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Earthquakes
U4L5 & 6
• Faults occur because forces inside Earth cause
Earths plates to move.
• Rocks will bend, compress, stretch, and possibly
break.
• An earthquake is ground movements that occur
when blocks of rock move suddenly and release
energy
• Applied forces cause popsicle stick
(and rocks) to undergo elastic
deformation. Even if the stick (rock)
returns to its original shape, energy is
released.
• When the elastic limit is passed,
breakage occurs.
Bending & changing shape
Breaking if elastic limit is reached.
Focus
• Earthquake’s point of energy release.
• Where the first motion of earthquake occurs
• Most foci are within 65 km of Earth’s surface. A
few are as deep as 700 km.
Epicenter
• Point on Earth’s surface directly above
earthquake focus.
• When rocks break they move along surfaces
called faults.
• Rocks on either side of fault can move up, down,
or sideways.
Where do Earthquakes Happen?
1.Normal Fault
* Tension Force =“pulling apart”
* Divergent plate boundary
*crust at divergent boundaries tends
to be thin – earthquakes are shallow
2.
Reverse Fault
* Compression force = squeeze rock
* Convergent plate boundary
* Compression squeezes
rock above fault up &
over rock below fault.
*earthquakes tend to be very strong & can be deep
3. Strike-slip fault
* Shear stress = push on rock in horizontal & opposite directions
* Transform plate boundary
* Rocks on either side of fault move past each other without much up or
down motion.
* relatively shallow – upper 50km of crust
Big one – San Andreas Fault in CA
Types of Earthquake (Seismic) Waves
• Body waves – travel
through earth’s interior
• P waves – a.k.a. pressure
waves
• Travel fastest – also called
primary waves since they
are the first to be detected
• Through solid, liquid &
gasses
• Cause rock to move back &
forth in direction wave is
traveling
• S waves – a.k.a. shear
waves
• Slower, arrive second, so
called secondary waves
• Can’t travel through
liquid mantle or liquid
core
• Cause rock to move side
to side or up and down
Compare P waves and S waves
• Surface waves – L waves – travel along surface of earth, not through it
• make motion on earth’s surface only
• Cause more damage than body waves
• Up & down motion AND back and forth motion
• Do p. 256 & 257
• Difference between P and S waves = lag time;
• used to find epicenter with triangulation
• Read and do p. 258 & 259
Seismometer
• Instrument that measures
seismic waves.
• Consists of rotating drum of
paper and pendulum with
attached pen.
• When seismic waves reach
seismometer, the drum vibrates
but pendulum remains at rest.
Vertical
Motion
Seismograph
Horizontal Motion Seismograph
Seismogram p. 306
• Paper record of seismic events.
Show video P “Seismic Waves
Measuring the magnitude (amount of energy
released by an earthquake)
• Richter Scale – each time magnitude goes up one unit,
ground motion is 10 times greater
• EQ with magnitude of 5.0 has 10 times as much motion as a 4.0
EQ
• Moment Magnitude Scale (used more now)
• Based on size of area the fault moves, distance that fault moves,
and rigidity of rocks
Measuring the intensity (effects of EQ)
• Modidifed Mercalli Scale
• See pg. 261