Seismic events

Question 1

Outline how earthquakes are formed?

Answer 1

At all plate boundaries, plates move due to gravitational sliding, slab pull and convection currents. Plates get stuck due to friction and tension and pressure builds up. When plates suddenly jerk past each other or crack forming fault lines, this releases the pressure sending shockwaves through the earth’s crust as earthquakes.

Question 2

How are seismic events distributed?

Answer 2

Found along plate boundaries
Particularly: destructive and conservative plate boundaries.
The Ring of Fire accounts for 90% of the world’s Earthquakes.
The Alpine-Himalayan belt accounts for 5-6% of the world’s earthquakes.

Question 3

How does the depth of focus affect nature and magnitude of earthquakes?

Answer 3

Deep focus earthquakes tend to be higher magnitude than shallow focus earthquakes. Deep focus earthquakes generally do less damage than shallow focus earthquakes as shock waves have to travel further to reach the surface reducing their power.

Question 4

Why are earthquakes higher magnitude at destructive plate boundaries?

Answer 4

High pressure builds up between plates as subduction.

Question 5

Why are earthquakes lower magnitude at constructive plate boundaries?

Answer 5

Fracturing (as plates move apart), injection of magma and eruption is frequent. So, tensional stresses do not have time to accumulate.

Question 6

What are the three factors affecting the magnitude of earthquakes?

Answer 6

Type of plate boundary
Depth of focus
Rate of movement

Question 7

Give three reasons why earthquakes may occur away from plate boundaries?

Answer 7

Reactivation of old fault lines - potentially due to deferred stress release OR…Large dams and reservoirs - pressure on underlying rocks reactivates old faults
Hydraulic fracturing
Subsidence of old deep mines

Question 8

What is a body wave?

Answer 8

Travel through the earth can be divided into primary (p) waves or secondary (s) waves.

Question 9

What is a surface wave?

Answer 9

Travel along the Earth’s surface, these cause the most damage, as cause more ground movement and travel more slowly so take longer to pass

Question 10

What is a primary wave?

Answer 10

alternately compress and expand, so particle motion is parallel to direction of wave movement. Can travel through ALL substances, fastest.

Question 11

what is a secondary wave?

Answer 11

transverse, movement of particles is perpendicular (90 degrees) from movement of wave (up and down). Cannot travel through air or water, slower but cause more damage (greater amplitude).

Question 12

What is a Rayleigh wave?

Answer 12

transverse, movement of particles is perpendicular (90 degrees) from movement of wave (up and down). Cannot travel through air or water, slower but cause more damage (greater amplitude).

Question 13

What are loves waves?

Answer 13

horizontal shear waves, move the ground from side to side at right angles to the direction of the movement. This can damage infrastructure and buildings.

Question 14

What does the Richter scale measure?

Answer 14

Logarithmic scale measures the magnitude of an earthquake based on the amplitude of secondary waves.

Question 15

What does the Moment Magnitude Scale measure?

Answer 15

Logarithmic scale measures earthquake magnitude based on the total amount of energy released. Distance a fault has moved x force required to move it.

Question 16

What does the Mercalli scale measure?

Answer 16

Intensity of an event and its impact - 12 point scale, subjective.

Question 17

What is the epicentre of an earthquake?

Answer 17

Point on the earth’s surface directly above the focus - where the earthquake is first felt.

Question 18

What is the focus of an earthquake?

Answer 18

Point in the earth’s crust where the earthquake starts.

Question 19

What is the relationship between magnitude and frequency of seismic events?

Answer 19

Negative relationship - as magnitude increases frequency decreases.

Question 20

What are the two primary impacts of seismic events?

Answer 20

Ground rupture (displacement of the earth’s surface along fault lines).
Ground shaking

Question 21

Give three factors affecting the severity of ground shaking from seismic events.

Answer 21

Dependent on magnitude
Depth of focus
Distance from the epicentre

Question 22

How do earthquakes result in tsunamis?

