Geography quiz 30 multiple questions

profileVincent666
Lecture3A-Earthquakes2.pdf

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.