Download Earthquake_Revised

Earthquakes
Stress Relief
Earthquakes & Plate Tectonics




Rocks break & move along surfaces called
faults
When plates move, stress is put on rocks.
A rubber band (just like rocks) can only be
stretched so far until it breaks; this is
known as the elastic limit.
Earthquakes are the Earth vibrating.
Where Do They Occur?




Most earthquakes occur
along tectonic plate
boundaries
Most faults are located
near plate boundaries
80% of all earthquakes
occur along edges of the
Pacific Plate
Pacific Ring of Fire is
an area where
earthquakes & volcanoes
are very common
Why Do We Have Quakes?
As plates bump and grind with one another,
deformation takes place.
 Deformation is the rocks response to stress
being applied to it.
 Forces at Plate Boundaries:

1) Compression
2) Tension
3) Shearing

Faults
3 types of faults
Normal
 Reverse
 Strike-slip

Types of Faults- Normal
Normal
– occur at
divergent plate
boundaries
– force of tension
is at work
– Rock above the
fault surface
moves downward
in relation to the
rock below the
fault surface
– Example: Sierra
Nevadas
Types of Faults- Reverse
REVERSE
–
–
–
–
occur at
convergent plate
boundaries
force of
compression at
work
The rocks above the
fault surface are
forced up & over the
rocks below fault
surface
Example: Himalayas
Types of faults- Strike-slip
– occur at
transform fault
boundary
– force of shearing
at work
– Rocks move past
each other without
much upward or
downward motion
– Rocks snag one
another & become
twisted & strained
– Example: San
Andreas fault
Boundary/Force/Fault
Boundary
Force
Fault
Convergent
Compression
Reverse
Divergent
Tension
Normal
Transform
Fault
Shearing
Strike-slip
How Strong are Earthquakes?

The strength of an
earthquake depends
on the type of plate
motion involved at
the different faults.
Faults & Earthquake Strengths
Produced
1.
2.
3.
Strike-slip faults:
because of the horizontal
motion of the plates,
earthquakes are typically
very SHALLOW and
MODERATE in strength
Reverse faults: due to
compression, earthquakes
are usually very DEEP and
very STRONG
Normal faults: due to
tension pulling plates
apart, the quakes are
SHALLOW and WEAK
Focus vs. Epicenter
The focus is the point
inside the Earth where
primary and secondary
waves originate.
 The epicenter is the
point directly above
the focus on the
surface where surface
waves are generated.

Seismic Energy


Primary waves
– fastest waves
– causes particles in rock to move back & forth in same
direction
Secondary waves

Move the earth by causing particles of rock to move at right
angles to direction of wave

Arrive 2nd at a seismograph station
Surface Waves

Surface Waves Move
by back & forth swaying motion
 Move by giving particles an elliptical
motion

travels out from epicenter
 Slowest
waves
 Cause the most damage on the earth’s
surface
Surface Waves
Surface waves travel on
the Earth’s surface and
cause the most damage
to buildings and roads.
 Their movement
resembles ocean waves,
moving the ground up
and down.
 They travel the slowest
of all seismic waves.

Locating an Epicenter




Need 3 seismograph station readings
Scientists draw circles around seismograph
stations; the radius of the circle is equal to the
station’s distance from the earthquake epicenter
The Intersection of the 3 circles is where the
epicenter is located
The farther apart the arrival of the primary,
secondary, & surface waves, the farther away
the epicenter
Seismic Waves





At certain depths, the speed & path of
seismic waves change.
The changes mark the boundaries of earth’s
layers.
Moho discontinuity- boundary between
crust & upper mantle
The Shadow Zone is an area where no
waves are detected.
Boundaries cause waves to change speed &
bend.