Answer 22

Earthquakes vertically move the seabed up by several meters. Displacing (moving) the water above. The greater the movement of the sea floor the greater the volume of water displaced and the bigger the wave produced - hence greater intensity at destructive plate boundaries as higher magnitude earthquakes.Large waves radiate outwards across the ocean away from the epicentre of the earthquake. In deep water no energy is lost to frictional drag with the seabed.When it reaches shallow water near the coast, as it approaches the shore, the water becomes shallower and the base is slowed down by friction, forcing the circular wave motion into an elliptical form, which heightens until it can no longer be maintained and breaks. This is called shoaling.A large wave hits the coast, a tsunami.

Question 23

How does the height of the wave affect the intensity of the impacts of a tsunami?

Answer 23

due to the level of movement of the sea floor - the greater the movement the greater the volume of water displaced and the bigger the wave produced.

Question 24

How does the distance the wave travel affect the intensity of the impact of a tsunami?

Answer 24

waves lose energy as they travel towards the land, so the closer to the coast the waves start, the less energy they will lose, therefore tsunamis tend to be more powerful when they start closer to the coast.

Question 25

How does the shape of the coastline affect the intensity of the impact of a tsunami?

Answer 25

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 26

How does the relief of a coastline affect the intensity of a tsunami?

Answer 26

Cliffs present a natural barrier to a tsunami.

Question 27

How does the presence of natural defences affect the intensity of a tsunami?

Answer 27

Coral and mangroves act as natural defences by dissipating wave energy through their large surface areas.

Question 28

How does the population density affect the the intensity of a tsunami?

Answer 28

High population density - greater intensity. Also, the young and old are the most vulnerable, and there may be a gender disparity.

Question 29

What is liquefaction?

Answer 29

Weakening of water saturated sediment during an earthquake causing it to act as a liquid.

Question 30

What dies liquefaction lead to?

Answer 30

erptions of pressurised water and sand at the surface called sandblow - may result in localised flooding.

Question 31

What conditions worsen the impacts of liquefaction?

Answer 31

Geology - loose sand and silt within 10m of the earth’s surface.Water table close to the surfaceLack of deep foundations to buildings. (other ways to reduce the risks associated with liquefaction include: stabilising the ground and improving water drainage).

Question 32

How do seismic events result in landslides (and avalanches)?

Answer 32

The earthquake destabilised a hillside and caused it to fall - e.g. 2008 Wenchuan earthquake caused about 45, 000 separate landslides.

Question 33

Why are landslides induced by earthquakes a hazard?

Answer 33

Cause debris flows

Debris flows may result in natural dams and resultant flash flooding when dams break.

Question 34

What are the four secondary hazards of seismic events?

Answer 34

Tsunamis
Landslides
Liquefaction
Fires

Question 35

Is prediction effective for seismic events?

Answer 35

No- although regions generally at risk can be identified using plate tectonics it is difficult to know when a hazard will occur.

Question 36

Give three methods used when attempting to predict seismic events?

Answer 36

Monitoring of groundwater levels
Release of radon gas
Unusual animal behaviour

Question 37

What is seismic gap theory?

Answer 37

Theory that over the long run all parts of the fault must average about the same level of movement per time. This can either happen through the cumulative efforts of a very large number of very small earthquakes. Therefore, 1 large and long standing ``seismic gap’‘suggests that a significant earthquake should be expected.

Question 38

Give two methods used to attempt to prevent earthquakes?

Answer 38

Nuclear explosions at depth

Pumping water/oil into a fault line to prevent plates from sticking

Question 39

Give three methods of creating hazard resistant structures

Answer 39

Concrete weights on the top of buildings to move in the opposite direction ti the force of the earthquake to counteract stress (with computer programme)
Rubber shock absorbers in the foundations
Cross bracing the structure to hold it together better when it shakes

Question 40

Give two pieces of evidence demonstrating the effectiveness of hazard resistant structures.

Answer 40

The comparison between the 1989 Loma Prieta earthquake 6.9MMS and 1988 6.8 MMS Armenia earthquake shows the effects of different types of buildings. The greater earthquake proof buildings in California resulted in only 63 deaths whereas in Armenia over 25,000 people died, many inside buildings that collapsed as a result of soft foundations and no earthquake proofing features. Haiti - substandard building design, shoddy construction and inasquate materials, no building code 7.0 magnitude 12.01.2020 - 230, 000 killed and 3 million affected vs Chile 2010 8.8 MMS 521 people killed.

