Seismic Hazards

Question 1

Define the focus

Answer 1

Point in the earth’s crust/lithosphere at which the earthquake occurs - where the plates suddenly slips and the energy is released

Question 2

Define the epicentre

Answer 2

The point at which the seismic waves reach the earth’s surface

Question 3

Define shockwaves (seismic waves)

Answer 3

Waves of energy released form the jolting/slipping of plates

Radiates from the focus and causes the earth’s crust to shake

Question 4

Define aftershocks

Answer 4

Usually smaller earthquakes that occur after the main seismic event.

Plates readjust back to their original state causing (usually) smaller earthquakes in the area - minuets, days, weeks post event

Question 5

What is the Seismic Gap Theory

Answer 5

Predicting the relative size and frequency of an earthquake in a given area, depending on the size and frequency of previous earthquakes in the area

(Areas which experience many small earthquakes will not likely experience a large one)

Question 6

What percentage of earthquakes occur on the Circus Pacific Belt

Answer 6

80%

Question 7

How are earthquakes formed

Answer 7

Plate movements produce huge levels of energy and their movements are not smooth, causing high levels of friction

Friction along the plate margins builds up stress in the lithosphere

When the strength of the rocks under stress suddenly overcome the elastic strength of the rock, they fracture along cracks (faults) sending a series of seismic shockwaves to the surface

A lot of the energy is transferred vertically to the surface and then moved outwards from the epicentre

Question 8

Other than the main cause of plate tectonics, how else are earthquakes formed

Answer 8

Mining
Fracking
Reservoir construction

Question 9

What was the most powerful earthquake ever recorded

Answer 9

Chilie 1960

9.5 on moment mag scale and 80ft tsunami

Question 10

How do earthquakes form at destructive boundaries (Collision)

Answer 10

Two continental plates collide.

Frictional stresses build up and energy is released suddenly in the from of an earthquake as the plates slip/jolt

Question 11

How do earthquakes form at destructive boundaries (Subductive)

Answer 11

O + C plate collide

As the Oceanic plate subducts, it grinds on the Continental crust. Huge friction and pressure builds and is eventually released

Continental plate can ‘ping’ upwards to form a tsunami

Question 12

How do earthquakes occur at conservative plate margins

Answer 12

Plates move in opposite direction or in the same direction at different speeds

Build up of pressure as the plates get stuck as they grind against each other, high levels fo friction

Plates suddenly slip along the strike slip fault Lin e

Energy is released which causes an earthquake

Question 13

How do smaller earthquakes form at constructive plate margins

Answer 13

As the plates move apart, they will vibrate and move which causes minor earthquakes

Little energy or pressure is built up so minor vibrations

Question 14

Why is the depth of the focus significant

Answer 14

Shockwaves have more distance to travel at deep focus earthquakes, therefore energy is disputed and reduced by the time it reaches the surface (epicentre)

Shallow focus - shockwaves have less distance to travel, so the earthquake is more powerful at surface, damage increased

Question 15

Give the depth of a shallow focus earthquake

Answer 15

0-70 km

Question 16

Give ht depth of a deep-focus earthquake

Answer 16

300-700km

Question 17

Give the two types of Seismic waves/shockwaves

Answer 17

Body waves (travel through the earth). Can be subdivided into Primary (P) and Secondary (S) waves

Surface waves (travel along the Earth’s surface. Can be subdivided into Love and Rayleigh waves

Question 18

Which type of seismic wave is more deadly

Answer 18

Surface waves cited to be most hazardous as they tend to cause the moist damage to buildings which collapse and kill people

Question 19

What is ground rupture

Answer 19

Movement in a. fault breaks through to the surface. Ruptures may occur suddenly during an earthquake of gradually in the form of fault creep

Fault ruptures almost always follow pre-exisiting faults which are zones of weakness

Question 20

Describe Primary Waves

AKA Compression/pressure waves

Answer 20

Fastest and reach the surface first (first to eb decade by seismometers)

High frequency and travel through both solid and liquids (mantle and core) to the opposite side of the earth at speeds of 5.5km/s

Rocks move back and forth and cause the ground to break up (slinky diagram)

Question 21

Describe Secondary Waves

AKA Shear/transverse waves

Answer 21

Half as fast as P waves and reach surface next

High frequency and can travel through solids but not liquids

Travel at speeds of 3km/s

Cause material to shake at right angles to the direction of trade travel. The greater the displacement the greater the amplitude of the wave.
Cause ground to shake up and down

