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SEISMIC WAVES
What is Seismology?
• Seismology is the
study of earthquakes
and seismic waves
that move through
and around the earth
• A seismologist is a
scientist who studies
earthquakes and
seismic waves
What Are Seismic Waves?
• Seismic waves are the waves of
energy caused by the sudden
breaking of rock within the
earth.
• They are the energy that travels
through the earth and is
recorded on seismographs.
• Seismic comes form the Greek
word meaning to “SHAKE”
Types of Seismic Waves
Earthquakes produce 3 basic
kinds of wave motions:
• P Waves
• S Waves
• L Waves
Primary Waves
aka compressional waves
• back and forth wave
motion
• this motion
alternates squeezing
and stretching the
rock through which
it passes
• can travel through
any material- solid
rock, magma, ocean
water, and air
• travel about twice as
fast as S waves.
S Wave-Secondary Waves
aka Shear Wave
• side to side waves
• cause particles to
move at right
angles to the
direction the
waves are
traveling
• can travel through
solids but not
through liquids or
gases
L Waves-Surface Waves
• When P and S
waves reach the
surface, they set
up a third wave
called L waves or
surface waves
• These waves
move like
“ripples” on a
pond and travel
more slowly than
P or S waves
P Waves S Waves L Waves
(Primary or (Secondary (Surface)
Compressional) or Shear)
Back & Forth Side to Side “Ripples” on a
motion motion pond
travel through: travel thru: Surface Waves
solid rock, Solids but not
magma, ocean liquids or
water, and air gases.
Fastest, first to Twice as Slow as Slowest, last to
reach P waves, reach reach
seismograph second seismograph
How Do Seismologists Measure
earthquake waves?
• Seismographs are instruments that
detect and record earthquake waves
• Earthquakes are measured according to
the RICHTER SCALE. This is based on the
size of the largest seismic wave produced
by the earthquake.
• The damage done by an earthquake is
measured according to the MERCALI
SCALE.
Modified Mercalli Scale
Inten Verbal Magnit
Witness Observations
sity Description ude
I Instrumental 1 to 2 Detected only by seismographs
II Feeble 2 to 3 Noticed only by sensitive people
III Slight 3 to 4 Resembling vibrations caused by heavy traffic
IV Moderate 4 Felt by people walking; rocking of free standing objects
Rather
V 4 to 5 Sleepers awakened and bells ring
Strong
VI Strong 5 to 6 Trees sway, some damage from overturning and falling object
VII Very Strong 6 General alarm, cracking of walls
VIII Destructive 6 to 7 Chimneys fall and there is some damage to buildings
IX Ruinous 7 Ground begins to crack, houses begin to collapse and pipes break
Ground badly cracked and many buildings are destroyed.There are some
X Disasterous 7 to 8
landslides
Very Few buildings remain standing; bridges and railways destroyed;water, gas,
XI 8
Disasterous electricity and telephones out of action.
8 or Total destruction; objects are thrown into the air,much heaving,shaking and
XII Catastrophic
greater distortion of the ground
What Do Seismologists Learn
From Seismic Waves?
• Seismologists use seismographs to record the
amount of time it takes seismic waves to travel
through different layers of the Earth.
• As the waves travel through different densities,
seismologists can deduce the type of material the
waves are travelling through.
• The results can provide a snapshot of the Earth’s
internal structure and help us to locate and
understand fault planes and the stresses and
strains acting on them.
Results of Seismic Waves
TRIVIA QUESTIONS
• What is the most earthquake prone state in the
US?
ALASKA- it’s one of the most seismically active
regions of the world
• Why does California have so many EQ’s and NY
does not?
CA is on a plate boundary
• What states have the least amount of EQ’s?
Florida and N. Dakota
• Where and when did the largest EQ occur in the
20th Century?
1960 Chilean EQ: Magnitude 9.6 and broke a fault
over 1,000 miles long!