9.1. Tsunamis

Question 1

Global Distribution of Tsunamis

Answer 1

• Up to 90% of the world’s tsunamis occur in the Pacific Ocean - as they are associated with subduction zones, most of which are found in the Pacific.
• generally caused by earthquakes, but can be caused by volcanoes (Krakatoa, 1883) and landslides (Alaska 1964)

Question 2

Tsunami

Answer 2

a long high sea wave caused by an earthquake, submarine landslide, or other disturbance.
- most are secondary hazards from earthquakes

Question 3

What factors cause Tsunamis

Answer 3

1) Submarine Earthquakes

2) Landslides

Question 4

How submarine earthquakes cause tsunamis

Answer 4

• majority of them occur this way
• most are generated at subduction - convergent plate

1) Near the source of submarine earthquakes the seafloor is “permanently” uplifted or down-dropped along a fault
2) Entire water column is pushed above the boundary up or down generating the initial wave of a tsunami
3) Once a tsunami is generated, the potential energy that results from pushing water above the mean sea level is then transferred to kinetic energy and the tsunami begins its race towards the shore
4) The amount of energy released can be very large because the energy released by a strong earthquake is very large
5) As the tsunami travels towards the nearest landmass, the depth of the ocean decreases and the height of the wave increases
6) The wavelength also decreases. As a result, the front of the leading wave becomes significantly steeper

Question 5

How landslides cause tsunamis

Answer 5

• can result from volcanic activity, especially the explosion of volcanic islands and landslides or rockfalls
• can result in a highly destructive localised tsunami
• can also be the result of earthquakes causing land to slide

1) Instead of an excess rise of water at the surface, a landslide takes down the water surface, producing a trough
2) Water rushes into the void, the trough rises from a swell and oscillations can go on for a very long time
3) Each oscillation produces waves which radiates outwards. It can be several minutes from one wave to the next

Question 6

Localised Tsunami

Answer 6

. A tsunami from a nearby source for which its destructive effects are confined to coasts within 100 km of the source

Question 7

Distant Tsunamis

Answer 7

a tsunami that originates from a distant source, defined as more than 1,000 km (620 mi) away or three hours’ travel from the area of interest, sometimes travelling across an ocean

Question 8

Factors determining the destructiveness of Tsunamis

Answer 8

1) Wave energy
2) Shape of coastline
3) Relief of coastline
4) Presence of natural defences
5) Demography
6) Lack of, or inadequate, warning systems and evacuation plans

Question 9

How wave energy determines the destructiveness of Tsunamis

Answer 9

• when a tsunami is generated out to sea, water displacement can form waves with wavelengths greater than 100km
• the longer the wavelength, the greater volume of water and more energy is conserved
• in deep water no energy is lost to frictional drag with the seabed
• as it approaches the shore, the water becomes shallower, forcing the circular wave motion into elliptical form, which heightens until it an no longer be maintained and breaks

Question 10

How shape of coastline determines the destructiveness of Tsunamis

Answer 10

• Indented coastlines with long, narrow bays concentrate energy on the bay head due to a funneling effect as the wave travels up the bay
• Irregular coastlines and offshore islands can set up interference patterns in the waves which, when they coincide perfectly, can accentuate the waveform

Question 11

How relief of coastline determines the destructiveness of Tsunamis

Answer 11

• cliffs present a natural barrier to a tsunami

- low-lying land allows seawater to incur for several kilometers

Question 12

How presence of natural defences determines the destructiveness of Tsunamis

Answer 12

• coral and mangroves act as natural defences by dissipating wave energy through their large surface areas
• Tourist developments have removed much mangrove vegetation and this exacerbated the impact in the Indian Ocean 2004 disaster

Question 13

How demography determines the destructiveness of Tsunamis

Answer 13

• in all natural disasters, the young and old are the most vulnerable due to their lack of mobility and strength
• In 2004 tsunami - 80% of deaths were female
• men were fishing, safest place to be during tsunami
• women were performing domestic roles and looking after children at home on the coast

Question 14

How lack of, or inadequate, warning systems and evacuation plans determines the destructiveness of Tsunamis

Answer 14

• better the warning systems and evacuation plans, the more prepared the people would be and so less death

Question 15

Tsunami Primary Hazards

Answer 15

• flooding of coastal areas by surging water
• destruction of buildings and infrastructure (powerlines, bridges, etc.)
• Human deaths on a large scale by impact of surging water
• objects carried offshore by wave backwash

Question 16

Tsunami Secondary Hazards

Answer 16

• spread of disease due to contaminated water
• stirring of sea bed sediment can cover coral reefs and leads to their destruction
• washing away of soil destroys farmland

Question 17

Tsunami Prediction and monitoring

Answer 17

1) Pacific Tsunami Warning Center (PTWC)
2) Open-Ocean Buoys and Coastal Tide Gauges
2) Deep-Ocean Assessment and Reporting of Tsunamis (DART)

Question 18

Pacific Tsunami Warning Center (PTWC)

Answer 18

• centre is established in Hawaii
• detect tsunami activity for the 24 nations of the Pacific Basin
• the seismic stations detect all earthquakes in the area, which are interpreted to see if there is a risk of tsunami
• The tidal gauges can give little or no warning of the tsunami since they are close to shore and can really only record details of the event.
• aim is to alert all areas within one area and this can be done as it takes a while for a wave to travel to travel across the Pacific

Question 19

Open-Ocean Buoys and Coastal Tide Gauges

Answer 19

• both report information to stations within the region
• Buoys - since buoys are always floating on the sea surface, by recording their own movements they are in fact recording the movements of the sea surface
• Tide stations measure minute changes in sea level, and seismograph stations record earthquake activity

Question 20

Deep-Ocean Assessment and Reporting of Tsunamis (DART)

Answer 20

• Uses unique pressure recorders that sit on the ocean bottom.
• These recorders are used to detect slight changes in the overlying water pressure.
• The DART system can detect a tsunami as small as a centimeter high above the sea level.

Question 21

Evaluation of Tsunami Predictions

Answer 21

PTWC

• Not all earthquakes result in tsunamis and it is difficult to decide whether or not to issue a warning.
• Usually, warnings are sent out by the Hawaiian headquarters when the magnitude of the underground earthquake is larger than 7.5.
• If there is a false alarm, then this results in a loss of $30 million alone for each false warning.
• Furthermore, if there are too many false alarms, then people will become complacent and ignore them.

DART

• this project is expensive and its success has been limited to the Pacific rather than the Indian Ocean.
• One of the reasons for the huge loss of life in the 2004 tsunami was the absence of such a system in the Indian Ocean.

Question 22

Tsunami Hazard Mapping

Answer 22

• can reduce hazardous nature of tsunamis
• rezoning of low-lying coastal land.
• buildings would not be built in low-lying coastal land. Instead, these areas would be turned into public parks and buildings would not be built near beachfronts.
• this reduces the hazardous nature of tsunamis as there would be less destruction and less people would go there.
• the buildings and businesses would be moved up to higher ground back from the shore.
• this can reduce hazardous effects of tsunamis, especially hydrodynamic effects.