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Chapter 14 Essentials of Geology by Pearson


The vibration of Earth produced by the rapid release of energy


The source of the energy that radiates in all directions from here in the form of waves


Large fractures in the Earth's crust that are associated with movements that produce earthquakes

Plate Tectonics Theory

Most of the motion along faults can be explained by this


The study of earthquake waves; dates back to the Chinese almost 2000 years ago


People who study earthquakes and their characteristics

H.F. Reid

First explained mechanism for earthquakes

Mechanism for earthquakes

Rocks on either side of fault are deformed by tectonic forces, bend and store elastic energy, frictional force is overcome

Elastic rebound

Slippage at the weakest point (the focus) occurs; vibrations occur as the deformed rock "springs back" to its original shape


Point at the depth where the rocks ruptured to produce earthquakes; place where quake waves originate


smaller quakes produced after a major quake caused by rocks shifting to new positions


small earthquakes that come before a major earthquakes

San Andreas

Most studied fault system

Fault creep

Slow gradual displacement of some portions of fault

50 to 200 years

Great earthquakes should occur about


Instruments that record seismic waves

Large hollow jar

Chinese used this to measure seismic activity with suspended free motion mass


Records obtained from seismographs

Types of seismic waves

Surface, Body

Surface waves

Travel along the Earth's surface; cause greatest destruction; slow & great

Two kinds of body waves

Primary (P) and Secondary (S)

Body waves

Travel through Earth's interior; two types

Primary (P) waves

travel 1.7 times faster than S waves; travel through whole interior; motion is back & forth

Secondary (S) waves

shaking motion at right angles; travels only through solids; slower velocity than P waves; slightly greater amplitude than P waves


Wavelength, crest, trough and amplitude


location on the surface directly above the focus; located using the difference in velocities of P & S waves


At least this many station recordings needed to locate an epicenter

Travel-time graph

used to determine each station's distance to the epicenter

Locating the epicenter

a circle with radius equal to distance to epicenter drawn around each station; point where all 3 circles intersect is epicenter


percent of energy released by earthquakes; originates in a few relatively narrow zones that wind around the globe

Major earthquake zones

Circum-Pacific belt, Mediterranean Sea region to Himalayan complex & oceanic ridge system (underwater mountains)

5 to almost 700 km

Range of depths of earthquakes

Types of earthquakes

Shallow, Intermediate & Deep Focus


surface to 70 km; occur along oceanic ridge system; nearly all damaging quakes originate at this depth; not much room so more damage


between 70to 300 km

Deep Focus

over 300 km; almost all occur in circum-pacific belt


Percent of all earthquakes occur at depths of less than 100 km

Two measurements of earthquake size

Intensity and Magnitude


a measure of the degree of quake shaking at a given locale based on amount of damage


estimates the amount of energy released at the source of the quake

Modified Mercalli Intensity Scale

developed using CA buildings; drawback not true measure of quakes actual severity; Guiseppe Mercalli; 1-12 in Roman numerals; I - least; XII - most; used after quake

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