Geography quiz 30 multiple questions
Vincent666
CH. 3 - EARTHQUAKES
https://www.usgs.gov/news/updat
e-magnitude-71-earthquake-
southern-california
Learning Objectives
• Compare and contrast the different types of faulting.
• Explain the formation of seismic waves.
• Summarize the processes that lead to an earthquake and the release of seismic waves.
• Differentiate between the magnitude scales used to measure earthquakes.
• Identify global regions at most risk for earthquakes, and describe the effects of earthquakes.
• Describe how earthquakes are linked to other natural hazards.
• Explain how human beings interact with and affect earthquake hazards.
• Propose ways to minimize seismic risk and suggest adjustments we can make to protect ourselves.
Basic Fault
Features
Footwall • Block below the fault plane • Miner would stand here
Hanging wall • Block above the fault plane • Hang a lantern here
Faults ≠ Plate boundaries
• However, most faults occur along plate boundaries
• Fault types
- Distinguished by direction of rock displacement
• Three basic types:
1. Dip-slip
a) Normal
b) Reverse
2. Strike-slip
a) right-lateral
b) left-lateral
3. Oblique slip
Seismic Waves
• Caused by a release of energy from rupture of a fault
• Body waves: travel through the body of the Earth:
• P waves, primary or compressional waves
- Move fast with a push/pull motion
- Can move through solid, liquid, and gas
• S waves, secondary or shear waves
- Move slower with an up/down motion
- Can travel only through solids
Seismic Waves, cont.
Surface waves: move along
Earth’s surface • P and S waves that reach the
surface
• Travel more slowly than body
waves
• Complex horizontal and vertical
ground movement
Rayleigh Waves • Rolling motion
• Responsible for most of the
damage near epicenter
• Shaking produces both
vertical and horizontal
movement
Seismic Waves, cont.
Surface waves: move along Earth’s surface
• P and S waves that reach the surface
• Travel more slowly than body waves
• Complex horizontal and vertical ground movement
Love Waves • Horizontal ground shaking
• Faster than Rayleigh waves
• Do not move through water or air
• Very hazardous!
Wave direction
Calculating Epicentral Distance
P wave has velocity VP ; S wave have velocity VS
VS < VP
Both originate at the same place – the hypocenter – and travel the same distance, but
the S wave takes longer to arrive than the P wave.
Time for S wave to travel a distance D:
Time for P wave to travel a distance D:
The time difference between them is:
Now solve for the distance D:
Time = Distance
Velocity
TS = D
VS
TP = D
VP
(TS -TP ) = D
VS - D
VP = D
1
VS -
1
VP
æ
è ç
ö
ø ÷=D
VP -VS
VPVS
æ
è ç
ö
ø ÷
𝐷 = 𝑉𝑃𝑉𝑆 𝑉𝑃 − 𝑉𝑆
𝑇𝑠 − 𝑇𝑝
Earthquake Shaking
• Shaking experience depends on:
1. Earthquake magnitude
2. Location in relation to epicenter and direction of rupture
3. Local soil and rock conditions
• Strong shaking from a moderate magnitude or higher
Fault Rupture
• Slip: displacement between two rock blocks
• Rupture area: surface where rocks have moved
• Both parameters used in advanced magnitude calculations because related to the energy needed to move large blocks of rock
Earthquake Magnitude
Richter scale • Not typically used anymore
• Recorded with a seismograph
- Measures maximum amount of ground shaking due to S wave
• Local magnitude
- Depends on where it is located
- Specific to only one location
• For events with shallow hypocenters that are located less than 500 km from seismograph stations
• Based on idea that the bigger the earthquake, the greater the shaking of the Earth
Is this always true though?? Photo credit: SSA
Richter Scale – Local earthquake magnitude (ML)
• For local earthquakes in Calif.
• Recorded on Wood-Anderson.