Question 22

Describe love waves

Answer 22

Slowest of the seismic waves and cause the most damage

Shaking is at right angles to the direction of movement

Moves ground side to side, damaging infrastructure and buildings

Question 23

Describe Rayleigh waves

Answer 23

Waves radiate from the epicentre in complicated low frequency rolling motions

Produce both horizontal nd vertical ground movements (like at sea)

Rocks move in an ellipsis as the wave passes and breaks up the surface

Question 24

What might affect the way in which surface waves travel

Answer 24

Type of material they re travelling through

Unconsolidated sediment (sand/silt) can cause seismic waves to be amplified, less so than shaking of solid bedrock

Question 25

How are tsunami’s formed (5)

Answer 25

Earth’s crust separated into masses of tectonic plates that spread apart ad collide, creating earthquakes (submarine)

The energy released from an earthquake can be so powerful that it lifts millions of tonnes of water above it, casing tsunami waves

Tsunami waves move in big flat waves across the open ocean - in the deep ocean these waves are barley noticeable at the surface. Travels 550mph+, with a time period of 10-60 mins between each wave

As they approach shallower water, they slow down, become compressed and grow in hight

Sudden retreat/drawback of water from shore

Question 26

What are some human impacts of a tsunami

2004 Indonesia

Answer 26

Widespread homelessness (500,000 faced int refugee camps in Achech province, Indonesia)

Economies devastated - fishing, agriculture and tourism. 44% in Achech province lost their livelihoods

Loss of deep sea trawlers resulted in fewer catches. The few fish that are caught are less likely to reach market because of damaged infrastructure

Water supplies and soil contaminated by seawater

Gap between rich and poor increases

Question 27

Effects of tsunami’s vary accordingly to its……and…………….but are also influenced by the population……………, costal relief and…….use of the costal region

Answer 27

Size
Displacement
Density
Land

Question 28

Describe liquefaction

Answer 28

Occurs in unconsolidated sediments that are saturated with water.

Shaking moves the sediment material apart, soil stability declines and soil behaves like a liquid due to an increase in water pressure

Question 29

Describe the key features of landslides

Answer 29

Occurs as a result of the ground shaking, causing particles and sediment in the soil to become loose, causing slope failure - even on gentle slopes

Involves a block of rock moving very rapidly downhill along a planar surface.
Unlike a mudflow where the moving material becomes mixed, the block of material remains largely intact

Question 30

What are the impacts of landslides

Answer 30

Further deaths and injuries

Flooding from blocked rivers creating quake lakes

Permanent disruption to natural drainage patterns

Loss of farmland for food production

Infrastructure and equipment destroyed

Question 31

What is the richer scale

Answer 31

Formula based on the amplitude of the largest recorded seismic wave and the distance between the earthquake focus and the seismometer

Richter scale is logarithmic, so magnitude 7 is 10x larger than magnitude 6

Energy released is proportional to the magnitude

Question 32

What is the Mercalli scale

Answer 32

Qualitative descriptor scale of earthquakes, based on obersvatiosn by humans on the amount of damage that has been caused nearest to the epicentre

Question 33

Give some pros of the Mercalli scale

Answer 33

Reflects the impacts, easily comparable

Clear parameters for each level

Internationally used, easily accessible, doesn’t require specific skills/specialised equipment

Tells us where aid needs to be directed

Question 34

Give some cons of the Mercalli scale

Answer 34

Subjective - may not be a standardised scale that can be compared easily

Outdated (last updated 1956), technology and development has since increased in durability + tech

Doesn’t tell us the magnitude

Only works in urban areas, with witnesses and infrastructure

Upper limit

Question 35

Give some Pros of the Richter scale

Answer 35

Useful form small scale earthquakes

Doesn’t have an upper limit

Non subjective, easy to classify and compare.

Question 36

Give some cons of the Richter scale

Answer 36

Less useful for larger earthquakes over mag 8

Doesn’t show the amount of energy released

Doesn’t measure the impacts/damage caused

Requires skilled workers and expensive scientific equipment - not accessible to everyone

Question 37

What is the Moment Magnitude Scale

Answer 37

Most commonly used scale globally - adaptation of the Richter scale

Combines the amount of movement on the fault, rock strength, and size of the rupture area

Measures the total moment release of the earthquake where moment is a products of the distance the fault has moved, and the force required to move it

Question 38

Gove some Pros of the Moment Magnitude Scale

Answer 38

For events greater than magnitude 8, significantly more accurate

Most commonly used - sea easily comparable world-wide

Enhances understanding of the characteristics of the earthquake (future area etc)

Accurately calculates the time, location, energy released and magnitude

Question 39

Describe the pattern of the largest earthquakes in th eWorld

Answer 39

Nearly all major earthquakes occur along plate boundaries (Notably the Pacific Ring of Fire)

Mostly at destructive plate boundaries (subjective

Some clusters around mountain ranges (Andes, Himalayas)

Large concentration of earthquakes on the Alpide Bet

Question 40

Frequency and regularity of earthquakes shows no……………….

