Tsunami Flashcards
Tsunami Definition
A series of shallow water waves generated by the sudden displacement of a large body of water, usually an ocean but may occur in seas, bays, lakes, rivers, fjords
Not “tidal waves” which would imply tsunami are related to the tides of the Earth
Not “seismic waves” as tsunami may be triggered by other mechanisms
Classification of Waves
Wavelength
Period
Cause
-For example, tides are generated by gravity, wind-generated waves by fetch, and tsunami by the displacement of large bodies of water
Wavelength
the distance between two identical points on a wave
Height
measured from the base of the trough to the crest of the wave
Amplitude
the height of the wave measured from the still water level line (equal to ½ the wave height)
Period
refers to the time between two successive waves at a stationary point
Velocity
refers to the speed at which the wave travels (dependent on water depth)
Wind-Generated Waves
Affect the uppermost layer of water only
Caused by wind
Wavelength in metres
Period in seconds
Travel at low speeds
Break when the reach the shore, dissipating their energy
Tsunami Waves
Involve the motion of the entire water column from surface to sea-floor
Caused by the large displacement of water
Wavelength in kilometers
Period in minutes
Travel at high speeds
Do not break when they reach the shore, resulting in a wall of water that runs over normally dry land
Causes of Tsunami
Tsunami are most commonly generated by underwater shallow-focus earthquakes which cause the rise and fall of the ocean floor.
This movement triggers the displacement of large bodies of water which travel as a series of waves thousands of kilometers from their source.
In addition to earthquakes tsunami may be triggered by:
landslides, submarine slumps, rock falls, and avalanches
explosive volcanic eruptions or flank collapses
human-caused explosions
meteorite impacts
There are 4 important stages to consider from the time the tsunami is generated to its arrival on land:
Generation
the upward or downward movement of the ocean floor produces waves that spread outward from the source
There are 4 important stages to consider from the time the tsunami is generated to its arrival on land:
Propagation
the waves spread out in all directions from the point of initiation
There are 4 important stages to consider from the time the tsunami is generated to its arrival on land:
Inundation
how tsunami waves behave as they approach land and inundate coastlines
There are 4 important stages to consider from the time the tsunami is generated to its arrival on land:
Aftermath
how tsunami waves behave on land including risk factors and mitigation strategies
Generation
Vertical motion associated with underwater faults sets in motion tsunami waves that transmit energy outwards and upwards from the source
Thrust and reverse faults at subduction zones may displace large volumes of water in this way, resulting in tsunami
Normal faults may also displace large volumes of water and generate tsunami
Horizontal movement on strike-slip faults does not displace water to produce tsunami
The size of tsunami waves thus depends on the following factors:
magnitude of the shallow-focus earthquake (M7 and above)
area of the rupture zone
rate and volume of water that is displaced
depth of water above the rupture
nature of motion of the ocean floor
vertical offset or displacement of the fault
Propagation
Propagation refers to any of the ways in which waves travel
From a hazards perspective we are most interested in understanding how fast and how far tsunami waves travel so we can anticipate impact on coastlines
Celerity refers to the velocity of wave propagation
Tsunami waves have been known to travel across the Indian Ocean in less than one day (e.g. Indian Ocean tsunami of 2004)
Wave directions may change as the waves reflect or diffract in response to the topography
The rate at which waves lose their energy is inversely related to their wavelength
The velocity of shallow water waves such as tsunami depends on the water depth and gravity:
C = √(g*d)
where C = velocity in meters per second,
g = gravitational acceleration (9.8 m/sec2),
d = depth in meters
Tsunamis travel much faster in deep ocean than closer to shore
Bathymetry
(from the Greek “bathus” or deep and “metron” or measure) is the study of landforms of the ocean floor
Bathymetric data is used to help predict the coastal regions that will be most affected as well as the arrival times of tsunami traveling across the ocean
Shoaling
means that tsunami waves are very destructive when they arrive on shore, even thousands of km away from their origin
Inundation
Inundation refers to how tsunami waves behave as they approach land and inundate coastlines
Tsunami hazard is evaluated by maximum wave run-up which may be measured as:
Inundation: refers to the horizontal distance that the waves flood inland
Run-up: refers to the vertical inundation or the height of the incoming waves
Inundation and run-up are affected by:
Shoaling: amplitude and height of the waves increase as the waves reach the shoreline
