Earthquakes

Question 1

Outline the parts of an earthquake

Answer 1

Focus - the point which this pressure release occurs within the crust - 3 categories:
1) shallow focus - 0-70km deep - tend to cause the greatest damage and account for 75% of all the earthquake energy released
2) intermediate focus - 70-300km deep
3) deep focus - 300-700km deep

Epicentre - the point immediately above the focus on the earth’s surface

Question 2

How do earthquakes occur on conservative boundaries

Answer 2

because of friction, the plates cannot simply glide past each other. Rather, stress builds up in both plates and when it exceeds the threshold of the rocks, the energy is released – causing earthquakes

Question 3

How do earthquakes occur on constructive boundaries

Answer 3

Oceanic divergence forms chains of submarine mountain ridges and regular breaks called transform faults cut across the ridges. These faults occur at right angles to the plate boundary and may widen at different rates, leading to frictional stressed shallow focus earthquakes.

Question 4

How do earthquakes occur at destructive boundaries

Answer 4

The descending oceanic plate starts to melt - this zone of melting is called the Benioff zone. Melting is the result of increased heat at depth and friction. This friction my also lead to tension building up, which may be suddenly released as an earthquake.

Question 5

What are the seismic shockwaves

Answer 5

• Primary or pressure (P) waves are the fastest and reach the surface first. High frequency. They travel through both the mantle and core to the other side of the Earth.
  
  • Secondary or shear (S) waves are half as fast and reach the surface next. They are also high frequency but shake.They can travel through the mantle, but not the core, so cannot be measured at a point opposite the focus or epicentre.
  • Surface Love (L) waves are the slowest and cause most of the damage.
  • Rayleigh (R) waves radiate from the epicentre in complicated low frequency rolling motions.

Question 6

How is the magnitude and frequency of earthquakes measured using the richter scale?

Answer 6

Richter scale - logarithmic scale - each number 10x the magnitude of the one before it, slight increase in value causes great effect on ground - major earthquakes = above 7

Question 7

How is the magnitude and frequency of earthquakes measured using the moment magnitude scale?

Answer 7

• based on total amount of energy released by an earthquake - logarithmic - no upper limit - more accurate than richter scale especially for larger earthquakes

Question 8

How is the magnitude and frequency of earthquakes measured using the mercalli scale?

Answer 8

• measures impacts of an earthquake using observation of the event - scale 1-12, 1=earthquake only detected by instruments, 12= earthquake that causes total destruction

Question 9

Advantages and disadvantages Richter scale

Answer 9

ADV
- It provides accuracy in comparing the strength of earthquakes and the location
- Mathematical means - quantitative data - not subjective
- Records both surface and body waves

DISADV
- Only valid for certain frequency and distance ranges
- Falls between 6-9 - not always easy to tell the difference
- Values above 8 on the Richter scale would send seismographs into a frenzy
- Sometimes logarithmic nature of each level is lost because many people don’t realise the significance of the differences between levels

Question 10

Advantages and disadvantages of moment magnitude scale

Answer 10

ADV
- Its more accurate in calculating earthquakes over magnitude 8 than Richter scale - more effective record
- More effective with the higher magnitude earthquakes

DISADV
Inaccurate for small earthquakes

Question 11

Advantages and disadvantages of mercalli scale

Answer 11

ADV
- More effective representing the earthquake’s impact on people and therefore may be easier for people to understand the impact of different earthquakes
- Easier to compare impacts of earthquakes

DISADV
- Subjective - different people might interpret it in different ways
- Only measure impacts not the strength - cant compare earthquakes - lower magnitude earthquakes may have greater impacts depending on location

Question 12

How are tsunamis created?

Answer 12

• large waves caused by shallow focus earthquakes, volcanic eruptions, underwater debris slides and large landslides into the sea
  -long wave length, low wave height in the open ocean and travel quickly - over 700km per hour but when reaching shallow water bordering land, increase rapidly in height

Question 13

How do earthquakes trigger tsunamis?

Answer 13

Tsunamis are large waves caused by the displacement of large volumes of water and can be triggered by underwater earthquakes causing the seabed to move which displaces water. Waves radiate out from the epicentre of the earthquake and the greater the movement of the sea floor, the greater the volume of water displaced and the bigger the wave produced

Question 14

Tsunami warnings

Answer 14

Quite often the first warning given to coastal populations is the wave trough in front of the tsunami which results in a reduction in sea level known as drawdown. The tsunami follows. The event usually consists of a number of waves, the largest not necessarily being the first.

Question 15

Where are most tsunamis found

Answer 15

around 90% generated within the Pacific Basin all associated with tectonic activity taking place around its edges. Most generated at convergent plate boundaries - subduction taking place particularly on Western side of the Pacific and the bordering eastern side of the Indian ocean.

