ERTH2404 L10 Earthquakes Upload

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ERTH2404

Lecture 11: Earthquakes

Dr. Jason Mah


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Review

Peaceful versus Explosive eruptions Defined by viscosity, volatiles, volume Mafic and felsic Peaceful: icelandic, hawaiiian, strombolian Explosive: vulcanian, plinian, caldera

Hazards Primary: pyroclastic flow (nue ardente) & fall, gas Secondary: lahar

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Reading assignment

Please read Kehews book to complement thematerial presented in this lecture:

Chap. 8 p. 272-315;

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Lecture contents

Plate tectonics & the relation to earthquakes Earthquakes at spreading centers Earthquakes at transform faults Earthquakes at convergent zones

Intraplate earthquakes Seismic waves and their characteristics Locating the epicenter of an earthquake Estimating the size of an earthquake

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Fundamentals definitions Earthquakes : movement of the Earth along

breaks in crust Faults : breaks in crust Displacement : motion on faults Seismic waves : energy propagation, motion

generated when fault releases energy Wavelength Amplitude Frequency

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Plate tectonics and earthquakes

6Ref.: Abbott, P.L. 2004. Natural Disasters.

4th Edition. Fig. 2.12. Shown with permission.


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Plate tectonics and earthquakes Earthquakes do not occur randomly Relation between tectonic environment,

deformation forces and earthquake size

No significant earthquake activity associatedwith hot spots

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Environment Deformationforce

Earthquakesize

Spreading centers Tension SmallTransform faults Shear Large

Convergent zones Compression Gigantic


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1. Earthquakes at spreading centers

Frequent Shallow

Small

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Formation of new oceanic lithosphere1. Centering: moving lithosphere centers about a

"static" hot region in the mantle

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Ref.: Abbott, P.L. 2004. Natural Disasters.4th Edition. Fig. 2.24. Shown with permission.


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Formation of new oceanic lithosphere2. Doming: increase in heat causes the Earths

lithosphere to buldge up into a dome

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Tension


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Formation of new oceanic lithosphere3. Rifting: Area is pulled-apart by tensional forces.

Rocks fracture. The central area sags. Volcanismis common.

11Ref.: Abbott, P.L. 2004. Natural Disasters.


Tension


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Formation of new oceanic lithosphere4. Spreading: New ocean floor is formed in pulled-

apart area.

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Ref.: Abbott, P.L. 2004. Natural Disasters.

4th

Edition. Fig. 2.24. Shown with permission.

Tension


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East African rift system "Rifting" stage Rifting started 25 Ma ago

Three zones of tension meet at theAfar triple junction Red Sea Gulf of Aden East African rift system

Frequent, shallow earthquakes Current seismic activity in Africa:

http://neic.usgs.gov/neis/current/africa.html

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East African rift system

14 R e

f . :

A b b o t t

, P . L . 2

0 0 4

. N a t u r a

l D i s a s t e r s .

4 t h

E d i t i o n

. F i g

. 2 . 2 6 .

S h o w n w i t

h p e r m i s s i o n

.

Afar triple junction


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15 S o u r c e :

U S G S


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2. Earthquakes along transform faults

Sporadic Deep

Potentially large

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San Andreas fault Transform fault accommodating horizontal

movements between the Pacific and NorthAmerican plates

Complex system of sub-parallel faults Shear stress dominant

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San Andreasfault

18 R e

f . :

A b b o t t

, P . L . 2

0 0 4

.

N a t u r a

l D i s a s t e r s

.

4 t h

E d i t i o n

. F i g

. 4 . 2 .

S h o w n w i t


.