Question 41

Give two factors limiting the effectiveness of hazard resistant buildings in reducing risk to a population.

Answer 41

Need to retrofit even if new strict building regulations are put in place the older buildings remain vulnerable to an earthquake - e.g. Kobe 1995 Japan where it was mainly due to the collapse of older wooden buildings that 6,300 people were killed that were the homes of the poorest people.Need to enforce building regulations effectively e.g. 1985 Mexico city where many multi-storey buildings collapsed leading to a total death toll of 30,000 - building regulations had not been enforced

Question 42

What doesFEMA stand for?

Answer 42

Federal emergency management agency’s (FEMA)

Question 43

Give the four objectives of FEMA to reduce hazard risk in the USA.

Answer 43

To promote understanding of earthquakes and their effects
To work to better identify earthquake risk
To improve earthquake-resistant design and construction techniques
To encourage the use of earthquake safe policies

Question 44

How does education minimise the risk of seismic hazards?

Answer 44

minimise loss through use of drills (so the population knows how to respond effectively reducing injury/death - e.g. staying away from buildings where possible and finding strong door frames etc. to shelter under if inside) - Disaster Prevention Day (1st September) Japan. Ensure people have emergency supplies prepared - e.g. The Red Cross issued a list of supplies e.g. food stuff, clothing, first aid kits post 1989 Loma Prieta earthquake.

Question 45

How does fire prevention minimise the risk of seismic hazards?

Answer 45

smart meters cut off the gas if an earthquake of a sufficient magnitude occurs reducing the risk of death by fire. E.g. In Tokyo the gas company has a network that transmits seismic information to a computer which then informs employees where to switch off major pipelines.

Question 46

How does emergency planning minimise the risk of seismic hazards?

Answer 46

careful organisation to tackle the impacts of a hazard more efficiently and effectively reducing the intensity of impacts. E.g. heavy lifting gear needs to be available and many people should be given first aid training as it could be some time after an event that trained medical personnel can arrive. Much of the preparation in California involves the establishment of computer programs to identify which areas the emergency services should be sent to first.

Question 47

How does early warning systems/ tsunami protection minimise the risk of seismic hazards?

Answer 47

while not entirely predictable automated systems can give out warnings if foreshocks or if undersea earthquakes increase tsunami risk so people can take evasive action. E.g. The pacific warning system.

Question 48

How does sea walls minimise the risk of seismic hazards?

Answer 48

prevention walls along the coast to protect the land behind from oncoming tsunamis - e.g. Japan. N.b. often ineffective as large tsunamis are likely to overwhelm them.

Question 49

How does insurance minimise the risk of seismic hazards?

Answer 49

people take out insurance to cover their losses and help recover faster.

Question 50

How does land-use planning minimise the risks of seismic hazards?

Answer 50

Identifies the most hazardous areas and regulates land use - e.g. should not build hospitals etc. and should ensure sufficient open space as this forms a safe area away from fires and aftershock damage to buildings.

Question 51

What is micronzonation?

Answer 51

mapping out the variability of hazards through urban areas to identify areas that are more or less hazardous with respect to some geological and geophysical characteristics. As these will affect the likelihood/severity of: ground shaking, liquefaction susceptibility, landslides and flooding.

Question 52

What factors should be considered when land-use planning?

Answer 52

Known fault
Geology - soft sediment layers can amplify wave magnitudes and result in liquefaction (1985 Mexico City)
Topography - e.g. Port Au Prince Haiti 2010 some of the most dramatic damage was associated with amplification by small scale topographical features such as hills and ridges.

Question 53

Describe the frequency of seismic events

Answer 53

Earthquakes are frequent around the world and occur every day at boundaries. N.b. Negative relationship between magnitude and frequency.

Question 54

Describe the regularity of seismic events

Answer 54

Earthquakes follow no pattern and are random so there is irregularity between events.

Question 55

Describe the predictability of seismic events

Answer 55

Earthquakes are almost impossible to predict. Microquakes may give some indication but the magnitude cannot be predicted as how strong they are is random - see above for methods used