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EARTHQUAKES

Definition of Earthquake

Ground trembling or shaking due to sudden

release of energy accumulated in deformed

rock.

Rocks at relatively shallow depths in the Earth’s

crust (cool and brittle) deform until they reach a

yield point and break forming faults.

At new ruptures or movement along older faults,

strain energy is released suddenly and the earth

quakes.

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Aftershocks and Foreshocks

Aftershock — a series of smaller quakes that

occur after a major earthquake as the crust

readjusts to the change in stress.

Aftershocks are usually smaller but are very

destructive.

Foreshocks — small earthquakes that may (or

may not) precede a major quake by days to

years.

It is hoped that monitoring foreshocks may be

useful

as a prediction tool.

Location of an Earthquake

Focus (Hypocenter): precise underground location at

which rocks begin to rupture.

Epicenter: point on the Earth’s surface directly above

the focus.

The energy of an earthquake is

released from the slippage along a

fault and radiates in all directions.

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Seismology

Seismology is the study of earthquakes.

Seismographs are instruments that detect and record earthquakes.

In the seismograph, inertia tends to

keep the suspended mass motionless

while the recording surface (on a

rotating drum) vibrates with the

bedrock.

Thus the seismograph measures the

displacement or movement of the

ground as seismic waves pass through

the station.

QuickTime™ and a Cinepak decompressor are needed to see this picture.

Typical seismographs consist of

rotating drums with recording

paper.

Most modern seismographs

now record data digitally and

are available in near real time

on the internet.

USGS

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Some Uses of the Seismograph

Measurement of the energy released by an earthquake (Richter Magnitude)

Measurement of the location of an earthquake (epicenter)

Interpretation of the Interior of the Earth (Chapter 11)

Detection of underground nuclear bomb testing

Numerous spin-off apps

Thousands of seismographs are

deployed in national and international

networks to record earthquakes.

Seismographs

This extensive network

permits us to determine

the location of an

earthquake and to

study the earth’s

interior.

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Seismograms record the arrival of seismic energy at the recording

station.

Careful analysis of seismograms indicates that several different types

of seismic waves are recorded.

•Body waves travel through the

Earth’s interior and provide useful

information about the earthquake

and the interior structure of the

Earth.

•Surface waves move along the

surface of the Earth. They tend to be

the most destructive.

Seismic Waves – Seismic Energy Traveling

How does seismic energy propagate through the Earth?

earthquake

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Two types of body waves:

P-waves - primary waves -

compress and extend

material in the direction of

wave travel.

S-waves - secondary

waves move the material in

a direction that is normal to

the direction of wave travel.

Body Waves: P-waves

P-waves travel ~6 km/sec. They are

compressional waves and particle

motion is in the travel direction.

QuickTime™ and a Cinepak decompressor are needed to see this picture.

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Body Waves: S-waves

S-waves travel in the crust ~3.6 km/sec

(slower than p-waves). They

propagate through the Earth by

displacing particles perpendicular to

the direction of travel.

QuickTime™ and a Cinepak decompressor are needed to see this picture.

P-waves travel ~1.7 times

faster than s-waves.

Hence, the farther the

seismograph is from the

location of the earthquake

the greater the difference

in arrival times between

the p-wave and s-wave.

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Seismograms

From the arrival times of the different types of waves, we can see that

VelocityP-waves > VelocityS waves > Velocitysurface waves

The amplitude shows relative destructive force(energy) of each wave:

Energysurface waves > EnergyS-waves > EnergyP-waves

•Note that the S-wave arrives at this seismograph station ~240

seconds after the arrival of the P-wave.

• The greater the difference between the arrival of the P- and S-waves

(the S-P interval), the more distant the earthquake from the recording

station.

S-P interval

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Locating the Distance to an Earthquake

We can determine the distance to the earthquake epicenter from the

seismograph by the lag time between the P-waves and S-waves.

Locating the Epicenter of an Earthquake

If we have the

distance to the

earthquake

determined for 3

different seismograph

stations, we can

locate the epicenter

by triangulation.

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Measuring the Size of an Earthquake

Richter Scale

Moment-Magnitude Scale

Modified Mercalli Intensity Scale

Earthquake Magnitude

The magnitude of an earthquake is related to the amount of energy

released during a seismic event.

The Richter scale is used to describe earthquake magnitude. It is

determined from deflections recorded on seismographs and uses a

logarithmic scale - each unit is a 10-fold increase in the amplitude of

the seismic waves and a 32-fold increase in energy.

Earthquakes with magnitudes less than ~2.0 are not commonly felt by

people. Although there is no upper limit to the Richter scale, the

largest earthquakes have magnitudes of ~8.9. The energy released by

an earthquake of this size is equal to the detonation of 1 billion tons of

TNT.

