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Nov 29, 2013 (3 years and 9 months ago)

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Earthquakes

Chapter 5

Created by Rachel Joseph

An Earthquake is…

the shaking and trembling
that results from the
movement of rock beneath
Earth's surface


The movement of Earth's
plates produces strong
forces that squeeze or pull
the rock in the crust

This is stress, a force that
acts on rock to change its
volume or shape
(deformation)

This stores energy in the
rock.

When the rock snaps from all
the stress, it releases
energy as seismic waves.


Where Do Earthquakes Occur?


Most earthquakes take place near the edges
of tectonic plates. This figure shows the Earth’s
tectonic plates and the locations of recent
major earthquakes.


Stress

There are three
different types of
stress that occur on the
crust, shearing, tension,
and compression

These forces cause some
rocks to become fragile
and they snap

Some other rocks tend
to bend slowly like road
tar softened by the suns
heat

(

獨敡物湧

)

(

捯浰牥獳

)

(

瑥湳楯t

)

Faults

A fault is a break or crack in
the crust where slabs of
crust slip past each other.
The rocks on both sides of a
fault can move up or down or
sideways


When enough stress builds on
a rock, the rock shatters,
releasing energy, creating an
earthquake

Faults usually occur along
plate boundaries, where the
forces of plate motion
compress, pull, or shear the
crust too much so the crust
finally breaks.

Section

1
What Are Earthquakes?

Chapter 8

Elastic Deformation and Elastic Rebound



Click below to watch the Visual Concept.


Visual Concept

San Andreas Fault


San Andreas fault in California. Notice it is not one
long fault, but many, somewhat connected smaller
ones. This line will parallel the transform boundary
between the North American Plate and the Pacific
Plate.


Transform boundaries tend to have the most
earthquakes.

Hanging Wall? Foot Wall?


See the block farthest to the
right that is shaped kind of
like a foot? That’s the foot
wall. Now look at the block
on the other side of the fault.
See how it’s
resting

or
hanging

on top of the
footwall block? That’s the
hanging wall.



Here’s another way to think
of it: the hanging wall block
is always above the fault
plane, while the foot wall
block is always below the
fault plane.


Strike
-
Slip Faults


Shearing creates
this fault

In this fault, rocks
on both sides of the
fault slide past each
other with a little up
and down motion

When a strike
-
slip
fault forms the
boundary between
two plates, it
becomes a transform
boundary



Normal Faults

Tension forces in Earth's
crust causes these types
of faults

Normal faults are at an
angle, so one piece of rock
is above the fault, while
the other is below the
fault

The above rock is called
the hanging wall, and the
one below is called the
footwall

Tension forces cause the
hanging wall to slip
downward


Normal faults occur along
the Rio Grande rift valley
in New Mexico, where two
pieces of Earth's crust
are diverging



Reverse Faults


Compression forces
produce this fault

This fault has the same
set up as a normal fault,
but reversed, which
explains it’s name

So the hanging wall gets
pushed up the footwall.

This fault produced part
of the Appalachian
Mountains in the eastern
United States


How Do Mountains Form?

The forces of plate
movement can build up
Earth's surface
,
so over
millions of years,
movement of faults can
change a perfectly flat
plain into a gigantic
mountain range

Sometimes, a normal
fault uplifts a block of
rock, so a fault
-
block
mountain forms

When a piece of rock
between two normal
faults slips down, a
valley is created




Mountains Formed by Folding



Sometimes, under
current conditions, plate
movement causes the
crust to fold

Folds are bends in rock
that form when
compression shortens and
thickens part of Earth's
crust


The colliding of two
plates can cause folding
and compression of crust

These plate collisions can
produce earthquakes
because rock folding can
fracture and lead to
faults




Anticlines and Synclines





Geologists use the
terms syncline and
anticline to describe
downward and upward
folds in rock


An
a
nticline is a fold in a
rock that
a
rcs upward

A
s
yncline is a fold in a
rock that points
downward (
s
inks)

These folds in rocks are
found on many parts of
the earths surface
where compression
forces have folded the
crust



Plateaus

The

forces that
elevate mountains
can also raise
plateaus, a large
area of flat land
elevated high above
sea level

Some form when a
vertical fault pushes
up a large flat piece
of rock

Like a lasagna, a
plateau consists of
many layers, so it is
wider than it is tall




Seismic Waves

Seismic Waves:
vibrations
that travel through Earth
carrying the energy released
during an earthquake

an earthquake produces
vibrations called waves

that
carry energy while they
travel out through solid
material

During an earthquake,
seismic waves go out in all
directions from the
focus

They ripple like when you
through a stone into a lake or
pond

Epicenter
: the point on the
surface directly above the
focus.











