Lecture 12 Flashcards
Tsunami
A body wave generated in an ocean or large lake by seismic activity.
Long wavelength (often >100km)
High speeds (>800 km/h)
Amplitudes of ~10cm until approach continental shelf (>10m)
Distinguished from regular ocean waves by long wavelength
Period is long enough that loss of energy due to viscous dampening is minimal (even over long distances)
Displacement wave
Waves in an ocean or lake that result from a displacement of water caused by shifts in the basin floor or movement of material into the water (such as a rock avalanche). They may be seismogenic
Ways to generate a tsunami (5)
1) Rapid tectonic strain of the sea floor during an earthquake with significant vertical component (> 1 m) over a large area (hundreds of km2)
2) Submarine landslide
3) Subaerial landslide
4) Submarine volcanic eruption
5) Meteorite impact
1929 grandbanks EQ and tsunami
Most deadly EQ and tsunami in Canada
2.5 hours after EQ, tsunami strikes southern end of Burin Peninsula in NFLD
3 main pulses
28 lives
Depth
1/2 wavelength
Crest
Amplitude of wave over sea level
Trough
Amplitude of wave under sea level
Wavelength
Distance from a crest to next crest, or distance from trough to next trough
Amplitude
Difference from sea level wave rises or falls
Height of displacement wave if caused by subaerial or submarine landslides
May be greater than 100m
Wavelength of displacement wave caused by seismicity
Can be 1000s of metres
Tidal waves
Waves resulting from the gravitational force between Earth and the Moon or Sun. Have a period of two waves per day (semi-diurnal)
Water is not being displaced, but pulled
Other ocean waves
For example from storm surges
Lower heights and shorter periods
Wave generated by wind (period, wavelength, height on sea)
Period: 1-25s
Wavelength: 1-1000m
Height on sea (2xA): < 3m
Wave generated by tsunami (period, wavelength, height on sea)
Period: 10-60 min
Wavelength: >100km
Height on sea (2xA): <1m
Celerity
Wave speed
Celerity equation in shallow water
Where d/L is <0.05
sqrt(gD)
g: 10m/s^2
D: depth (m)
Shoaling
As wave approaches decreasing water depths the leading wave will slow, but the water behind it will “build up”an increase the wave height
Focusing
As a wave enters a concave bathymetry, the wave front bends due to refraction of the waves, and they converge onto each other
Resonance
As a wave enters a closed bay, waves will reflect and oscillate in the bay. If the period of the oscillation is in phase with the tsunami wave, then a larger amplitude will result, similar to a standing wave
What causes amplification?
Shoaling
Focusing
Resonance
Wave energy loss
Inversely proportional to wavelength
Tsunamis have long wavelengths so maintain energy over long distances
Tsunami monitoring
Buoy warning systems
Deep Ocean Assessment and Reporting of Tsunami (DART)
Run-up heights
Maximum height reached on land
Can be determined through deposits
Inundation distances
How far inland
Can be determined through deposits
Tsunami prediction
Models
Historical examples
