tsunamis

Indian Ocean earthquake & tsunami

• earthquake on Sumatra triggered tsunami

• fault surface slipped along subduction zone

• 30 km³ water displaced

• waves 30 m high

Hilo, Hawaii tsunami

• waves 12 m high

• cause by 8.6 M earthquake in Aleutian Islands

currents, tides, wind-drive waves

currents: circulate water around global ocean

tides: cause the sea surface to rise & fall, generally twice daily

wind-drive waves: moving air shares the water surfaces

cause of tsunamis

• waves generated by sudden movement of mass against water

  • sea floor (when moved by earthquake)

  • submarine landslide

  • subaerial landslide that falls into a body of water

  • pyroclastic flow from a volcano

  • air blast from an explosively erupting volcano

  • meteorite

tsunamis are defined by…

their origin

how do tsunamis form?

1. displacement of sea floor in ‘quake

2. water rushes in and overcorrects

3. creates long, low waves

trough vs crest

trough: line along which water depth is lowest

crest: line along which water depth is highest

wave height, amplitude, length, period, velocity

height: vertical distance between crest and trough

amplitude: vertical distance between equilibrium level and a crest (= half the wave height)

length: horizontal distance between successive crests (or successive troughs)

period: time between passage of successive crests (or successive troughs)

velocity: horizontal speed at which crest (or trough) moves

near-field vs far-field

near-field/local: reach shore close to source; higher wave heights

fair-field/distant: reach shore far from their source; lower wave heights

shoaling

• changes a wave undergoes as they approach shore & shallow water

• as waves approach shallower water, they increase in height

• velocity = √ (gravity x depth)

normal shoreline

intersection of sea level before the tsunami arrives

drawback/drawdown

if a trough of a tsunami arrives first, the drawback will lower the sea surface below sea level

tsunami vs run-up elevation

tsunami elevation: the greatest vertical distance between the crest of the tsunami and sea level

run-up elevation: vertical distance between normal shoreline and inundation limit

inundation depth, limit, distance

depth: the vertical distance between the ground and water surface

limit: the line on land which water stops

distance: horizontal distance between the normal shoreline and inundation limit

high vs low relief areas

high: will cause water to not flow inland (i.e. steep cliffs)

low: may have water flow kms inland

Sulawesi (Indonesia) earthquake & tsunami

major soil liquefactions & mudflows

wind-drive waves vs tsunami waves

• wind-drive waves

  • 100 m wavelength

  • water motion follows elliptical path; storm wave can move at 30-50 km/h (open ocean) and 6-7 km/h (near shore)

• tsunami waves

  • 200 km wavelength (in open ocean) and 10 km wavelength (in shoreline)

  • water motion flows horizontally 30-40 km/h (near shore)

tsunami damage

• location dependent

  • infrastructure (i.e. boats, houses, piers) present

  • relief of area

  • substrate (sediment/lithified rock)

Tohoku (Japan) tsunami

• 8.9 M earthquake, 4th strongest in history

• led to Fukushima Daichi nuclear disaster

fault, active, normal, reverse, thrust

fault: fractures along which movement has taken place

active fault: movement with last 11,000 years

normal fault: hanging wall block moves downward relative to footwall block (e.g. rift valleys)

reverse fault: hanging wall block moves upwards relative to footwall block

thrust fault: reverse fault with very low angle fault plane (<45°)

hanging wall vs footwall block

hanging wall block: overlies an inclined fault plane

footwall block: underlies an inclined fault plane

fault slips

• can trigger earthquakes and displace the sea floor

• often occur at convergent plate boundaries

normal vs thrust fault

normal: hanging wall slides downward and suddenly produces a depression

thrust: hanging wall moves upward and pushes the overlying water up

subaerial vs submarine landslide-generated tsunamis

subaerial landslides: fall into the sea push down on its surface, producing large depression, generating tsunami as the sea surface bounces up & down

submarine landslides: generate tsunamis partly because of the downward motion of a solid mass underwater pulls the surface of the ocean down

Lituya Bay (Alaska) tsunami

• 7.8 M earthquake released about 30 million m³ of rock and debris into the bay moving at ~300 km/h

• water had nowhere to go but the bay itself creating a run-shot of up to 524m

Grand Banks (NFL) tsunami

• 7.2 M earthquake

• turbidity current recorded by cable breaks on sea floor - flow velocity 15-60 km/hr

• flow traveled over 700 km

frank collapse of volcanic islands

Islands of Hawaii are prone to frank collapse, a sudden catastrophic slump that removes part of a volcano

Krakatoa

• tsunami due to volcanic eruption

• 40 m high tsunami

• huge ash cloud produced

• wave heights were ~15-35m

• global temp reduced 0.5C for 10 years after eruption