Chapter 6: Earthquakes
Earthquakes and Plate Tectonics
• Vibrations of the
earth’s crust are
• Earthquakes usually
occur when rocks
suddenly shift along a
Elastic Rebound Theory
• Elastic Rebound Theory – theory that rocks
that are strained past a certain point will fracture
and spring back to their original shape.
• As fractures occur and rocks slip into new
positions, rocks along a fault release energy in
the form of vibrations called seismic waves.
• This release of energy often increases the stress
in other rocks along the fault, causing more
fracture and spring back. This leads to
Elastic Rebound Theory
• Aftershocks – a tremor that follows and is
smaller than a major earthquake.
• Focus – area along a fault at which slippage first
occurs, initiating an earthquake.
• Epicenter – point on the earth’s surface directly
above the focus of an earthquake.
• During an earthquake, seismic waves radiate
outward in all directions from the focus.
3 types of focus
– Shallow-focus: earthquakes occur within 70
km of the surface.
– Intermediate-focus: earthquakes occur
within 70-300 km of surface.
– Deep-focus: earthquakes occur in
subduction zones at a depth of 300-650 km.
• Most earthquakes that usually cause the
most damage have a shallow focus.
• Most earthquakes do not originate very
deep within the earth because the high
pressure and high temperature at these
depths cause rock to exhibit plastic
behavior rather than to fracture.
Major Earthquake Zones
• The earth has three major earthquake zones:
– Pacific Ring of Fire: includes west coast of N/S
America, the east coast of Asia, the western Pacific
islands of the Philippines, Indonesia, New Guinea,
and New Zealand.
• Many earthquakes occur along this zone because the plate
movements cause stress to build up in rocks. The rocks
fracture and shift causing earthquakes.
– Mid-ocean ridges: Earthquakes occur along mid-
ocean ridges because oceanic crust is pulling away
from both sides of each ridge.
– Eurasian-Melanesian mountain belt: mountains
caused by the collision of the Eurasian plate with the
African and Indian plates.
• Fault zones – groups of interconnected faults.
• Fault zones form at plate boundaries because of the
intense stress that results when the plates separate,
collide, subduct, or slide past each other.
• Not all earthquakes result from movement along plate
• Earthquakes have been recorded in the middle of the
continent as early as 1812 in New Madrid Missouri. This
location is not near active plate boundaries.
• In the 1970’s, studies of the Mississippi River region
revealed an ancient fault zone deep within the earth’s
• Seismic waves can be detected and recorded by using a
• Seismograph – instrument used to detect and record
• A seismograph consists of 3 separate sensing devices:
– One device records the vertical movement of the ground.
– The other two record horizontal motion in the east-west and
• A seismograph records the seismic activity by tracing
wave-shaped lines on paper or electronic signals.
• The electronic signals can be recorded on magnetic tape
or loaded into a computer that analyzes the seismic
• A seismograph records the seismic activity
by tracing wave-shaped lines on paper or
• The electronic signals can be recorded on
magnetic tape or loaded into a computer
that analyzes the seismic waves.
Types of Seismic Waves
• Each type of wave travels at a different speed
and causes different movements in the earth’s
• Primary waves (P-Waves), move the fastest
and are therefore the first to be recorded. P
waves can travel through solids and liquids. The
more rigid the material, the faster the P waves
travel through it.
• P waves are compressional waves, causes
rock particles to move together and apart along
the direction of the waves.
• Secondary waves (S-Waves), are slower
than p waves and can only travel through
solids. S waves are shear waves,
meaning they cause rock particles to move
at right angles to the direction in which
the waves are traveling.
• Surface waves are the result of P waves
and S waves reaching the surface and
their energies being converted. Surface
waves are the slowest waves. Surface
waves cause the surface of the crust to
rise and fall, causing the most destruction
when traveling through the surface
Locating an Earthquake
• Scientists analyze the difference between the
arrival times of the P waves and the S waves in
• To determine location of earthquakes, scientists
plot the difference between the arrival times of
the two waves. Then they consult a standard
graph that translates the differences in arrival
times into distance from the epicenter. (see
Figure 6-5 p.104)
• Magnitude is a measure of the energy released by an
earthquake and is also described as the amount of
• The Richter scale or the moment magnitude scale is
used to express the magnitude of earthquakes.
• The largest earthquake so far recorded registered a
magnitude of 8.9
• A major earthquake is a 7.0 or higher.
• A moderate earthquake is between a 6-7.
• A minor earthquake is between 2.5-6 on the Richter
• Earthquakes with magnitudes of less than 2.5
are called microquakes and usually are not felt
• Mercalli scale – scale that expresses the
intensity of an earthquake with a Roman
numeral and a description. The Mercalli scale
uses Roman numerals from I-XII to describe the
extent of earthquakes. I would be weak and XII
being total destruction.
• Intensity – the amount of damage caused by an
• Most injuries result from the collapse of buildings
and other structures or from falling objects and
• Other dangers include:
– Explosions caused by broken electric and gas
– Flooding waters released from collapsing dams
• Duration can affect the damage caused
by an earthquake.
• A moderate earthquake that continues for
a long time often causes more damage
than an earthquake of higher magnitude
that last only a short time.
Destruction to Buildings and
• Most buildings are not designed to withstand the
swaying motion caused by earthquakes.
• The type of ground beneath a building can affect
the way the building responds to seismic waves.
• A building constructed on loose soil and rock is
much more likely to be damaged during an
earthquake than one built on more solid ground.
• A major earthquake with an epicenter on the ocean floor
sometimes causes a giant ocean wave called a
• Name comes from the Japanese word for “harbor
• Scientists believe that most tsunamis are caused by two
events related to undersea earthquakes:
– Underwater landslides
• Disastrous earthquakes and tsunamis have encouraged
the expansion and improvement of the Pacific Tsunami
Warning Center (PTWC). This network of seismograph
stations around and in the Pacific Ocean alerts scientists
to the location and magnitude of earthquakes.
• A destructive earthquake may occur in any place in the United States.
• People living or visiting near active faults should be ready to follow a few
simple earthquake safety rules:
– Before an earthquake occurs, be prepared:
• Canned food, bottled water, flashlights, batteries, portable radio.
• Plan what you will do with family, coworkers, etc
• In school, follow the instructions of teachers and administration.
• Learn how to turn off the gas, water, and electricity to your house.
– During an earthquake, stay calm:
• Move to a safer position when possible.
• Protect yourself from falling debris by standing in a doorway or under a desk or table.
• Do not run outside.
• If driving, stop in a place away from tall buildings, tunnels, power lines, and bridges, and
remain in the car until the tremors cease.
– After an earthquake, be cautious:
• Check for fire and fire hazards.
• Always wear shoes.
• Avoid downed powerlines and objects touched by downed wires.
Earthquake Warnings and
• One of the earliest means of predicting earthquakes was
to observe the behavior of animals.
• To make more accurate predictions, scientists are
trying to detect changes in the earth’s crust that can
signal an approaching earthquake.
• Faults near many population centers have been located
• Instruments placed along these faults measure small
changes in rock movement around the fault and can
detect an increase in strain.
• Seismic gaps – a place where the fault is
locked and unable to move.
• The strain around seismic is placed on the
surrounding rock. This continued stress
and strain could lead to future
• Sometimes there is a detection of slight
tilting of the ground shortly before an
• Detection of strain and cracks in rocks caused
by the stress that builds before earthquakes.
• Scientists can also detect changes in well levels
and increased natural gas seepage from
strained or fractured rocks.
• Some earthquakes are preceded by a decrease
in the speed of local P waves. The P waves
being measured have traveled from distant