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Natural Hazards > Tsunami > Flashcards

Tsunami Flashcards

1
Q

What are tsunamis

A

A series of large waves

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2
Q

What are tsunamis generates by

A

Submarine earthquakes
Volcanic eruptions under or into the sea.
Landslides into or under the sea.
Comet or meteor rite impacts into the sea.

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3
Q

What is a comet

A

Icy body from outer regions of solar system

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4
Q

What is a meteorite

A

From asteroid belt near mars and Jupiter

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5
Q

What do tsunamis need

A

Vertical displacement of water column, over large area (100-1000km^2). Gravity helps it adjust to the displacement causing the series of waves

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6
Q

What is the equation for tsunamis velocity

A

Water velocity (v) controller by earths gravity (g 9.8m/m^2) and the depth of the sea (H)

V= square root of gH

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7
Q

What is the equation for tsunami amplitude related to the velocity

A

A^2v= constant

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8
Q

How do tsunamis form

A

As wave travels to the shore, depth decreases causing velocity to decrease which in turn caueee amplitude to increase. Wave slows down but get bigger.

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9
Q

How does the tsunami wave change as it gets closer to coast

A

In middle of ocean the depth of the sea is 5000m, velocity is 224m/s and amplitude is 2m. Unrecognisable.
10km from coast the depth is 100m, velocity is 32m/s and amplitude is 5.3m.
100m from coast the depth is 1m, beockity is 3.3m/s and amplitude is 16.5m.

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10
Q

What are the tsunami characteristics offshore

A

Velocity - FAST. 900km/h
Amplitude - SMALL. Less than 2m.
wavelength - LONG. Hundreds of km.
Hazard level - LOW. Hardly noticeable

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11
Q

Tsunami characteristics onshore

A

Vecolity = SLOW. Few tens of km/h.
Amplitude INCREASED. Several tens of m.
Wave length DECREASES to tens of km. wave crests closer.
Hazard level INCREASED.

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12
Q

How is a tide gauge useful in measuring tsunamis

A

Usually measure predictable undulating increase and decrease of sea level due to rises. 12 and half hours is the cycle of tides. Before tsunamis the red line follows prediction closely then there’s a huge variation. Blue-red to get tsunami irrespective of tide.

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13
Q

What isrun up

A

Height above mean sea level that the wave reached as it rushes onsure. Usually greater than incoming wave amplitude.

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14
Q

What is inundation

A

Horizontal distance reached, measured inland from the mean shoreline

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15
Q

How can run up and inundation vary considerably along a shoreline

A

Depends upon local bathymetry and topography

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16
Q

Example of run up shown by scored vegetation

A

2004 tsunami at Banda Aceh , Sumatra

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17
Q

Where do tsunamis occur

A

Subjection zones with large earthquakes and volcanic eruptions

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18
Q

Where has the highest tsunami hazard level

A

The Pacific Ocean due to the ring of fire

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19
Q

What is the percentage of tsunamis that happen in the Pacific

A

59%

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20
Q

What is the percentage of tsunamis in the Mediterranean Sea

A

25%

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21
Q

Where else contains relatively short subduction zones

A

Indian and Atlantic oceans

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22
Q

What are the hazards of a tsunami

A

waves can knock down trees, buildings, bridges etc.
People killed by drowning or hit by debris.
Afrtermath brings water borne diseases - malaria from stagnant breeding mosquitos and sewage and decomposing bodies

23
Q

What are the ways of reducing vulnerability of tsunamis

A

Assess hazard level.
Land use planning and defence.
Education, training and emergency planning.
Tsunami warning systems.

24
Q

How to assess hazard level

A

Map deposits and erosion from previous tsunamis.
Identify source zones.
Estimate recurrence intervals -> probabilistic forecasting.
Tsunami modelling.
Hazard mapping.

