tsunami and wind waves
- wind waves: wind drives water, as waves approach shore, exp friction of the front part against seafloor and slows down, back part climbs and falls down as a breaking wave, creating circular waves with troughs and crests of 10s of m wavelengths
- tsunami waves: waves, hundreds of km wavelength, large masses of water that may have crests for long distance with water flowing straight, v. heavy
tsunami at shoreline
1. overview
2. signs
3. what to do
- very rapid rising tide rushing inland, sometimes water retreats first
- mild shaking > 25 sec powerful distant earthquake may have gen tsunami, immediately evacuate
- people need to evacuate fast to travel far inland and to higher ground, if not possible get into high building, since tsunami can travel as fast as plane, knowing signs and knowing to leave is why edu is important
Sumatra, Indonesia, 2004
1. overview
2. consequences
- tsunami created at subduction zone, generated big tsunami
- no warning in Indian ocean, killed many people with no warning, started warning sys for Indian and Atlantic oceans following fatalities, effects felt elsewhere as oceans all connected
4 causes of tsunami
- downward moving mass indents water surface and sends waves to sides as in meteorite impacts and pyroclastic flows
- seafloor rising due to bouncing up of subducting plate pushes water surface up
- submerged mass from submarine slump, debris flows, or turbidity currents slide down slope and pushes water in front of it and draws in water from behind
- dropping of seafloor pulls water surface down when caldera collaspe after volcanic explosion
tsunami wave height
- varies but gen near-field tsunami (close to source) will have higher waves, as further, far-field tsunami lower waves
- island, shorelines, and seafloor can cause interference which is complicated and hard to model
- most disruptive, tsunami beams are narrow band of high waves oriented perpendicular to source caused by elongated source such as fault
shoaling and tsunami waves
1. open ocean
2. shallow water
- not high crossing open ocean bc seafloor deep, wave velocity 700 km/h, boats may not feel much motion and may be much safer in deep water
- slow in shallow water, continetal shelf 160 km/h, approaching beach 40 km/h, back of wave catches to slower front, increasing height
coastal shape and bathymetry
steep cliffs may prevent water from flowing inland, water flows many km into gently sloping coasts, shallow sand bars and narrow bays/estuaries increase wave height
tsunami trains
series of waves, individual wave heights and arrival intervals vary, wave train typically include 6-25 waves over 3-6 hours, first wave is not always the highest, first drawback not always the lowest
Cascadia subduction zone earthquake tsunami
1. overview
2. historic evidence
3. preparation
- offshore of south B.C
- sequences of tsunami laid sediment indicate historic tsunami along Pacific northwest coast through mud with remains of marine plants and sand, ghost Sitka spruce forest where trees sawed by tsunami in Oregon
- warning systems with speaker of diff languages along B.C coast
tohoku tsunami, 2011
1. overview
2. destruction
3. consequences
- Mw 9 subduction zone earthquake, large fault slippage causing 30 m high wavefront
- seawalls, towns, and farms destroyed, Fukushima nuclear power plant destroyed causing no power, drowned backup gen on first floor, unable to properly shut down, explosion contaminated area with radioactive debris
- be cautious on where nuclear reactors are built and how to limit nuclear debris
turbidity currents
sediment disperses and forma turbulent cloud, scarps and sediment flows visible on bathymetric maps
tsunami relief and recovery
dmg to roads and communication systems thus rely on hovercraft and debris removal to access communities, prioritize freshwater, food, shelter, and healthcare
tsunami prediction
1. seismic activity
2. DART buoys
- earthquakes most common cause, thus most important to monitor seismic information in subduction zone
- DART buoys by NOAA have ocean floor pressure sensors connected to each buoy, tsunami pressure pulse displaces buoy, triggering radio alert
watch vs. warning
watch is when there is very probable evidence and chance that hazard may happen, thus should be aware and best to evacuate; warning is when already is happening