Coastal/Bathymetric Topography: this includes factors such as:
- variations in elevation as the tsunami moves from deep ocean to shore
- interaction of tsunami waves with steep coastlines (reflection)
- diffraction that occurs around reefs, and other barriers
the period of a bay, basin, inlet, or harbor (resonance; interference)
interference of wave patterns as tsunami waves interact with edge waves and each other
There are 4 types of behaviour when waves interact with coastal or bathymetric topography:
Reflection- depends on the shape of the coastline and the presence/absence of barriers
Refraction- as waves move from deep to shallow water their velocity and wavelength decrease, wave height increases and the direction of wave motion changes
Diffraction- occurs when the waves encounter a barrier; the waves bend and change direction as they travel around the barrier
Interference- occurs when two waves interact with each other, forming new wave patterns (also causes resonance)
Some locations along the coast are prone to more inundation or run-up than others
exposed ocean or barrier beaches (inundation)
cleared land for agriculture or development (smooth topography) (inundation)
river deltas (run-up)
headlands (run-up)
bays and harbors (resonance)
Resonance
Resonance occurs in bays and harbors due to the long periods of tsunami waves
In most cases when tsunami waves enter a bay or harbor their energy is dissipated around the whole bay
If, however, the period of the tsunami wave is a multiple of the natural resonance frequency of the harbor, then interference will occur
This will result in seiche, or very large waves produced by many waves combining together
The word tsunami literally means “harbor wave” because of this phenomenon (Bryant, 2008)
Hilo Bay, on the Big Island of Hawaii is famous for tsunami resonating within its harbor:
Hilo Bay naturally resonates with a period of 30 minutes
Any tsunami with multiples of this period (i.e. 15 min, 30 min, or 1 hour) will resonate within Hilo Bay
Resonance in harbors can occur for as long as 6 to 24 hours
Aftermath of Tsunami
A popular misconception is that a tsunami is a single wave
Instead, tsunami are a series of waves separated by long periods from 10 minutes to 2 hours
The first wave is commonly not the highest; interaction with edge waves increases amplitude
Edge waves are created by refraction that occurs along the shoreline
When tsunami waves arrive on shore, they may appear either as a series of waves or a bore
Bore
A bore is a step-like wave with a steep breaking front created when one wave overtakes another
Drawdown
if the trough of the wave hits the shoreline first (as opposed to the crest), the water may withdraw with a hissing or roaring noise and the seafloor may be exposed
Drawdown may occur anywhere from 1 minute to 1 hour before the arrival of the first wave
Other Causes of Tsunamis
Earthquakes (72%)
Landslides (10%)
Volcanoes (5%)
Other or Unknown (13%)
Landslide-Triggered Tsunami
Landslides, submarine slumps, rock falls, and avalanches may trigger tsunami if the debris displaces a large enough volume of water (oceans, rivers, bays, lakes, fjords)
These mass wasting events are often triggered by earthquakes
Typically these tsunami are localized and much smaller than the tsunami that occur in the ocean
The risk of tsunami is the greatest along steep coasts where large volumes of debris accumulate at high altitudes (e.g. British Columbia)
Famous examples include Lituya Bay, Alaska in 1958 (rockfall) and Grand Banks, Nfld in 1929 (submarine slump)
Volcano-Triggered Tsunami
Less commonly, tsunami are produced in association with volcanic activity
The most common mechanism is when pyroclastic flows are blasted or flow down the flanks of the volcano displacing large volumes of water (e.g. Krakatau, 1883 generated a tsunami with waves up to 30 m)
These tsunami rapidly decrease in size away from volcano
Tsunami may also be generated due to landslides that could occur on the submerged flanks of volcanoes (Clague, Munroe, and Murty, 2003)
Human Caused Explosions
Nuclear testing in the U.S. in the 1940s and 1950s has generated tsunami in the past
Halifax explosion, 1917 generated small localized waves
Meteorite Impacts
No historic examples
However, there are tsunami deposits that are associated with the Yucatan meteorite impact at the end of the Cretaceous period, ~ 65 million years ago
The Dangers of Tsunami
Tsunami waves will travel long distances without losing their energy
Intense hydrodynamic forces can destroy or lift buildings and houses from their foundations
Tsunami naturally occur at the same time as other natural hazards (large earthquakes, landslides, volcanic eruptions)
Tsunami can have unanticipated secondary effects
Tsunami have long recurrence intervals
Tsunami “Stones”
“At the edge of Aneyoshi, a small village on Japan’s northeastern coast, a 10-foot-tall stone tablet stands, carved with a dire warning to locals…”
“High dwellings are the peace and harmony of our descendants,” the rock slab says. “Remember the calamity of the great tsunamis. Do not build any homes below this point.””
