Module 5 Flashcards
CAN YOU DIG IT?
By the end of this unit, you are expected to be able to:
Describe how waves such as tsunami are classified
Compare and contrast wind-generated waves with tsunami waves
Explain how earthquakes, landslides, volcanic eruptions, and other events trigger tsunami
Describe how tsunami waves propagate in the deep ocean including how bathymetry may affect them
Discuss how shoaling and coastal topography affect inundation and run-up at specific locations
Describe how tsunami waves arrive onshore and a hazard associated with drawdown
Tsunami
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
A wave may be classified into a particular type based on its: 3
- Wavelength
- the distance between two identical points on a wave (e.g. between wave crests or troughs)
2.Period
refers to the time between two successive waves at a stationary point
3.Cause:literally what caused it
Wavelength
Height
Amplitude
Period
Celerity
Wavelength: the distance between two identical points on a wave (e.g. between wave crests or troughs)
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
Celerity refers to the speed at which the wave travels (dependent on water depth)
Wind Generated Waves vs. Tsunami Waves
In the deep ocean, wind-generated waves have greater wave heights (3m) and slower velocities (16-32km/hr) than tsunami waves (
How are tsunamis normally triggered?
Name 3 other possible causes
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
Why would a strike slip fault not cause a tsunami?
You need vertical displacement of crust to cause Tsunami, so Strike-slip doe not work(not completely sure this is right)
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
Propagation: the waves spread out in all directions from the point of initiation
-tsunami moves rapidy in deep ocean with wave height of under a metre
Inundation: how tsunami waves behave as they approach land and inundate coastlines
-tsunami slows as it reaches shallower waters and wave height is squeezed upwards
Aftermath: how tsunami waves behave on land including risk factors and mitigation strategies
Describe how a tsunami is created by a stick-slip
This is common around ring of fire:
1.build up of strain
2.coastal bulge
- stuck area ruptures during earthquake releasing land upwards
- vertical change
4.Tsunami formed
The size of tsunami waves thus depends on the following factors: 6
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
T OR F
The size of tsunami waves is related to the size of the earthquake, with larger tsunami generated by larger earthquakes
T
Celerity
propogation
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)
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
-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
Since tsunami waves have very large wavelengths, they will lose little energy as they propagate across the ocean
Describe Inundation and give both types
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 two things
Inundation and run-up are affected by:
1.Shoaling: amplitude and height of the waves increase as the waves reach the shoreline
2.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
Shoaling
Shoaling: amplitude and height of the waves increase as the waves reach the shoreline
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)
occurs when two waves interact with each other, forming new wave patterns
-causes two things
resonance and interference
MOST OFTEN WAVES APPROACH SHORES ________
OBLIQUELY
“After Refraction”: when one wave hits another called ___________ INTERFERENCE
CONSTRUCTIVE
- if trough hits another trough of another wave the trough deepens
- if crest hits crest the crest increases(I think)
Some locations along the coast are prone to more inundation or run-up than others (Bryant, 2008): 5
- 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)
If, however, the period of the tsunami wave is a multiple of the natural resonance frequency of the harbor, then interference will occur
RESULTING IN A ________
This will result in seiche, or very large waves produced by many waves combining together
Hilo Bay Ex
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
-also an example of people coming back to shore after only one way, unfortunately there were more coming
Aftermath of Tsunamis
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
Edge Waves
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 _____
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
-Rapid drop in sea level is called DRAWDOWN
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)
Alpine Landslide demonstration video: https://www.youtube.com/watch?v=v4TQLIj9Oog
Famous examples include Lituya Bay, Alaska in 1958 (rockfall) and Grand Banks, Nfld in 1929 (submarine slump)
Lituya Bay ex
BEST EXAMPLE OF LANDSLIDE TRIGGERED TSUNAMI
- steep banks means when rocks fell they
- the water went up over the landscape and completely cleaned of the land(see pic on bottom right)
-Earthquake-Landslide-Tsunami
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)
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: 3
1.Greatest hazard=Ring of Fire(subduction zone)
Return period <500 years
Located within or directly in the path of tsunami from active subduction zones (M9 earthquakes)
2.Significant hazard
Return period of 500-2000 years
Located adjacent to active continental faulting or in regions of moderate distance from active subduction zones
3.Low hazard
Return period of 2000+ years
Coastal areas subject to effects from submarine slides, volcanic landslides, or infrequent but large earthquakes
Which of these is most likely for Tsunamis?
oceans: Atlantic, Pacific, Indian, Arctic
Pacific and Indian- ring of fire
Least Likely=Atlantic and Arctic
2 types of Tsunami
1.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)
2.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
Teletsunami
1.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)
Port Alberni ex
-anywhere with coastal embankments can make tsunami’s so much worse
Geologic Evidence for Subduction Zone
Earthquakes and Tsunami
-describe process where tsunami sand layers are added to areas
- Coastal Forest
- Great Earthquake, Land Sinks flooding forest
- Within an hour Tsunami rushes ashore
- Following tsunami forest is dead in tidal flat covered in tsunami sand layer
- “Ghost” Forest
Primary Effects of Tsunamis
Primary effects diminish with distance from the coast and include: 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
Primary Effects of Tsunamis
Primary effects diminish with distance from the coast and include: 1.Impact from the onrushing waves and debris Human impact (deaths and injury) Hydrostatic forces Buoyancy Hydrodynamic forces Debris impact 2.Flooding and erosion Damage of ecosystems Coseismic subsidence
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.
Open area under structure minimizes ________ impacts
hydrostatic
-the foundation is deeply rooted in bedrock
Buildings must be constructed to withstand
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
General Mitigation strategies for reducing damage to buildings
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
Natural Barriers to Tsunami Waves
.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
Flooding
Coseismic 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 and tertiary effects occurring in the days to weeks after the tsunami could include:
Secondary effects:
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:
Disease outbreaks (cholera, malaria)
Loss of shelter, crime, mental trauma
Economic vulnerability of communities dependent on tourism, fishing, or agriculture
Reducing Tsunami Hazards: 4
1.Early detection and warning
Earthquake monitoring
Tsunami warning systems
2.Mitigation strategies for land use and structural controls
Building codes for susceptible coastline areas
Use of natural vegetative barriers
3.Probability analysis
Identify potential earthquake sources
Map prehistoric tsunami deposits (local and teletsunami)
Tsunami inundation maps
4.Education and tsunami readiness
3 types of tsunami warning systems
Three types of warning systems:
- Ocean-wide (e.g.) Pacific Tsunami Warning Center, Hawaii
- Regional (e.g. West Coast & Alaska Warning System)
- Local (e.g. BC Provincial Emergency Program)
Warning systems use tsunameters to detect small changes in pressure when a tsunami passes
tsunameters
Warning systems use tsunameters to detect small changes in pressure when a tsunami passes
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