25
Q

How can you see erosion from previous tsunamis

A

Sometimes big boulders that have been deposited and you can’t tell how else they got there

26
Q

How does identity source zone help

A

Look at past earthquakes, slope stability of underwater volcanoes and can get a rough idea of areas more vulnerable. Difficult bc big tsunamis from far away that was small when it gets to you or big ones that started close by

27
Q

How do you get estimated recurrence intervals

A

Records and geological records . Look at past and extrapolate to the future for a rough idea of how frequent tsunamis are

28
Q

How does tsunamis modelling help

A

Computer models that represent their behaviour - energy, grow in height, interactions with features on sea floor like islands can make them retract or bounce off coastlines they’ve hit. Give an idea of coastline with big run up and inundation

29
Q

What does hazard mapping do

A

Shows evacuation routes, population, areas that need to be protected

30
Q

What is land use planning and defences like

A

Use areas closest to sea for recreation only. Tsunami control forests absorb wave energy. Trees can absorb tsunami energy to try and reduce tsunamis

31
Q

mangroves and coral reefs are effective natural defences but what’s wrong

A

They are often degraded or destroyed by human activities such as aquaculture, tourism, pollution

32
Q

Why are mangroves important

A

Dense network of tree roots partially in and partially out of water. Important ecosystem for fish breeding in the roots. For birds and other animalsX

33
Q

How much of the mangroves have been lost in last century

A

50%

34
Q

When was there an increase in recognition of mangroves

A

2004

35
Q

What do engineering solutions include

A

Breakwaters and sea walls but these are expensive and ugly and not always high enough

36
Q

Example of place with high tsunami barrier

A

10m in Taro Japan but in March 2011 the tsunami was more than 10n high flooding taro and many other areas. Water is trapped then

37
Q

When can tsunamis be detected

A

Several hours before they reach a coast

38
Q

What are tsunamis often preceded by

A

A withdrawal of the sea at this stage there may still be time to escape by running or driving inland or uphill

39
Q

Examples of tsunamis warning systems

A

PTWS
NEAMTWS
IOTWS
CARIBE-EWS

40
Q

What can dart bouys do

A

Detect the difference between long wave lengths of tsunamis and short of wind driven. If they confirm it the watch becomes a warning and triggers emergency response

41
Q

Example f the national tsunami warning centre

A

Earthquake
Calculate epicentre and magnitude - issue bulletin.
If 6.5:
Check sea level data is tsunami is generated.
If yes confirm warning.
Danger? If yes national emergency responses

42
Q

What does dart bpr stand for

A

Deep ocean assessment and reporting of tsunamis. Bottom pressure recorder

43
Q

What does PTWS stand for

A

Pacific Tsunami Warning System

44
Q

What does the PTWS cover

A

28 countries, control centre in Honolulu Hawaii - 15 staff.

45
Q

What happens during the PTWS

A

Global seismograph network detects potentially tsunamigenic earthquakes (usually submarine, shallow affects deformation of sea bed, M>7.5)
Dart buoys used.
Alert vulnerable communities so emergency plans can be activated.

46
Q

How long does it take to analyse data and issue warnings

A

10-20 minutes: means blind zones for tsunamis generates close to the coast

47
Q

What does TWS mean

A

Japanese Tsunami Warning System

48
Q

How does the Japanese tsunami warning system work

A

Warning issued within 3 minutes of a tsunamigenic EQ. Quicker bc of dense coverage of equipment.
Communities well prepared.
Successful at reducing death toll.
Highest risk by coast if tsunami is generated too close for warning to reach ppl.

49
Q

Tsunami case study

A

Mw=9 megathrust EQ near Sumatra (Indonesia). Indian plate subducting under the Burma Microplate.
20m max displacement on the fault plane.
Highest intensity (IX) felt in Banda Aceh, Sumatra.
Resulting tsunami caused extreme destruction around Indian Ocean especially in Sumatra (run up 30m, inundation 1.5km).
No warning system in place at the time. Subsequent funding put forward.
290,000 were killed.
1.1 million displaced by EQ and tsunami.

50
Q

What are three other notable tsunamis

A

Aegean Sea - 1500 BC.
Indonesia - 1883.
Japan - 2011.

51
Q

What was the cause and effects of the Aegean Sea tsunami

A

Cataclysmic eruption of Santorini.

Demise of advanced Minoan civilisation on Crete (source of Atlantic legend).

52
Q

What was the cause and effects of the Indonesia tsunami

A

Cataclysmic eruption of Krakatoa.

Up to 35m run up. 36,700 killed

53
Q

What was the cause and effects of the Japan earthquake

A

Me= 9 megathrust earthquake.

Up to 40m high run up, 20,000 killed.

Natural Hazards (7 decks)

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