• ML = log A(mm) + 3 log[R] – 2.92
• Reference :
• Should expect Amplitude of 1 mm
• At Distance of 100 km
• From ML=3
Use the above equation or the graphical method
• Three vertical axis; R (or S-P), M, A
• Measure (S-P)
• Find R : R= 8(S-P)
• Mark R on R vertical axes
• Measure A, mark on axis
• Draw a line between two marks
• Obtain M
Example:
• Amplitude = 23 mm
• S-P time = 24 s
• Thus Ml=5
Earthquake Magnitude
• Richter scale - Not typically used anymore
- Recorded with a seismograph (or seismometer) • Measures maximum amount of ground shaking due to S wave
- Local magnitude • Depends on where it is located
• Specific to only one location
• Moment magnitude scale - Absolute size of earthquake (compare multiple locations)
- Measurement of actual energy released • Determined from area of rupture, amount of slippage, and the rigidity of
the rocks
- Estimates can take days to calculate
Seismic Moment
• Current method of measuring earthquake size
• Relies on the amount of movement along the fault that
generated the earthquake
Where A is the fault area (W x L), μ is the shear modulus
(rigidity) and D is the amount of slip
Units of
or [N ×m]
[dyne ×cm]
M0 = mAD
Total Slip in the M7.3 Landers Earthquake
Rupture on a Fault
Moment Magnitude
• Magnitude scale that uses the seismic moment of an
earthquake
• More accurate for large earthquakes since it is tied to the
physical parameters of the fault such as rupture area, slip
and energy release
• Measures amount of energy released by the movement
along the whole rupture surface
MW = 2
3 log(M0 )-6.1 No units!
Global Frequency of Earthquakes
Descriptor Average Magnitude Annual Number of Events
Great 8 and higher 1
Major 7–7.9 14
Strong 6–6.9 146
Moderate 5–5.9 1344
Light 4–4.9 13,000 (estimated)
Minor 3–3.9 130,000 (estimated)
Very Minor 2–2.9 1,300,000 (estimated)
(approx. 150 per hour)
Source: U.S. Geological Survey, “Earthquake Statistics,” 2000–2015, available at
https://earthquake.usgs.gov/earthquakes/browse/stats.php. Accessed 7/21/2017.
Ground motion and energy comparison
• Magnitude is on a logarithmic scale
• When magnitude increases by 1, the ground motion
(amount of shaking) increases 10 times
- A M6 earthquake has 10 times larger ground motion than a M5
- A M6 has 100 times more than a M4
• Energy is different
- When magnitude increases by 1, amount of energy released
increases ~32 times
Relationship between physical fault
characteristics and Mw?
Magnitude versus Fault Length
10
100
1000
10000
6 7 8 9 10
Magnitude
F a
u lt
L e
n g
th (
k m
)
Magnitude of earthquake is controlled by fault length
(or area) that ruptures
Magnitude versus
fault length
(determined from
aftershock zone
length) for various
earthquakes (Alaska,
1964; Denali, 2002;
Landers, 1992; Loma
Prieta, 1989;
Northridge, 1994,
etc.). Results were obtained using Seismic/Eruption views.
Alaska, 1964
Denali, 2002
Landers, 1992
Sumatra, 2004
Magnitude versus fault length
Northridge, 1994
Loma Prieta, 1989
Equivalent Mw of a
variety of seismic
events, human-
made events, and
other phenomena
Earthquake Intensity
• Measured by Modified Mercalli Scale • Qualitative scale (I-XII) based on damage to structures and
people’s perceptions • Can vary within an earthquake with a single magnitude
• Can vary from country to country
• Modified Mercalli Intensity Maps • show where the damage is most severe
• Based on questionnaires sent to residents, newspaper articles, and reports from assessment teams
• Recently, USGS has used the internet to help gather data more quickly
Abbreviated Modified Mercalli Intensity Scale
Maps of Intensity Shake Maps use high-quality seismograph data to show areas of intense shaking
Useful in crucial minutes after an earthquake
- Show emergency personnel where greatest damage likely occurred
- Locate areas of possible damaged gas lines and other utilities
1994 M6.7 Northridge earthquake 2001 M6.8 Nisqually earthquake
1994 Northridge 2001 Nisqually
Magnitude
is a measure for the size or energy release
of an earthquake
Intensity
is a measure for the degree of shaking
Factors Affecting Intensity
An earthquake has only one magnitude.
The same magnitude earthquake can have different effects in different areas.
Why??
