TSUNAMIS - Geography 12royalbaygeography12.weebly.com/.../nh7-tsunamis.pdf · DISTANT TSUNAMIS...
Transcript of TSUNAMIS - Geography 12royalbaygeography12.weebly.com/.../nh7-tsunamis.pdf · DISTANT TSUNAMIS...
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
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