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Chapter 6 Earthquakes and Earth's Interior


General features

Vibration of Earth produced by a sudden release of energy

Associated with movements along faults

Explained by the plate tectonics theory

Mechanism for earthquakes was first explained by H. Reid

Early 1900’s

Rocks " spring back "

Phenomenon called elastic rebound

Vibrations (earthquakes) occur as rock elastically returns to its original shape

Fault creep

Often preceded by foreshocks

Often followed by aftershocks


Earthquake waves

Study of earthquake waves is called seismology

Earthquake recording instrument

Records movement of Earth

Instrument is a seismograph

Record is called a seismogram


Types of earthquake waves

Surface waves

Body waves

Primary waves

Secondary waves


Surface waves

Complex motion

Slowest velocity of all waves


Body waves

Primary (P) waves

Push-pull (compressional) motion

Travel through




Greatest velocity of all earthquakes waves

Secondary waves (S) waves

" Shake " motion

Travel only through solids

Slower velocity than P waves


Locating an earthquake

Focus-the place within Earth where earthquakes waves originate


Point on the surface, directly above the focus

Located using the difference in the arrival times between P and S wave recordings, which are related to distance

Three station recordings are needed to locate an epicenter

Circle equal to the epicenter distance is drawn around each center

Point where three circles intersect is the epicenter


Earthquake zones are closely correlated with plate boundaries

e.g., Circum-Pacific belt

e.g, Oceanic-ridge system


Earthquake intensity and magnitude

Mercalli intensity scale

Assesses damage at a specific location

Depends on

Strength of the earthquake

Distance from the epicenter

Nature of the surface material

Building design



Concept introduced by Charles Richter in 1935

Measured using the Richter scale

Earthquake magnitude scale

Amplitude of largest wave recorded

Largest earthquakes near magnitude 8.6

Magnitudes less than 2.0 are usually not felt

Each unit of magnitude increase corresponds to

A tenfold increase in wave amplitude

About a 30-fold energy increase


Earthquake destruction

Factors that determine destruction

Magnitude of the earthquake

Proximity to population

Destruction from

Ground shaking

Liquefaction of the ground

Saturated material turns fluid

Underground objects may float to surface

Tsunamis, or seismic sea waves

Landslides and ground subsidence




Short-range -- no reliable method yet devised for predicting


Premise is that earthquakes are repetitive

Region is given a probability of a quake


Earth’s interior

Most of our knowledge of the interior comes from the study of P and S waves

Travel time of P and S waves through Earth vary depending on the properties of the materials

S waves travel only through solids


Earth’s interior structure


Thin outer layer

Varies in thickness

70 km in some mountainous regions

Less than 5 km in oceanic regions

Two parts

Continental crust - lighter, granitic rocks

Oceanic crust - basaltic composition



Crust and uppermost mantle (about100 km thick)

Cool, rigid, solid

Mohorovicic discontinuity ( Moho) separates the crust from the mantle



Below crust

2885 km thick

Composition similar to the igneous rock peridotite



Upper mantle

At a depth from 100 - 350 km

Hot, weak rock

Easily deformed

Up to 10 percent is molten

Key to explaining plate movement


Outer core

Below mantle

2270 km thick

Mobile liquid

Does not transmit S waves

Mainly iron and nickel composition

Related to Earth’s magnetic field


Inner core

1216 km radius


Iron and nickel composition

High density

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