The Richter scale is not used to express damage like the Mercalli

scale.

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The Richter scale is quick-n-dirty

The Richter magnitude is

determined from the

maximum amplitude of

displacement measured on

seismogram at a known

distance from the

epicenter.

In this example, the

magnitude is 5.0 if the

max. amplitude is 23 mm

and s-p interval is 24

seconds (distance 215

km).

Earthquake wave amplitude decreases with distance from the epicenter.

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Earthquake magnitude describes the

size of an earthquake.

Earthquake Magnitude

Earthquake magnitude is calculated

on a logarithmic scale, so...

The difference in energy-release from

one magnitude to the next is very

large.

Magnitude 4 1x

Magnitude 5 32x

Magnitude 6 1000x

Magnitude 7 32,000x

Magnitude 8 1,000,000x

Relative amount of energy

released during earthquake:

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Moment-Magnitude Scale

Actually what is currently used.

Based on “seismic moment”

– Moment= total length of fault rupture

X (times) depth of fault rupture

X total amount of slip along rupture

X strength of rock

– Sources of measurements

Field observations

Location of foci of shock and aftershocks

Geologic cross sections

Earthquake Intensity

Earthquake intensity describes the

strength of ground shaking at a

location.

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The local intensity of an earthquake

depends on:

1. The magnitude of the earthquake

2. The distance from the epicenter

3. The type of ground

Earthquake Intensity

Earthquake Intensity and Magnitude

The intensity of an earthquake is based on the observed effects of the

earthquake — it is an assessment of the damage caused by an

earthquake at a specific location. Thus the intensity of an earthquake

depends upon the strength of the earthquake, but also on the distance

from the epicenter — it varies from place to place with respect to the

earthquake's epicenter.

The modified Mercalli intensity

scale is composed of 12 increasing

levels of intensity that range from

imperceptible shaking to

catastrophic destruction. It does not

have a mathematical basis but is

arbitrary and based on observed

effects.

Figure shows Mercalli intensity of Tamiskaming,

Quebec earthquake (M6.2) of 1935

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The following is an abbreviated description of the 12 levels of Modified Mercalli intensity.

I. Not felt except by a very few under especially favorable conditions.

II. Felt only by a few persons at rest, especially on upper floors of buildings.

III. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not

recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a

truck. Duration estimated.

IV. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors

disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars

rocked noticeably.

V. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned.

Pendulum clocks may stop.

VI. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage

slight.

VII. Damage negligible in buildings of good design and construction; slight to moderate in well-built

ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys

broken.

VIII. Damage slight in specially designed structures; considerable damage in ordinary substantial buildings

with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns,

monuments, walls. Heavy furniture overturned.

IX. Damage considerable in specially designed structures; well-designed frame structures thrown out of

plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.

X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with

foundations. Rails bent.

XI. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.

XII. Damage total. Lines of sight and level are distorted. Objects thrown into the air.

Destructive Effects of

Earthquakes

Direct Hazards:

1. Ground shaking

2. Tsunamis

3. Fire

4. Landslides & Ground

Subsidence

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1. intensity

2. duration

3. nature of rocks and soil on

which a structure is located

4. structural design

Ground Shaking

Seismic vibrations damage structures. The

amount of structural damage is controlled by

Liquefaction is when soil turns to a mobile fluid, resulted in buildings

settling and collapsing under their own weight.

Buildings with foundations in bedrock are generally more earthquake

ready.

Much of the "flat lands" in the East Bay are susceptible to liquefaction.

Left photo from Marina District in S.F.

Above photo where bridge piers sink into

ground

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Tsunami

Tsunamis are giant water waves that usually result from the vertical

displacement of the seafloor during

an earthquake.

An earthquake can occur in one

area and the tsunami may inundate

another thousand of kilometers

away.

The tsunami that occurred as a result of

the M9.1 earthquake on Dec. 26, 2004

inundated coastlines around the Indian

Ocean resulting in ~275,000 fatalities.

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Fire

Although ground shaking can be

devastating, fires that occur after an

earthquake can be responsible for even

more deaths and damage.

The photos on the right show the fire

that occurred after the 1906 San

Francisco earthquake and burned down

500 blocks of the city.

The left photo shows the fire in

the Marina district after the

Loma Prieta earthquake (1989).

Landslides and Ground Subsidence

Earthquakes are a trigger for landslides - this is a particular hazard in

the S.F. Bay Area.

In addition, ground shaking and liquefaction can cause the ground to

subside - this is particularly damaging to buildings.

Landslide after 1999 Chi-Chi

earthquake in Taiwan blocking a

river.

Homes damaged in Alaska by ground

subsidence during an earthquake.