Intensity at a given location depends on:
- Earthquake magnitude
- Epicenter location
- Distance from epicenter
- Hypocenter depth
- Direction of rupture
- Duration of shaking
- Local soil conditions
- Building style
Depth of Focus
• Focus is the place within the Earth where the earthquake starts
• Depth of earthquake influences the amount of shaking • Deeper earthquakes
cause less shaking at the surface
• Lose much of the energy before reaching surface • Loss of energy is called
attenuation
Direction of Rupture
• Direction that the
rupture moves along
the fault influences the
shaking
• Path of greatest
rupture can intensify
shaking
• Directivity contributes
to amplification of
seismic waves
Direction of
Rupture
toward the
Area of Most
Intense
Shaking
Local Geologic Conditions
• Nature of the ground materials affects the earthquake energy
• Different materials respond differently to an earthquake
• Depends on their degree of consolidation - Seismic waves move faster through consolidated bedrock
- Move slower through unconsolidated sediment
- Move slowest through unconsolidated materials with high water content
• Material amplification - Energy is transferred to the vertical motion of the surface waves
Material
Amplification
of Shaking
Material
Amplification of
Shaking
El Salvador, 2001L’Aquila, 2009
Pakistan, 2005 China, 2008
Ground Motion During Earthquakes
• Buildings are designed to handle vertical forces (weight
of building and contents)
>>> vertical shaking in earthquakes is usually safe
• Horizontal shaking during earthquakes
>>> can do massive damage to buildings
• Acceleration
– Measured as acceleration due to gravity (g)
– Weak buildings can be damaged by as little as 0.1g
– At isolated locations, peak ground acceleration can be
as much as 1.8g (Tarzana Hills in 1994 Northridge, CA)
Periods of Buildings and Responses of Foundations:
• Buildings have natural frequencies and periods
• Periods of swaying are about 0.1 second per story
– 1-story house shakes at about 0.1 second per cycle
– 30-story building sways at about 3 seconds per cycle
• Building materials affect building periods
– Flexible materials (wood, steel) → longer period of shaking
– Stiff materials (brick, concrete) → shorter period of shaking
• Velocity of seismic wave depends on material through
which it is moving
– Faster through hard rocks/materials
– Slower through soft rocks/materials
Ground Motion During Earthquakes
Geographic Regions at Risk from Earthquakes
• Earthquakes not randomly distributed
• Most along plate boundaries
Geographic Regions at Risk from Earthquakes
• Earthquakes not randomly distributed
• Most along plate boundaries
Plate tectonics and earthquakes
Exists relation between tectonic environment, deformation
forces and earthquake characteristics
Divergent Zone
• Dominant deformation force: tension
• Stress is released in frequent, strong, and shallow earthquakes
• E.g., East Pacific Rise (MOR), East African Rift System (no oceanic litho yet!)
Tension
Plate Boundary Earthquakes
• Subduction Zones
- Site of the largest earthquakes
- Megathrust earthquakes
- Example: Cascade Mountains
- Convergence between a continental and oceanic plate
- Example: Aleutian Islands
- Convergence between two oceanic plates
• Transform Fault Boundaries
- Example: San Andreas Fault in California, Loma Prieta
earthquake
- Boundary between North American and Pacific plates
Earthquakes at Convergent Zones
Convergent Zones
• Region where two tectonic plates collide
• Dominant deformation force – compression
- Infrequent and great earthquakes
- Immense amount of energy is stored, then suddenly released
- Shallow, intermediate and deep earthquakes
Megathrust
earthquakes due to
shear stress at the
contact between the
overriding and
subducting plates
2010 Chile Megathrust Earthquake
1960 M9.5 Chile
Earthquake –
largest known
Earthquake
1957 1964
1960
Magnitude 9+
2011
1700
All the M9.0 earthquakes are
along subduction zones!
All have generated tsunamis
Rupture surfaces for the Pacific Rim subduction zones
Potential areas for
M9.0 earthquakes.
Subduction zones
generate the really
large earthquakes.
Earthquakes in continent-
continent collisions
Active Faults in Tibet, China, Southeast Asia
What is the cause of all these faults?
Earthquakes along Transform Faults
• Dominant deformation force: shear
• Stress released in infrequent, major and shallow
earthquakes
• San Andreas fault, CA
- Transform fault accommodating horizontal movement
between the Pacific and North American plates
- NO material created nor consumed
- Fault created as a result of the subduction of the
Farallon plate under the North American plate in the last
30 Ma
20 Ma 10 Ma
Earthquakes along Transform Faults
Earthquakes and
hot spots
• Dominant deformation
forces: tension
(mainly) and shallow
earthquakes
• Frequent, strong and
shallow earthquakes
Earthquakes occur at shallow
depth due to magma movement
beneath the volcano
Hot spots and earthquakes
Hypocenters beneath
Kilauea volcano, Hawaii
When magma is on the
move at shallow depths,
it can generate a nearly
continuous swarm of
relatively small near-
surface earthquakes
Intraplate Earthquakes
• Earthquakes that occur within plates
• Example: New Madrid seismic zone
• Located near St. Louis, MO
• Historic earthquakes similar in magnitude
to West Coast quakes
• Often smaller M than plate boundary
quakes, but
• Can cause considerable damage due to
lack of preparedness
• Can travel greater distances through
stronger continental rocks
Plate Boundary vs. Intraplate Earthquakes
Why the difference in shaking areas?
