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Earthquakes Flashcards

1
Q

What is an earthquake

A

It occurs when the earths crust or whole lithosphere is elastically strained until it suddenly breaks

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2
Q

What is seismic waves

A

Energy involved in rupture is propagated through the earth as a series of seismic waves - energy through the earth

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3
Q

What are the steps of earthquakes

A

Crystal blocks at rest - fault in the middle.
Deformation during stress build up.
The instant of rupture.
Rebounding to a new equilibrium.

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4
Q

What is the primary effect of earthquakes on the ground

A

The blocks on each side of the fault are permanent displaced from each other. The displacement may be horizontal or vertical. Over geological time, these displacements can add up to significant plate tectonic motions.

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5
Q

How do we get really big movement of tectonic plates

A

If displacement happens a lot

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6
Q

Why do different fault zones had different frequency and cycles of earthquakes

A

It depends on the strength of rocks

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7
Q

What are the secondary effects of earthquakes

A

As the fault ruptured, the rocks vibrate until they settle into their new position. This causes ground shaking, damage to buildings, landslides, liquefaction. More hazardous

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8
Q

What is the fault plane

A

The area that ruptures (can cover thousands of km2)

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9
Q

What is the focus (hypocentre) of the earthquake

A

The point on the fault plane at which the rupture status is called

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10
Q

How deep can the focus get

A

Usually happens within the first few 10s of Kms. Could be 700km deep

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11
Q

What three co-ordinates can thefocus be defined by

A

Latitude (N or S)
Longitude (E or W)
Focal depth (km)

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12
Q

What is the epicentre

A

The point on the earths surface directly above the focus

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13
Q

What is a thrust fault

A

If the fault is inclined (not vertical) then the point above could be away from the fault at the surface I.e at a cliff face

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14
Q

Where do earthquakes occur

A

95% at plate boundaries

5% are intraplate

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15
Q

What plate boundaries are earthquakes on

A

Constructive plate boundaries (MORs)
Conservative plate boundaries (transform fault)
Destructive plate boundaries (island arcs and active continental margins)
Can be away from plate boundaries.

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16
Q

What are the focal depth of EQ at constructive and conservative plate boundaries

A

Shallow - 0-15km

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17
Q

What is the focal depth of EQ at destructive plate boundaries

A

0-700km

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18
Q

What does the lithosphere do as it subduction into the mantle

A

Stays rigid

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19
Q

Intraplate EQs are not well understood but what may they be related to

A

Crustal loading and unloading due to climate change erosion

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20
Q

What is the Wadati-Benioff zone

A

The distribution of earthquake foci at a convergent plate boundary

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21
Q

What do seismographs do

A

Measure ground displacement, velocity or acceleration vs time.

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22
Q

What do modern seismographs have

A

Digital output instead of pen/paper drum

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23
Q

What can high sensitivity instruments detect and amplify

A

Displacements as small as 10-10m. As we use electronic seismographs

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24
Q

What do strong motion instruments record

A

High amplitude displacements close to EQ epicentres. Need 6 seismographs

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25
Q

What seismographs do you need to fully record ground motion

A

Need 1 vertical and 2 horizontal (N-S and E-W)

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26
Q

What are the four types of seismic wave

A

P wave
S wave
Love wave
Rayleigh wave

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27
Q

Facts about p waves

A

Fastest wave.
6km a second.
Compression and expansion of crust.
Motion is each part of rock is moved backwards and forwards (dilutations)

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28
Q

Facts about the S waves

A

Each part in the rock is being bent out of shape, volume stays the same but it’s moving from original position.
4km per second.
Wavy

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29
Q

What are p waves and S waves

A

Body waves that go through the earth and can be detected on other side of the world

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30
Q

What is the motion of love waves

A

Side to side

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31
Q

What is the motion of Rayleigh waves

A

Rolling motion of the ground

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32
Q

Facts about love and Rayleigh waves

A

Roughly both 3km per second. Surface waves. Confined to crust and dissipate with depth. Move on surface causing damage.

