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
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
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.
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.
5
Q
How do we get really big movement of tectonic plates
A
If displacement happens a lot
6
Q
Why do different fault zones had different frequency and cycles of earthquakes
A
It depends on the strength of rocks
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
8
Q
What is the fault plane
A
The area that ruptures (can cover thousands of km2)
9
Q
What is the focus (hypocentre) of the earthquake
A
The point on the fault plane at which the rupture status is called
10
Q
How deep can the focus get
A
Usually happens within the first few 10s of Kms. Could be 700km deep
11
Q
What three co-ordinates can thefocus be defined by
A
Latitude (N or S)
Longitude (E or W)
Focal depth (km)
12
Q
What is the epicentre
A
The point on the earths surface directly above the focus
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
14
Q
Where do earthquakes occur
A
95% at plate boundaries
5% are intraplate
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.
16
Q
What are the focal depth of EQ at constructive and conservative plate boundaries
A
Shallow - 0-15km
17
Q
What is the focal depth of EQ at destructive plate boundaries
A
0-700km
18
Q
What does the lithosphere do as it subduction into the mantle
A
Stays rigid
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
20
Q
What is the Wadati-Benioff zone
A
The distribution of earthquake foci at a convergent plate boundary
21
Q
What do seismographs do
A
Measure ground displacement, velocity or acceleration vs time.
22
Q
What do modern seismographs have
A
Digital output instead of pen/paper drum
23
Q
What can high sensitivity instruments detect and amplify
A
Displacements as small as 10-10m. As we use electronic seismographs
24
Q
What do strong motion instruments record
A
High amplitude displacements close to EQ epicentres. Need 6 seismographs
25
What seismographs do you need to fully record ground motion
Need 1 vertical and 2 horizontal (N-S and E-W)
26
What are the four types of seismic wave
P wave
S wave
Love wave
Rayleigh wave
27
Facts about p waves
Fastest wave.
6km a second.
Compression and expansion of crust.
Motion is each part of rock is moved backwards and forwards (dilutations)
28
Facts about the S waves
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
29
What are p waves and S waves
Body waves that go through the earth and can be detected on other side of the world
30
What is the motion of love waves
Side to side
31
What is the motion of Rayleigh waves
Rolling motion of the ground
32
Facts about love and Rayleigh waves
Roughly both 3km per second. Surface waves. Confined to crust and dissipate with depth. Move on surface causing damage.
33
What do the seismic waves look like on a graph
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
34
How to locate the epicentre
The time difference between the first p and S waves arrivals on a seismogram is proportional to the distance between EQ and seismograph
35
Why do you need to use three seismograph stations to calculate the epicentre
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
36
How to locate the focus using the P-S interval
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
37
How do seismologists usually determine depth of focus
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
38
What is the pP-P
The first reflected pP arrival time minus the first direct P wave arrival time
39
What is the measurement for the size of an earthquake
Seismic Moment (M0)
40
How is seismic moment determined
By the magnitude of the force which acts on the earths lithosphere and the strength of the block that eventually fractures
41
What is the seismic moment equation
```
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
42
What is the modulus of rigidity
Strength of the rock.stress applied before it breaks.
43
What is the area of fractured fault plane
How much it has been ruptured. Length by depth to get area.
44
What is usually the average displacement along the fault
A couple of cm
45
What does Mo range from
10^10 to 10^23 N m
46
How can the modulus of rigidity be measured
In the laboratory (stress/strain) or in the field (from the speed of seismic waves)
47
How to calculate the area of fractured fault plane
Estimated from the distribution of aftershocks as the rocks are settling down into new equilibrium. Roughly tells us which area ruptured
48
How to measure the average displacement of the fault
In the field for big EQ that rupture the earths surface in an accessible area. Or use GPS or InSAR
49
Nowadays what do we do to determine Mo directly
Calibration with seismograph data - means we can assign a moment magnitude to each EQ
50
What is the scale used for earthquakes
The moment Magnitude (Mw) scale
51
How can modern seismographs with microprocessors calculate seismic moment directly
Form the amplitudes and frequencies of seismic waves produced by an EQ
52
How to obtain a Moment Magnitude (Mw)
Using a conversion formula - Mw=2/3((log10Mo)-9.1)
| To scale down large values to more user friendly
53
Nowadays what does earthquake magnitude usually mean
The moment magnitude NOT the Richter scale
54
Why is Mw preferred to Richter scale
Because the richter couldn’t distinguish between EQs with magnitudes >7.
