Great Landslides - Tsunami - page 131

wave propagated down the bay towards the ocean. This tsu-
nami will be described in detail subsequently. Miller ( 1960 )
describes many similar slides. For example, there have been
seven tsunamigenic events in Norway, which together have
killed 210 people. The heights of these tsunami ranged
between 5 and 15 m with run-ups of 70 m above sea level
close to the source. Five of these events occurred in Tafjord in
1718, 1755, 1805, 1868, and 1934. In the April 7, 1934 event,
1 9 10 6 m 3 of rock dropped 730 m from an overhang into the
fjord. The initial tsunami was 30-60 m high, decaying to
10 m several kilometers away. It traveled at a velocity of
22-43 km hr -1 . Historically, a slide sixteen times this vol-
ume has occurred in Tafjord. Loen Lake in Norway has also
been subject to slide-induced tsunami, with three events
occurring in 1936 alone. The largest of these produced run-
ups of 70 m above lake level, killing 73 people. In the United
States, slides have also generated tsunami in Disenchantment
Bay, Alaska on July 4, 1905, and several times in Lake
Roosevelt, Washington. The Disenchantment Bay event
produced maximum run-up of 35 m, 4 km from the source,
while slides into Roosevelt Lake have generated run-ups of
20 m on at least two occasions. One of the more unusual
tsunami produced by a rockslide occurred in the Vaiont
Reservoir in Italy in October 1963. Here, 0.25 9 10 9 m 3 of
soil and rock fell into the reservoir sending a wave 100 m high
over the top of the dam and down the valley, killing 3,000
people. Even small earthquakes can generate tsunamigenic
submarine landslides, sometimes hours after the event. The
Kona earthquake of 1951 in Hawaii had a magnitude of only
6.7; yet, it produced a tsunami with a maximum run-up of
1.5 m as the result of a landslide that occurred 3 h afterwards.
Tsunami generated by submarine slides have been common
historical occurrences. For example, the Grand Banks earth-
quake of November 18, 1929 triggered a submarine landslide
that is famous for the turbidity current that swept downslope
into the abyssal plain of the North Atlantic (Heezen and Ewing
1952 ). Less well known is the devastating tsunami that swept
into the Newfoundland coast (Whelan 1994 ). Similarly, the
11 m high tsunami that swept the foreshores of Sagami Bay
after the Great Tokyo Earthquake of September 1, 1923 is now
thought to have been produced by submarine landslides
(Moore 1978 ; Bryant 2005 ). Major tsunami generated by
submarine slides triggered by earthquakes have also occurred
at Port Royal, Jamaica, in June 1692; at Ishigaki Island, Japan,
on April 4, 1771; and at Seward, Valdez, and Whittier, Alaska,
following the Great Alaskan Earthquake of 1964. The Port
Royal Tsunami flung ships standing in the harbor inland over
two-story buildings and killed 2,000 inhabitants, while the
Ishigaki Island Tsunami carried coral 85 m above sea level
and killed 13,486 people.
Historically, 66 tsunami events involving either sub-
merged or terrestrial landslides have taken 14,661 lives in
the Pacific and Indian Ocean regions (Intergovernmental

Oceanographic Commission 1999 ; National Geophysical
Data Center 2013 ). In the twentieth century, submarine
slides have occurred off the Magdalena River, Colombia,
and the Esmeraldas River, Ecuador, in the Orkdals Fjord,
and off several Californian submarine canyons. In 1953,
submarine landslides cut cables in Samoa and produced a
2 m high tsunami. On July 18, 1979, a tsunami generated by
a landslide, unaccompanied by any earthquake or adverse
weather, destroyed two villages on the southeast coast of
Lomblen Island, Indonesia, killing at least 539 people.
Smaller tsunami were produced by submarine slides at
Nice, France, on October 16, 1979; at Kitimat Inlet, British
Columbia, on April 27, 1975; and in Skagway Harbor,
Alaska, in November 1994. In the latter two cases, run-ups
of 8.2 and 11 m respectively were observed.

7.4.1 The Lituya Bay Landslide of July 9, 1958

Lituya Bay is a T-shaped, glacially carved valley lying
entirely on the Pacific Plate. Glaciers reach almost to sea level
in Crillon and Gilbert Inlets at the head of the Bay (Fig. 7.4 ).
The main bay measures 11.3 km long and 3 km wide, with a
220 m maximum depth. Cenotaph Island obstructs the center
of the bay, and La Chaussee Spit, which is the remnant of an
arcuate terminal moraine left over from the last glaciation,
blocks the entrance to the sea. The bay has been subject to
giant waves geologically (Miller 1960 ). For example, run-ups
of 120 and 150 m above sea level were produced by events in
1853 and on October 27, 1936 respectively. The July 9, 1958
event is the largest, however, reaching an elevation of 524 m
above sea level. The trigger for the event was an earthquake
with a magnitude, M s , of between 7.9 and 8.3 that occurred
around 10:16 PM along the Fairweather Fault at the junction
of the Pacific and North American Plates. The earthquake's
epicenter was 20.8 km southeast of the head of Lituya Bay.
Vertical and horizontal ground displacements of 1.1 and
6.3 m respectively occurred along the fault and reached the
surface in Crillon and Gilbert Inlets at the head of Lituya Bay.
Vertical and horizontal accelerations reached 0.75 and 2.0 g
respectively in the region. None of these disruptions was
sufficient to generate a giant wave. The earthquake triggered
landslides in the northern part of the embayment. Terrestrial
landslides are inefficient mechanisms for tsunami generation
because only about 4 % of their energy goes into forming
waves. No known landslide has ever produced a wave the size
of the Lituya Bay wave, which reached eight times the height
above sea level of the largest slide-generated wave recorded
in any Norwegian fjord. A Jökulhlaup or water burst from an
ice-dammed lake high up on Lituya Glacier also could have
caused the wave. The water level in this lake dropped 30 m
following the earthquake, and the hydraulic head was cer-
tainly sufficient to generate a giant wave. However, neither