Global Regions at Risk
Greatest Hazard
Return period <500 years
Located within or directly in the path of tsunami from active subduction zones (M9 earthquakes)
Global Regions at Risk
Significant Hazard
Return period of 500-2000 years
Located adjacent to active continental faulting or in regions of moderate distance from active subduction zones
Global Regions at Risk
Low Hazard
Return period of 2000+ years
Coastal areas subject to effects from submarine slides, volcanic landslides, or infrequent but large earthquakes
Distant tsunami (teletsunami)
tsunami that originate from distant sources, generally more than 1,000 km away (e.g. 1964 Vancouver Island)
Teletsunami are capable of producing both distant and local effects (e.g. 1700 Cascadia)
Local tsunami-
tsunami that originate from nearby sources, generally within 100km (e.g. Lituya Bay, Alaska)
Generally includes tsunami generated by landslides and volcanic eruptions
Local tsunami have shorter periods and do not last as long as distant tsunami
Tsunami Risk in Canada
Pacific Coast
Tsunami produced by M8 earthquake at the Cascadia subduction zone
Teletsunami generated within the Pacific Ring of Fire (e.g. 1964 Vancouver Island)
Local tsunami triggered by landslides (e.g. 1975 Kitimat Inlet submarine slide)
Greatest risk is to communities located in inlets or along the coast of western Vancouver Island including Tofino, Ucluelet, and Port Alberni
Tsunami Risk in Canada
Atlantic Coast
Halifax Explosion, 1917
1929 Grand Banks submarine slump and tsunami
Very little evidence of prehistoric tsunami deposits
It’s possible that teletsunami generated by large earthquakes in the Atlantic Ocean could affect the Atlantic coast (e.g. 1755 Lisbon, Portugal)
Tsunami Risk in Canada
Arctic Coast
Low risk (no historical tsunami or prehistoric deposits)
Presence of sea ice reduces risk
Tsunami Risk in Canada
Interior Waterways
Steep unstable slopes in alpine areas
Lakes containing unstable delta sediments (e.g. in 1908, a landslide on the Liève River, western Quebec produced a tsunami that killed 27 people)
Primary Effects of Tsunamis
Impact from the onrushing waves and debris
- Human impact (deaths and injury)
- Hydrostatic forces
- Buoyancy
- Hydrodynamic forces
- Debris impact
Flooding and erosion
- Damage of ecosystems
- Coseismic subsidence
Human Impact
Majority of tsunami deaths are by drowning or physical impacts with stationary or floating debris
Often floating debris or people will be swept out to the ocean by the outflow
Tsunami waves are usually tens of meters in height
Compared to a small structure which is ~ 8m (25 ft) in height
Vertical evacuation shelters
like these have sufficient height to elevate evacuees, and are structurally designed to resist the effects of tsunami waves. They are most useful when there is not enough evacuation time prior to the tsunami warning.
Buildings must be constructed to withstand:
Hydrostatic forces may cause pressure on walls from variations on water depths on either side
Buoyancy may cause flotation or uplift
Hydrodynamic forces are caused by the impact of the waves on the building and the drag/overturning forces produced as the waves flow around the building
Debris impact is caused by floating objects
Scour erodes around the foundations of buildings
Mitigation Strategies
Reduce land development or change zoning practices in tsunami inundation zones
Reduce the amount of critical infrastructure (e.g. roads, hospitals, schools, etc) in areas <300m from the coast
Raise existing buildings above expected inundation levels (e.g. raising homes on stilts)
Build multistory buildings (if necessary) in inundation zones that are made with steel and reinforced concrete
Anchor buildings to foundations
To protect against hydrostatic forces, provide openings in buildings so water can reach equal heights within and outside of buildings
Use deep piles and piers to protect against scour
Mitigation Strategy
A change to zoning regulations could allow for low density development in tsunami inundation zones.
Hydrodynamic Forces
The power of the waves is immense (Chang 2011):
(e.g.) a wall of water 3 feet X 3 feet X 3 feet = 1700 pounds weighs about the same as a smart car
Now imagine this wall of water traveling at 48 km/hr towards you!
