Geoscience Reference
In-Depth Information
Fig. 7.1
Woodcut portraying
the great flood of January 30,
1607 in the lowlands surrounding
Bristol Channel, UK. Analysis of
historical records and field
evidence lends support for a
tsunami as the cause of the
flooding. A likely source would
be a submarine slide off the
continental shelf edge near
Ireland. Source White (
1607
)
Table 7.1
Area and volume of large submarine slides and their associated tsunami
Area (km
2
)
Volume (km
3
)
Location
Tsunami features
Hawaiian Islands
Nuuanu slide
23,000
5,000
Alika 1 and 2
4,000
600
May be responsible for the Lanai event, run-up [365 m
Storegga slides, Norway
112,500
5,580
First
52,000
3,880
Second
88,000
2,470
Third
6,000
Included in above
Maximum wave height of 5 m swept east coast of Scotland
Agulhas, South Africa
-
20,000
Sunda Arc, Burma
3,940
960
Saharan Slide
48,000
600
Canary Islands
40,000
400-1,000
Grand Banks, 1929
160,000
760
3 m high tsunami wave, Burin Peninsula, Newfoundland
Bulli, SE Australia
200
20?
May explain tsunami features in Sydney-Wollongong area
Source Based on Moore (
1978
), Harbitz (
1992
) and Masson et al. (
1996
)
In this equation, pore water pressure is a crucial deter-
minant in failure. Sediments that are water-saturated are
more prone to failure, while factors that temporarily increase
pore water pressure such as the passage of seismic waves can
reduce the term (r-n) to zero. At this point, the strength of a
soil becomes completely dependent upon the cohesion within
loose sediment or within stratified rocks. Material on sub-
merged continental shelves and island flanks are water sat-
urated and devoid of vegetation. Their slopes were drowned
and became prone to failure because of a Holocene rise in sea
level amounting to 100-130 m in the last 3,000-6,000 years.
This resulted in an additional weight on the seabed of
103-133 9 10
6
tkm
-2
. Pore water pressure also increased
internally within sediment lying at deeper depths with this
rise in sea level. Slides in these environments usually occur
close to the time of initial saturation. The Storegga slides off
Norway may be the best example of such a process (Dawson
et al.
1988
; Dawson
1999
).
Changes in sea level do not have to be large to induce
failure. Storm surges associated with the passage of a tropical
cyclone can load and deload the shelf substantially (Bryant
2005
). For example, along the east coast of the United States,
7 m high surges are common. The resulting increase in
weight on the seabed can be 7.2 9 10
6
tkm
-2
.Ifthisis
preceded by deloading due to the drop in air pressure as the
cyclone approaches shore, than the total change in weight
canamountto109 10
6
tkm
-2
. In areas where the Earth's
crust is already under strain, this pressure change may be
