a maximum theorized velocity of
50 m s
-1
or initially underwent rapid acceleration, then the
calculated run-up heights of the second slide were 13.7 m
along the east coast of Iceland and Norway, 11.5 m along
the east coast of Greenland, and 18-5.3 m along the north
and east coasts of Scotland respectively. These latter figures
agree with field evidence in Scotland showing that the wave
reached 4 m above contemporary sea level at most locations
in northeastern Scotland, 10 m at the northern tip of Scot-
land, and 24.8 m on the Shetland Islands (Smith et al.
2004
). Along the Norwegian coast, tsunami deposits indi-
cate that run-ups reached 10-11 m above sea level along the
coastline adjacent to the headwall of the slide, and up to
4 m elsewhere (Bondevik et al.
1997a
,
b
).
The most prominent signature of the Storegga Tsunami
is the presence of thin sand layers sandwiched between silty
clays and buried 3-4 m below the surface of the raised
estuarine plains or carseland of eastern Scotland (Fig.
7.10
).
These deposits have been described at over 17 sites and are
best developed in the Firth of Forth region, where they can
be found more than 80 km inland (Long et al.
1989
;
Dawson et al.
1988
; Smith et al.
2004
). This long penetra-
tion up what was then a shallow estuary is beyond the
capacity of even the largest storms in the North Sea and
requires tsunami wave amplitudes at the maximum range of
those modeled. Generally, the tsunami deposited these sand
layers 4 m above the high-tide limit. The presence of these
buried sand layers has been known since 1865; however, it
wasn't until the late 1980s that researchers realised that the
sands were evidence of tsunami originating from the Stor-
egga submarine slides. The basic characteristics of the sands
which are gravelly, include marine and brackish diatoms
and peat fragments from the underlying sediments. The
most common diatom species is Paralia sulcata (Ehren-
berg) Cleve, which constitutes over 60 % of specimens.
Many of the diatoms are broken and eroded—features
indicative of transport and deposition under high-energy
conditions. Generally, the anomalous layer comprises grey,
micaceous, silty fine sand less than 10 cm thick, although
thicknesses of 75 cm have been found. In places, multiple
layers exist, consisting of a series of layers of moderately
sorted sand. Grain size fines upwards both within individual
units and throughout the series. Detailed size analysis
indicates that as many as five waves may have reached the
coast, with the first and second waves having the greatest
energy (Fig.
3.4
).
This sequence of sands is also well preserved in raised
lakes along the Norwegian coast (Bondevik et al.
1997a
,
b
).
As the tsunami raced up to 11 m above sea level, it first
eroded the seaward portion of coastal lakes around Bjugn
and Sula, located adjacent to the headwall of the Storegga
slides (Fig.
7.10
). The wave then laid down a graded or
massive sand layer containing marine fossils. As the wave
lost energy upslope, it deposited a thinner layer of fining
sand. Only one wave penetrated the upper portions of the
lakes, but closer to the sea, up to four waves deposited
successively thinner layers of sediment in deposits
20-200 cm thick (Fig.
7.11
). As each wave reached its
maximum limit inland, there was a short period of undis-
turbed flow during which larger debris such as rip-up clasts,
waterlogged wood fragments; and fine sand and organic
debris, torn up and mulched by the passage of the tsunami
wave, accumulated over the sands. Fish bones from the
marine species Pollachius vierns (coalfish) and buds from
Alnus spp. (alder) were found in the organic mash. Both the
length of the fish bones and the stage of development of the
buds indicate that the event occurred in the late autumn.
Each backwash eroded into the freshly deposited organic
layer, but because water was ponded in lakes, velocities
were not as high as in the run-up. Finally, when the waves
abated, fine silt and sand settled from suspension in the
turbid lake waters and capped the tsunami deposit with thin,
muddy layers of silt termed gyttja.
The buried sand is not the only evidence of the Storegga
Tsunami. The 1 m to 3 m open ocean amplitude of the
wave ensured that its effects were widespread throughout
the North Atlantic. Besides the buried sands, a raised dump
deposit of sand and cobbles has been found at Bitrufjorour,
Iceland (Dawson
1994
). Large aligned boulders have also
been found along the Skagerrak coast of Sweden pointing
towards the slides. Finally, a buried splay of sand, sand-
wiched between two peat layers, rises to the surface of an
infilled embayment on the west coast of Scotland at
Mointeach Mohr (Price et al.
1999
). The extent and shel-
tered location of this site rules out storm surge as the
mechanism of deposition. The contact between the sand and
the older, lower peat unit is erosional, with pieces of the
lower peat incorporated into the sand. It appears that the
sands in the tsunami deposit originated from the beach and
dunes at the mouth of the embayment and were deposited
rapidly landward in a similar fashion to the model proposed
This evidence certainly indicates that the tsunami was
large enough to have been very erosive along exposed rocky