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San Andreas fault, 1989 Loma Prieta

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Source: California Geological Survey

Shear stress


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San Andreas fault Animation: possible future movement along the

San Andreas fault

http://visearth.ucsd.edu/VisE_Int/aralsea/bigone.html

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http://visearth.ucsd.edu/VisE_Int/aralsea/bigone.htmlhttp://visearth.ucsd.edu/VisE_Int/aralsea/bigone.htmlhttp://visearth.ucsd.edu/VisE_Int/aralsea/bigone.htmlhttp://visearth.ucsd.edu/VisE_Int/aralsea/bigone.html


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3. Earthquakes at convergent zones

Sporadic Shallow to deep

Potentially gigantic

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Subduction zones

Plates under different stress regimes atdifferent depthsShallow earthquakes:

Compression: plates pushing against each other Tension: subducting plate bending downwards Shear: plates rubbing against each other

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Subduction zones

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Tension

CompressionShear


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Subduction zones have 3 stress regimes! Therefore they may contain the most energy

Megathrust fault: largest boundary betweenthe subducting and overriding plate

The worlds largest earthquakes aremegathrust earthquakes

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Shallow, intermediate, and deep earthquakesobserved

Intermediate and deep earthquakes aregenerated when high mantle T causes coldrocks in the subducting plate to yield

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Juan de Fuca plate subducting at a rate of afew cms per year

Cascadia megathrust Contact between Juan de Fuca and North

American plate

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Juan de Fuca plate

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4th

Edition. Fig. 4.9. Shown with permission.Source: Earthquakes Canada


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3. Earthquakes at convergent zones 1700 Cascadia earthquake Magnitude 9

Largest in Canadian history Recall that Loma Prieta was only M6.9

3-5 minutes of ground shaking People could not stand and felt sick

Tsunami across the Pacific

Japanese record dates event to 26 January 1700 21:00 Completely destroyed the winter village of the Pachena

Bay people of Vancouver Island with no survivors

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Oral record: first nation people of coastal BC:

In the period not long before European contact, astrong earthquake occurred at night. It wasfollowed by a large tsunami that destroyed thevillage of Pachena Bay

Canoes came down in the trees

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Oral record from: Quileute first nation, Washington State

Story describing an epic battle along the coastbetween the Thunderbird and the Whale

During the struggle there is a "shaking, jumpingup and trembling of the earth beneath, and arolling up of the great waters"

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Japan, Thoku March 11, 2011 Megathrust earthquake M9 Pacific plate thrusts under North American Plate

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3. Earthquakes at convergent zones Japan, Thoku

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USGS


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3. Earthquakes at convergent zones Japan, Thoku

Tsunami Animation

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NOAA


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Stats for Japan, Thoku (National PoliceAgency and other news agencies)

15 878 deaths 129 225 buildings collapsed 254 204 buildings half collapsed Nuclear power plant meltdown Tsunami wave 10m to 40.5m in height

Half way up Dunton tower Reached 10k inland

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4. Intraplate earthquakes Earthquakes occurring far from any plate

boundary Often associated with zones of lithospheric

weakness Failed rifts Impact craters Hot spot track

Re-activated as earthquake zones by latertectonic forces

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4. Intraplate earthquakes: Canada

36Source: M. Lamontagne, NRCan


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St. Lawrence River rift Major rift (500-600 Ma) Presently buried under younger rocks Rift coincides with several Eastern Canada

intraplate earthquakes

Rift : extensional plate tectonics Crust and lithosphere are pulled apart

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Rifted margin

Source: J. Adams, Earthquakes Canada


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4. Intraplate earthquakes: Canada Charlevoix impact crater (357 15 Ma)

Semi-circular area on the north shore of the St.Lawrence river

Differs from regional topography Shatter cones discovered during regional mapping Heavily eroded

All crater-filling products removed Associated with earthquake activity

Impact "scars" act as zones of weakness in the Earthscrust

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Charlevoix impact crater (357 15 Ma) 54 km dia

40 S o u r c e :

M . L

a m o n t a g n e

. S h o w n w i t


.


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Charlevoix: events from Jan 78 Sept 99

41 R e

f . :

N R C a n

. S h o w n w i t

h p e r m i s s i o n .


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Monteregian Hills, QC 10 mountains aligned E-W over 200 km Intrusive bodies

Less resistant host rock eroded away Formed by a Cretaceous hot spot (150 Ma)

Overlying plate moving west Oldest mountain at Oka

Cause of present-day seismic activity?

