5: Tsunami Hazard

Question 1

Name the five generation methods of a tsunami

Answer 1

(1) Large sub-marine earthquake
(2) Large sub-marine slide
(3) Volcanic sector collapse into/under sea
(4) Explosive volcanic eruption into/under sea
(5) Meteorite/comet impact

Question 2

Name an example of a large sub-marine earthquake

Answer 2

Sumatra, 26th December, 2004

Question 3

Name an example of a large sub-marine slide

Answer 3

Storegga Slide, North Sea, ~8000 b.p.

Question 4

Name an location where volcanic sector collapse into/under the sea has happened previously

Answer 4

Hawaii/Canary Islands

Question 5

Name an example of an explosive volcanic eruption into the sea

Answer 5

Krakatoa, 26th August, 1883

Question 6

Name an example of a meteorite impact into the sea that was thought to be the cause of a tsunami

Answer 6

Chixulub impact, 65 ma

Question 7

What is the typical offshore velocity of a tsunami?

Answer 7

Up to 900km/hr

Question 8

What is the typical offshore amplitude of a tsunami?

Answer 8

<2m

Question 9

What is the typical offshore wavelength of a tsunami?

Answer 9

Hundreds of km

Question 10

What is the typical offshore hazard level of a tsunami?

Answer 10

Low – hardly noticeable by ships

Question 11

What is a typical wave period onshore for a tsunami?

Answer 11

5-60 minutes

Question 12

What is the leading wave and what does this look like at first impact onshore?

Answer 12

If the leading wave is a depression, the first wave on shore will be sucked right out into the sea.

Question 13

To correct the maximum double tsunami amplitude, what must be removed?

Answer 13

The superimposed tide signal

Question 14

When tsunamis make landfall, velocity is dependent on what?

Answer 14

Water depth

Question 15

What is the equation for velocity of a tsunami onshore?

Answer 15

v = √(gd)

Question 16

What is a typical velocity for a tsunami onshore?

Answer 16

Few 10’s km/h

Question 17

What is the equation for velocity of a tsunami offshore?

Answer 17

v = wavelength/period

Question 18

What is the relationship between amplitude and velocity of an onshore tsunami?

Answer 18

a2 x v ≈ constant

Question 19

What is the approximate wavelength of an onshore tsunami?

Answer 19

Tens of km

Question 20

What happens to the wavelength of a tsunami from offshore to onshore?

Answer 20

Decreases

Question 21

What happens to the hazard level of a tsunami as it approaches the shore?

Answer 21

Increases

Question 22

What is the definition of tsunami run-up?

Answer 22

The height above mean sea level that the wave reaches as it comes onshore

Question 23

Run-up is usually ___ than incoming wave height

Answer 23

Greater

Question 24

What is the definition of tsunami inundation?

Answer 24

Horizontal distance traveled inland measured from mean shoreline

Question 25

Run-up and inundation can vary considerably along a single coastline depending on local ___ and ____.

Answer 25

Bathymetry and topography

Question 26

On average, how many tsunamis a year will inflict damage on coastlines?

Answer 26

2

Question 27

How frequently does an ocean-wide tsunami tend to occur?

Answer 27

Every 15 years

Question 28

What is involved in probabilistic forecasting of tsunamis (two things)?

Answer 28

Identification of source zones and estimation of reoccurrence intervals based on historical/geological evidence

Question 29

What four main factors does tsunami modeling need to account for?

Answer 29

Local bathymetry, topography, wave diffraction, energy dispersion

Question 30

What are the two main ways in which we can assess hazard for tsunamis?

Answer 30

Probabilistic forecasting and tsunami modeling

Question 31

Give an example of erosional signatures given by tsunamis?

Answer 31

Impact marks, drill holes, v-shaped grooves, sculptured headlands

Question 32

Give an example of depositional signatures given by tsunamis?

Answer 32

Sand laminae, foraminifera/diatoms, boulders, dump deposits, ridges, mounds and dunes

Question 33

What are the four main risk mitigation strategies against tsunamis?

Answer 33

Land use planning, coastal defenses, preparedness plans, tsunami warning systems

Question 34

What are the magnitude brackets for the tsunami warning system, and what does each bracket imply as the best action?

Answer 34

6.5-7.0: gives information
7.1-7.5: on watch
7.6-7.8: on regional watch
>7.8: ocean-wide warning

Question 35

What are the four technological processes in detecting a tsunami?

Answer 35

Sea-floor pressure sensors acoustically transmit to moored buoys at surface, which relay the data to a satellite, which relays information to the ground stations.

Question 36

Seabed pressure sensors can detect a tsunami as small as what height?

Answer 36

1 cm

Question 37

What is the series of events on the ground involving tsunami warning from the initial earthquake detected?

Answer 37

Earthquake detected -> initial warning -> water level checked -> second warning

Question 38

Name a few ways that tsunami warning is communicated to the government and local people

Answer 38

Email, phone, text messages, radio, TV, sirens

Question 39

Name the six main limitations to tsunami warning systems

Answer 39

(1) False alarms
(2) Not in place in some countries such as Myanmar
(3) Inaccuracy in the system
(4) Close to epicenter may not receive warning in time
(5) People need to be aware of how to respond
(6) Tourists and remote communities biggest risk