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Earthquakes and the Earth’s Interior
Ag Earth Science – Chapter 8.1
earthquake
 The vibration of the
Earth produced by the
rapid release of energy.
focus
 The point within the
Earth where the
earthquake starts
epicenter
 The location on the
surface directly above the
focus
fault
 Fractures in the Earth
where movement has
occurred
elastic rebound hypothesis
 The explanation stating
that when rocks are
deformed, they break,
releasing the stored
energy that results in the
vibrations of an
earthquake.
aftershock
 A small earthquake that
follows the main
earthquake
foreshock
 A small earthquake that
often precedes a major
earthquake
Earthquakes
 Each year, more than
30,000 earthquakes
occur worldwide that are
strong enough to be felt.
 Most earthquakes are
minor tremors, and
about 75/year are
considered major
earthquakes.
Earthquakes
 Earthquakes – the
vibration of the Earth
produced by the rapid
release of energy

The point within (inside)
the Earth where the
earthquake starts is called
the FOCUS.

The EPICENTER is the
location on the surface
directly above the focus.
Earthquakes
 Earthquakes are usually
associated with large
fractures in the earth’s
crust and mantle called
FAULTS.
 Faults – fractures in the
earth where movement
has occurred.
Elastic Rebound Hypothesis
 The springing back of the rock
into its original form is called
“elastic rebound” (example – a
stretched rubber band).
 The explanation says that when
rocks are deformed, they first
bend and then break, releasing
stored energy.
 Most earthquakes are produced
by the rapid release of elastic
energy stored in rock that has
been subjected to great forces.
When the strength of the rock is
exceeded, it suddenly breaks,
causing the vibration of the
earthquake.
Earthquakes
 The movements that
follow a major
earthquake often
produce smaller
earthquakes called
aftershocks.
 Small earthquakes called
foreshocks often come
before a major
earthquake.
Ag Earth Science – Chapter 8.2
seismograph
 An instrument that
records earthquake
waves
seismogram
 A record made by a
seismograph
surface wave
 A seismic wave that
travels along the surface
of Earth
S wave
 A seismic wave that
shakes particles
perpendicular to the
direction the wave is
traveling
P wave
 Earthquake wave that
pushes and pulls rocks in
the direction of the wave,
also known as a
compression wave.
moment magnitude
 A more precise measure
of earthquake magnitude
than the Richter scale
which is derived from the
amount of displacement
that occurs along the
fault zone and estimates
the energy released by an
earthquake
Measuring Earthquakes
 The study of earthquake
waves, or seismology, dates
back almost 2000 years.
 Seismographs –
instruments that record
earthquake waves.
 Seismogram – modern
seismographs that amplify
and electronically record
ground motion.
Earthquake Waves
 The energy from an
earthquake spreads
outward as waves in all
directions from the
focus.
 Seismograms show that
two main types of
seismic waves are
produced by an
earthquake – surface
waves and body waves.
Earthquake Waves
 Surface Waves

Seismic waves that travel
along Earth’s outer layer.
Earthquake Waves
 Body Waves

Waves that travel through
the Earth’s interior

P waves – “push – pull”
waves. The push (compress)
and pull (expand) rocks in
the direction the waves
travel.

S waves – shake the
particles at right angles to
the direction of travel.
Earthquake Waves
 A seismogram shows all
three types of seismic
waves – surface waves, P
waves, and S waves.
Locating an Earthquake
 The difference in
velocities of P and S
waves provides a way to
locate the epicenter.
Locating an Earthquake
 The winning car is always
faster than the losing car. The
P wave always wins the race,
arriving ahead of the S wave.
The longer the race, the
greater the difference in
arrival times of the P and S
waves at the finish line
(seismic station). The greater
the interval measured on a
seismogram between the
arrival of the first P wave and
the first S wave, the greater
the distance to the earthquake
source.
Locating an Earthquake
 Travel-time graphs from
three or more
seismographs can be
used to find the exact
location of an earthquake
epicenter.
Locating an Earthquake
 Earthquake Zones

95% of the major
earthquakes occur in a few
narrow zones

circum-Pacific belt: Japan,
Philippines, Chile, and
Alaska’s Aleutian Islands

Mediterranean – Asian belt
Measuring Earthquakes
 Historically, scientists have
used two different types of
measurements to describe
the size of an earthquake.