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33
Q

What do the seismic waves look like on a graph

A

First blip is p wave.
Second bigger one is S wave.
Surface wave is when the amplitude skyrockets.
Time intervals allow us to locate the earthquakes

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34
Q

How to locate the epicentre

A

The time difference between the first p and S waves arrivals on a seismogram is proportional to the distance between EQ and seismograph

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35
Q

Why do you need to use three seismograph stations to calculate the epicentre

A

You can create circles of how far they S-P time interval was and where the circles intersect is where the epicentre is. The radii are equivalent to distances from the EQ

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36
Q

How to locate the focus using the P-S interval

A

Focal depth can be determined by using he P-S intervals in 3D I.e the EQ focus is where 4 or more spheres intersect

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37
Q

How do seismologists usually determine depth of focus

A

By identifying the pP phase on seismograms. pP is a P wave that has been reflected from the surface of the earth at a point relatively near the focus.
The time interval pP-P is used to compute depth of focus tables

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38
Q

What is the pP-P

A

The first reflected pP arrival time minus the first direct P wave arrival time

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39
Q

What is the measurement for the size of an earthquake

A

Seismic Moment (M0)

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40
Q

How is seismic moment determined

A

By the magnitude of the force which acts on the earths lithosphere and the strength of the block that eventually fractures

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41
Q

What is the seismic moment equation

A 
Mo = uAd
u= modulus of rigidity (N/m2)
A= area of fractured fault plane (M2)
d= average displacement along the fault (m)

Units of Mo are Nm

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42
Q

What is the modulus of rigidity

A

Strength of the rock.stress applied before it breaks.

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43
Q

What is the area of fractured fault plane

A

How much it has been ruptured. Length by depth to get area.

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44
Q

What is usually the average displacement along the fault

A

A couple of cm

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45
Q

What does Mo range from

A

10^10 to 10^23 N m

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46
Q

How can the modulus of rigidity be measured

A

In the laboratory (stress/strain) or in the field (from the speed of seismic waves)

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47
Q

How to calculate the area of fractured fault plane

A

Estimated from the distribution of aftershocks as the rocks are settling down into new equilibrium. Roughly tells us which area ruptured

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48
Q

How to measure the average displacement of the fault

A

In the field for big EQ that rupture the earths surface in an accessible area. Or use GPS or InSAR

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49
Q

Nowadays what do we do to determine Mo directly

A

Calibration with seismograph data - means we can assign a moment magnitude to each EQ

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50
Q

What is the scale used for earthquakes

A

The moment Magnitude (Mw) scale

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51
Q

How can modern seismographs with microprocessors calculate seismic moment directly

A

Form the amplitudes and frequencies of seismic waves produced by an EQ

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52
Q

How to obtain a Moment Magnitude (Mw)

A

Using a conversion formula - Mw=2/3((log10Mo)-9.1)

To scale down large values to more user friendly

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53
Q

Nowadays what does earthquake magnitude usually mean

A

The moment magnitude NOT the Richter scale

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54
Q

Why is Mw preferred to Richter scale

A

Because the richter couldn’t distinguish between EQs with magnitudes >7.

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55
Q

What is the theoretical upper limit of The Mw scale

A

10

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56
Q

What is the theoretical upper limit of the Mw set by

A

The elastic properties of rocks, and dimensions of biggest subduction zones

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57
Q

For each increase of 1 on the Mw scale what does energy increase by

A

Factors of 32 so not a linear relationship

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58
Q

What happens as magnitude increases

A

The frequency decrease

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59
Q

What is the frequency magnitude relationship law

A

Gutenberg-richter law

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60
Q

What is the Gutenberg-richter law

A

LogN = a - bM

A and b are constants, different for each source region.
N is the number of earthquakes per year.
M is the moment magnitude.

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61
Q

What was the biggest earthquake ever measured

A

In chile 1960 Mw=9.3-9.5

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62
Q

Why have all the biggest earthquakes occurred at subduction zones

A

Displacement can be big here so plates subducting gets locked for a while and suddenly the subducting plate will move down and the one on top will spring up and displacement is a lot. Area is also so much bigger as it goes much deeper

63
Q

How many earthquakes on subduction zones have been measured

A

5

64
Q

Why are the biggest magnitude earthquakes not necessarily the ones that do the most damage

A 
Economic development
Vulnerability of humans
Isolated
Focal depth (shallow more destructive)
Different hazards being present 
Different infrastructure
65
Q

What does the strength with which an EQ is felt (intensity) depend on

A

Magnitude.
Distance from the earthquake epicentre.
Focal depth.
Group type - soft rock or sediment shakes more.

