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Lecture 6: Tsunami Waves
Key Questions1. What are four sources for tsunami waves?
2. What are the wavelength, period, velocity, frequency, height relations for a wave?
3. What does “feeling” bottom have to do with the height amplification of a wave?
4. What are typical wavelengths and speeds for tsunami waves?
5. What three methods does NOAA use to mitigate tsunamis?
6. What is DART?
Magnitude 6.9 - SOUTHERN QINGHAI, CHINA(depth of 6.2 miles)
India-Asia continent-continent plate convergence
Source unknown
Tsunami Hazards
Figure 6.B
A tsunami wave is created by some impulsive disturbance.
There are four possibilities.
1. Submarine earthquake
1. Submarine earthquake
2. Submarine landslide
Failure of the Nooksack Delta could cause a tsunami
3. Volcanic landslide
4. Asteroid impact
The Cretaceous-Paleogene boundary ~65.5 million years ago marks one of the three largest mass extinctions
L = wavelength
H = heightcalm water surface
seafloor
d = depth to the seafloor
L = wavelength
H = heightcalm water surface
seafloor
d = depth to the seafloor
Wave Period = T = time for one wave length to pass a point
LVelocity = v = T1Frequency = f = T
L = wavelength
H = height
seafloor
d = depth to the seafloor
Velocity ~~ g x d works if d < L/10
g = acceleration of gravity 9.81 m/s2
L = wavelength
H = height
A tsunami wave can have a wavelength (L) on the order of 100s of kilometers
and velocities as high as 800 km/hr!
Seattle Spokane450 km
Lperiod = T = v450 km
750 km/hr= = 0.60 hr
L = wavelength
H = height
Seattle Spokane450 km
Deep in the ocean the wave height may only be 1 meter or so.
Shoaling (wave amplification)
Rotating particles start sensing the seafloor when
depth = d = L2
d
where L is the wavelength
Shoaling (wave amplification)
when waves “feel” the bottom, they slow down
Shoaling (wave amplification)
when waves slow down, the wavelength gets shorter
Lthe period stays constant = T = v
Shoaling (wave amplification)
wavelength gets shorter and the wave gets higher
Shoaling (wave amplification)
If the wave becomes too high it will break when
HL
17
H
≥
Note: tsunami waves don’t break—their wave lengths are too long!
misconception
L = wavelength
H = height
Seattle Spokane450 km
Deep in the ocean the wave height may only be 1 meter or so.
Tsunami Mitigation
NOAA has taken a three-pronged approach:
1) Use computer models to produce inundation maps showingworse-case scenarios.
Tsunami Mitigation
NOAA has taken a three-pronged approach:
1) Use computer models to produce inundation maps showingworse-case scenarios.
Tsunami Mitigation
NOAA has taken a three-pronged approach:
2) Use the maps to educate the public.
Modeling predicts that a Cascadiasubduction zone earthquake may produce a tsunami that will arrive in Bellingham Bay about 2.5 hours after the EQ
Note: It would take about 20 minutes to reach the coast of Washington
Tsunami Mitigation
NOAA has taken a three-pronged approach:
2) Use the maps to educate the public.
Modeling predicts that a Cascadia subduction zone earthquake may produce a tsunami wave having an amplitude of 2-4 meters.
Tsunami Mitigation
NOAA has taken a three-pronged approach:
2) Use the maps to educate the public.
The tsunami will flood most of the Nooksack River delta and lower flood plain.
Tsunami Mitigation
NOAA has taken a three-pronged approach:
2) Use the maps to educate the public.
Tsunami Mitigation
2) Use the maps to educate the public.
Note: It would take about 20 minutes for a tsunami from a Cascadia subduction zone EQ to reach the coast of Washington
Tsunami Mitigation
2) Use the maps to educate the public.
Tsunami Mitigation
2) Use the maps to educate the public.
Earthquakes in the subduction zone along the Aleutian Islands also pose a risk.
Earthquakes in the subduction zone along the Aleutian Islands also pose a risk.
3:45 min
Earthquakes in the subduction zone along the Aleutian Islands also pose a risk.
March 28, 1964 Prince William Sound, Alaska
About 4 hrs to reach Neah Bay
Tsunami Mitigation
NOAA has taken a three-pronged approach:
3) A sophisticated detection system to warn the public.
DART® (Deep-ocean Assessment and Reporting of Tsunamis
