TSUNAMIS

Series of waves caused by a displacement of a large volume of water, typically in an ocean or large lake

Japanese term,“tsu” means harbour “nami” means wave

Very long wavelength (approx. 200 kilometers) compared to regular wind-driven waves (10s of meters)

Serval types of events can trigger a tsunami:

1. Large earthquake

2. Landslides

3. Explosive or volcanic eruption

4. Asteroid or comet impact

Can displace the seafloor or the floor of a large lake, or by triggering a large landslide

Typically needs to be a magnitude of 7.5 or larger to generate a damaging tsunami

Upward or downward movement of the seafloor displaces the overlying water and initiates a four-stage process…

A tsunami has four general stages: 1. initiation, 2. split, 3. amplification, 4. run-up.

The 4 Stages of a Tsunami

Displacement of seafloor during earthquake sets waves in motion that transmit energy outward and upward from the source

These waves intercept with the ocean surface and spread outward

In the deep ocean, waves move rapidly and are spaced far apart

capable of racing across an ocean at speeds up to 950 km/hr (faster than a jet passenger aircraft!)

Spacing between wave crests is 200km and height of waves is less than 1 meter

You would not notice a passing tsunami in a boat in the deep ocean!

As the tsunami nears land, both the water depth and the velocity of the tsunami decrease

Near land, the speed may be about 45 kilometers per hour – too fast to outrun but not nearly as fast as out in the open ocean

Spacing between waves decreases and the wave height increases as the energy is compressed into a smaller space

As the first tsunami wave approaches the shore, it transforms into a turbulent, surging mass of water- rapidly moves inland

Wave doesn't’t curl over and crash on land, instead the waves are typically turbulent, onrushing surges of water

Trough of the wave may arrive first, causing sea to recede and exposing the seafloor

Not a single wave! Rather a series of

waves that are separated by

minutes or hours

https://youtu.be/7O1wzLMpHtg

Indian Ocean Tsunami 2004, Thailand

https://youtu.be/1ikus_TEaGI

Tsunami Hits Japan Port Town

Once a wave has reaches its farthest extent inland, the water returns back to the open ocean in a strong, turbulent flow

DISTANT TSUNAMIS

Travels thousands of kilometers across the open ocean and strikes remote shorelines with little loss of energy

Distance of the affected shoreline from the source is generally greater than 1000 km or 3 hours tsunami travel time

LOCAL TSUNAMIS

Affects shorelines near the source of the earthquake

The distance of the affected shoreline from the source is within 100 km or less than 1 hour tsunami travel time

Arrive at the shoreline soon after the quake therefore little or no warning

Great subduction earthquakes can cause both kinds of tsunamis

Submarine landslides on the seafloor OR

Large landslides that fall from mountain or volcano slopes into a large body of water

Both submarine and regular landslides of this size are typically result from earthquakes

• 7.8 magnitude earthquake

triggered a rockslide at the head

of the bay

• Rockslide plunged into the bay &

displaced seawater that ran up

the opposite valley wall

• Tsunami destroyed forest on the

other side

• Caused destruction as high as

524 meters above the surface of

the bay

Part of the volcano can collapse and fall into the water during an eruption

Underwater volcano can erupt

Smaller tsunamis can be caused by volcanic mudflows entering the water

Much less common than earthquake or landslide- triggered tsunamis

Massive eruption 1883 (10x bigger than Mt. St. Helens)

21 km3 of fragmented rock and ash

Volcano collapsed into the sea generating 37 meter tall tsunami

Waves overwhelmed nearby islands, 165 villages destroyed

36,000 people died (tsunami, debris & pyroclastic flow)

Most impacts occur in the world’s ocean, some can displace enough water to cause a tsunami

Asteroid hit Yuncatan Peninsula 65 million years ago

Coasts in proximity to major subduction zones

Coasts directly across from the ocean basin from a subduction zone capable of generating a magnitude 9 earthquake

85% of recorded tsunamis have been in the Pacific Ocean (Ring of Fire…again)

Dec 26, 2004

9.1 Earthquake ruptured 250 km off Sumatra, Indonesia

Banda Aceh, a provincial capital on Sumatra, before and after the tsunami

The tsunami reached

heights of more than

30 meters in some

places around the

Indian Ocean.

Witnesses say the

surge of water was like

a flood rather than a

wave.

Satellites

before and

after showing

coastal

agriculture

near Banda

Aceh

December 26, 2004

9.1 magnitude quake (third largest in recorded history)

Waves reached nearby Indonesian Islands in just minutes

Some countries experienced tsunamis hours later

Countries boarding the Indian Ocean did not have warning systems in place

230,000 people died

No warning system in place