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Geophysics

Seismic Waves: Deep Earth ProbesThe Navy Reveals The Sea Floor

CH 17 & 18

26 Dec 2004 Banda Aceh Earthquake

Seismic Waves

WV Geological Survey Morgantown Seismograph Record 2 New Madrid Seismic zone Quakes in Illinois April 2008

5.2

4.6

2

P Waves and S waves diagram

Body Waves

Body WavesPRIMARY or Push-Pull Wave

(Compression) Motion Parallel to Wave Motion

Body Waves

SECONDARY or Shear WaveMotion Perpendicular to Wave Motion

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Go to http://www.eas.purdue.edu/~braile

/edumod/slinky/slinky.htm

Look at figure 13 for P wave

Then look at figure 14 for S wave

Image from

Seismic Waves and the Slinky: A Guide For Teachers

(v.2.2, December, 2000)

Prof. Lawrence W. BraileãDepartment of Earth and Atmospheric Sciences

Purdue UniversityWest Lafayette, IN 47907

braile@purdue.edu

Image from

Seismic Waves and the Slinky: A Guide For Teachers

(v.2.4, December, 2003)

Prof. Lawrence W. BraileãDepartment of Earth and Atmospheric Sciences

Purdue UniversityWest Lafayette, IN 47907

braile@purdue.edu

4

See Figure 16.5, 10th edition

Includes Tsunamis (700-800

km/h)

Surface Waves = Long (L) Waves

5

26 Dec 2004 Sumatra Earthquake killed >290,000

Measuring EarthquakesSeismograph

Seismograph Record

P WaveP WaveArrivalArrival

S WaveS WaveArrivalArrival

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Finding the Location of QuakesP-S Interval

P-S Interval

P-S Interval Tells How Far Away,

but in What Direction?

Use 3 Different Seismograph Locations, Plot Arcs from Each Station on a Map

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SEISMIC WAVES:DEEP EARTH PROBES

SEISMIC WAVES:DEEP EARTH PROBES

Some Waves Reflect Some Waves Refract(Bend)

P Wave Paths

Through Earth

P Wave P Wave Refraction Refraction Produces Produces

Shadow ZoneShadow Zone

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S Wave S Wave Paths Paths

Through Through EarthEarth

S Waves S Waves Absorbed by Absorbed by

the Outer the Outer Liquid CoreLiquid Core

Seismic Waves Reveal Earth’s Layers

Seismic Waves Reveal Earth’s Layers

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Details of Upper Layers

Isostacy~=

Buoyancy

Mountains Have Roots

Mountains Have Roots

Isostacy

10

Erosion (Unloading) Causes Uplift; Deposition (Loading) Causes Subsidence

Whole Earth

Structure

Temperatures & Pressures in the Earth

11

Rock Strength with Depth

LithosphereAsthenosphere

Earth’s Magnetic Field

May Stem from Convection in Liquid Outer

Core!

Inclination: Deviation from Horizontal

Declination: Deviation from True N-S

Earth’s Changing Magnetic Field

Variable Strength & OrientationNormal: Magnetic N ~ True NReversal: Magentic N ~ True S (last one: 788,000 y)Chrons: Paleomagnetic Time Interval

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How Can We Study PaleomagneticFields? Remnant Magnetism

Remnant Magnetism

Magnetic Anomalies

13

Gravity is NOT Constant

High Low“Normal”

Gravity Reveals Underlying Rock

Bouger Anomaly

North American Gravity

Bouger Anomalies

14

Seismic network reports 'jolt' on WV-VA border• Tuesday April 14, 2009

• by The Associated Press• ATHENS, W.Va. (AP) -- The Southeast United States

Seismic Network says a small earthquake occurred over the weekend along the West Virginia-Virginia border.

• The network says the quake had a magnitude of 2.4 and occurred at approximately 11:09 a.m. Saturday. The quake struck about 9 miles northeast of Athens and 10 miles north of Glen Lyn, Va.

• There were no reports of damage or injuries. Local officials in the area said there were no reports that anyone even felt the quake.

• Earthquake analyst Don Blakeman with the National Information Center in Colorado says a quake of that magnitude is at most a "jolt'' that won't do any damage.

Modified Richter (Total Energy) Magnitude ScaleModified Richter (Total Energy) Magnitude Scale

Sumatra 2004

Alfred Wegener’s Evidence for Continental Drift

1. Continental “Fit”

15

Include Continental Shelves for Best “Puzzle” Fit

Continental Drift

• Bacon, 1620

Continental Drift

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Alfred Wegener’s Evidence for Continental Drift

1. Continental “Fit”2. Similar Rocks, Ages

Wegener’s Evidence:

Similar Rocks,Similar Ages

Batholith

Alfred Wegener’s Evidence for Continental Drift

1. Continental “Fit”2. Similar Rocks, Ages

3. Similar Fossils

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Similar Fossils

Glossopteris (Common Tree in Coal Seams)Fresh-Water Fish

Mammal-Like Reptiles

http://toad.asu.edu/rschmag/gloss.html

http://www.dme.qld.gov.au/gsd/fossils/perm.htm

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Similar Fossils: Different Continents

Alfred Wegener’s Evidence for Continental Drift

1. Continental “Fit”2. Similar Rocks, Ages

3. Similar Fossils4. Widespread Glaciation

Widespread GlaciationGondwana Glaciation of Antarctica,

South America, Africa, Australia and India during Permian

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Strange Climate Zones

Gondwanaland: Antarctica, South America, Africa,

Australia and India

Laurasia: North America, Europe,

most of Asia

Source:http://vishnu.glg.nau.edu/rcb/Jur.jpg

Laurasia

Gondwanaland

Source: http://vishnu.glg.nau.edu/rcb/160_1st.jpg

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Source:http://vishnu.glg.nau.

edu/rcb/Perm.jpg

Pangaea = Gondwanaland

+ Laurasia

Ice Sheet

Pangaea = Gondwanaland

+ Laurasia

Source:http://vishnu.glg.nau.edu/rcb/Perm.jpg

Source:http://vishnu.glg.nau.

edu/rcb/Penn.jpg

Pangaea = Gondwanaland +

Laurasia

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Source:http://vishnu.glg.nau.

edu/rcb/Penn.jpg

Pangaea = Gondwanaland +

Laurasia

Alfred Wegener’s Evidence for Continental Drifts Was NOT

Accepted in USA.

Problem: No Mechanism for Continents to Plow Through

Oceanic Crust

Answer lies on the Sea Floor & in Ch. 18 + 19