Natural Hazards: Causes and Effects - Disaster Management Center ...

the Pacific between the years 1900 and 1970. (See Fig. 3.2.) Of these, 35 caused casualties
and damage near the source only, whereas nine spread destruction throughout the Pacific.
Natural Preconditions for Disasters
Tsunamis are believed to originate as vertically displaced columns of ocean water. Seismic or
volcanic action on the ocean floor may cause tsunamis by creating a pulse or force on a wall of
water equal to the depth of the ocean at the point of the movement. Tsunamis spread outwards
in all directions from the point of origin, traveling at a speed proportional to the square root of
the depth of water and reaching 1,000 kilometers per hour (600 miles per hour) in the deep
ocean. The distance between successive wave crests may be as much as 500 kilometers (310
miles). As the waves reach coastal areas, this speed decreases, though the interval of time
between the passage of successive waves remains unchanged (usually between 20 and 40
minutes). A single tsunami may comprise up to 12 large wave crests.
The destructive power of tsunamis derives from the fact that the amplitude of the waves, which
is usually less than one meter (three feet) in the deep ocean, increases sharply as the waves
reach shallow water near the coast, and may be further enhanced by funnelling or resonance
effects on bays and estuaries. In extreme cases, wave heights may reach as much as 20 or 30
meters (65 or 100 feet). In such cases, waves may sweep a considerable distance inland. 3
Approximate Epicenters of Tsunami Generating Earthquakes
1900 – 1969
Figure 3.2