GEOL105 Chapter Notes - Chapter 4: Probabilistic Analysis Of Algorithms, Magnetometer, Canadian Meteorological Centre
31 May 2018
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CH 4: Tsunamis
4.1 Introduction to Tsunamis
● Tsunami: a series of gravity waves caused by the large-scale displacement of the entire
water column of the ocean or rarely a large lake
○ Most commonly the result of a large eq deforming the seafloor
○ May also be caused by a large underwater or sea-cliff landslide. Collapse of part
of a volcano into the ocean, a submarine volcanic explosion, or the ocean impact
of an asteroid
○ Not tidal waves, and in a few cases, are not seismic sea waves
● How do earthquakes cause a tsunami?
○ Eq rupture in seafloor pushes water upwards starting the tsunami
○ Tsunami moves rapidly in deep ocean reaching speeds of up to 900 km/hr (wave
height < 1 m)
○ As the tsunami nears land, it slows to about 45 km/hr but is squeezed upwards,
increasing in height
○ Tsunami heads inland destroying all in its path (trough of wave may arrive first,
exposing seafloor)
● When an eq ruptures and uplifts the seafloor close to land, both distant and local
tsunamis may be produced
○ Distant tsunami: a series of gravity waves in the ocean originating from a
source typically thousands of kms away from the shoreline that is inundated.
These waves are produced by the displacement of the entire water column of the
ocean by an underwater eq, landslide, volcanic eruption, or extraterrestrial
impact
○ Local tsunami: a series of gravity waves in the ocean originating from a source
that is close to the shoreline that is inundated
■ Can arrive quickly
■ Ex. island of Okushiri in western Japan on July 12, 1993
● Landslides cause a tsunami
○ Submarine landslides: landslides that take place underwater
○ Or they can be large rock avalanches that fall from mountains into the sea
○ In most cases, the landslides are triggered by an eq
4.2 Geographic Regions at Risk from Tsunamis
● The heightened risk for tsunamis comes from the geographic location of a coast in
relation to potential tsunami sources, such as eqs, landslides, and volcanoes
● Coasts in near proximity to a major subduction zone, or directly across the ocean basin
from a major subduction zone are at the greatest risk
4.3 Effects of Tsunamis and Linkages with Other Natural Hazards
● Primary effects- are related to the inundation of the water and the resulting flooding and
erosion
find more resources at oneclass.com
find more resources at oneclass.com
○ Much of the damage to both the landscape and human structures results from
the tremendous amount of debris carried by the water as it moves inland and
then back out again to the ocean
○ What is left behind is often bare, eroded ground and areas covered with all sorts
of human and natural debris
● Secondary effects- occur in the hours, days, and weeks following the event
○ Fires
○ Water supplies may become polluted
○ Disease outbreaks
● Several linkages exist between tsunamis and other natural hazards
○ Tsunamis are closely linked to submarine and coastal eqs and landslides
○ Island volcanic explosions
○ Oceanic impacts of asteroids and comets
4.4 Natural Service Functions of Tsunamis
● Movement of vast amounts of seawater on land undoubtedly brings many chemicals
from the ocean to the land
● May have long-term effects on ecosystems
● Bring ashore a large volume of sediment that, over a long period, contributes to the
general development of the landscape
4.5 Human Interaction with Tsunamis
● Magnitude and freq are not in any way tied to human activity
● We can plant buffer zones of trees along a coast that will absorb some of the impact of
incoming tsunami waves
● Might consider constructing our buildings to withstand the onslaught of moderate
tsunamis
● Moving other structures inland where they are less likely to be damaged
4.6 Minimizing the Tsunami Hazard
● Detection and warning
○ First warning comes from an eq in an offshore area that is large enough to
produce a tsunami
○ Tsunami warning system using technology and information
○ Tsunami warning system components
■ A network of seismographs to accurately locate and determine the depth
and magnitude of submarine and coastal eqs
■ Automated tidal gauges to measure unusual rises and falls of sea level
■ A network of sensors connected to floating buoys
○ Surface buoys with a bottom sensor, known as a tsunameter, detect small
changes in the pressure exerted by increased volume of water as a tsunami
passes overhead
● Structural control
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Tsunami: a series of gravity waves caused by the large-scale displacement of the entire water column of the ocean or rarely a large lake. Most commonly the result of a large eq deforming the seafloor. May also be caused by a large underwater or sea-cliff landslide. Collapse of part of a volcano into the ocean, a submarine volcanic explosion, or the ocean impact of an asteroid. Not tidal waves, and in a few cases, are not seismic sea waves. Eq rupture in seafloor pushes water upwards starting the tsunami. Tsunami moves rapidly in deep ocean reaching speeds of up to 900 km/hr (wave height < 1 m) As the tsunami nears land, it slows to about 45 km/hr but is squeezed upwards, increasing in height. Tsunami heads inland destroying all in its path (trough of wave may arrive first, exposing seafloor) When an eq ruptures and uplifts the seafloor close to land, both distant and local tsunamis may be produced.
