Earthquakes Chapter 15 - Belmont ESC11esc11.weebly.com/uploads/5/9/3/7/593742/_web_-_chapter15.pdf · See Chapter 15 Worksheet Package How is an Earthquake’s Epicenter Located?

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Chapter 15…what’s the connection?

Earthquakes and Plate Tectonics…

As with volcanoes, earthquakes are not randomly distributed over the globe

Figure showing the distribution of earthquakes around the globe

At the boundaries friction causes plates to stick together.

Earthquakes occur when built up energy causes them to break apart.

Figure showing the distribution of earthquakes around the globe

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Where Do Earthquakes Occur and How Often?

• ~80% of all earthquakes occur in the Ring of Fire(most of result from convergent margin activity)

• ~15% occur in the Mediterranean-Asiatic belt

• remaining 5% occur in the interiors of plates and on spreading ridge centers

• more than 150,000 quakes strong enough to be felt are recorded each year

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Where do earthquakes form?

Figure showing the tectonic setting of earthquakes

Where do earthquakes form?

Figure showing the tectonic setting of earthquakes

ADD ARROWS TO DIAGRAM

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What is the Elastic Rebound Theory?

•Explains how energy is stored in rocks

•Rocks bend until the strength of the rock is exceeded

•Rupture occurs and the rocks quickly reboundto an undeformed shape

•Energy is released in waves that radiate outward from the fault

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What is an Earthquake?Ground movement caused by the sudden release of seismic energy due to tectonic forces.

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The Focus and Epicenter of an Earthquake

• The point within Earth where faulting begins is the focus or hypocenter

• The point directly above the focus on the surface is the epicenter

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Fault Traces Southern

Vancouver Island Wrangellia

Crescent Terrane

Pacific Rim Terrane

Fault Traces Southern

Vancouver Island

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What are Seismic Waves?

• Response of earth material to the arrival of energy fronts released by rupture

• Two types:– Body waves

• P and S– Surface waves

• R and L

Body Waves: P and S wavesP or primary waves• fastest waves• travel through solids,

liquids, or gases• compressional wave,

material movement is in the same direction as wave movement

S or secondary waves• slower than P waves• travel through solids only• shear waves - move

material perpendicular to wave movement

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P WavesTwo Types of Body Waves

P-WavesS WavesTwo Types of Body Waves

S-Waves

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Surface Waves: R and L waves

Surface Waves• Travel just below or along the ground’s surface• Slower than body waves• Rolling and side-to-side movement• Especially damaging to buildings

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Two Common Types of Surface Waves

Functioning of Seismograph

Seismographs record earthquake events on a seismogram.

At convergent boundaries, focal depth increases along a dipping zone called a Benioff zone

How is an Earthquake Recorded?

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Using Seismic Wave Behavior• P waves arrive first, then S waves, then L and R• Average speeds for all these waves is known• The difference in arrival times at a seismograph station can be

used to calculate the distance from the seismograph to the epicenter.

How is an Earthquake’s Epicenter Located?

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Earthquake Waves: Interpreting a Seismogram WS

1. Identify(i)the first P-waves _______ (ii)the first S-waves _______(iii)the first L-waves _______2. Describe (i)P-waves _________________________________________________________(ii)S-waves ________________________________________________________(iii)L-waves _______________________________________________________3. Which type of earthquake/seismic waves (i)will not pass through the earth's outer core? _______(ii)cause the most damage to surface buildings? _______(iii)pass through all the earth's internal zones? _______4. What is the difference in arrival times for the P and S-waves? ___________

See Chapter 15 Worksheet Package


Time-Travel Graphs

Show the average travel times for

P-waves and S-waves.


The farther away a seismograph is from the focus of an earthquake, the longer the interval between the arrivals of the P-and S- waves.

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Map of Epicenter, using Time-Travel

differences of P-waves and S-waves


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• Three seismograph stations are needed to locate the epicenter of an earthquake

• A circle where the radius equals the distance to theepicenter is drawn

• The intersection of the circles locates the epicenter

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Earthquake Magnitude

1 every 5-10 yearsGreat earthquake. Can be totally destructive near the epicentre.

> 8.0

20Major earthquake. Serious damage.

7.0-7.9

100May cause a lot of damage in very populated areas

6.1-6.9

500Slight damage to buildings and other structures

5.5-6.0

30,000Often felt, only minor damage2.5-5.4

900,000Usually not felt, but recorded< 2.5

Approx. number each year

Earthquake EffectsMagnitude Earthquake Depth and Plate Tectonic Setting

Can Earthquakes be Predicted?

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Earthquake Depth and Plate Tectonic Setting


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Can Earthquakes be Predicted?Earthquake Prediction Programs– include laboratory and field studies of rocks before,

during, and after earthquakes– monitor activity along major faults– produce risk assessments

Earthquake Precursors: – changes in elevation or

tilting of land surface– fluctuations in

groundwater levels, – magnetic field, electrical

resistance of the ground– seismic dilatancy model– seismic gaps


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Earthquake HazardsThese are important hazards to understand:

EQ’s commonly strike without warningno time for evacuationnot a predictable trend to number of earthquakes, their magnitude or location • 1000's of large earthquakes every year • ~ 20 are > M7.0 and these account for 90% of

the energy released and 80% of all the fatalities

EQ Tsunamis and building collapse kills thehighest number of people per year(> 1 million during the past century)

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What are the Destructive Effects of Earthquakes?