66
Q

What is ground acceleration

A

The relative intensity of shaking in different places. Can be horizontal (damaging) or vertical

67
Q

What is the main cause of damage to buildings

A

Ground acceleration

68
Q

What is one way of indicating the damage causing potential of an EQ

A

Peak ground acceleration

69
Q

What is the equation for acceleration

A

acceleration = Force/mass

70
Q

How is ground acceleration usually expressed

A

As a fraction of the acceleration due to gravity (9.8m/s^2)

71
Q

What is the smallest ground acceleration that can be felt

A

0.001g (1000th of g, felt at rest and probably at top of building)

72
Q

What can 0.05g do

A

Damage weak buildings

20th of g

73
Q

What does greater than 1g do

A

Usually completely flatten buildings

74
Q

Example of a structural damage to homes bc of EQ

A

In California after the 1971 San Fernando EQ (6.6 Mw). Bricks not suitable for EQ and pancakes houses. The roofs cracked in half and parts of the building has slid from the structure

75
Q

Besides ground acceleration what are the other factors that influence EQ damage

A 
Duration of ground shaking.
Types of building.
Frequency of oscillation.
Direct surface fault breakage.
Landslides.
Ground liquefaction.
Fires.
Floods.
76
Q

How does the duration of ground shaking factor in EQ damage

A

The shorter the better.
Closer means shorter shaking periods due to the waves not having long to spread out, near epicentre means more intensity however - a trade off

77
Q

How does the type of building factor in EQ damage

A

Height and shape.
Design ‘soft storey’.
Materials and strength.
Foundations attached to ground.

78
Q

example of a soft storey

A

Big open plan area with a few pillars like shopping centres

79
Q

How does the frequency of oscillation of the ground factor in EQ damage

A

Long period/ low frequency vibration do more damage to tall buildings (further away) and short periods/ high frequency vibrations do more damage to short buildings. (Close to earthquake)

80
Q

What is resonance

A

All objects have a natural frequency of vibrations. If the ground motion matches the natural frequency than the effect is heightened. They reinforce each other

81
Q

Example of frequency of oscillation

A

Mexico City EQ.

Low frequency last 3 minutes and tall buildings swayed, topped and crashed. Small buildings sustained less damaged.

82
Q

How does the direct surface fault breakage factor in EQ damage

A

When the fault plane reaches the surface but the type of breakage depends on fault

83
Q

What are the different kinds of surface fault breakage

A

Thrust fault
Normal fault
Strike-slip fault

84
Q

What is a thrust fault

A

Blocks of crust move together, one rises above the other. Typical near subduction zones

85
Q

What is a normal fault

A

Extension of the crust where one slides down under - offset

86
Q

What is a Strike-slip fault

A

Neither expansion or compression but a horizontal sliding. Everything is relatively flat

87
Q

Where are landslides a problem

A

Problems in mountainous areas. EQ can cause part of the ground to detach and slide away

88
Q

Example of landslide

A

My Huascaran in Peru, 1970 mobilised 13 million km of rock. Triggered huge debris flow which killed 66,000

89
Q

What is ground liquefaction

A

Quicksand effect.
Sediment that’s not consolidated rock, lose particles, high water content, grains of minerals touching each other and spaces in between them filled with water. Shaking causes particles to move around and jostle each other and water becomes dominant and things can sink and loses cohesiveness.

90
Q

Example of liquefaction

A

Port Royal in Jamaica.
Shaking due to a M~8 EQ -> liquefaction of sand -> lateral spreadin -> sand bar sank and harbour rose -> one third of the town disappeared beneath the sea -> 2500

91
Q

What are fires caused by

A

Overturned stoves.
Broken gas Mains.
Short circuited electricity cables.
Debris in contact with naked flames.

92
Q

Examples of fires

A

SAN Francisco
Tokyo
Managua
Kobe

93
Q

How does floods factor in EQ damage

A

Due to diverted rivers, burst dams, changes in land elevation, tsunamis

94
Q

Example of flood

A

SAN Fernando EQ damaged the Van Norman dam. 80,000 people evacuated. The reservoir wasn’t full, and the dam did not fail.

