Environmental Engineering Reference
In-Depth Information
Commission on Large Dams (ICOLD, 1975) has recommended the dynamic analysis
approach for high embankment dams whose failure may cause loss of life or major damage.
Design proceeds first by conventional methods followed by dynamic analysis to inves-
tigate any deficiencies which may exist in the pseudostatic design. A simplified procedure
for estimating dam and embankment earthquake-induced deformations is presented by
Makdisi and Seed (1978). Dynamic analysis is also discussed by Brandes (2003).
11.3.5
Tsunamis and Seiches: Response of Large Water Bodies
Tsunamis
General
Tsunamis are long sea waves that can reach great heights when they encounter shorelines,
where they represent a very substantial hazard. Generally associated with earthquakes,
they also result from underwater landslides. Seismologists generally agree that they usu-
ally reflect some sudden change in seafloor topography such as up-thrusting or down-
dropping along faults, or less frequently the sliding of unconsolidated material down
continental shelves. They are potentially very damaging.
Occurrence : Synolakis (2003) lists 152 tsunamis that have occurred in the past 100 years,
Geographic Occurrence
Pacific Ocean regions : The regions of most frequent occurrence include the Celebes Sea, Java
Sea, Sea of Japan, and the South China Sea, but in recent years, tsunamis have struck and
caused damage in Crescent City, California and Alaska from the 1964 event; and in Hilo,
Hawaii, Japan, and Chile from the 1960 Chilean quake. Japan probably has the greatest inci-
dence of tsunamis of any country, and has been subjected to 15 destructive tsunamis since
1956, eight of them disastrous (Leggett, 1973). One of the worst tsunamis in history
occurred along the northeast coast in June 1896 when a wave 76-100 ft (23-30 m) above sea
level rushed inland, destroying entire villages and killing more than 27,000 people. The
cause was considered to be a nearby earthquake. The Hawaiian Islands are subjected to a
serious tsunami about once in every 25 years (Leggett, 1973).
Atlantic Ocean : Occurrence is very infrequent. Following the Lisbon earthquake of 1755,
the sea level was reported to have risen to 6 m at many points along the Portuguese coast,
and in some locations to 15 m. The Grand Banks event of 1929, with the epicenter located
about 400 km offshore, caused a great tsunami that was very destructive along the
Newfoundland coast and took 27 lives. It may have been caused by the turbidity current
described in Section 11.3.4.
Indian Ocean (Bay of Bengal): On December 26, 2004, as this topic was going to print, a
Magnitude 9 earthquake occurred 155 miles off the coast of Aceh Province of Sumatra.
Located about 6 miles below the seabed of the Indian Ocean, where the Indo-Australian Plate
subducts beneath the Philippine Plate, it caused a tsunami that resulted in sea waves reach-
ing heights of 35 ft or more, devastating shorelines in Sri Lanka, India, Indonesia, Malaysia,
Thailand and Maldives. Deaths were reported to exceed at least 50,000, many of which
occurred in Sri Lanka, 1000 miles distant, where the sea wave arrived 2 h after the earthquake.
A number of tsunamis have occurred in recent years in the East Indian Ocean including Flores
in Indonesia in 1992 resulting in more than 2000 deaths, as reported by Synolakis (2003).
Mediterranean Sea : Occasional occurrence.
Characteristics
At sea : Tsunamis can be caused by nearby earthquakes, or as often occurs, by earthquakes
with epicenters thousands of kilometers distant from the land areas they finally affect. They
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