• Ground Shaking - amplitude, duration, and damage increases in poorly consolidated rocks

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• Building collapse• Fire• Tsunami• Ground failure• Economic and Societal

Impacts

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Seismic Risk Map – North America Seismic Risk Map – Greater Victoria

Belmont

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How are the Size and Strength of an Earthquake Measured?

Modified Mercalli Intensity Map1994 Northridge, CA earthquake

Magnitude 6.7

Modified Mercalli Scale(Intensity)

• subjective measure of damage done and people’s reactions

• isoseismal linesidentify areas of equal intensity

Modified Mercalli Scale

(Intensity)


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Richter Scale (Magnitude)

• measures total amount of energy released by an earthquake; independent of intensity

• Amplitude of the largest wave produced by an event is corrected for distance and assigned a value on an open-ended logarithmic scale


Nomogram

S.17Seismic Wave Paths Through the Earth

S-waves

P-waves

Earth’s Layered Structure

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Seismic Wave Paths Through the Earth

S-waves

P-waves


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Layers Defined by Composition


Crust• Continental crust

- Upper crust = granitic rocks- Average density is about 2.7 g/cm3

- Up to 4 billion years old• Oceanic crust

- Basaltic composition - Density about 3.0 g/cm3

- Young -180 million years or less

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Layers Defined by Composition


Mantle• Below crust to a depth of 2900 kilometers• Composition of uppermost mantle is the igneous

rock peridotite (changes at greater depths).Core • Below mantle• Sphere with a radius of 3486 kilometers• Composed of an iron-nickel alloy• Average density of nearly 11 g/cm3

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Layers Defined by Physical Properties


Lithosphere• Crust and uppermost mantle (about 100 km thick)• Cool, rigid, solidAsthenosphere• Beneath the lithosphere• Upper mantle• To a depth of about 660 kilometers• Soft, weak layer that is easily deformed

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Layers Defined by Physical Properties


Lower Mantle• 660–2900 km, more rigid layer• Rocks are very hot and capable of gradual flow.Inner Core• Sphere with a radius of 1216 km• Behaves like a solid

Outer Core• Liquid layer, 2270 km thick• Convective flow of metallic iron within generates

Earth’s magnetic fieldS.20b


Discovering Earth’s Layers


• Velocity of seismic waves increases abruptly below 50 km of depth

• Separates crust from underlying mantleShadow Zone• Absence of P waves from about 105 degrees to

140 degrees around the globe from an earthquake• Can be explained if Earth contains a core

composed of materials unlike the overlying mantle

Moho

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The Shadow Zone

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The Shadow Zone

NO S-wavesNO

P-waves or S-waves

ONLYP-waves

Shadow Zone

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The Shadow Zone

NO S-wavesNO

P-waves or S-waves

ONLYP-waves

Shadow Zone

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Earthquake Waves: The Shadow Zone WS


Earthquake Waves: The Shadow Zone WS

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Earthquake Waves: The Shadow Zone WS Earthquake Waves: The Shadow Zone WS


Earthquakes and Tsunamis

How does an earthquake form a tsunami?

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Earthquakes and Tsunami’s• An earthquake under the ocean has the potential

to form a tsunami.• The earthquake must vertically displace overlying

water (extensional or compressional faults - not shearing)

Extension Compression Shearing

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Generation of a Tsunami

2004 South Asian Boxing Day Event

• Biggest earthquake in 40 years!

• Magnitude 9.2• 150 km off the west of

Northern Sumatra• Generated a disastrous

tsunami in 12 countries

Above: Countries most effected by the 2004 tsunami

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• second largest recordedearthquake

• occurred at a subduction zone

• 1,600 km of faultlineslipped about 15 m

• released energy equivalent to 23,000 Hiroshima atomic bombs

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Before and After photographs from Phuket, Thailand

2004 Boxing Day tsunami

A village near the coast of Sumatra lays in ruin after the tsunami.

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How does an earthquake form a tsunami?

We CANNOT stop the geologic processes.We CANNOT stop the population growth/expansion.

Therefore, we must try to reduce (mitigate) the hazards

How do we mitigate the hazards from earthquakes?

• Scientific study• Reinforce buildings• Education• Disaster plans

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How do we mitigate the hazard from tsunamis?

• Monitoring– technology-intensive

• high costs for many poorer countries

– often no technology available• reliant on the Pacific Tsunami

Warning Center• tsunami in 1998 was not

detected• Education

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Exercise 6

Seismic Waves!Complete Worksheets

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UNIT TEST - Chapters 13-16

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Assignments:Chapter 13 – Plate Tectonics Notes & Vocab √Chapter 14 – Plate Tectonics & Volcanoes Notes & Vocab √Chapter 15 – Plate Tectonics & Earthquakes Notes & Vocab √Chapter 16 – Plate Tectonics & Mountain Building Notes & Vocab

For Review and Test Prep:Go to esc11.weebly.com

• Flashcard Library (home page)• volcano photo review• link to practice quizzes and review PowerPoints• Quizlab – Chapters 13-16 Practice Test

Documents

Earthquakes Chapter 15 - Belmont ESC11esc11.weebly.com/uploads/5/9/3/7/593742/_web_-_chapter15.pdf · See Chapter 15 Worksheet Package How is an Earthquake’s Epicenter Located?