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Monteregian Hills, QC

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Ref.: Grice, J. 1989

Direction of plate motion


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Monteregian Hills, QC

44Source: J. Adams, Earthquakes Canada


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Summary of Earthquake Environment

Environment Force regime Earthquake size Depth Example

SpreadingCenter

Tension SmallLow magnitude

Shallow< 70 km

East Africa,Afar triple junction

Transformfault Shear LargeTypically < M8 Deep300 km to 700km

San Andreas,CA

Convergentzones

Compression(tension &shear aresecondary)

GiganticPotentially > M8

Shallow to Deep< 70 km to700km

Cascadia,Japan, Thoku

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Seismic waves Earthquakes are caused by sudden

movements along faults

1. Stress from deformational forces build upuntil rocks fail Stress must build up over several years before

enough energy is stored to cause rupture2. Rocks fracture and shift3. Energy is released as seismic waves

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Fault rupture

Rupture is initiated at the hypocenter Point of weakness along the fault Point of origin of an earthquake in the subsurface

Epicenter : the hypocenter projected to theEarths surface

directly above hypocenter Fault propagate along the fault surface in a

few seconds

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Fault rupture

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R e

f . :

A b b o t t , P . L . 2 0 0 4

. N a t u r a l D i s a s t e r s

.

4 t h

E d i t i o n . F i g . 3

. 1 2

. S h o w n w i t h p e r m i s s i o n

.


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Seismic waves

Two types: Body waves: propagate through the whole body of

the Earth Large earthquakes generate body waves recorded

all over the world Surface waves: propagate only near the Earths

surface

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Body waves0.5 - 20 HzFast

Surface waves0.005 - 0.1 HzSlow

Seismicwaves

Rayleigh waves

Love waves

Primary (P) waves

Secondary (S) waves


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Seismic waves

Seismic waves: waves caused by the release of energy in the Earth

How are seismic waves recorded? Seismometer : sensor that detects Earths motions Seismograph : an instrument that records these

motions Seismogram : output from seismography

paper record or digital file of an earthquakes motion

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Seismic waves

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Animation: Seismographs

USGS

Dragon SeismoscopeZhang Heng (78-139AD)


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Seismic waves Modern seismograph

station Quiet environment

Good contact with bedrock Three seismometersdetect the threecomponents of groundmotion

East WestNorth SouthUp Down

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P h o t o :

G . A

t k i n s o n

, C a r

l e t o n U .


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Seismic waves1. Body waves : propagate through the whole body of

the Earth Large earthquakes generate body waves recorded

all over the world

2. Surface waves : propagate only near the Earthssurface

Tend to have stronger vibrations, higher wave amplitudeand cause the most damage

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Body waves

0.5 - 20 HzFast

Surface waves0.005 - 0.1 HzSlow

Seismicwaves

Rayleigh waves

Love waves

Primary (P) waves

Secondary (S) waves


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Seismic waves: Body waves

Primary (P) waves : Compressional energy Small amplitude Travel fastest, recorded first Propagate in solids, liquids, gases Typical velocities

1500 m/s water 2500 m/s sediments 5000 m/s hard rock

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Secondary (S) waves : Shear energy

More destructive than P waves because they shake

buildings sideways Larger amplitude than P waves Travel 1.7 times slower than P waves

Propagate in solids only

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R e f

. : A

b b o t t

, P . L . 2

0 0 4

. N a t u r a

l D i s a s t e r s

.

4 t h

E d i t i o n

. F i g

. 3 . 1 8 . S

h o w n w i t

h p e r m i s s i o

n .


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Seismic waves: Surface waves

Love waves : Cause horizontal shifting Slower than P and S-waves but faster than Raleigh

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USGS


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Seismic waves: Surface waves

Rayleigh waves : Longitudinal and transverse motions Slowest velocity

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USGS


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Seismic waves

Animation: Seismic wave motion

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Epicenter Location

Question How do we determine the location of the

epicenter?

