The Great Wave
What is a Tsunami?
A tsunami is a series of waves (called a "wave train") generated in a body of water by an abrupt
disturbance that vertically displaces the water column. Earthquakes, landslides, volcanic eruptions,
volcanic flank collapse/submarine landslides, and large asteroid impacts all have the potential to
generate a tsunami.
The term tsunami comes from the Japanese language meaning ‘harbor wave’.
Tsunamis are also called ‘seismic sea waves’.
Tsunamis have been historically referred to as tidal waves.
How a Tsunami Forms
When the water is displaced, water from the surrounding rushes in to fill the depression, forming a
series of high speed (up to 870km/hr), flat, spread out waves (average wavelength of 360km). In deep
water tsunami waves are nearly undetectable.
As the leading waves of a tsunami approach a shoreline, friction with the sea floor slows the waves
down. This compresses the wavelength and increases the wave height. The waves surge onto shore as
a rapidly rising flood of water with great destructive power.
Earthquake Generated Tsunami
Most tsunami are generated during shallow focus underwater earthquakes associated with sudden rise
or fall of the seafloor, most commonly along subduction zones.
Most destructive tsunamis occur in Pacific Ocean. The borders of the Pacific Ocean are dominated by
active subduction zones that produce frequent violent earthquakes .
Velocity and Wave Height
Tsunami waves in the open ocean are low and far apart but move at velocities of several hundreds of
kilometers per hour. They slow and build much higher in shallow water near the coast, especially in
Run up is the height to which a tsunami wave rushes up onshore. Driftwood, trees, and the remains of
boats, houses and cars are swept up by the incoming wave, and commonly mark the upper limit of
tsunami run-up. The first run-up of a tsunami is often not the largest.
The inundation can extend inland by 1000 feet (305 m) or more, covering large expanses of land with
water & debris.
As the wave recedes into the trough before the next wave, the onshore water and its debris flow
back offshore. The time between the trough and the next tsunami wave is often more than a half
Chile Tsunami - May 1960
On May 22, 1960 the largest earthquake on record struck the coast of Chile with a Mw of 9.5. The
earthquake ruptured along a 1,000 km length of the subduction zone. In Chile, the earthquake and the
tsunami that followed took more than 2,000 lives. From Chile the tsunami radiated outward, killing 61
people in Hilo, Hawaii and 122 on the island of Honshu, Japan.
South East Asia Tsunami - December 26, 2004
On December 26, 2004 at 07:58:53 local time in the Indian Ocean there was an undersea earthquake;
known as the Sumatra-Andaman earthquake. With the earthquake, an estimated 745 miles of
faultline slipped along the subduction zone. The vertical rise of the seafloor by several meters during
the earthquake produced the tsunami.
The tsunami devastated the shores of Indonesia, Sri Lanka, Thailand, India, Malaysia, Burma, and
Bangladesh with waves up to 100 feet. Maldives, Somalia, Kenya and Tanzania were also affected. The
2004 tsunami was the deadliest in recorded history.
Because of the distances involved, the tsunami took anywhere from fifteen minutes to seven hours to
reach the various coastlines.
• Drowning in the incoming waves.
• Being thrown against solid objects.
• Being carried back out to sea in the outgoing wave.
• Being hit by debris carried by the wave.
The Relevance of Hazard Prediction and Mitigation for the 2004 Tsunami
A simple program of public education and awareness of the potential hazards could have saved many
The magnitude of the tsunami disaster could have been mitigated with a proper disaster preparedness
plan and a functioning early warning system.
Human destruction of coral reefs, coastal mangrove trees, and sand dunes that had formerly protected
some coastal areas was believed to be a significant factor in the loss of life and damage.
Tsunami from Great Earthquakes in the Pacific Northwest
Convergence of the North American and Juan de Fuca plates causes bulging of the N.A. plate off the
coasts of Oregon, Washington, and S. British Columbia. Uplift rates are ~4mm/yr. and eastward
transport is ~30mm/yr.
A sequence of peat, sand, and mud are a geologic record of tsunami. Radiocarbon dating of organics in
buried soils along the coast of the Pacific Northwest indicate tsunami with recurrence intervals
ranging from 300 to 900 years. Radiocarbon dating places the last of those events around 1700.
How a Tsunami Forms
Volcano Generated Tsunami
Tsunamis can be caused by volcanic processes that displace large volumes of water including
• Volcanic earthquakes,
• undersea eruptions,
• pyroclastic flows,
• caldera collapse,
Krakatau Volcano, 1883
Landslide and Rockfall Generated Tsunami
When major fast-moving rockfalls or landslides enter the ocean, they can displace large amounts of
water and generate tsunami.
Lituya Bay, Alaska, 1958
Tsunami from Volcano Flank Collapse and Submarine Landslides
The ridges that radiate outward from the top of a shield volcano and become the sites of most
eruptions break the volcano into three enormous segments. Rapid collapse of one of these segments
into the ocean can displace thousands of cubic kilometers of water and generate tsunami hundreds of
meters high. None have happened in historic time.
Tsunami from Asteroid Impact
The impact of a large asteroid into the ocean would generate large tsunami that would radiate outward
from the impact site…CANNONBALL!
Scientists have found traces of an asteroid-collision event 3.5BYA that they say would have created a
giant tsunami that swept around the Earth several times, inundating everything except the mountains
and exterminating all primitive life.
Tsunami Hazard Mitigation
Mitigation of tsunami destruction involves two main approaches:
tsunami warning systems, and inundation maps.
International Tsunami Warning System
ITWS includes 31 seismic stations & > 60 tide stations
• A world network of seismographs locates the epicenter of major earthquakes.
• Pressure sensors pick up subtle pressure changes as tsunami waves pass by and Buoys transmit the
data to warning centers via satellite.
The ITWS issues watches and warnings to the media and to local, state, national, and international
officials and NOAA Weather Radio broadcasts tsunami information directly to the public.
Mapping of potential inundation areas.
An inundation map shows areas of potential tsunami flooding, based on elevation and orientation with
respect to the open ocean.