Intraplate Earthquakes of Eastern Canada
Ancient St.
Lawrence River Rift
and its two failed
arms
Earthquake Effects
• Primary – caused directly by fault movement
• Ground shaking
• Surface rupture
• Secondary
• Liquefaction of ground
• Regional changes in land elevation
• Landslides
• Fire
• Tsunamis
• Disease
Shaking and Ground Rupture
• Ground rupture
- Displacement along the fault causes cracks in surface
• Fault scarp
• Shaking
- Causes damage to buildings, bridges, dams, tunnels,
pipelines, etc.
- Measured as ground acceleration
- Buildings may be damaged due to resonance
• Matching of vibrational frequencies between ground and building
Site Amplification
Liquefaction
• A near-surface layer of water-saturated sand changes rapidly from a solid to a liquid
• Causes buildings to “float” in earth
• Common in M 5.5 earthquakes in younger sediments
• After shaking stops, ground re-compacts and becomes solid
Liquefaction, cont.
Regional Changes in Land Elevation
• Vertical deformation linked to some large earthquakes
• Regional uplift
• Subsidence
• Can cause substantial damage on coasts and along streams
• Can raise or lower
the ground-water table
http://www.nzgs.org/library/uc-geologists-key-
contributors-to-fault-rupture-mapping-following-
the-m7-8-kaikoura-earthquake/
Landslides
• Most closely linked natural hazard with earthquakes
• Earthquakes are the most common triggers in mountainous areas
• Can cause a great loss of human life
• Can also block rivers creating “earthquake lakes”
2002 Nov 3: Denali M 7.9 earthquake
Caused landslides in mountains (unpopulated areas)
Mosaic view of rock avalanches across Black Rapids Glacier.
Photo by Dennis Trabant, USGS; mosaic by Rod March, USGS
2002 Nov 3: Denali Mw7.9 earthquake
2002 Nov 3: Denali Mw7.9 earthquake
Fires • Shaking and surface displacements
• Cause power and gas lines to break and ignite
• Knock over appliances, such as gas water heaters, and leaks ignite
• Threat even greater due to
• Damaged firefighting equipment
• Blocked streets and bridges
• Broken essential water mains
Tsunamis
• Long wavelength sea waves (160km)
• 800 km/hr (500 mi/hr)
• Long wave periods [160km/(800km/hr)] =12 min.
• Generated by
- Earthquake displacement of seafloor
- Submarine mass slides
- Explosive volcanic eruptions
- Impacts
2004 Sumatra Earthquake
and Tsunami
Disease
• Causes: - Loss of sanitation and housing
- Contaminated water supplies
- Disruption of public health services
- Disturbance of the natural environment
- Rupture of sewer and water lines • Water polluted
Groundwater and Energy Resources
• Geologic faults from earthquakes greatly influence underground flow of: - Water
- Oil
- Natural gas
• Fault zones can act as preferential paths
• Can create natural and underground dams - Slow or redirect flow
- Oases in some arid areas
- Accumulation of oil and gas
Mineral Resources
• Faulting may be responsible for accumulation or
exposure of economically valuable minerals
• Mineral deposits develop along fault cracks called
veins
- Can be the source of precious metals
- Those on large fault zones may produce enough to be
economically viable for extraction
Earthquakes Caused by Human Activity
• Loading Earth’s crust, as in building a dam and reservoir - The weight from water reservoirs may create new faults or
lubricate old ones
• Injecting liquid waste deep into the ground through disposal wells - Liquid waste disposals deep in the earth can create pressure
on faults
• Creating underground nuclear explosions - Nuclear explosions can cause the release of stress along
existing faults
Induced Seismicity?
