🌋3.1.5.4 - Seismic Hazards

Question 1

What are seismic hazards?

Answer 1

Earthquakes, tsunamis, landslides and liquefaction

Question 2

What is seismicity?

Answer 2

The Earth’s shaking, can be human induced or natural

Question 3

How are seismic waves formed?

Answer 3

Pressure builds up at a point where plates meet
Sudden release due to rock failure creates waves
The waves cause the ground to shake
Intensity of the shaking is dictated by the depth of focus and energy release

Question 4

What are tsunamis caused by?

Answer 4

Earthquakes under the sea, moving a column of water above it, displacing it

Question 5

What are the characteristics of tsunamis?

Answer 5

Waves less than 3ft tall, but are compress towards land, gaining height. A wave train which propagates form the earthquake.

Question 6

What is liquefaction?

Answer 6

The mixing of sand or soil with groundwater during the shaking of a moderate or strong earthquake. Ground becomes very soft and similar to quicksand.

Question 7

When is land susceptible to landslides?

Answer 7

It has experienced heavy rain
Fractured or unconsolidated rock
Fold mountains, unstable plate boundary
Steep relief

Question 8

How can earthquakes be predicted?

Answer 8

Past seismic events
Remote sensing
Radon gas emissions

Question 9

How can past seismic events be used to predict earthquakes?

Answer 9

Looking into past frequencies and magnitudes to infer future events can be used for prediction

Question 10

How can remote sensing be used to predict earthquakes?

Answer 10

GPS can be used to monitor the smallest of movements by the plates, stationary plates suggest build up of pressure

Question 11

How can radon gas emissions be used to predict earthquakes?

Answer 11

Radon escapes form cracks in the earth’s crust and a sudden increase may suggest that an earthquake may be imminent

Question 12

Are earthquakes predictable?

Answer 12

No

Question 13

What type of boundary to most earthquakes originate from?

Answer 13

Destructive - at subduction zones

Question 14

Where is liquefaction a particularly dangerous hazard?

Answer 14

In areas where groundwater is near the surface and the soil is sandy

Question 15

What is subsidence?

Answer 15

When the ground surface is lowered often during earthquakes

Question 16

What are the characteristics of P waves?

Answer 16

Fastest waves
Earth moves backwards and forwards but surface stays flat
Moves through solids and liquids

Question 17

What are the characteristics of S waves?

Answer 17

A ripple, up and down movement
Water wave, surface doesn’t stay flat
Shakes earth at right angles

Question 18

What are the characteristics of Rayleigh waves?

Answer 18

Up and down movement and side to side, moves like a snake
Low frequency, rolling motion
Radiate along the surface

Question 19

What are the characteristics of Love waves?

Answer 19

Slowest waves, surface
Causes sideways motion and most damage
One pulse, up and down and some side to side

Question 20

What is a seismograph used for?

Answer 20

To measure ground movement

Question 21

How does a seismograph work?

Answer 21

Pen attached to weight to track movements

Question 22

What is a seismometer used for?

Answer 22

Recording ground movement data digitally

Question 23

What are the issues with a seismometer?

Answer 23

So small that waves must be amplified

Question 24

What does the Mercalli scale range from and to?

Answer 24

1-12

Question 25

What is a 1 on the Mercalli scale?

Answer 25

Instrumental - not felt by many people unless in favourable conditions

Question 26

What is a 12 on the Mercalli scale?

Answer 26

Cataclysmic - total destruction

Question 27

What are the positives of the Mercalli scale?

