Geography 2152F/G Lecture 3: Lec3 - Tsunamis
19 Feb 2019
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Geography of Hazards 2152B
Lecture 3 – Tsunamis
Tsunamis
• Tsunami is Japanese for “harbor wave”
• They are produced by the sudden displacement of water
• Events capable of triggering tsunamis:
- Earthquakes that cause uplift of the seafloor
- Landslides
- Volcano flank collapse
- Underwater volcanic eruptions
- Meteorites
Historic Tsunamis
Earthquake-Triggered Tsunamis
• Earthquakes can cause tsunamis in two ways:
1. By displacement of the seafloor
2. By triggering a landslide that enters water
• Generally, an earthquake must be of at least M 7.5 in order to trigger a tsunami
• Tsunamis develop in a 4-stage process
• STAGE 1:
- Displacement of the seafloor sets waves in motion that transmit energy upward and outward
- When the waves reach the surface of the water, they spread outward
- First is the pulse of energy going up and water going to the surface, hits the surface, and the wave
moves out
• STAGE 2:
- The waves move rapidly across the open ocean (they can reach speeds of over 500 km/h)
- The spacing of the wave crests is very large (it can be more than 100 km)
- The height (amplitude) of the waves is often small (less than 1 m)
- Passengers on ships in the ocean rarely even notice tsunamis passing beneath them
• STAGE 3:
- As the tsunami approaches land, the water depth decreases
- This results in the water ‘piling up’ and causes these effects
- A decrease in wave speed
- A decrease in spacing of the waves
- An increase in wave amplitudes
• STAGE 4
- As the tsunami impacts land, waves can reach heights of dozens of meters
- The wave speed at this time can be up to 50 km/h making them impossible to outrun
- During some tsunamis, the water first recedes from the shore and exposes the seafloor
- Water might pull back before the big wave comes in because water is displaced and pulled back to
create the larger wave. Looks like tide going out.
find more resources at oneclass.com
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Geography of Hazards 2152B
- The exposure of the beach is receding
Tsunami Event
• A tsunami event consists of a series of large waves reaching shore that can last for several hours
• Run Up: the maximum vertical distance that the largest wave of a tsunami reaches as it travels inland
• Run-Up measuring how far the wave travels in elevation, maximum vertical distance on topography
Types of Tsunamis
Distant Tsunami:
• A tsunami that travels thousands of kilometres across the open ocean
• On remote shorelines across the ocean, reduced energy lessens its impact
• They are also called tele-tsunamis (distant tsunami)
Local Tsunami:
• A tsunami that affects shorelines a few kilometers to about 100 km from its source
• Because of this short distance, local tsunamis provide little warning
Regions at Risk
• Coasts located near subduction zones or across oceans from subduction zones are most at risk
• Areas at greatest risk are the Pacific Ocean and the Mediterranean Sea
Effects of Tsunamis
Primary Effects:
• Flooding and erosion destroy beaches, coastal vegetation, and infrastructure
• After the tsunami retreats to the ocean, scattered debris is left behind
• Most tsunami deaths from drowning. Injuries result from physical impacts with debris
Secondary Effects
• These are effects that generally occur after the event is over
• Fires may develop due to ruptured gas lines or from ignition of flammable chemicals
• Water supplies may become contaminated and water-borne diseases (cholera) may spread
• Secondary are not as obvious, linger over time
• Cholera spreads through dirty water – in developing countries without filtering systems have more
problems
Indian Ocean Tsunami of 2004
• This catastrophic event occurred on December 26th
• The source was a M 9.1 earthquake off the west coast of Sumatra (an island in Indonesia)
• It was the 3rd strongest earthquake in world history
• The earthquake occurred in a subduction zone (sinking, one plate pushed up) between the Burma and
Indian-Australian plates. Cause pulses of energy
• These plates had been locked for over 150 years thus allowing strain to build
• The rupture causes some land areas along the coastline to subside below sea level
• The tsunami reached nearby Indonesian islands within minutes of the earthquake
• Many coastal communities in Indonesia and surrounding countries were heavily damaged during the
event
• Countries bordering the Indian Ocean did not have a tsunami warning system like those bordering the
Pacific Ocean
• People were caught by surprise and over 230,000 died
• Many were unfamiliar with tsunamis and some were intrigued by the approaching waves
• Most people in the area were ignorant of an early warning sign (the receding sea)
Lessons from the 2004 Tsunami
• Effective tsunami warning systems are needed around all oceans where tsunamis can occur
• In 2006, a new warning system became operation in Indian Ocean
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find more resources at oneclass.com
Geography of Hazards 2152B
• A warning system by itself is not enough. Why?
- Emergency officials must have an organized plan for evacuating residents during a warning
- Earthquake and tsunami education is necessary for people who live along or visit coastlines
Detecting Tsunamis
• The Pacific Ocean warning system uses a network of seismographs to estimate earthquake magnitude
• Sensors electronically connected to buoys verify that a tsunami was produced
• They rest on the seafloor and measure changes in water pressure passing over them
• These are sensors are called tsunameters → monitor pressure of the water above the sensor which
indicates larger volume of water moving over, not monitoring the wave just monitoring body of water
Structural Control
• Damage can minimize through regulations on buildings and structures
• Some cities in Hawaii require flood proofing measures such as basement window sealing and bolting
homes to their foundation
• Concrete levees are other preventative measures but can be very expensive
• Offshore barriers are only feasible outside cities with very large populations
Inundation Maps
• Maps showing the geographic area that can be potentially impacted by tsunamis are created to help plan
for future events
• Historical records, geologic data, and aerial photography aid in making the maps
• Many North American cities on Pacific coast have such maps and development restrictions may exist
there in areas at high risk of tsunamis
Land-Use
• Vegetation plays a role in determining tsunami damage
• In areas impacted by smaller waves, trees and dense vegetation protect areas farther inland
Japan Tsunami of 2011
• This catastrophic event occurred on March 11th
• The source was a M 9.0 earthquake 70 km off the east coast of Japan (local tsunami)
• A tsunami warning was issued nearly an hour before its arrival
• Over 15,000 people died and damage to Japan’s infrastructure was extensive
EXAM: Why are tsunamis rare in the Atlantic Ocean
Answer: The Atlantic splitting and becoming more rare, there are plates beneath = lack of subduction zones
Definitions
• Local Tsunami – areas affect up to 100km away
• Indian Ocean – location of 2004 tsunami
• Amplitude – height of the wave
• Tele-tsunami – another name for distant tsunami
• Primary Effects – first effects of tsunami
• Tsunameter – sensor to detect possible tsunami
• Subduction zones – large tsunamis occur here
• Tsunami – a series of giant waves
• Run-Up – vertical distance tsunami travels
• Inundation Map – maps out the run-up
Categories of Adjustment
Modify the Loss Burden
• Loss sharing
• Spread the burden beyond immediate victims
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find more resources at oneclass.com
Document Summary
They are produced by the sudden displacement of water. Earthquakes can cause tsunamis in two ways: by displacement of the seafloor, by triggering a landslide that enters water, generally, an earthquake must be of at least m 7. 5 in order to trigger a tsunami. Displacement of the seafloor sets waves in motion that transmit energy upward and outward. When the waves reach the surface of the water, they spread outward. First is the pulse of energy going up and water going to the surface, hits the surface, and the wave moves out. The waves move rapidly across the open ocean (they can reach speeds of over 500 km/h) The spacing of the wave crests is very large (it can be more than 100 km) The height (amplitude) of the waves is often small (less than 1 m) Passengers on ships in the ocean rarely even notice tsunamis passing beneath them.
