Earthquakes and tsunamis

Question 1

On average, how many people die per year because of earthquakes?

Answer 1

10,000 (although this can vary a lot)

Question 2

What in earthquakes causes the ground to shake?

Answer 2

When plates move against each other, they sometimes stick, causing large amounts of pressure. When the pressure is too much, the rock fractures along faults, and the energy is suddenly released as seismic waves, causing the ground to shake

Question 3

What is the name of the point inside the crust from where pressure is released?

Answer 3

The focus, or the hypocentre

Question 4

What is the name of the point on the surface directly above the point where pressure has been released?

Answer 4

epicentre

Question 5

What are the three different types of seismic waves?

Answer 5

Primary waves (body waves- travels through the earth’s body)
Secondary waves (body waves)
Love waves (surface waves -travel along the surface)

Question 6

What are P waves like?

Answer 6

Primary waves are the fastest and first to reach the surface. In solids, they are generally twice as fast as S waves, traveling at about 8km/sec
-They travel through both solids and liquids
-They are compressional waves so shake in a backwards and forwards motion
-They are only damaging in the most powerful earthquakes

Question 7

What are S waves?

Answer 7

Secondary waves are slower than primary waves - they travel at around 4km/sec
-They only travel through solids (unlike P waves which can travel through liquid), meaning they cannot travel through the outer core
-They move in a sideways motion, shaking at right angles to the direction of travel, because its a transverse wave
-They do more damage than P waves

Question 8

What are L waves?

Answer 8

Love waves are the slowest, but cause the most damage as they shake the ground from side to side. They are larger (have a large amplitude) and focus all their energy on the earth’s surface

Question 9

What is the destructiveness of a wave dependent on?

Answer 9

It’s amplitude and frequency

Question 10

What are the two different things to consider when measuring an earthquake?

Answer 10

The magnitude - the amount of energy released by the event
Intensity- a measure of the ground shaking and the effect it has on people, structure and the natural environment

Question 11

What measures can be used for measuring the magnitude of earthquakes

Answer 11

Moment magnitude scale (generally preferred) or the Richter scale

Question 12

How does the moment magnitude scale work?

Answer 12

It measures the total energy released by an earthquake at the seismic moment (moment it occurs), using the:
-size of seismic waves
-amount of slippage or rock movement
-area of fault surface broken by the earthquake
-resistance of the rocks affected
The scale goes from 1 as the smallest and is infinite, but generally stops at 10. The scale is logarithmic (each number is ten times the magnitude of the number before)

Question 13

What was the largest earthquake ever measured on the moment magnitude scale?

Answer 13

a magnitude of 9.5 in Chile in 1960

Question 14

What are the positives of the moment magnitude scale?

Answer 14

-accurate at measuring infinitely large earthquakes
-uses a variety of indicators

Question 15

What are the negatives of the moment magnitude scale?

Answer 15

Accurate for only the largest earthquakes and doesn’t consider impacts for people

Question 16

How is earthquake intensity measured?

Answer 16

Using the Modified Mercalli Intensity Scale

Question 17

What is the Modified Mercalli Intensity Scale?

Answer 17

This takes observations from people who experienced the earthquakes and rates them on a scale from I (hardly noticed) to XII (catastrophic). It’s a relative scale because people experience different amount of shaking in different places

Question 18

What are the positives of the Mercalli scale?

Answer 18

It uses real examples and assess intensity, which may be more useful than a simple statistic of magnitude

Question 19

What are the negatives of the Mercalli Scale?

Answer 19

It’s subjective, because it doesn’t use quantitative data and it’s open to bias. It also is dependent on the built up area, so it’s difficult to compare

Question 20

What is the Richter scale?

Answer 20

It’s a measurement of the amplitude of the waves produced by an earthquake, where the largest wave amplitude on the seismograph is recorded.
It’s an absolute scale: wherever an earthquake is recorded, it will measure the same on the Richter Scale and is logarithmic

Question 21

What are the positives of the Richter Scale?

Answer 21

-accurate
-easy to understand

Question 22

What are the negatives of the Richter Scale?

Answer 22

an absolute scale so doesn’t take into account other things such as intensity
Logarithmic may cause it to lose accuracy

Question 23

What are the primary effects of an earthquake?

Answer 23

-ground shaking, which causes buildings, bridges, roads and infrastructure to collapse
-crustal fracturing- when energy released during an earthquake causes the Earth’s crust to crack

Question 24

What are the secondary effects of an earthquake?

Answer 24

Liquefaction
Landslides and avalanches
Tsunamis

Question 25

What is liquefaction?

Answer 25

The violent shaking during an earthquake causes surface rocks to lose strength and become more liquid than solid. The subsoil loses its ability to support building foundations, so buildings and roads tilt or sink. It can make rescue efforts more difficult, and disrupt underground power and gas lines (leading to fires)

Question 26

How are landslides and avalanches caused?

Answer 26

The ground shaking places stress on slopes, so that they may fail (resulting in landslides and avalanches). Many of these effects account for a large proportion of the damage and injuries caused by an earthquake

Question 27

How much of an impact do landslides have?