95
Q

What describes the severity of earthquake damage

A

earthquake intensity scales using a numerical scale

96
Q

Why are degrees of institution given Roman numerals

A

To avoid confusion with magnitude scales

97
Q

What was the first intensity scale to be used

A

The Rossi-Forel scale devised in Italy in the early 19th century

98
Q

What is the intensity scale used for earthquakes

A

Modified Mercalli scale

99
Q

What do most modern scales expend up to

A

Intensity XII (except the JMA used in Japan)

100
Q

How can trained EQ observed estimate the intensity using the scales

A

Can use apps to get ppl to classify how they felt

101
Q

Difference between intensity and magnitude

A

Intensity relates directly to Peoples experiences during EQs than magnitude does.
EQ can have lots of intensities but only one magnitude.

102
Q

When is intensity highest

A

At the epicentre

103
Q

When can you calculate the approximate intensity at epicentre

A

If magnitude and focal depth are known

104
Q

What is the equation for intensity

A

Intensity = K1M+K2/D+K3/R+G

Ks are constants determines from experience
M is moment magnitude
G is ground factor (zero for solid rock, higher for soft ground)
D is the focal depth of the earthquake
R is the distance from the epicentre

105
Q

Where do most really big EQs (M>7.5) occur

A

Subduction zones

106
Q

What depths typically reduces intensity but increase area

A

100-250 km

107
Q

What is the most hazardous earthquake region with low risk due to low population and vulnerability

A

Kermadec islands

108
Q

Where is EQ risk highest

A

In densely populated regions where most earthquakes have focal depths of <15km e.g California, Turkey and China

109
Q

Case study of earthquake

A

Kobe EQ, Japan 1995

110
Q

Facts about Kobe EQ

A

16th Jan, early morning so most at home.
M - 6.9 depth. 20km. 1.5 max displacement.
Max acceleration 0.8g - strong shaking of reclaimed land around Osaka Bay.
100,000 buildings destroyed (1950s and 60s, post war, not retrofitted, pre building codes)
Elevated highways and trains collapsed.
Fires.
5,100 dead, 30,000 injured, 300,000 homeless.

111
Q

Do earthquakes have warning signal

A

No - unlike most other kinds of natural hazards so have to take a long term preparedness

112
Q

What must preparedness for EQ be

A

On a long term basis rather than relying on short term evacuation

113
Q

Why is long term preparedness not always implemented

A

In poorer parts of the world

114
Q

Two contrasting case studies for EQ preparedness

A

Newland 2010 had no deaths, damage with anti seismic buildings and good emergency planning.
Maximum intensity 10 and MM is 7. Haiti killed 230,000 and houses destroyed - no planning.

115
Q

What are the six steps to reducing vulnerability

A 
Assess hazard level.
Land use planning.
enforce anti-seismic design codes.
Education, training and emergency planning.
Earthquake insurance.
Earthquake early warning systems (EEWS).
116
Q

Where is probabilistic forecasting widely used

A

In EQ prone areas - good monitoring and historical record to see size and time frame of earthquakes

117
Q

What does probabilistic forecasting form the basis of

A

Land use planning, building regulations, insurance premiums, preparedness plans

118
Q

What does earthquake hazard assessment aim to assess

A

Maximum intensity which has a probability of occurring within a period of years at a particular location

119
Q

What is the intensity in Lancaster

A

VI

120
Q

What is the intensity for san fransico

A

X - San Andreas fault

121
Q

What do hazard assessments study

A

Frequencies, locations, magnitudes and intensities of historical EQs and uses this data to estimate expected level of ground shaking

122
Q

What is mapped onto a map of the region in hazard assessments

A

EQ hazard values e.g max intensity, peak ground acceleration or recurrence period

123
Q

What do microzonation maps show

A

High level of detail - incorporating geological and topographic into

124
Q

What is land use planning based on

A

Seismic hazard microzonination maps

125
Q

Where to avoid building in land use planning

A

On faults , soft ground and steep topography if possible. Avoid building dams, nuclear power stations, hospitals, elevated roads in areas of high hazard

126
Q

What are the basic principles of anti seismic design

A

Understand how the ground shakes during EQs - in a range of directions, amplitudes and frequencies.
Understand how built structure respond to shaking e.g resource.