55
What is the theoretical upper limit of The Mw scale
10
56
What is the theoretical upper limit of the Mw set by
The elastic properties of rocks, and dimensions of biggest subduction zones
57
For each increase of 1 on the Mw scale what does energy increase by
Factors of 32 so not a linear relationship
58
What happens as magnitude increases
The frequency decrease
59
What is the frequency magnitude relationship law
Gutenberg-richter law
60
What is the Gutenberg-richter law
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.
61
What was the biggest earthquake ever measured
In chile 1960 Mw=9.3-9.5
62
Why have all the biggest earthquakes occurred at subduction zones
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
How many earthquakes on subduction zones have been measured
5
64
Why are the biggest magnitude earthquakes not necessarily the ones that do the most damage
```
Economic development
Vulnerability of humans
Isolated
Focal depth (shallow more destructive)
Different hazards being present
Different infrastructure
```
65
What does the strength with which an EQ is felt (intensity) depend on
Magnitude.
Distance from the earthquake epicentre.
Focal depth.
Group type - soft rock or sediment shakes more.
66
What is ground acceleration
The relative intensity of shaking in different places. Can be horizontal (damaging) or vertical
67
What is the main cause of damage to buildings
Ground acceleration
68
What is one way of indicating the damage causing potential of an EQ
Peak ground acceleration
69
What is the equation for acceleration
acceleration = Force/mass
70
How is ground acceleration usually expressed
As a fraction of the acceleration due to gravity (9.8m/s^2)
71
What is the smallest ground acceleration that can be felt
0.001g (1000th of g, felt at rest and probably at top of building)
72
What can 0.05g do
Damage weak buildings
| 20th of g
73
What does greater than 1g do
Usually completely flatten buildings
74
Example of a structural damage to homes bc of EQ
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
Besides ground acceleration what are the other factors that influence EQ damage
```
Duration of ground shaking.
Types of building.
Frequency of oscillation.
Direct surface fault breakage.
Landslides.
Ground liquefaction.
Fires.
Floods.
```
76
How does the duration of ground shaking factor in EQ damage
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
How does the type of building factor in EQ damage
Height and shape.
Design ‘soft storey’.
Materials and strength.
Foundations attached to ground.
78
example of a soft storey
Big open plan area with a few pillars like shopping centres
79
How does the frequency of oscillation of the ground factor in EQ damage
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
What is resonance
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
Example of frequency of oscillation
Mexico City EQ.
| Low frequency last 3 minutes and tall buildings swayed, topped and crashed. Small buildings sustained less damaged.
82
How does the direct surface fault breakage factor in EQ damage
When the fault plane reaches the surface but the type of breakage depends on fault
83
What are the different kinds of surface fault breakage
Thrust fault
Normal fault
Strike-slip fault
84
What is a thrust fault
Blocks of crust move together, one rises above the other. Typical near subduction zones
85
What is a normal fault
Extension of the crust where one slides down under - offset
86
What is a Strike-slip fault
Neither expansion or compression but a horizontal sliding. Everything is relatively flat
87
Where are landslides a problem
Problems in mountainous areas. EQ can cause part of the ground to detach and slide away
88
Example of landslide
My Huascaran in Peru, 1970 mobilised 13 million km of rock. Triggered huge debris flow which killed 66,000
89
What is ground liquefaction
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
Example of liquefaction
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
What are fires caused by
Overturned stoves.
Broken gas Mains.
Short circuited electricity cables.
Debris in contact with naked flames.
92
Examples of fires
SAN Francisco
Tokyo
Managua
Kobe
93
How does floods factor in EQ damage
Due to diverted rivers, burst dams, changes in land elevation, tsunamis
94
Example of flood
SAN Fernando EQ damaged the Van Norman dam. 80,000 people evacuated. The reservoir wasn’t full, and the dam did not fail.
95
What describes the severity of earthquake damage
earthquake intensity scales using a numerical scale
96
Why are degrees of institution given Roman numerals
To avoid confusion with magnitude scales
97
What was the first intensity scale to be used
The Rossi-Forel scale devised in Italy in the early 19th century
98
What is the intensity scale used for earthquakes
Modified Mercalli scale
99
What do most modern scales expend up to
Intensity XII (except the JMA used in Japan)
100
How can trained EQ observed estimate the intensity using the scales
Can use apps to get ppl to classify how they felt
101
Difference between intensity and magnitude
Intensity relates directly to Peoples experiences during EQs than magnitude does.
EQ can have lots of intensities but only one magnitude.