An understanding of plate tectonics allows seismologists to know which areas are at risk, however it is…………….to predict when or……….a seismic disaster will strike

Answer 40

Predictability

Impossible
Where

Question 41

Is earthquake activity increasing

Answer 41

NO

On average there are about 15 earthquakes every year with a. magnitude of 7+

As with any random phenomena, the number of earthquakes every year varies slightly, but in general there are no dramatic variations

Question 42

Why might it seem like there a more earthquakes recently (5)

Answer 42

Technology - Recording and prediction constantly being improved - more earthquakes are now being recorded

Earthquake clustering - although long term averages ae fairly constant, in any quasi-random process, you get clustering in time. People notice the clusters, they don’t notice the gaps in between

Population increases mean that more people than ever live in earthquake prone regions. So although number of earthquakes remain the same, the impact increases

Earthquakes in populated areas, far more apparent than those in rural areas

Global communication - now have near instant pictures of eventuating earthquakes worldwide. More people are aware of the earthquake and it impact

Question 43

Can we predict earthquakes (yes)

Answer 43

Geoscientists able to identify particular areas at risk and make probabilistic forecasts about the likelihood of earthquakes happening in a specified area over a specified period

These forecasts are based on date gathered through local seismic monitoring networks, local monitoring, geological fieldwork and historical records

Forcsts are improved as our theoretical understanding of earthquakes grow

Long term forecasts (years to decades) are currently much more reliable than short term forecasts

Question 44

Give some main short-term predictors of earthquakes and the issue with this

Answer 44

Micro-quakes before main tremor

Bulging of the ground

Rising levels of radon gas in concentrations of water

Electric + magnetic changes within local rocks

irrational animal behaviour

However none have found to be a good indicator as hey can all occur without being followed by an earthquake

Question 45

Can we predict earthquakes (no)

Answer 45

Not currently possible to make deterministic predictions of when and where earthquakes will happen

Predictions have a long timescale and do not allow for movements of minor/unknown faults

For this to be possible it would be necessary to identify a diagnostic precursor

However, so far the search for diagnostic precursors have been unsuccessful and most geoscientists don’t believe there is a realistic prospect of accurate prediction in the foreseeable future

Question 46

Why is it difficult to predict earthquakes

Answer 46

The crust’s response to changing stress is not linear, (making prediction of behaviour more difficult) and is dependant on the crust’s complex and highly variable geography

As a result, very difficult t build accurate simulations which predict tectonic events. Laboratory experiments cannot accurately reflect the complexities of real-world geological settings

Earthquakes originate deep beneath the ground - so data gathering depends on remote observation techniques

Question 47

How can earthquake hazards be mitigated

Answer 47

Seismic monitoring research provides the basis for forecast monitoring and hazard mapping.

Geoscientists can engage with local authorities and communities to ensure that this information s used effectively

Failing buildings are the greatest causes casualties so essential that new buildings are constructed to withstand their effects

Local populations should be aware of the risks and how two act during an earthquake

Question 48

Why is evacuation not an effective strategy for earthquakes

Answer 48

Not possible to predict precise timing, location, and size of an earthquake to a sufficiently high degree of accuracy

Question 49

How common are earthquakes in the UK

Answer 49

Fairly common though most not noticed by people

20/30 earthquakes felt by people each year, with a few hundred smaller ones only recorded by sensitive instruments

Question 50

What doesn’t the UK experience large earthquakes

Answer 50

Most happen at plate boundaries, but the UK is located in the middle of the Eurasian plate so not subject to significant tectonic activity

Largest possible earthquake in UK = 6.5 (British Geological Society)

Question 51

What does the UK experience earthquakes

Answer 51

Eurasioan plate is riddles with fault lines, often slipping slightly casing tremors detectable only by specialist equipment

Fracking has the potential to cause mini earthquakes

Glacial rebound - uplift resulting from melting ie sheets that used to cover UK

North Sea oil extraction

Question 52

The UK is only likely to get a magnitude 4 earthquake every….years, and a magnitude 5 earthquake every…….years

Answer 52

2+

10-20