• Earthquake activity
that occurs above the
rate of naturally
occurring seismicity
due to human activity
• Injection of large
volumes of water at
high pressures ➔
hydraulic fracking
Future Earthquake Hazard Reduction
• Frequent small earthquakes
- May help prevent larger ones
- Release pent-up energy
- Reduction of elastic strain
• Scientists try to identify areas that have not
experienced earthquakes in a long time
- Greatest potential for producing large earthquakes
Minimizing the Earthquake Hazard
• Earthquakes strike without warning
- Great deal of research devoted to anticipating earthquakes
- Focus of minimization is on forecasting and warning
• Forecasts assist planners
- Considering seismic safety measures
- People deciding where to live
• Long-term forecasts
- Do not help anticipate and prepare for a specific earthquake
- Need predictions, but not there yet
The National Earthquake Hazard
Reduction Program
• Major goals
- Develop an understanding of the earthquake source
• Obtaining information about the physical properties and mechanical
behavior of faults
• Develop models of the physics of the earthquake process
- Determine earthquake potential
• Detailed study of active regions, determine rates of deformation
• Calculate probabilistic forecasts
- Predict effects of earthquakes
• Obtain information to predict ground rupture and shaking and effects on
structures
• Apply research results
The Canadian National Seismograph Network
Seismic Risk
RISK = VULNERABILITY x HAZARD
Where is vulnerability especially high?
How to determine seismic hazard?
1. Where in the past have we felt significant earthquakes?
2. Are there any geographic patterns? Magnitude repeats?
3. Which areas are most vulnerable?
Seismic Risk
RISK = VULNERABILITY x HAZARD
Where is vulnerability especially high?
How to determine seismic hazard?
What
patterns
do you
see?
Seismic Risk
National Building Code of Canada (NBCC)
Seismic guidelines used to design and construct buildings that are as
earthquake-resistant as necessary for the expected seismic hazard of their
setting
Classification based on velocity of shear waves in the top 30 m of material
Estimation of
Seismic Risk
• Hazard maps show earthquake risk
• Probability of a particular event or the amount of shaking
• Damage potential determined by how the ground moves and how the buildings within the affected region are constructed
http://earthquakescanada.nrcan.gc.ca/hazard-alea/simphaz-en.php
Estimation
of Seismic
Risk in
California
Earthquake Prediction
• The when and where of
a future earthquake
• Not currently possible
with our knowledge of
faults and stress
• Would require:
• The current stress stage of
a fault
• The maximum strength of
a fault
• The stress stage after an
earthquake
Earthquake Forecasting
• The likelihood of
earthquakes happening in
a specified area over a
specified period
• Can be determined from:
• Known faults
• Historical earthquakes
• Seismicity maps
• Paleoseismology from
trenches
• Geodetic strain rates from
GPS - crustal motion maps
Short-Term Prediction
• Pattern and frequency of earthquakes
- Foreshocks
• Deformation of ground surface
- Changes in land elevation
• Seismic gaps along faults
- Areas that have not seen recent quakes
• Geophysical and Geochemical changes
- Changes in Earth’s magnetic field, groundwater levels
Earthquake Warning Systems?
• Possible? Technically yes…
could develop system that
would provide up to 1
minute of warning • Network of seismometers and
transmitters along the San
Andreas Fault
• Earthquake warning system
NOT a prediction tool –
earthquake has already
happened
• Concern for liability issues • False alarms
• Failures
Community Adjustments to the Earthquake Hazard
• Location of critical facilities - Should be located in earthquake safe locations
- Need detailed maps of ground response (microzonation)
• Structural protection - Buildings must be designed and/or retrofitted to withstand
vibrations
- “Earthquakes don’t kill people, buildings kill people”
• Education - Could include pamphlets, workshops, information on internet
- Earthquake and tsunami drills
• Increased insurance and relief measures - Vital to help recovery from an earthquake
Earthquake Hazard Model Design
Earthquake
Hazard
Model
Design
example
2011 Tohoku Earthquake
• M 7.2 earthquake detected two days before - This was a size that was predicted to happen any day
- Did not expect it to be a foreshock to a larger earthquake
• Pacific plate slid under the Eurasian plate - Earthquake 500 times more powerful than history suggested
- Also produced larger than predicted tsunami
• Automatic alert went out 8 seconds after P waves confirmed - Allowed 10 seconds of preparation
2011 Tohoku
Earthquake, cont.