Answer 27

Over the last 40 years, around 70% of earthquake caused deaths are caused by landslides

Question 28

What’s an example of geology having a major impact in an earthquake?

Answer 28

1989 earthquake in San Francisco, with the Marina District suffering the worst damage. Built on a man-made landfill, the area’s soft, sandy soil amplified ground shaking, increasing damage of buildings and causing liquefaction. The Cyprus Freeway also collapsed, causing 42 of the 67 earthquake related deaths, as the part of the freeway that collapsed was built on soft mud, whereas adjoining parts built on firmer ground remained standing

Question 29

What are aftershocks?

Answer 29

They are the result of the Earth ‘settling down’ or readjusting along the part of the fault that slipped at the time of the main earthquake. They can happen weeks, months or even years after the initial earthquake. In general, the larger the earthquake, the larger and more numerous the aftershocks

Question 30

What’s an example of an aftershock that caused large levels of damage?

Answer 30

The 2011 magnitude 6.3 aftershock that struck Christchurch New Zealand caused more damage and loss of life than the initial 2010 earthquake. This was because it was very shallow focused and much closer to the city itself

Question 31

Can an earthquake be accurately predicted?

Answer 31

No
All we know is that earthquakes happen along plate boundaries and areas that have had one big one and likely to have another - they can predict where but not when. For example, there is a 67% chance of another serious earthquake striking San Francisco in the next 30 years

Question 32

What is the depth of earthquakes at constructive boundaries?

Answer 32

shallow

Question 33

Example of a constructive ridge?

Answer 33

East African Rift Valley

Question 34

What is the depth like of destructive earthquakes?

Answer 34

Intermediate and deep earthquakes occurring in the subduction zone, also known as the Benioff zone

Question 35

What is a strike-slip fault?

Answer 35

The two plates move past each other (San Andreas Fault

Question 36

What is a normal fault?

Answer 36

The two plates are being pulled apart due to tension and the hanging wall moves downward relative to the foot wall (Constructive plates are always normal faults)

Question 37

What is a reverse fault?

Answer 37

The hanging wall moves up the foot wall as a result of compression

Question 38

What is the main cause of tsunamis?

Answer 38

Most are caused by large underwater earthquakes along subduction zones. Energy released causes the sea floor to uplift, displacing the waver column above.
They can also be caused by underwater volcanoes or landslides

Question 39

Where do tsunamis occur?

Answer 39

Because they’re linked to tectonic events, they tend to occur along plate boundaries - particularly the Pacific Basin’s Ring of Fire

Question 40

What is stage one of tsunamigenesis?

Answer 40

Generation of a tsunami in deep ocean. As one plate is subducted, strain causes one plate to become distorted. This eventually ruptures, releasing energy in an earthquake and displaces a large amount of sea water

Question 41

Discuss the second stage of tsunamigenesis

Answer 41

This is the tsunami run up
Waves radiate from the source in all directions. They have a long wavelength and small wave height; they travel very quickly
The waves get smaller as they travel further away from the epicentre

Question 42

What are the characteristics of waves like in the run-up stage?

Answer 42

Wave height: 0.5-5m
Wave length: 150-250m
Usually travel at around 805km per hour

Question 43

What factors affect the tsunami run up?

Answer 43

-cause of the wave e.g. eruption or earthquake
-distance travelled from source because energy is lost as they travel
-water depth affects energy loss through friction
-offshore topography and coastal orientation

Question 44

What is the third stage of tsunamigenesis?

Answer 44

Landfall
As the water shallows, waves:
-slow down due to friction
-increase in height (up to 30m)
-wave energy is crowded into a smaller volume of area, which is what causes the wave to increase in height

Question 45

What is the landfall event like?

Answer 45

May last for several hours
4th or 5th wave is often the largest one
40% of wave energy is scattered back to sea, while 60% is expended at or near the coast

Question 46

What factors impact the destruction level of a tsunami?

Answer 46

-physical landscape
-drainage
-presence of ecosystem protection, such as mangroves and coral reefs
-population density
-infrastructure and land use

Question 47

How does tsunami early warning systems work?

Answer 47

They use seismic sensors to detect deep underwater earthquakes. But not all earthquakes cause tsunamis so they use a system called DART (Deep-ocean Assessment and Reporting of Tsunami). This uses seabed sensors and surface buoys to monitor changes in sea level and pressure. When tsunami waves are detected, the system sends this information to land via satellite to be reviewed by scientists who use computer modelling to estimate the size and direction of tsunami
Those at risk are then informed

Question 48

What are 2 examples of DART being used?

Answer 48

The Pacific Warning system in Hawaii gives warnings to countries around the Pacific rim
In 2011, after a 9.0 earthquake, the system failed as the size of the earthquake was underestimated. As a result, some people thought the tsunami was small and so didn’t prepare

Question 49

How can the tsunami event be modified?

Answer 49

There are no technologies to prevent or divert tsunamis
Research has found planting around coasts can reduce the impact as coastal mangroves create a buffer zone and dissipate energy

Question 50

How can the loss of a tsunami event be modified?

Answer 50

Insurance

Question 51

How can modify vulnerability before a tsunami?

Answer 51

-warnings given so people can evacuate
-land use planning
-hard engineering and earthquake proof buildings.