Natural Barriers
Mangroves are naturally growing trees and shrubs in the intertidal coastal zone
In some cases, houses and other small structures are spared damage due to the protection from mangroves or rows of trees
A mitigation strategy would be to locate infrastructure inland and adjacent to natural vegetative barriers
However, many regions repeatedly remove mangroves to allow for development of homes, hotels, and tourist facilities on the beach
Man-Made Barriers
May be costly to build and upkeep
Could protect large populations
Could be overtopped by large tsunami
Subject to scouring at the base
Damage to Ecosystems
Crops could be destroyed due to saltwater
Oil leaks (e.g. 8 million litres of oil escaped from oil plants in Indonesia during the 2004 Indian Ocean tsunami)
Natural vegetative barriers along the coast may be destroyed
Coral reefs and coastal wetlands could also be damaged
Flooding and Subsidence
Flooding may occur up to 300 metres inland
Mitigation strategies could include: raising buildings above inundation levels, locating mechanical and electrical equipment at higher levels in buildings, protecting critical infrastructure with sea walls (e.g. hazardous material storage facilities)
Coseismic subsidence of 1-2 metres was seen along the northwest coast of Sumatra during the 2004 Indian Ocean tsunami
Secondary effects of Tsunamis
Fires (caused by spread of liquid contaminants)
Radiation release (e.g. Fukushima 2010)
Contamination of water and soils
Environmental impacts of floating rafts of debris
Tertiary effects of tsunamis
Disease outbreaks (cholera, malaria)
Loss of shelter, crime, mental trauma
Economic vulnerability of communities dependent on tourism, fishing, or agriculture
Early detection and warning
Earthquake monitoring
Tsunami warning systems
Mitigation strategies for land use and structural controls
Building codes for susceptible coastline areas
Use of natural vegetative barriers
Probability analysis
Identify potential earthquake sources
Map prehistoric tsunami deposits (local and teletsunami)
Tsunami inundation maps
Reducing Tsunami Hazards
- Early detection and warning
- Mitigation strategies for land use and structural controls
- Probability Analysis
- Education and Tsunami Readiness
Three types of warning systems for Tsunamis
Ocean-wide (e.g.) Pacific Tsunami Warning Center, Hawaii
Regional (e.g. West Coast & Alaska Warning System)
Local (e.g. BC Provincial Emergency Program)
Lessons Learned from Past Tsunamis
An early detection warning system could have potentially saved many thousands of lives during the Indian Ocean tsunami
Warning systems need to be coupled with earthquake and tsunami education and preparedness (e.g. Japan, 2011)
Inundation Maps
Risk may be assessed by assessing the size, frequency, and probable impact of tsunami on coastal communities (Clague, Munro, and Murty, 2003)
Inundation maps have been produced for high risk areas in Canada to increase hazards awareness and to inform land planning
Inundation and maximum run-up values are calculated from numerical models and based on tsunami deposits in the geological record
Tsunami Readiness
Establish 24-hour emergency operation centres
Clearly communicate what tsunami warnings mean
Have ways to alert the public
Develop a preparedness plan with emergency drills
Promote community awareness programs through education
Protecting Yourself & Others
Be aware of natural warning signs
Heed official warnings; tell others about the danger
Play it safe, even after you think the danger has passed
Expect many waves
Move uphill, inland and away from harbors
Don’t wait or watch the wave- tsunami move fast!
Prepare for blocked or broken roads
If trapped go to the top or roof of a building, climb a tree, or climb onto something that you can use as a raft
Stay clear of debris
Be aware of inundation zones and hazards when you are in tsunami country
Tsunami Strikes Thailand
December 24th 2004 M9.1 Earthquake Generated Tsunami
Other areas with high risk include eastern Indian Ocean
The past is the key to the present axiom doesn’t always work when dealing with long time scales
No warning system in place like Pacific Ocean
Why was the Sumatra tsunami so deadly?
High population density on low-lying areas of Indian Ocean
Short distance from tsunami source to populated low lying coasts, leaving little time for warning
Distant tsunami (tele-tsunami): source of the tsunami more than 1,000 km away from the area of interest
Regional/local tsunami: source of the tsunami within 1,000 km of interest
No tsunami warning system in Indian Ocean in 2004
Poor and developing countries with vulnerable infrastructure
Low awareness/preparedness of tsunami hazard
Japan Tsunami
Occurred on March 11, 2011, killing ~ 16,000 people
Source was a M 9.0 earthquake beneath the seafloor
Subduction zone east of Honshu Island
The direct damage from the earthquake and tsunami was U.S. $235 billion
Most expensive natural disaster in history
Occurred on March 11, 2011, killing ~ 16,000 people
Source was a M 9.0 earthquake beneath the seafloor
Subduction zone east of Honshu Island
The direct damage from the earthquake and tsunami was U.S. $235 billion
Most expensive natural disaster in history
