7.1 Rocks Move Along Faults
A fault is a break or fracture in the lithosphere,
where blocks of rock move past each other.
Along parts of a fault, rocks may stick or lock
together and bend as stress is put on them.
Stress is the force exerted when an object
presses on, pulls on, or pushes against another
object.
As stress increases, the rocks break free.
The sudden release of stress in the lithosphere
is called an earthquake.
An earthquake is a shaking of the ground
caused by the sudden movement of large blocks
along a fault.
*Most earthquakes occur along plate boundaries
because large amounts of stress build up as
plates move.
• Compression: the force that occurs
when two tectonic plates collide
Most earthquakes occur in the lithosphere
because the tectonic plates are rigid and
brittle, can break easily, and can move
suddenly.
7.2 Earthquakes Release Energy
All earthquakes start beneath Earth’s surface.
A focus is the point underground where rocks
first begin to move. (where the earthquake
begins)
The epicenter is the point on Earth’s surface
directly above the focus.
As energy is released, it travels as seismic waves, outward
in all directions.
Seismic waves are the vibrations caused by earthquakes.
A WAVE IS A DISTURBANCE IN A MEDIUM. Examples
are a rock thrown in a pond, a sound, or light.
Shallow vs. Deep Focus
• In general, if two earthquakes with equal
strength have the same epicenter, the
SHALLOW FOCUS will cause MORE
DAMAGE, as seismic waves from the deepfocus earthquake lose more energy as they
travel up to the surface.
Questions:
1.Why do the waves lose energy as they
move to the surface?
2. If the plates are pulling apart in the
picture on this page, what sort of fault is
this?
3. Is this type of fault likely to have
deep earthquakes?
There are 3 types of Seismic Waves THAT WE’LL DISCUSS:
1. Primary Waves (P waves)
2. Secondary Waves (S waves)
P and S waves are known as BODY WAVES
since they go through the BODY of the
Earth, instead of along its surface.
3. Surface Waves (There are Love and Rayleigh surface waves, but we will simply call
them surface waves.)
As these waves travel through Earth, they reflect (bounce
off of) boundaries between different layers.
The waves can also refract (bend) as they enter a new layer.
Whether the waves reflect or refract depends on the angle
that they hit the new layer(s) and the density of the new
layer(s). (Same is true for light and sound waves!)
Properties of Primary Waves (P waves):
*Fastest and 1st to arrive (we use 5km/sec)
*Can move through Solids, Liquids, and Gases
*Particles are slightly pushed together or pulled apart
in the direction of the waves’ travel.
*Buildings may experience push/pull in response
*One of the two types of body waves (with S waves)
Properties of Secondary Waves (S waves):
*Second to arrive
*Travel ½ the speed a primary waves (that’s 2.5 km/sec)
*Particles move perpendicular (right angle) to the
direction of the wave, causing small buildings to rock
back and forth
*ONLY TRAVEL THROUGH SOLIDS!
(This is how
scientists realized our outer core is NOT solid!)
*One of the two types of body waves (with P waves)
Properties of
Surface Waves:
*Move along the SURFACE of the Earth
*As depth increases, motion from surface waves
decreases, then stops
*Make ground roll up and down or shake side to side
*SLOWEST!
*BIGGEST GROUND MOVEMENT/MOST
DAMAGE!!
Seismic Stations are placed all over the world
to detect and measure ground movements.
Seismographs are instruments that constantly
record ground movements, both up and down and
side to side.
Recordings produced by the seismograph are
known as seismograms.
*Scientists use these seismograms to determine
locations and strengths of earthquakes. They
need at least 3 to locate an EXACT location.
There are 2 types of seismographs, one to
measure up and down movement and one to
measure side to side movement of the ground.
7.3 Earthquake Damage Can Be Reduced
There are 2 scales that measure earthquake strength:
*The Richter Scale
*The Moment Magnitude Scale
The Richter Scale is based on how fast the ground moves
(amount of shaking) at the seismic station. Has accuracy
issues since it was developed for the ground in southern
California.
The Moment Magnitude Scale is based on the total amount
of energy that is released by earthquakes.
Both scales show a top value of 10, but neither has a
maximum value. Most scientists today prefer the Moment
Magnitude Scale.
32 times more
An increase in one whole number indicates an increase of
energy
*GREATER MAGNITUDE = GREATER DAMAGE*
Moment Magnitude
0-3.9
Very Minor to Minor
Effects Near Epicenter
rarely noticed
4.0-4.9 Light
slight damage
5.0-5.9 Moderate
some structures damaged
6.0-6.9
major damage to structures
Strong
7.0-7.9 Major
some well-built structures
destroyed
8.0 and above Great
major to total destruction
Damage from Earthquakes
Aftershock:
a smaller earthquake that
follows a more powerful earthquake.
Liquefaction: process
where shaking of the
ground causes certain
soils to act like a liquid,
causing buildings to
topple or just sink into
the ground. SEATTLE
may look like the photo
on the right some day.
Tsunami: large water wave caused by an earthquake.
• Japan’s recent tsunami killed thousands and destroyed a nuclear reactor, spreading
contamination worldwide.
• A tsunami in Indonesia
Earthquakes Cause Damage, cont’d
A lot of damage from earthquakes is caused as
the earth is ripped apart and fires start due to
broken natural-gas lines, broken electrical power
lines, or overturned stoves.
Most injuries and deaths are not caused by
direct movement of the ground hurting people, but
by collapsing buildings and other structures,
fires, and the occasional tsunami.
Structures can be designed to resist
earthquake damage
Many structures are shaken off of their foundations
during an earthquake.
Building engineers use base isolators (left) to absorb
ground motion and cross braces (right) for support and
structure to buildings in locations of fault zones.