102
When is intensity highest
At the epicentre
103
When can you calculate the approximate intensity at epicentre
If magnitude and focal depth are known
104
What is the equation for intensity
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
Where do most really big EQs (M>7.5) occur
Subduction zones
106
What depths typically reduces intensity but increase area
100-250 km
107
What is the most hazardous earthquake region with low risk due to low population and vulnerability
Kermadec islands
108
Where is EQ risk highest
In densely populated regions where most earthquakes have focal depths of <15km e.g California, Turkey and China
109
Case study of earthquake
Kobe EQ, Japan 1995
110
Facts about Kobe EQ
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
Do earthquakes have warning signal
No - unlike most other kinds of natural hazards so have to take a long term preparedness
112
What must preparedness for EQ be
On a long term basis rather than relying on short term evacuation
113
Why is long term preparedness not always implemented
In poorer parts of the world
114
Two contrasting case studies for EQ preparedness
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
What are the six steps to reducing vulnerability
```
Assess hazard level.
Land use planning.
enforce anti-seismic design codes.
Education, training and emergency planning.
Earthquake insurance.
Earthquake early warning systems (EEWS).
```
116
Where is probabilistic forecasting widely used
In EQ prone areas - good monitoring and historical record to see size and time frame of earthquakes
117
What does probabilistic forecasting form the basis of
Land use planning, building regulations, insurance premiums, preparedness plans
118
What does earthquake hazard assessment aim to assess
Maximum intensity which has a probability of occurring within a period of years at a particular location
119
What is the intensity in Lancaster
VI
120
What is the intensity for san fransico
X - San Andreas fault
121
What do hazard assessments study
Frequencies, locations, magnitudes and intensities of historical EQs and uses this data to estimate expected level of ground shaking
122
What is mapped onto a map of the region in hazard assessments
EQ hazard values e.g max intensity, peak ground acceleration or recurrence period
123
What do microzonation maps show
High level of detail - incorporating geological and topographic into
124
What is land use planning based on
Seismic hazard microzonination maps
125
Where to avoid building in land use planning
On faults , soft ground and steep topography if possible. Avoid building dams, nuclear power stations, hospitals, elevated roads in areas of high hazard
126
What are the basic principles of anti seismic design
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
How do anti seismic design reduce damage
Strengthening structures.
Using appropriate materials.
Securing building contents and services.
Hi tech engineering to rescue building response.
128
Example of appropriate materials
Strong, light weight and flexible but sometimes compromises need to be made like reinforced concrete and not timber bc of deforestation
129
Examples of building contents and services
Water and gas
130
Example of hi tech engineering
Base isolation
Damping systems
Dynamic control system
Have to replace after an earthquake
131
What is base isolation
Rubber pads
132
What is damping system
Deliberately absorbs seismic energy. Pistons filled with liquid tjay compresses and takes some swaying out of the building
133
What is dynamic control system
Systems designed to counter act swaying of the building, minismies the amplitude of the building
134
Characteristics of anti seismic structure
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
Why do we retrofit buildings
Older buildings are more numerous and vulnerable.
Cheaper than rebuilding.
Affordable, accessible and culturally acceptable.
136
In poor areas what is it best to concentrate on
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
What are regulatory building codes
Anti-seismic design codes based on acceptable risk
138
What is good and bad about regulatory building codes
No compliance is illegal but enforcement is expensive as someone has to check on the builders
139
Examples of education training and emergency planning
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
What has been estimated about w good emergency response
Can cut those affected y a factor of 10
141
What is earthquake insurance
Sold separately to other insurance in EQ prone countries and is has big deductible (only cover 85%). Take up is low
142
What is the take up of earthquake insurance in California and Japan
25%
143
What is EEWS
Earthquake early warning systems
144
What are the EEWS approaches
Single station approach.
| Network approach.
145
What is the single station approach
Detect first p wave from an earthquake and sound an alarm to warn that stronger shaking is imminent - more false alarms but quick
146
What is the network approach
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
How much warning can EEWS give
Up to a minute but if close to epicentre there may be nothing
148
Examples of responses to earthquake early warning systems
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
What would a deterministic prediction specific
Time, epicentre, depth and magnitude of a future EQ
150
Where has been intensively researched over the last few decades
Japan and California
151
What are some new ways they are trying to detect earthquakes
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
What are the precursor stages
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
Is there anyway to predict earthquakes
No despite decades of intensive research
154
What do some seismologists argue
It’s better to put more resources into long term preparedness rather than short term prediction
