Tsunamis - Extreme Events: A Physical Reconstruction and Risk Assessment - page 110

or reefs surrounding oceanic islands or atolls. Hence, they travel as deep-water
wavesfor much of their existence and travel at relatively low velocities across
oceans. Tsunamis travel much faster than storm- or wind-generated waves and
can attain velocities of up to 800 km h − 1 in the deepest ocean basins. Unlike
storm waves, tsunamis often don't break until they reach shore. Hence, their
greater velocity and period, and unbroken state until shore means that tsunamis
are able to penetrate further inland. This is why they are able to wreak consid-
erably greater damage to human communities and structures on the coast, and
also the coastal landscape, than storm waves.
Tsunamis can be generated by a number of mechanisms including:
submarine earthquakes;
landslides into and below the ocean surface;
volcanic eruptions;
meteorite/asteroid impact into the ocean; and
atmospheric disturbances either generated by volcanic eruptions or
weather related phenomena such as squall lines and tropical cyclones.
Each of these mechanisms can produce different tsunami characteristics. Earth-
quakes generate tsunami through the rapid displacement of the ocean floor and
hence the water column above. Earthquakes typically occur due to movements
along faults, but not all fault movements and earthquakes produce tsunami.
Faults vary in their structural characteristics. Faults that generate a vertical dis-
placement of the ocean floor and water column such as dip/slip faults and thrust
faults produce larger tsunamis than strike faults. Most larger and destructive
tsunamis are generated by earthquakes greater than 7 and particularly 7.5 on
theRichter scale. Often the magnitude of the earthquake, particularly if it occurs
at shallow depth (<100 km), determines the size of the tsunami. Certain types
of earthquakes appear more effective at producing tsunamis. Those that do not
display a large peak energy release, but rather release their energy slowly due to
theslow rupturing of the fault, can cause the formation of very large tsunamis.
Such earthquakes are called tsunami earthquakes and can often occur with-
out many people noticing them, or at least not becoming too alarmed because
of their relatively low intensity. Earthquake-generated tsunamis are often com-
posed of a number of individual waves that can be separated by distances of
several hundred kilometres. It is not uncommon for more than five waves, and
sometimes ten or more waves, to occur during the one tsunami event (Fig. 5.1).
Notall tsunamis are the same size, and the largest wave can occur in any
position in the wave train. Like most tsunamis, earthquake-generated tsunamis
are often less than 1 m high in the open ocean, but the wave height increases
substantially as it travels across the continental shelf and slows in velocity. The