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Epicenter Location: Fundamental Principle

P waves travel faster than S waves Further away from the epicenter, the greater

the difference betweenthe P and S arrival times

Arrival time : time at which a particular seismicwave is recorded by a seismometer

A minimum of 3 seismograph stations isrequired to locate the epicenter

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Epicenter Location: Step 1

Determining the distance between theepicenter and one seismograph station

Step 1.1 Identify the P arrival time Step 1.2 Identify the S arrival time Step 1.3 Compute the difference between

the P and S arrival times

Step 1.4 Read the corresponding distance onthe travel time vs distance curves

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Difference between P and S arrival times is3 minutes 45 seconds

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Travel time vs distance curves

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The corresponding epicentral distance is 2250 km R e

f . :

A b b o t t , P . L . 2 0 0 4

. N a t u r a l D i s a s t e r s

.

4 t h

E d i t i o n . F i g . 3

. 2 1

. S h o w n w i t


.


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The epicenter could be located anywhere at adistance of 2250 km from the seismographstation

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2250 km

Station 1Map view


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Epicenter Location: Repeat for 3 stations

Determining the direction from where theseismic energy came

Repeat step 1 for a minimum of two additional

seismograph stations Select stations covering evenly the area of interest

Example: Station 2: epicentral distance = 1750 km Station 3: epicentral distance = 1500 km

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Epicenter Location: 2 nd Station

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2250 km

1750 km

Station 1

Station 2

?

?

Map view


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Epicenter Location: 3 rd Station

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2250 km

1750 km

1500 km

Station 1

Station 2

Station 3

Map view

Additional stations canbe used to improvethe accuracy


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Estimating the Magnitude

Earthquakes are related to the energy releaseby a fault

Larger faults (length, width) can have larger

earthquakes Bigger the earthquake

Greater the ground shaking Greater the amplitude recorded on seismograms

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How should the "size" of an earthquake beestimated?

Suggestions: Cost of damage? Lives lost? Length of rupture of the earthquake fault? Amount of ground shaking?

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Modified Mercalli Intensity Scale Developed by Mercalli, 1902 Based on extent of damage

Problems with the Mercalli Intensity Scale Depends on distance from epicenter Depends on surface materials Building design It is subjective!

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Richter Magnitude scale

Magnitude : total amount of energy releasedduring fault rupture

Developed by Frank Richter, 1935 M = log10Amplitude + constant

Constant is a wave attenuation factor

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Logarithmic scale Increasing the magnitude by 1 is an increase in

amplitude by a factor of 10

Magnitude 2 stamping your foot on the floor

Magnitude 2.5 smallest earthquake felt by people

Magnitude 3 amplitude of 1 mm measured100 km from the epicenter

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Logarithmic scale

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Magnitude 4

Magnitude 5

Magnitude 6 100mm

10mm

1mm

10 times larger

10 times larger

S o u r c e :

M . L

a m o n t a g n e

, N R C a n


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Richter Magnitude: Measurement

Based on measurements made onseismograms

Calculations summarized visually in anomograph

Difference between P and S arrival times Plot on the left column

Maximum amplitude of seismic waves Plot on the right column Read magnitude in the central column

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Richter Magnitude scale: Station 1

77 R e f

. : A

b b o t t

, P . L . 2

0 0 4

. N a t u r a

l D i s a s t e r s

.

4 t h

E d i t i o n

. F i g

. 3 . 2

4 . S h o w n w i t


.


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M=7 : "Major" earthquake, causes seriousdamage up to ~100 km

San Francisco, Loma Prieta M6.9

79USGS


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M=8 : "Great" earthquake, great destruction,loss of life over several 100 km 1906 San Francisco >3000 deaths 225 000 homeless 28 000 buildings $400M in 1906 dollars $9 857M in 2011 dollars

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1906 San Francisco, M8

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USGS

Financial district, http://www.sfmuseum.org


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M=9 : Rare great earthquake, major damageover a large region over 1000 km

Chile 1960 M9.5, Alaska 1964 M9.2, Sumatra 2004

M9.1, Japan 2011 M9

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Chile 1960 M9.5 1 655 killed 3 000 injured 2M homeless $550M in Chile $75M in Hawaii 138 killed, $50M in Japan 32 killed in Philippines

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References

Animations http://www.iris.edu/hq/programs/education_

and_outreach/animations#CC

Current world seismicity//neic.usgs.gov/neis/current/

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