Lecture 7 - Tsunamis

What are the four main causes of tsunamis?

Earthquakes

Landslides

Explosive Volcanic Eruptions

Meteorite Impacts

What is the most common cause of tsunamis?

Earthquakes.

What does the Japanese word tsunami mean?

Harbor wave.

How frequently do moderate and large tsunamis occur?

Moderate: about twice per year. Large: every 15–20 years.

Tsunami Triggers

Earthquakes

Landslides

Explosive Volcanic Eruptions

Meteorite Impacts

How are tsunamis generated?

Sudden movement of a large mass displaces the sea surface, creating waves.

Masses that may suddenly move include

• Seafloor shifting up / down

• submarine and / or subaerial landslide

• air blast from an explosive volcanic eruption

• meteorite impact

Tsunamis are not _____ _______ or ________ __________ _________

Tidal Waves

Large Storm Waves

Why are tsunamis classified as gravity waves?

• Water surface horizontal @ equilibrium

• Equilibrium level upset if mass pushes water up or down.

Gravity restores displaced water toward equilibrium, transferring energy through the water.

T/F A single mass produces several gravity waves.

True

What is a tsunami wave train?

A group of several waves produced by a single disturbance.

What are a wave crest and trough?

Crest: line along which water depth is highest.

Trough: line along which water depth is lowest.

What is wave height?

The vertical distance between a crest and a trough.

What is wave amplitude?

The vertical distance from equilibrium level to crest

→ = half the wave height.

What is wavelength?

The horizontal distance between successive crests or troughs.

What is wave period?

The time between successive crests or troughs passing a point.

What is wave velocity?

The horizontal speed at which a crest or trough moves.

What are tsunami beams and what are they caused by?

• Narrow band of high tsunamis oriented perpendicular to source

• Caused by elongated source → such as a fault.

Tsunami beams generated which earthquakes?

1960 Chile Earthquake

2011 Tohuku Earthquake

• both due to slip along a convergent boundary.

T/F Tsunami waves are small while crossing the open ocean.

True. Shoaling creates towering waves as they approach shore.

What is shoaling?

The slowing and heightening of tsunami waves as they enter shallow water.

As water depth decreases, the wavelength and speed of a tsunami ______, and its height _______

Decrease, Grow

How fast do tsunamis travel in deep ocean, on continental shelves, and near beaches?

About 700 km/h, 160 km/h, and 40 km/h, respectively.

Why do tsunami waves increase in height near shore?

Shallow water slows the wave front, allowing faster water behind it to catch up.

• wave grows taller to accommodate the water volume

• water at rear wave catches up to slower water

What is tsunami drawback?

When an approaching tsunami trough lowers sea level

What is tsunami elevation?

The greatest vertical distance between the tsunami crest and sea level.

What is inundation distance?

The horizontal distance from normal shoreline to the inland inundation limit.

• how far inland tsunami water travels from the shoreline

What is run-up elevation?

The vertical distance from normal shoreline level to the inundation limit.

How many waves typically occur in a tsunami wave train, and over what time?

6–25 waves over 3–6 hours.

Is the first tsunami wave always the highest?

Not always. Later waves may be higher.

First drawback may not be the lowest.

• individual wave heights and arrival intervals vary

How do storm-wave and tsunami periods compare?

Storm waves:

• limited drawback & inundation distance

• wave periods of 5-20 seconds

• shallow wave base moves small volume of water

Tsunamis

• extensive drawback & inundation distance

• wave period 10 minutes to 2 hours

• deep waves move large volume of water

How do storm waves and tsunamis differ in drawback and inundation distance?

Storm waves - limited drawback & inundation distance

Tsunamis - extensive drawback & inundation distance

How do wave periods differ between storm waves and tsunamis?

Storm waves - 5-20 seconds

Tsunamis - 10 minutes to 2 hours

How do storm waves and tsunamis differ in wave base and water volume?

Storm waves - shallow wave base & small volume of water

Tsunamis - deep wave base, large volume of water.

Why do tsunamis have more dynamic pressure than storm waves?

Faster velocity

Why do tsunamis cause more extensive flooding than storm waves?

They have deep wave bases, move large water volumes, and produce extensive inundation.

Why do tsunamis have more dynamic pressure?

Faster velocity

How do normal and thrust faults displace water?

Normal faults pull water downward; thrust faults lift water upward.

Wind driven waves are _____, as measured _____ to the crest, so the volume of water in a single wave is relatively _____, even if wave is high. Wave runs out of water when it reaches top of beach.

Narrow, Perpendicular, Small

Destruction from tsunamis

Advancing tsunamis

Drawing back tsunamis

Normal faults develop in

Sinking lithospheric plates

Thrust faults develop in

accretionary prism.

• sudden vertical movement displaces water

Normal falls _____ water _____ equilibium level

pull, below

Thrust faults ______ water ________ equilibrium level

Lift, Above

Why do strike-slip faults rarely generate tsunamis directly?

They generally produce little vertical seafloor displacement.

What characterizes megathrust earthquakes in this lecture?

Mw greater than 8.6 at convergent margins.

What happens to an accretionary prism @ convergent margin before and during megathrust slip?

Before: shortens, thickens, rises.

During slip: face moves seaward and land elevation sinks.

What is the difference between near-field and far-field tsunamis?

Near-field waves reach nearby shores; far-field waves travel into the open ocean.

What caused the 1946 Hilo, Hawaii tsunami?

An Mw 8.6 earthquake near Unimak, Alaska; waves traveled about 3,850 km in under 5 hours.

