Transcript Earthquakes and Volcanoes - Tenafly Public Schools

Earthquakes
Chapter 11
P. Lobosco
Forces Inside the Earth
Chapter 11, Section 1
• Objectives:
• Explain how earthquakes result from the
buildup of energy in rocks.
• Describe how compression, tension and
shear forces make rocks move along the
faults.
• Distinguish among normal, reverse and
strike-slip faults.
Elastic Limit
• There is an elastic
limit to the amount a
rock can bend and
stretch before it will
break.
• Once the limit is
passed, the rock will
move along a fault
line.
Earthquakes
• An earthquake is the shaking and
trembling that results from the sudden
movement of the rock along the fault.
• Energy is released. The earthquake will
continue until the energy is used up.
Fault Line
Three Types of Force
• Compression is the
force that squeezes
rock together.
• Tension is the force
that pulls rocks apart.
• Shearing is the force
that causes rocks on
either side of a fault to
slide past each other.
Faults
• Tension causes rocks
to be pulled apart and
the rocks move down
along the fault line.
• Compression causes
rocks to squeeze
together and rocks
move up along a fault
line.
• Strike slip is side by
side.
Himalayan Mountains
• The Himalayan
Mountains which
were formed by the
convergence
(compression) of
Indo-Australian Plate
and the Eurasian
Plate has many
reverse and thrust
faults.
Features of Earthquakes
Chapter 11, Section 2
• Objectives:
• Explain how earthquake energy travels in
seismic waves.
• Distinguish among primary, secondary and
surface waves.
• Describe the structure of Earth’s interior.
Seismic Waves
• The waves of energy generated by an
earthquake are called seismic waves.
• There are three main types of seismic
waves.
• Primary (P waves)
• Secondary (S waves)
• Surface (L waves)
San Andreas Fault
• The San Andreas fault extends 960 km
from Mexico to the north of California.
• The land to the west is moving north. The
land to the east of the fault is moving
south. All the rocks do not move at the
same time so earthquakes occur in one
area and then another.
Focus
• Most faults occur
between the surface
and a depth of 70
kilometers.
• The point beneath the
surface where the
rocks break and move
is called the focus.
The focus is the
underground origin of
an earthquake.
Epicenter
• Directly above the
focus, on the Earth’s
surface is the
epicenter.
Earthquake waves
reach the epicenter
first. During an
earthquake, the most
violent shaking is
found at the
epicenter.
Primary Waves
• Seismic waves that travel fastest are P
waves. They travel through solids, liquids
and gases.
• They move at different speeds depending
on the density of the material through
which they are moving. As they move
deeper in the Earth they move faster.
• P waves are push-pull waves.
Secondary Waves
• Seismic waves that do not travel through
the Earth as fast as P waves do are called
secondary or S waves.
• S waves travel through solids but not
liquids or gases.
• S waves cause particles to move from side
to side. They move at right angles to the
direction of the wave.
P and S Waves
Surface Waves
• The slowest moving seismic waves are
called surface waves or L waves.
• L waves originate on the Earth’s surface at
the epicenter. They move along the
surface the way waves travel in the ocean.
The Earth’s surface moves up and down
and side to side with each L wave.
• L waves cause most of the damage.
The Seismograph
• Invented in 1893 by John Milne, a
seismograph detects and measures
seismic waves.
• A weight attached to a spring remains
nearly still even when the Earth moves. A
pen attached to the weight records any
movement on a roll of paper on a
constantly rotating drum. The drum moves
with the Earth and affects the line.
Seismograph
Seismologists
• Seismologists study earthquakes. They
can determine the strength of an
earthquake by the height of the wavy line
recorded on the paper.
• The seismograph record of waves is called
a seismogram.
Scales Used to Measure
Earthquakes
• The Richter scale is used to calculate the
strength (magnitude) of an earthquake.
• The Modified Mercalli is used to measure
the intensity (amount of destruction) of an
earthquake.
Seismograph Stations
• Each type of wave reaches a seismograph
station at a different time based on its
speed.
• Primary waves arrive first. Secondary
waves travel slower and arrive later.
• The difference in arrival time is used to
estimate the distance from the station to
the epicenter.
Locating an Epicenter
• Scientists need readings
from three or more
stations to determine the
location. A circle is
drawn around each
station. The radius of
each circle is equal to the
station’s distance from
the epicenter.
• The point of intersection
is the location of the
epicenter.
Interior of the Earth
• Seismic waves
change speed as they
move through the
earth’s layers. They
speed up when they
pass through the
bottom of the crust
and enter the mantle.
This boundary is
known as the Moho.
The Moho
• The boundary is
named for the
scientist, Andrija
Mohorovicic who
discovered this
information.
• It is called the
Mohorovicic
discontinuity or Moho.
Seismic Speed in the Mantle
• The mantle is divided into layers based on
changes in seismic wave speed. Both P
and S waves slow down again when they
reach the asthenosphere.
• They speed up when they move through a
more dense part of the mantle.
Predicting Earthquakes
• Scientists need to know the amount of
stress applied as well as the elastic limit of
the rock to determine when an earthquake
will occur.
Predicting Earthquakes
• Scientists have identified warning signals
the help predict earthquakes with greater
accuracy.
• Often changes occur in the speed of P and
S waves before an earthquake occurs.
• Sometimes slight changes in the tilt of the
Earth’s surface can be detected.
• Some scientists believe animals behavior
is affected.
Seismic Speed in the Core
• The core is divided into two layers based
on how they travel through it.
• Primary waves slow down and secondary
waves do not travel through the liquid
outer core.
• Primary waves speed up again when they
reach the solid inner core.
People and Earthquakes
Chapter 11, Section 3
• Objectives:
• Explain where most earthquakes in the
United States occur.
• Describe how scientists measure
earthquakes.
• Discuss earthquake safety procedures.
Seismic Risk Map of the U.S.
The Richter Scale measures the
strength or magnitude
• An increase of one
magnitude on the
Richter scale means
that 32 times more
energy is released.
• An earthquake of
magnitude 6 is 32 x
32 x 32 times greater
than an earthquake
with a magnitude of 3.
Magnitude
• The height of the lines
traced on the paper of
a seismograph is a
measure of the
energy that is
released, or the
magnitude of an
earthquake.
The Modified Mercalli Scale measures the
amount of damage
• Earthquakes can also
be described by the
amount of damage
they cause. The
Modified Mercalli
scale describes the
intensity of an
earthquake using the
amount of structural
and geologic damage
in a location.
Earthquake Damage
• The amount of damage created by an
earthquake depends on several factors.
• The earthquake’s strength
• The kind of rock and soil that underlies an
area
• The population of the area
• The kind of buildings in the area
• The time at which the earthquake occurs
Earthquake Belts Worldwide
Tsunamis
• Earthquakes which occur on the ocean
floor produce giant sea waves called
tsunamis. Tsunamis can travel at speeds
of 700 to 800 km per hour. As they
approach the coast, they can reach
heights of greater than 20 meters.
• The Tsunami Warning Center is located in
Hilo, Hawaii.
Earthquake Belts