Intensity – measure of the
amount of shaking at a given
location / based on damage

Magnitude – measure of the
seismic waves or the amount
of energy released at the
source of the earthquake
Measuring Earthquakes
 Richter Scale

Based on amplitude of
largest seismic wave (P, S, or
surface) recorded on a
seismogram.

A tenfold of increase in wave
amplitude equals an increase
of 1 on the magnitude scale.

Example – a 5.0 magnitude
is 10 times greater than a
4.0 magnitude
Measuring Earthquakes
 Moment Magnitude

Derived from the amount
of displacement that
occurs along a fault zone.

Moment magnitudes is the
most widely used
measurement for
earthquakes because it is
the only magnitude scale
that estimates the energy
released by earthquakes.
Measuring Earthquakes
 Mercalli Scale

Rates an earthquake’s
intensity in terms of the
earthquake’s effects at
different locations.

Example –
Earthquake barely felt is a
“I”

Earthquake that causes
near/total damage is a “XII”
Ag Earth Science – Chapter 8.3
tsunami
 Japanese word for a
seismic sea wave
liquefaction
 A phenomenon,
sometimes associated
with earthquakes, in
which soils and other
unconsolidated materials
saturated with water are
turned into a liquid that
is not able to support
buildings
seismic gap
 An area along a fault
where there has not been
any earthquake activity
for a long period of time
Seismic Vibrations
 The damage to buildings and
other structures from
earthquake waves depends on
several factors.
 These factors include the
intensity and duration of the
vibrations, the nature of the
material on which the
structure is built, and the
design of the structure.
 Liquefaction - soils and other
unconsolidated materials
saturated with water are
turned into a liquid that is not
able to support buildings.
Tsunamis
 Tsunami – seismic sea
wave
 A tsunami triggered by an
earthquake occurs where a
slab of the ocean floor is
displaced vertically along a
fault.
 A tsunami can also occur
when the vibration of a
quake sets an underwater
landslide into motion.
Other Dangers
 With many earthquakes,
the greatest damage to
structures is from
landslides and ground
subsidence, or the
sinking of the ground
triggered by vibrations.
Predicting Earthquakes
 The goal of short-range
prediction is to provide an
early warning of the
location and magnitude of
a large earthquake.
 So far, methods for short-
range predictions of
earthquakes have not been
successful.
 Long-range forecasts give
the probability of a certain
magnitude earthquake
occurring within 30-100+
years.
Predicting Earthquakes
 Scientists study historical
records and look for
seismic gaps (area of
fault where no activity
has taken place for a long
time).
 Scientists don’t yet
understand enough
about how and where
earthquakes will occur to
make accurate long-term
predictions.
Ag Earth Science – Chapter 8.4
crust
 The thin, rocky outer
layer of the Earth
mantle
 The 2890 km – thick
layer of earth located
below the crust
lithosphere
 The rigid outer layer of
earth including the crust
and upper mantle
asthenosphere
 A weak plastic layer of
the mantle situated
below the lithosphere
outer core
 A layer beneath the
mantle about 2260 km
thick
inner core
 The solid innermost layer
of the earth, about 1220
km in diameter
Moho
 It is the boundary
separating the crust from
the mantle, discernable
by an increase in the
velocity of seismic waves
Layers Defined by Composition
 Earth’s interior consists of
three major zones defined by
its chemical composition –
the crust, mantle, and core.
 Crust – the thin, rocky outer
layer of Earth, is divided into
oceanic and continental crust.
 Mantle – 82%+ of the earth’s
volume is contained in the
mantle – a solid, rocky shell
that extends to a depth of
2890 km.
 Core – the core is a sphere
composed of iron-nickel alloy
Layers Defined by Physical Properties
 Earth can be divided into layers
based on physical properties –
the lithosphere, asthenosphere,
outer core, and inner core.
 Lithosphere – Earth’s outermost
layer that is about 100 km in
thickness. Consists of the crust
and uppermost mantle.
 Asthenosphere – lies beneath the
lithosphere and is a softer,
weaker layer due to it’s high
temperature “waxy” texture.
 The core is divided into the outer
(liquid) core and inner (solid)
core.
Discovering Earth’s Composition
 Early seismic data and
drilling technology indicate
that the continental crust is
mostly made of lighter,
granitic rocks.
 The crust of the ocean floor
has a basaltic composition
 Earth’s core is thought to be
mainly dense iron and nickel,
similar to metallic meteorites.
The surrounding mantle is
believed to be composed of
rocks similar to stony
meteorites.