Answer 27

``` Can be used for damage comparison Relates to epicentre location No equipment required Idea of damage done, builds a picture Perfect for response planning Allows links for human need ```

Question 28

What are the negatives of the Mercalli scale?

Answer 28

Subjective, qualitative measure Uncertainty and bias Location of observer alters results Compares effects, not the event

Question 29

What does the Richter scale measure?

Answer 29

Ground deformation and energy release

Question 30

What type of scale does the Richter follow?

Answer 30

Logarithmic

Question 31

What is the highest recorded value on the Richter scale?

Answer 31

8.9

Question 32

Seismicity

Answer 32

Earth shaking Can be human induced through fracking, mining and reservoir construction

Question 33

Earthquake Formation

Answer 33

. Friction along margins = stress in lithosphere . Stress suddenly overcome . Plates fracture along faults . Sends a series of shock waves to the surface . Tremors can be followed be several weeks of after shocks whilst crust settles

Question 34

What factors make the damage done by an earthquake worse?

Answer 34

Time of day Depth of focus Population density Level of development Building quality

Question 35

P Waves

Answer 35

Fast moving, first to reach the surface Travel through solids and liquids (crust, mantle, core) High frequency

Question 36

S Waves

Answer 36

Second waves to reach the surface Travel through liquids (mantle) Sideways movement, shaking earth at 90° to direction of travel Do more damage than P waves

Question 37

Love Waves

Answer 37

Slowest moving Do most damage because of sideways movement

Question 38

Rayleigh Waves

Answer 38

Radiate from epicentre in complicated, low frequency rolling motions

Question 39

Tsunami

Answer 39

Large sea wave generated by earthquakes on the ocean floor, large landslides and submarine volcanic eruptions

Question 40

Tsunami Formation

Answer 40

. Pressure between 2 plates suddenly released . Large waves of energy displace water . Water train created, spreads in concentric circles to shore . Upon reaching shore, wave shoaling occurs (friction = highly destructive wave) . Wave retreats taking debris, forming the next wave

Question 41

What factors make the damage done by a tsunami worse?

Answer 41

Distance travelled Focus depth (shallower = larger) Population density Level of development Warning given

Question 42

Landslide

Answer 42

Movement of rock, debris or earth down a slope

Question 43

Landslide Formation

Answer 43

Ground shaking due to earthquake destablishes cliffs and steep slopes Causes material to quickly move down slope

Question 44

What factors make landslides worse?

Answer 44

Steep gradient High relief rainfall (saturates ground = unstable) Shallow soil Unstable rock

Question 45

Liquefaction

Answer 45

Jelly like state of silts and clays resulting from intense ground shaking

Question 46

Liquefaction Formation

Answer 46

Sand or soil mixes with ground water Becomes very soft, causing subsidence and building collapse

Question 47

What factors make liquefaction worse?

Answer 47

High ground water Reclaimed soil Sandy soil (loosely packed grain = more pore spaces to fill with water)

Question 48

Richter Scale

Answer 48

Logarithmic 1 to 10+ Easy comparison between disasters Quantitative

Question 49

Modified Mercalli Scale

Answer 49

Qualitative 0 to XII Different levels for different areas Only useful with buildings around

Question 50

Preparedness: Name some of the events that occur in the minutes before an earthquake

Answer 50

Microquakes before main tremor Magnetic changes in local rock Unusual animal behaviour

Question 51

Prevention: Although almost impossible to prevent, what have some scientists suggested?

Answer 51

Putting water and oil in margins Lubricates movement Plates will slide not stick

Question 52

Adaptation: What form does protection mainly take?

Answer 52

Preparation by modifying the human and built environments to decrease vulnerability and reduce loss

Question 53

Adaptation: 2 features of hazard resistant structures

Answer 53

Concrete weight on roof- moves in opposite direction to force of earthquake, counteracting stress Rubber shock absorbers in foundations- allows some movement

Question 54

Adaptation: What can people in old buildings do?

Answer 54

Retrofit them to make them more aseismic

Question 55

Mitigation: How does education help?

Answer 55

Minimises loss of life, through earthquake drills

Question 56

Adaptation: 2 ways fires can be prevented

Answer 56

Smart meters- cut off gas in an earthquake over a certain magnitude Tokyo gas companies- switch off pipelines, reducing the number of fires

Question 57

Adaptation: 2 ways of protecting against tsunamis

Answer 57

Tsunami warning systems Build 12m high tsunami walls (ineffective)

Question 58

Protection: How can the emergency services be ready?

Answer 58

Heavy lifting gear available Carefully organised and planned response People in communities given first aid training to treat injured before help arrives

Question 59

Mitigation: Why are people urged to take out insurance in richer countries?

Answer 59

To cover their losses- expensive

Question 60

Adaptation: Land use planning

Answer 60

Schools and hospitals built in low risk areas Green space put into high risk areas, forming a safe area away from fires and aftershock debris + distribution areas

Question 61

Prevention: How can any impacts from liquefaction be prevented?

Answer 61

Authorities preventing building on land prone to liquefaction

Question 62

Mitigation: How to earthquake and tsunami warning systems reduce damage caused?

Answer 62

Detect weak shockwaves TV / SMS warnings given Aids evacuation

Question 63

Predicting earthquakes: past seismic events

Answer 63

. Seismic gap theory . Regular pattern can help predict the next earthquake . The seismic gap states that earthquake hazard increases with time since the last large earthquake on certain faults or plate boundaries

Question 64

Animals in prediction

Answer 64

. They may detect seismic activity and notice smaller vibrations and exhibit unusual behaviour . They may sense ionisation of air caused by the large rock pressure in earthquake zones in their fur . May smell gases

Question 65

Lazar beams

Answer 65

Can detect plate movement by directing the beam across the fault line

Question 66

Pros of mercalli scale

Answer 66

. Useful in determining damage caused to an area and responses can be planned . Structural engineers contribute information for assigning values of VIII which is useful for rebuilding phases

Question 67

Cons of mercalli scale

Answer 67

. Can only measure earthquakes in inhabited ares . Not very scientific . Damage caused may not accurately reflect an earthquake’s strength . Only measure effects not energy released . Witnesses that scale is based on may not be accurate

Question 68

Mercalli scale vs Richter scale

Answer 68

. Unlike the mercalli scale which looks at damaged caused the Richter scale measures ground deformation and energy released . In richter as you move up the magnitude frequency decreases . Large earthquakes are rare as it takes many years for the pressure and energy to build up

Question 69

Do scientist prefer the Richter scale or Mercalli scale?

Answer 69

Scientists prefer the Richter scale as it is more scientifically based, logarithmic scale, rather than human observation

Question 70

Steps to seismic waves

Answer 70

1) Pressure builds at the point where 2 plates meet 2) Sudden release (due to rock failure creates waves) 3) The waves cause the ground to shake 4) The intensity of the shaking is dictated by the depth of the focus and the energy release 5) The result of this is the earthquake hazard

Question 71

P-waves (primary)

Answer 71

Fastest waves, moving through solid and liquids causing the earth to move backwards and forwards

Question 72

S-waves (secondary)

Answer 72

Slower waves which move through a sideways motion, shaking the Earth at right angles to the direction of travel. Can’t move through liquids but do much more damage then P-Waves.

Question 73

Love waves

Answer 73

Surface waves which are the slowest. Cause a sideways motion and the most damage.

Question 74

Rayleigh waves

Answer 74

Radiate along the sirface in a low-frequency rolling motion.