How Earthquakes Form

Everyday, about 8,000
earthquakes hit Earth, but
most of them are too little
to feel

Earthquakes will always begin
when the stress builds up the
rock in a fault.

Friction keeps the rocks
from moving, so the stress
builds up.

Eventually the energy is so
great the friction is
overcome and the rock
breaks or moves.

This release of energy and
movement of rock is an
earthquake


Focus:
the point beneath
Earth's surface where rock
that is under stress breaks




Aftershocks

An earthquake
that occurs
after a larger
earthquake in
the same area

May strike
hours, days, or
even months
later


Seismic Waves Ctd.

There are three
different types of
seismic waves: P waves,
S waves, and surface
waves

An earthquake sends out
two of those waves, P
and S waves (called body
waves as they travel
through the interior of
the Earth).

When they reach the
top of the epicenter,
surface waves form


Primary Waves

Also known as P Waves

The first waves to
come are these waves
(fastest)

P waves

are
earthquake waves that
compress and expand
the ground like an
accordion
(compressional)

P waves cause
buildings to expand
and contract

Secondary Waves

Also known as S Waves

After P waves, S waves
come next

S waves are earthquake
waves that vibrate from
one side to the other as
well as down and up
(transverse)

They shake the ground
back and forth

When S waves reach the
surface, they shake
buildings violently

Unlike P waves, which
travel through both
liquids and solids, S
waves cannot move
through any liquids

Surface Waves (Love/Rayleigh Waves)

When S waves and P
waves reach the top,
some of them are
turned into surface
waves (L
-
waves)


Surface waves move
slower than P waves and
S waves, but they can
produce violent ground
movements

Some of them make the
ground roll like ocean
waves

Other surface waves
move buildings from side
to side


Section

1
What Are Earthquakes?

Chapter 8

Seismic Waves: Surface Waves



Click below to watch the Visual Concept.


Visual Concept

Detecting Seismic Waves

Geologists use
instruments called
seismographs

to
measure the
vibrations of seismic
waves

Seismographs
records the ground
movements caused
by seismic waves as
they move through
the Earth


Mechanical Seismographs

Until just recently, scientists
have used a mechanical
seismograph

a mechanical seismograph
consists of a heavy weight
connected to a frame by a
wire or spring

When the drum is not moving,
the pen draws a straight line
on paper wrapped around the
drum


Seismic waves cause the drum
to vibrate during an
earthquake

the pen stays in place and
records the drum's vibrations

The higher the jagged lines,
the more severe earthquake

The weight on the
seismograph never
moves. The machine
moves back and
forth.


Locating the Epicenter


Since the P waves
travel faster than
the S waves,
scientists can use
the difference in
arrival times to see
how far away the
earthquake
occurred.


It does not tell the
direction however.

Determining Direction


One station can only
learn how far away
the quake occurred.


They would draw a
circle at that radius.


Two stations can
isolate the epicenter
to two possible
locations.


If three stations
combine their data,
the quake occurred
where the three
circles overlap.

Section

2
Earthquake Measurement

Chapter 8

S and P Time Method: Finding an Epicenter



Click below to watch the Visual Concept.


Visual Concept

Measuring Earthquakes

There are many ways to
measure an earthquake

There are at least 20
different types of scales.

3 of them are the Mercalli
scale, Richter scale, and
the Moment Magnitude
scale

Magnitude is a
measurement of
earthquake strength
(energy) based on seismic
waves and movement along
faults


The Mercalli Scale

Developed in the twentieth
century to rate
earthquakes according to
their intensity.

The intensity of an
earthquake is the strength
felt by people and the
damage caused.

Is not a precise
measurement, the rating is
based on the damage done.

But, the 12 steps explain
the damage given to people,
land surface, and buildings

The same earthquake could
have different Mercalli
ratings because of the
different amount of
damage in different spots



The Mercalli scale uses Roman numerals
to rank earthquakes by how much
damage they cause

The Richter Scale


The Richter scale is a
rating of the size of
seismic waves as measured
by a particular type of
mechanical seismograph


Developed in the 1930’s

All over the world,
geologists used this for
about 50 years

Electric seismographs
eventually replaced the
mechanical ones used in
this scale

Provides accurate
measurements for small,
nearby earthquakes

Does not work for big, far
away ones



Richter Magnitude v. Energy

The Moment Magnitude Scale

Geologists use this scale
today

It’s a rating system that
estimates the total energy
released by an earthquake
(basically the Richter scale
modified for distance)

Can be used for any kind of
earthquakes, near or far

Some news reports may
mention the Richter scale,
but the magnitude number
they quote is almost always
the moment magnitude for
that earthquake


We are in a place of moderate danger of earthquakes
happening


Section

3
Earthquakes and Society

Chapter 8

Earthquake
-
Hazard Level



Click below to watch the Visual Concept.