Question 16

Describe the process of liquefaction

Answer 16

When soil is saturated with water, the vibrations of an earthquake can cause it to act like a liquid. This makes the soil weaker and easier to deform so it’s more likely to subside, especially where it has a heavy weight on top of it such as a building

Question 17

How do earthquakes cause landslides and avalanches

Answer 17

Shaking of the ground can dislodge rock, soil or snow, causing landslides or avalanches that move downslope quickly. Shaking can also loosen ground material, making it easier for water to infiltrate. The weight of the extra water may trigger a landslide even after ground shaking has stopped.

Question 18

How can earthquakes be predicted?

Answer 18

Earthquake forecasting
- identify particular areas of risk and make probabilistic forecasts about the likelihood of earthquakes happening in a specified area over a specified period.
- These forecasts are based on data gathered through global seismic monitoring networks, high-density local monitoring in known risk areas, and geological field work, as well as from historical records
- can’t make deterministic predictions as need a diagnostic precursor - a characteristic pattern of seismic activity or some other physical, chemical or biological change, which would indicate a high probability of an earthquake happening in a small window of space and time

Question 19

Why are earthquakes difficult to predict?

Answer 19

• Most earthquakes result from the sudden release of stress in the earth’s crust, which has built up gradually due to tectonic movement, usually along an existing geological fault.
• crust response to changing is not linear and dependent on geology
• Laboratory experiments which attempt to reproduce these physical processes can add to our understanding, but cannot accurately reflect the complexities of real-world geological settings.
• originate beneath the ground, often many kilometres down, so data gathering depends on remote observation techniques and measuring effects at the surface.
• earthquakes tend to occur in sequences but the patterns of these vary greatly

Question 20

How can communities prepare for earthquakes?

Answer 20

Education
→ Instructions issues by authorities in how to prepare for such events by securing homes, appliances and heavy furniture and assembling ‘earthquake kits’
→ Earthquake drills in schools, offices and factories
→ Issue a list of supplies that people should have at hand in case of an earthquake - three day supply of water, range of foodstuffs, clothing and bedding, first aid kit, and tools and supplies - this was issued following the Loma Prieta event (1989) by American Cross

Emergency services
→ Heavy lifting gear needs to be available
→ Many people should be given first aid training as it could be some time after the event that trained medical personnel arrive
→ Much of the preparation in California involves the establishment of computer programmes that identify the areas the emergency services should be sent to first

Land use planning
→ Certain types of buildings should be put in areas of low risk e.g. schools and hospitals
→ Important to have sufficient open space, as this forms a safe area away from fires and aftershock damage to buildings

Question 21

How can earthquakes be mitigated against?

Answer 21

• Evacuation is rarely an effective strategy for mitigating earthquake hazard, because it is not possible to predict the precise timing, location and size of an earthquake to a sufficiently high degree of accuracy.
• However, early warning systems can be used to minimise risk to people and property.
• Currently, the rail system in the San Francisco Bay Area (BART) uses an earthquake early warning system to automatically slow trains when an earthquake occurs. The system, called ShakeAlert, is now being implemented in other states.
• Falling buildings are by far the greatest cause of casualties during earthquakes = earthquake proof buildings - but this is much more expensive than doing so at the time of construction.
• During the past 30 years, engineers have constructed skyscrapers that float on systems of ball bearings, springs and padded cylinders. Acting like shock absorbers in a car, these systems allow the building to be decoupled from the shaking of the ground.
• Another technique to dampen the swaying of a tall building is to build in a large (several tons) mass that can sway at the top of the building in opposition to the building sway. Known as “tuned mass dampers”, these devices can reduce the sway of a building up to 30 to 40 percent.

Question 22

how does the magnitude and depth of the earthquake affect consequences?

Answer 22

the higher the magnitude the more severe consequences will be as there is more energy, however the deeper earthquakes are less damaging because their energy dissipates before it reaches the surface and seismic waves travel less distance

Question 23

how does the distance from the epicentre affect consequences?

Answer 23

the effects are more severe closer to the epicentre - Seismic energy is lost as waves travel through the Earth, so the further you are from the epicentre of an earthquake, the less shaking you will feel

Question 24

how does the geological conditions affect consequences?

Answer 24

The nature of the ground at the surface of an earthquake can have a profound influence on the level of damage. Loose, sandy, soggy soil can liquefy if the shaking is strong and long enough - liquefaction

Question 25

how does the population density, preparedness and education affect consequences?

Answer 25

The more densely populated an area, the more likely there are to be deaths and casualties. Being prepared and educated can reduce the fear, anxiety and losses that accompany disasters. Communities should know what to do in the event of an earthquake and where to seek shelter during a fire. They should be ready to evacuate their homes and take refuge in public shelters and know how to care for their basic medical needs.

Question 26

how does the design and strength of buildings affect consequences?

Answer 26

Simple changes reduce the cost of repair and minimise risk of injury. For example, houses are built with steel rod foundations which can bend and yield in an earthquake, which reduces the risk of collapse. Reduces financial losses and lives lost.

Question 27

How does the time of day affect consequences?

Answer 27

a severe earthquake at night means many more people might be trapped in their houses or worse killed by their collapsing homes. Similarly, an earthquake during rush hour can see many trapped, injured or killed in cars or trains.