• Seafloor suddently slipped, pushing up against overlying water

• Unimuk experienced 40m high tsunami

• Waves travlled 3850 km to Hawaii in under 5 hours

What were the key facts of the 1960 Chile tsunami?

• Mw 9.5 Valdivia earthquake = largest recorded earthquake

• 25 m high near-field waves and 3 m high far-field tsunamis in Japan 22 hours later.

What happened during the 1964 Alaska tsunami?

Mw 9.2 earthquake; crust uplifted 12 m

• Parallel band of crust subsided 2.5 m

• near-field tsunami killed 120.

• Area size of california displaced

What do Cascadia's ghost forests reveal?

A megathrust event around 1700 caused sudden coastal subsidence, saltwater flooding, and forest death.

What evidence is found in Cascadia’s ghost forests?

Dead cedar trunks rise above grass in coastal Washington, with tree roots in soil covered by peat.

What do Cascadia’s ghost forests reveal about past land movement?

• Land Subsided below sea level, forming coastal wetlands and peat, before rising again

How was Cascadia subsidence event dated, and when did it occcur?

• Charcoal from fire pits used to date subsidence event to approximately 1700

Cascadia’s ghost forest formed in 3 stages. Whats the first?

Continental Margin Sticks

• Margin shortens horizontally + rises vertically

• Forests grew on dry land

Cascadia’s ghost forest formed in 3 stages. What’s the 2nd?

Continental Margin Slips

• Margin moves westward from megathrust earthquake

• Land surface subsides and becomes salt marsh

• Forests die in salty water

• Peat forms in salt marshes.

Cascadia’s ghost forest formed in 3 stages. What’s the third?

Continental Margin Sticks

• Margin shortens horizontally and rises vertically

• Grass grows on uplifted land.

What caused the 2004 Indian Ocean tsunami?

Mw 9.3 Earthquake on the Sunda Trench, off Sumatra

• Sudden displacement of sea floor (overriding plate launched west 15 metres)

• Near field tsunami: 30 m wave front, 7 km inundation, about 230,000 deaths.

How did the overriding plate and accretionary prism move during the 2004 Indian Ocean Tsunami?

Overring plate lurched west 15 metres.

Accretionary prism rose a few metres near the trench and subsided 1m near the coast

What caused the 2011 Tohoku tsunami and Fukushima disaster?

Mw 9.0 earthquake of Japan’s NE coast

• 60,000 km2 area slipped as much as 40m.

• 30m high wave front

• destroyed and innundated seawalls, towns, farms.

• 30 m waves flooded generators, stopping cooling pumps at Fukushima Daiichi.

What happened to the Fukushima Daiichi nuclear power plant during the 2011 Tohuku Tsunami?

• No power

• Backup generators failed

• Cooling pumps failed

• Explosions contaminated area with radioactive debris

How do subaerial and submarine landslides generate tsunamis?

Subaerial slides push water down

Submarine slides pull water down while their toes push water up.

What happened at Lituya Bay in 1958?

• Mw 7.8 earthquake triggered a subaerial rockslide in Lituya Bay Fjord

• 30 million m3 rock fell into bay @ 300kmh

• Narrow fjord → focused a 524 m high wave front.

• Mega tsunami wave front >100m

1929 Grand Banks, Newfoundland

Earthquake triggered a submarine slump that generated a tsunami and damaged trans-Atlantic cables.

What generated 1998 Papau New Guinea tsunami?

• Mw 7.0 Earthquake off coast of Papua New Guinea

• Minimal earthquake damage, massive tsunami impact

• 15m high wave front devastated norther coastline.

• tsunami generated by submarine landslide

What is a mega-tsunami, and what is one volcanic flank-collapse example?

Wave front over 100 m.

• Fogo Island flank collapse about 73,000 years ago generated a 250 m high wave front

• Fogo Island 60km west of Santiago island.

• Mega tsunami hit Santiago & moved giant rocks

What is the only explanation for giant rocks from sea cliffs being on Santiago island? (650m inland and 220m above sea level)

Mega tsunami

What volcanic events have generated tsunamis?

Thera eruption (1600 BCE) in Mediterranean → decline of Minoan civilization on Crete.

Krakatau eruption 1883) in Sunda Strait → killed 36000 on Sumatra and Java

What meteroite impact generated a tsunami?

Chicxulub asteroid impact (66 Ma).

→ asteroid struck Yucatan Peninsula

→ created 150km diameter Chicxulub crater

→ impact ended Mesozoic Era and Age of Dinosaurs

→ mega tsunamis generated from impact of falling debris

Debris cloud collapse may trigger _____ because debris displaces _____ _____ when it falls

Tsunamis, sea surface

Most common cause of tsunmais?

Earthquakes

• not all earthquakes generate tsunamis.

T/F Seismographs can confirm tsunami generation.

False. They CANNOT.

DART buoys deployed to detect tsunamis.

What does DART stand for and how does it detect tsunamis?

Deep-Ocean Assessment and Reporting of Tsunamis

• Ocean floor pressure sensors connected to each buoy detect pulses and trigger buoy alerts.

• 60 DART buoys in the pacific

T/F 2004 Indian Ocean Tsunami was caused by a megathrust earthquake and generated a megatsunami

False.

→ Yes, generated by Megathrust Earthquake

→ Generated a 30m Near-Field Tsunami

Which examples from lecture are megatsunamis?

• 1958 Lituya Bay

• Fogo Island, Cape Verde