Visual Concept

Earth’s Liquid Core


This was one of the
proofs that the Earth
has a liquid core.


You have an S
-
wave shadow
where the S waves
do not pass through
liquids.

How Earthquakes Cause Damage

The severe shaking
provided by seismic
waves can damage or
destroy buildings and
bridges, topple utility
poles, and damage gas
and water mains


With their side to side,
up and down movement,
S waves can damage or
destroy buildings,
bridges, and fracture
gas mains.

Earthquake Hazard


Earthquake hazard is a measurement of how likely an
area is to have damaging earthquakes in the future.




An area’s earthquake
-
hazard level is determined by
past and present seismic activity.




The greater the seismic activity, the higher the
earthquake
-
hazard level.



Earthquake Forecasting


Forecasting when and
where earthquakes will
occur and their
strength is difficult.




By studying areas of
seismic activity,
seismologists have
discovered some
patterns in earthquakes
that allow them to
make some general
predictions.



Earthquake Forecasting,
continued


Strength and Frequency

Earthquakes vary in
strength. The strength of earthquakes is
related to how often they occur.



This table shows more detail about the
relationship.


Earthquake Forecasting,
continued


Another method of forecasting an earthquake’s strength,
location, and frequency is the gap hypothesis.



The gap hypothesis

is based on the idea that a major
earthquake is more likely to occur along the part of an active
fault where no earthquakes have occurred for a certain
period of time.



Earthquake Forecasting,
continued


An area along a
fault where
relatively few
earth
-
quakes
have occurred
recently but
where strong
earthquakes
have occurred in
the past is called
a
seismic gap.



Earthquake Forecasting,
continued


Using the Gap
Hypothesis

Not all
seismologists believe the
gap hypothesis is an
accurate method of
forecasting
earthquakes.




But some seismologists
think the gap hypothesis
helped forecast the
approximate location
and strength of the 1989
Loma Prieta earthquake
in California.



Section

3
Earthquakes and Society

Chapter 8

Gap Hypothesis and Seismic Gaps



Click below to watch the Visual Concept.

Visual Concept

Earthquakes and Buildings


Earthquakes can easily
topple buildings and
destroy homes. Today,
older structures in
seismically active
places, such as
California, are being
made more
earthquake resistant.




Retrofitting

is the name
given to the process of
making older structure
more earthquake
resistant.



Earthquakes and Buildings,
continued


A common way of
retrofitting an older
home is to securely
fasten it to its
foundation.




Steel is often used
to strengthen
buildings and
homes made of
brick.



Earthquakes and Buildings,
continued


Earthquake
-
Resistant
Buildings

A lot has been
learned from building
failure during
earthquakes.




With this knowledge,
architects and engineers
use new technology to
design and construct
buildings and bridges to
better withstand
earthquakes.




The next slide shows some
of the technology used to
make earthquake
-
resistant buildings.




Earthquakes and Buildings,
continued


Are You Prepared for an Earthquake?


Before the Shaking Starts
The first thing should do
safeguard your home
against earthquakes.




Place heavier objects on
lower shelves so they do
not fall during an
earthquake.




Find safe places within
each room of your home
and outside of your home.




Make a plan with others
to meet in a safe place
after the earthquake is
over.



These students are
participating in an
earthquake drill.

Earthquake Preparations,
continued


When the Shaking Starts
If you are indoors,
crouch or lie face down
under a table or desk.




If you are outside, cover
your head with your
hands and lie face
down away from
buildings, power lines, or
trees.




If you are in a car on an
open road, you should
stop the car and remain
inside.



Earthquake Preparations,
continued


After the Shaking Stops
Try
to calm down and get
your bearings.




Remove yourself from
immediate danger, such
as downed power lines,
broken glass, and fire
hazards.




Do not enter any
damaged buildings unless
you are told it is safe by
someone in authority.




Beware that aftershocks
may cause more damage.



San Francisco: after 1906 quake