Animation of Tidal Elevations in the Pacific

Tides and Tsunamis

• Gravitational forces of moon and sun• Equilibrium theory of Tides• Dynamic theory of Tides (Reality): tidal

patterns, confined basins

• Tsunamis: generating forces• Effects of tsunamis• Warning systems, defenses

Equilibrium Theory

Tides are always in equilibrium withthe gravitational pull of the moon andEarth is a planet covered in water.

Assumption

Gravity and centrifugal force(also called “tractive forces”)

Together:The earth-moon system

Equilibrium theory of tides

Tides: The moon and the sun together

Spring tide

Neap tide

semidiurnal

diurnal

composite

+

=

Dynamic Theory

Waves travel at a fixed waves speedThere are continents and rotation

Needs to account for:

See video

Tidal circulation

• Tides progress around basins, counterclockwise in S hemisphere and clockwise in N hemisphere

Animation of Tidal Elevations in the Pacific

Inertia + continents cause the tidal motion on the planet to differ markedly from the “motion” of the forces.

We can calculate the water motion knowing the forces, but we cannot say that the shape of the water is the same as the “shape” of the forces.

For this reason tides must not be visualized as bulges standing under the sun and moon.

But rather as very long waves over the sea forced by the gravitational-centrifugal forces associated with the moon-sun-earth system.

Tides in confined basins• Increase tidal range (the difference

between high and low tide)

• Examples--Bay of Fundy, Canada--Northern Gulf of California, Mexico

• Tidal bores - wave of water moving upstream - result of high-tide crest entering confined inlet

Bay of Fundy: map

• 2.416

Bay of Fundy tides

• Extreme tides (10m or more) found where small marine basin adjoins large ocean– Bay of Fundy, Nova Scotia– Gulf of California

• (in most places, tides are 1 to a few meters in range)

Shock Waves and Tidal Bores are similar!

Tidal bore: Severn River, England

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Tidal ecosystems

• Rise and fall of tides creates stressful environments for intertidal marine organisms

Tidal ecosystems

• Others take refuge in tide pools, where water remains even at low tide

TsunamiJapanese for harbor (tsu) wave (nami)

Caused by displacements of water•landslides into the sea•submarine earthquakes•submarine volcanoes•asteroid impacts

“Shallow-water” wave: disturbs water all the way to bottom

Tsunami of April 1, 1946

Earthquake triggers tsunami with devastating local and distant effects

The speed of the tsunami wave

C = sqrt(g d)

C = speed,

g = acceleration due to gravity (9.8 m/sec/sec)

d = depth (depth of Pacific ~4,600 m)

C= sqrt(9.8 * 4,600)

Speed = 212 meters per second; 472 mph

Alaska to Hawaii in 5 hours!

Locally, the tsunami washed away the 5-story lighthouse at Scotch Cap, Alaska

Before… and after

Hilo, Hawaii, 1946: Tsunami crossed the north Pacific to become one of Hawaii’s worst natural disasters

Hilo, Hawaii, 1946

Tsunami breaking over main pier in Hilo, 1946

This man did not survive

Chile earthquake, 1960numerical simulation of tsunami

8.5 hours

1.5 hours 16.5 hours

23.5 hours

Aftermath of local tsunami: Chiloe, Chile, 1960

Distant effects of the Chile earthquake: tidal wave aftermath, Hilo Hawaii

More tsunami damage in Hilo, 1960

Tsunamis: what can be done?• Early warning system for evacuation (if EQ is distant)• Coastal zoning. Get development out of the way. Example: Hilo,

Hawaii• Defense. Protective walls. Example: Taro, Japan