• Few buildings collapsed • Allowed occupants to
escape
• However, widespread superficial damage and minor structural damage
• Most damage due to tsunami damage, liquefaction, and landslides
• M 9.0 earthquake triggered a tsunami that was up to 120 m
• Fukushima nuclear disaster
Japan The most seismically active
country in the world. 2 or 3 large
earthquakes per century
Up to 5 M8 earthquakes per
century!
http://www.esri.com http://prezi.com
Mexico City, 1985
• Mw 8.3 megathrust earthquake broke ~350 km away from
Mexico City
• ~10,000 people died
• What caused this
earthquake?
• Michoacan
Seismic gap
North
American
plate
Cocos plate
Why so much shaking
here??
Observations - Mexico City, 1985
• Very strong motions produced 400 km from the fault rupture due to the
response of soft clays (i.e. near-surface geology)
• Motion and damage should have been minimal because of distance
from source
Resonance!
• Amplification of
surface waves
due to sediment
filled basin
• Specific
resonance of
medium-height
buildings
• Weak structures
Most severe damage to almost
400 buildings of between 7 and
18 storeys in height. (EEFIT,
1986)
Chapter 3 Summary
• Earthquakes are common along tectonic plate boundaries
where faulting is common.
• Faults are fractures where rocks on one side of the
fracture have been offset with respect to rocks on the
other side.
• Displacement is caused by compressional, tensional, or
shearing stresses and can be mainly horizontal or mainly
vertical.
Chapter 3 Summary, cont.
• A fault is usually considered active if it has moved during
the past 10,000 years and potentially active if it has moved
during the past 2 million years.
• Before an earthquake, elastic strain builds up in the rocks
on either side of a fault as the sides pull in different
directions.
• Released elastic strain energy radiates outward in all
directions from the ruptured surface of the fault in the form
of seismic waves.
Chapter 3 Summary, cont.
• Seismic waves are vibrations that compress (P) or shear
(S) the body of Earth or travel across the ground as
surface waves.
• Some faults exhibit tectonic creep, a slow displacement
not accompanied by felt earthquakes.
• Large earthquakes release a tremendous amount of
energy measured on a magnitude (M) scale.
Chapter 3 Summary, cont.
• Earthquake intensity varies with the severity of shaking
and is affected by proximity to the epicenter, the local
geological environment, and the engineering of structures.
• Buildings highly subject to damage are those that (1) are
constructed on unconsolidated sediment, artificially filled
land, or water-saturated sediment, all of which tend to
amplify shaking; (2) are not designed to withstand
significant horizontal acceleration of the ground; or (3)
have natural vibrational frequencies that match the
frequencies of the seismic waves.
Chapter 3 Summary, cont.
• Most earthquakes occur on faults near tectonic plate
boundaries.
• Intraplate earthquakes are also common various parts of
the United States.
• Some of the largest historic earthquakes in North America
occurred within the plate in the central Mississippi Valley in
the early 1800s.
Chapter 3 Summary, cont.
• The primary effect of an earthquake is violent ground
motion accompanied by fracturing, which may shear or
collapse large buildings, bridges, dams, tunnels, pipelines,
levees, and other structures.
• Other effects include liquefaction, regional subsidence,
uplift of the land, landslides, fires, tsunamis, and disease.
• Natural service functions include enhancing groundwater
and energy resources and exposing or contributing to
formation of valuable mineral deposits.
Chapter 3 Summary, cont.
• Human activity has locally increased earthquake activity
by fracturing rock and increasing water pressure
underground below large reservoirs, by deep-well disposal
of liquid waste, and by setting of underground nuclear
explosions.
• Understanding how we have caused earthquakes may
eventually help us control or stop large natural
earthquakes.
Chapter 3 Summary, cont.
• Reducing earthquake hazards requires detailed mapping
of geologic faults, the cutting of trenches to determine
earthquake frequency, and detailed mapping and analysis
of earth materials sensitive to shaking.
• Adjustments to earthquake hazards include improving
structural design to better withstand shaking, retrofitting
existing structures, microzonation of areas of seismic risk,
and updating and enforcing building codes.
Chapter 3 Summary, cont.
• To date, scientists have been able to make long- and
intermediate-term forecasts for earthquakes using
probabilistic methods but not consistent, accurate short-
term predictions.
• Early warning systems have been shown to be effective in
Japan, but no such system exists in the United States or
Canada.
• Warning systems and earthquakes prevention are not yet
reliable alternatives to earthquake preparedness.