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Earthquakes
Waves &
Seismograms
Lecture prepared by Mr. B
Topics
•Elastic rebound theory
•Seismic waves
•Seismograph and seismogram
•Finding distance to an earthquake
•Locating an earthquake
•Distribution of earthquakes
•Depth of earthquakes
•Earthquake intensity
•Earthquake magnitude
Earth In
Cross
Section
Under
the
Mantle,
it’s like a
Lava
Lamp
Can Earthquakes be Predicted?
Earthquake Precursors
1.
2.
3.
4.
5.
changes in elevation or tilting of land surface
fluctuations in groundwater levels
magnetic field
electrical resistance of the ground
Release of gases
Can Earthquakes be Predicted?
Earthquake Prediction Programs
– include laboratory and field studies of rocks before, during,
and after earthquakes
– monitor activity along major faults
– produce risk assessments
Elastic Rebound
Theory
Rocks bend under
stress while storing
elastic energy. When
the strain in the rocks
exceeds their
strength, breaking will
occur along the fault.
Stored elastic energy
is released as the
earthquake. Rocks“snap
back”, or rebound to
their original condition.
Spread of the Seismic Waves
Types of Earthquake (Seismic)
Waves
• Body Waves
– P-Waves (primary waves)
– S-Waves (secondary waves)
• Surface Waves
- L – Love Waves
- R – Raleigh Waves (surface, vertical)
P-Waves
P Wave Animation
S-Waves
S Wave Animation
Body Waves: P and S waves
•
Body waves
– P or primary waves
• fastest waves
• travel through solids,
liquids, or gases
• compressional wave,
material movement is
in the same direction
as wave movement
– S or secondary waves
• slower than P waves
• travel through solids
only
• shear waves - move
material
perpendicular to
wave movement
Surface Waves: R and L waves
•
Surface Waves
– Travel just below or along the ground’s surface
– Slower than body waves; rolling and side-to-side
movement
– Especially damaging to buildings
The Poor House 
Seismograph
(Horizontal)
Seismograph (Vertical)
Distribution of Quakes
A direct result of Continental Drift
Earthquake Risk
The Continental Plates
Profile of Subduction Zone
Tsunami
Tsunami
Seismogram
How is an Earthquake’s Epicenter Located?
Seismic wave behavior
– P waves arrive first, then S waves, then L and R
– Average speeds for all these waves is known
– After an earthquake, the difference in arrival times at a
seismograph station can be used to calculate the distance
from the seismograph to the epicenter.
How is an Earthquake’s Epicenter Located?
Time-distance graph
showing the average
travel times for P- and Swaves. The farther away a
seismograph is from the
focus of an earthquake,
the longer the interval
between the arrivals of
the P- and S- waves
Locating the Earthquake
How is an Earthquake’s
Epicenter Located?
•
•
•
Three seismograph stations
are needed to locate the
epicenter of an earthquake
A circle where the radius
equals the distance to the
epicenter is drawn
The intersection of the
circles locates the
epicenter
How are the Size and Strength of an Earthquake
Measured?
•
•
Modified Mercalli Intensity Map
– 1994 Northridge, CA earthquake,
magnitude 6.7
Intensity
– subjective measure
of the kind of
damage done and
people’s reactions
to it
– isoseismal lines
identify areas of
equal intensity
Mercalli Scale of
Earthquake Intensity
• Advantages:
– No high-tech instruments are required.
• Disadvantages:
– Damage depends on geologic materials and
type of structures in area
– Damage varies with distance from epicenter
– Subjective - different people may view
damage and effects very differently
The Goofy Mercalli Scale
I. People do not feel any Earth movement.
II. A few people might notice movement if they are at rest and/or on the
upper floors of tall buildings.
III. Many people indoors feel movement. Hanging objects swing back
and forth. People outdoors might not realize that an earthquake is
occurring
IV. Most people indoors feel movement. Hanging objects swing.
Dishes, windows, and doors rattle. The earthquake feels like a heavy
truck hitting the walls. A few people outdoors may feel movement.
Parked cars rock.
XI. Most buildings collapse. Some bridges are destroyed.
Large cracks appear in the ground. Underground pipelines are
destroyed. Railroad tracks are badly bent.
XII. Almost everything is destroyed. Objects are thrown into
the air. The ground moves in waves or ripples. Large amounts
of rock may move.
How are the Size and Strength of an Earthquake Measured?
• Magnitude
– Richter scale
measures total
amount of energy
released by an
earthquake;
independent of
intensity
– Amplitude of the
largest wave
produced by an
event is corrected
for distance and
assigned a value
on an open-ended
logarithmic scale
Richter Scale Math
• Increase by 1 whole number means a 10X
increase in the Magnitude of the quake
• For every increase on the Richter Scale, the
amount of energy released increases 30X
Compare a 5.0 to a 7.0 quake
7.0 has 10 X 10 = 100 times greater magnitude
7.0 has 30 X 30 = 900 times more energy!
Compared to a 1.0 quake, a 7.0 has:
10 X 10 X 10 X 10 X 10 X 10 = 1 000 000 greater strength
And
30 X 30 X 30 X 30 X 30 X 30 = 729 000 000 more energy
Earthquake Magnitude aand
Worldwide Occurrence
Magnitude
Less than 2.0
2.0-2.9
3.0-3.9
4.0-4.9
5.0-5.9
6.0-6.9
7.0-7.9
Greater than 8,0
Number/Year
600,000
300,000
49,000
6,200
800
266
18
1
Terminology Used in the
Study of Earthquakes
Earthquake intensity
Earthquake
magnitude
Richter Scale
Mercalli Scale
Elastic rebound
Fault
Focus
P-wave
Seismic wave
Seismogram
Seismograph
S-wave
Tsunami