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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
(Primary or
Compressional)
S Waves
(Secondary
or Shear)
L Waves
(Surface)
Back & Forth
motion
Side to Side
motion
“Ripples” on a
pond
travel through:
solid rock,
magma, ocean
water, and air
travel thru:
Solids but not
liquids or
gases.
Surface Waves
Fastest, first to
reach
seismograph
Twice as Slow as Slowest, last to
P waves, reach
reach
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
sity
Verbal
Description
Magnit
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
V
Rather
Strong
4 to 5
Sleepers awakened and bells ring
VI
Strong
5 to 6
Trees sway, some damage from overturning and falling object
VII
Very Strong
6
VIII
Destructive
6 to 7
IX
Ruinous
7
X
Disasterous
7 to 8
Ground badly cracked and many buildings are destroyed.There are some
landslides
XI
Very
Disasterous
8
Few buildings remain standing; bridges and railways destroyed;water, gas,
electricity and telephones out of action.
XII
Catastrophic
8 or
greater
Total destruction; objects are thrown into the air,much heaving,shaking and
distortion of the ground
Witness Observations
Felt by people walking; rocking of free standing objects
General alarm, cracking of walls
Chimneys fall and there is some damage to buildings
Ground begins to crack, houses begin to collapse and pipes break
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!