127
Q

How do anti seismic design reduce damage

A

Strengthening structures.
Using appropriate materials.
Securing building contents and services.
Hi tech engineering to rescue building response.

128
Q

Example of appropriate materials

A

Strong, light weight and flexible but sometimes compromises need to be made like reinforced concrete and not timber bc of deforestation

129
Q

Examples of building contents and services

A

Water and gas

130
Q

Example of hi tech engineering

A

Base isolation
Damping systems
Dynamic control system

Have to replace after an earthquake

131
Q

What is base isolation

A

Rubber pads

132
Q

What is damping system

A

Deliberately absorbs seismic energy. Pistons filled with liquid tjay compresses and takes some swaying out of the building

133
Q

What is dynamic control system

A

Systems designed to counter act swaying of the building, minismies the amplitude of the building

134
Q

Characteristics of anti seismic structure

A

Symmetrical, continuous structures with height: width <4.
Avoid extensions of different size/shape/material (could tear building apart).
Avoid soft storeys or x brace them - retrofit.
All structural elements must he securely connected together.
Test with shake tables, computational models or observations in EQs.

135
Q

Why do we retrofit buildings

A

Older buildings are more numerous and vulnerable.
Cheaper than rebuilding.
Affordable, accessible and culturally acceptable.

136
Q

In poor areas what is it best to concentrate on

A

Removing defects like rotten timbers and loose tiles.
Low cost strengthening of connections and walls like geomesh.
Education like short illustrated manuals and builder training.

137
Q

What are regulatory building codes

A

Anti-seismic design codes based on acceptable risk

138
Q

What is good and bad about regulatory building codes

A

No compliance is illegal but enforcement is expensive as someone has to check on the builders

139
Q

Examples of education training and emergency planning

A

Preparedness at home/work and school.
Emergency training.
Practice runs so ppl can swing into action and not panic.
Rapid mobilisation of emergency and medical services.

140
Q

What has been estimated about w good emergency response

A

Can cut those affected y a factor of 10

141
Q

What is earthquake insurance

A

Sold separately to other insurance in EQ prone countries and is has big deductible (only cover 85%). Take up is low

142
Q

What is the take up of earthquake insurance in California and Japan

A

25%

143
Q

What is EEWS

A

Earthquake early warning systems

144
Q

What are the EEWS approaches

A

Single station approach.

Network approach.

145
Q

What is the single station approach

A

Detect first p wave from an earthquake and sound an alarm to warn that stronger shaking is imminent - more false alarms but quick

146
Q

What is the network approach

A

Combine data from several seismograph stations to rapidly compute approx magnitude/ epicentre/ intensity map, and transmit warning via satellite to emergency services, critical facilities and public

147
Q

How much warning can EEWS give

A

Up to a minute but if close to epicentre there may be nothing

148
Q

Examples of responses to earthquake early warning systems

A

Controlling trains.
Controlling factory lines.
Prevent traffic accidents.
Controlling elevators.
Suspending work in progress to avoid mistakes.
Workers performing hazardous tasks to safety.
Altering schools and assembly halls for evacuation.

149
Q

What would a deterministic prediction specific

A

Time, epicentre, depth and magnitude of a future EQ

150
Q

Where has been intensively researched over the last few decades

A

Japan and California

151
Q

What are some new ways they are trying to detect earthquakes

A

Instruments deployed around active faults to try and detect signs the rocks are stressed and about to break e.g ground deformation, foreshocks, radongas emissions through micro cracks, changes in electrical conductivity of rocks and air.

152
Q

What are the precursor stages

A

Theory based on small scale lab studies but not supported by subsequent field investigations of real faults.
Stage 1- build up of elastic strain.
2- development of cracks
3- influx of water and unstable deformation.
5- sudden drop in stress followed t after shocks

153
Q

Is there anyway to predict earthquakes

A

No despite decades of intensive research

154
Q

What do some seismologists argue

A

It’s better to put more resources into long term preparedness rather than short term prediction

Natural Hazards (7 decks)

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