Great Earthquake-Generated Events - Tsunami - page 108

measured 6 m in relief. The earthquake was felt over an
area of 1.3 9 10 6 km 2 while land movement covered an
area of 520,000 km 2 . Most of Prince William Sound and the
continental shelf were affected by the latter deformation.
Approximately 215,000 km 2 of displacement contrib-
uted to the generation of the resulting tsunami over an area
measuring 150 9 700 km (Van Dorn 1964 ). The total
volume of crust shifted amounted to 115-120 km 3 . More
than 25,000 km 3 of water were displaced. Tsunami gener-
ation was also aided by some of the 52 major aftershocks
that occurred in the area of uplift. As a result, two main
tsunami-generating areas can be distinguished, one along
the continental shelf bordering the Gulf of Alaska and the
other in Prince William Sound (Lander and Lockridge
1989 ; Pararas-Carayannis 1998b ). As the tsunami moved
away from the Gulf of Alaska, it was forecast by the Pacific
Tsunami Warning System, which had been revamped fol-
lowing the 1960 Chilean Tsunami. Within 46 min of the
earthquake, a preliminary, Pacific-wide tsunami warning
was issued by the Pacific Tsunami Warning Center in
Honolulu. This was not sufficient to warn many Alaskan
communities of impending tsunami. For example, the
shores of Kenai Peninsula and Kodiak Island were struck by
tsunami within 23 and 34 min respectively of the earth-
quake (Von Huene and Cox 1972 ). Three major tsunami
developed in Prince William Sound. One was related to the
earthquake and had its origin near the west coast of Mon-
tague Island, at the southern end of the Sound. The second
was due to local landslides (Sokolowski 1999 ). The third
developed much later in the Port of Valdez region, probably
because of resonance within that part of the sound. Maxi-
mum positive crustal displacement in Prince William Sound
occurred along the northwest coast of Montague Island and
in the area immediately offshore. These earth movements
caused a gradient in hydrostatic level and numerous large
submarine slides in the area off Montague Island and at the
north end of Latouche Island. Bathymetric surveys later
showed that the combination of submarine slides and the
tilting of the ocean floor due to uplift created the solitary
wave observed in this region. The tsunami here did not
escape the sound. At Chenega, a solitary wave reached
27.4 m above sea level within 10 min of the earthquake.
Landslide-generated tsunami were confined to Prince
William Sound (Fig. 6.9 c), where communities generally
experienced wave run-ups of 12-21 m (Kachadoorian 1965 ;
McCulloch 1966 ). At the ports of Seward, Whittier, and
Valdez, docks, railway track, and warehouses sank into the
sea because of flow failures in marine sediments (Hansen
1965 ). The settlements were swamped by 7 to 10 m high
tsunami within an hour of the quake, but local run-ups were
greater. None of these communities had any warning of the
tsunami. At Seward, a swathe of the waterfront 100 m wide
dropped into the ocean over a distance of 1 km. Twenty

minutes later a 9 m high wave rolled into the town picking
up the rolling stock of the Alaska Railroad, tossing loco-
motives like Dinky toys (Fig. 6.1 ), shearing off the pilings
supporting the dock, destroying infrastructure and houses,
and killing 13 people. Near the harbor, the Texaco oil
storage tanks burst and caught fire, spilling flaming oil into
the receding sea. An hour later, a second, higher wave
roared in. It could not be missed in the dark because it
incorporated the flaming oil and raced towards the town as a
10 m high wall of flame. Eerily, the remains of the dock's
pilings had caught fire and bobbed like large Roman candles
in the waters of Resurrection Bay as successively smaller
waves raced in. At Whittier numerous slides occurred. One
of the resulting tsunami reached a height of 32 m above sea
level. At Valdez, a large submarine slide was generated at
the entrance to the port by collapse into the bay of the
terminal moraine at the end of Shoup Glacier. The resulting
tsunami lifted driftwood to an elevation of 52 m above sea
level and deposited silt and clay 15 m higher. In the town
itself, which is situated on an outwash delta with a steep
front, an area 180 m wide and 1.2 km long slid into the
fjord taking most of the waterfront with it. Within 2 or
3 min, a 9 m high wave swept inland through the town
(Fig. 6.10 ). Thirty-two people lost their lives, many as the
docks collapsed and were then swamped by water. Of the
106 deaths in Alaska due to tsunami related to the Good
Friday earthquake, up to 82 were caused by these localized
events. About 5-6 h after the earthquake, further tsunami
waves struck Valdez at high tide. The third wave came in at
11 AM March 27, and the fourth one at 1:45 AM March 28.
This last wave took the form of a tidal bore and inundated
the downtown section of Valdez. Apparently, these last
tsunami were produced by resonance that had built up in the
bay over a five hour period.
The main tsunami propagated southwards into the Pacific
Ocean within 25 min of the earthquake (Fig. 6.9 b) (Spaeth
and Berkman 1967 ; Lander and Lockridge 1989 ; Pararas-
Carayannis 1998b ). Its wave period was exceptionally long,
being an hour or more. This was caused by the long seiche
period of the shallow shelf in the region where the tsunami
originated. At many locations, the first wave arrived as a
smooth, rapid rise in sea level rather than as a distinct wave.
Seas then receded, to be followed by a bigger wave, often
coincident with high tide. On Kodiak Island, the third and
fourth waves were the highest and most destructive. Factors
such as reflection, wave interaction, refraction, diffraction,
and resonance had to be involved in the generation of this
tsunami wave train. Kodiak Island sustained heavy damage
with a maximum run-up of 10.6 m. The wave was focused
south down the west coast of North America. Along the
Canadian coast, the wave's height registered about 1.4 m on
tide gauges (Table 6.2 ). However, exposed locations such as
Shield Bay recorded a maximum run-up of 9.8 m elevation.