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Chapter 6 Earthquakes and Earth's Interior

Earthquakes

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

Solids

Liquids

Gases

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

Epicenter

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

 

Magnitude

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

Fires

 

Prediction

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

Long-range

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

Crust

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

 

Lithosphere

Crust and uppermost mantle (about100 km thick)

Cool, rigid, solid

Mohorovicic discontinuity ( Moho) separates the crust from the mantle

 

Mantle

Below crust

2885 km thick

Composition similar to the igneous rock peridotite

 

Asthenosphere

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

Solid

Iron and nickel composition

High density

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