Download Chapter 5: Earthquakes

Chapter 5: Earthquakes
5.1 Forces in Earth’s Crust
5.2 Earthquakes and Seismic Waves
5.3 Monitoring Earthquakes
Why do earthquakes occur more often
in some places than in others?
• BRAINSTORM.
– Then check your answer once we reach the end of
the chapter!
5.1 Forces in Earth’s Crust
How Does Stress Change Earth’s Crust?
• Think back to the movement of the plates. The
pieces of crust can be strong or weak, causing
bends and fold.
• Stress is a force that acts on rock to change its
shape or volume. This is expressed in unit per
area.
• As force increases, stress increases. Force is an
increase in energy
Kinds of Stress
• Types of stress act on rocks over millions of
years to change the shape and volume of rock.
– Tension: plates pull apart, crust becomes thinner
in the middle
– Compression: plates come together, plates
squeeze
– Shearing: plates slip past each other, grind past
each other
Earthquake Risk Around the World
• Most earthquake activity occurs near the Ring
of Fire in the Pacific Ocean
– West coast of Central America
– West coast of South America
• India, China and Pakistan due to the IndoAustralian Plate colliding with the Eurasian
Plate.
• Common where the Eurasian Plate meets the
Arabian and African Plates
How Do Faults Form?
• When enough stress (force) builds up, the rock
will break and a fault will form.
• A hanging wall is a piece of rock that sits over
a fault.
• A footwall is a piece of rock that lies under the
fault.
• Normal Fault: faults cut at
an angle so one block of
rock sits over the fault and
one sits under. Occur
where two plates diverge,
or pull apart. The hanging
wall slips down.
• Reverse Fault: same
structure as a normal fault.
Hanging wall moves up.
• Strike-Slip Fault: rocks on
either side of the fault slip
past each other. Think of a
transform boundary!
Types of Faults
How Does Plate Movement Create
New Landforms?
• Over millions of years, the forces of plate
movement can change a flat plain into
features such as anticlines and synclines,
folded mountains, fault-block mountains, and
plateaus.
Folding Earth’s Crust
• How Folds Form
– Compression shortens and thickens Earth’s crust
• How Anticlines and Synclines Form
– Compression forces fold crust upward into an arch
(anticline) and downward into a v-shape (syncline)
• How Folded Mountains Form
– Collision of two plates can cause compression and
folding of the crust over a wide area
Stretching Earth’s Crust
• Mountains that are separated by broad valleys
or basins have been formed from tension in
Earth’s crust that cause faulting.
• Footwalls move outward and hanging walls
slip downward.
Uplifting Earth’s Crust
• The forces that raise mountains can also raise
plateaus.
• A plateau is made of many flat layers and is
wider than it is tall.
• Famous Plateau: Four Corners in Arizona,
Utah, Colorado and New Mexico.
5.2 Earthquakes and Seismic Waves
• When you focus on something you are looking at
a specific point. The focus of an earthquake is the
point at which a rock beings to move, causing an
earthquake.
• An earthquake is the shaking and trembling that
results form movement of rock beneath Earth’s
surface.
– Some can be unnoticed, some can cause the earth to
break open, shift mountains and cause great damage.
– What are Earthquakes?
Cause of Earthquakes
• Forces of plate movement cause earthquakes.
Remember forces are transfers of energy. When
the energy builds so high, an earthquake occurs.
• Seismic waves are vibrations that are similar to
sound waves. They travel through Earth carrying
energy released by an earthquake.
Types of Seismic Waves
• Waves race out from the focus of an earthquake.
• The point on the surface directly above the focus
is the epicenter.
• Three types of seismic waves:
– P Waves: 1st waves to arrive, compress and expand
ground like an accordion, travel through liquids and
solids; damage buildings
– S Waves: 2nd waves to arrive, vibrate, only travel
through liquids, move left and right (think of swaying)
– Surface Waves: move more slowly but produce ground
movements that roll like ocean waves.
How Are Earthquakes Measured?
• The amount of earthquake damage or shaking
that is felt is rated using the Modified Mercalli
Scale. The magnitude, or size, of an
earthquake is measured on a seismograph
using the Richter scale or moment magnitude
scale.
• A seismograph is an instrument that records
and measures an earthquake’s seismic waves.
The Modified Mercalli Scale
• Rates the amount of shaking form an
earthquake.
• Rated by observations, not instruments
Rank
Description
I-III
People notice vibrations like those form a passing truck.
Unstable objects disturbed.
IV-VI
Some windows break. Plaster may fall.
VII-IX
Moderate to heavy damage. Buildings jolted off foundations.
X-XII
Great destruction. Cracks appear in ground. Waves seen on
surface.
The Richter
Scale
• The magnitude
is a single
number that
geologists
assign to an
earthquake
based on the
earthquake’s
size.
The Moment Magnitude Scale
• Used to rate the total energy an earthquake releases.
• Use seismographs
Comparing Magnitudes
• Each one-point increase in magnitude
represents the release of about 32 times more
energy.
• Magnitudes above 6 can cause great damage.
• Magnitudes of 8 or above are rare and the
most powerful.
How is an Epicenter Located?
• Geologists use seismic waves to locate an
earthquake’s epicenter.
– Use seismographs from all over the world.
– Observe the arrival of the P and S waves.
– Circles are drawn around three seismograph
stations to locate the epicenter of an earthquake.
The radius of each circle is the distance form that
seismograph to the epicenter. The point where the
three circles intersect is the location of the
epicenter.
5.3 Monitoring Earthquakes
• Seismic waves cause a simple seismograph’s
drum to vibrate, which in turn cause the pen to
record the drum’s vibrations.
Measuring Seismic Waves
• The pen of the seismograph moves and the
paper stays stationary. Since the pen is
suspended, the movement of the earthquake
causes the seismograph to move because it is
anchored to the ground and vibrates when the
seismic wave arrives.
• How Seismographs Work
Reading a Seismogram
• When an earthquakes seismic waves reach a
seismograph, the vibrations are recorded.
– P waves arrive first and the fastest
– S waves arrive shortly after the P waves
– Surface Waves produce the largest disturbances
on the seismogram.
– An AFTERSHOCK is a smaller earthquake that
occurs after a larger earthquake.
What Patterns Do Seismographic Data
Reveal?
• From past seismographic data, geologists have
created maps of where earthquakes occur
around the world. The maps show that
earthquakes often occur along plate
boundaries.
• Earthquake risk largely depends on how close
a given location is to a plate boundary.
– At risk: California, Washington, Alaska
So, why do earthquakes occur more
often in some places than in others?
• Earthquakes often occur along plate
boundaries, where stress caused by plate
movement stores energy in rock that makes
up the crust.