Measuring
Earthquakes
CH 5 Prentice Hall
p.154-159
Fault
• A break in Earth’s lithosphere (crust) where
plates slide or move past each other.
Earthquake (Focus)
• The point of first movement (where the
earthquake originates) usually underground
is called the Focus.
– Depth of the focus is related to the type of
boundary.
• Subduction- Deep 700m
• Divergent- 30m
• Transform- Shallow
Earthquake (Epicenter)
• The point directly above the focus on the
surface of the Earth is called the Epicenter.
Seismic Waves
• Vibrations caused by the energy of the
earthquake that travel through the Earth’s
interior and across the surface away from the
focus.
– P-Waves
– S- Waves
– Surface Waves
• .
Primary Waves
• First Wave to Arrive
– Squeeze and stretch rock, (Accordion like)
• Compressional Push/Pull
– Pass through
•
•
•
•
Solid rock
Magma
Ocean water
Air
Secondary Waves
• Arrive second after P-waves
– Vibrate from side to side as well as up and down.
– Shake the ground back and forth.
– Can not pass through liquids or gasses.
• Indicates that Earth has a liquid outer core.
Surface Waves
• Travel along the Earth’s surface
• Produced by S and P waves that reach the
surface.
– Produce severe ground movement
• Most damaging waves
Locating an Epicenter Lab
• Purpose: Geologists who study earthquakes
are called seismologists. If you were a
seismologist, you would receive data from all
across the country. Within minutes after an
earthquake, seismographs located in Denver,
Houston, and Miami would record the times
of arrival of the P waves and S waves. You
would use this data to zero in on the exact
location of the earthquake’s epicenter.
Locating an Epicenter Lab
• Seismograph Data:
City
Denver,
CO
Difference
in P & S
Wave
2min 10sec
Arrival
Times
Houston,
TX
Miami,
FL
4min
5min 40sec
Locating an Epicenter Lab
Locating an Epicenter Lab
200
400
600
800
1000 1200 1400 1600 1800
2000 2200 2400 2600 2800 3000 3200
3400 3600
3800 4000 4200 4400 4600 4800
5000 (km)
Seismograph
• An instrument used
to detect and record
waves produced by
earthquakes.
– seismic waves.
Seismograph
• Consist of a heavy
weight attached to a
frame by a wire.
• A pen is attached to a
heavy weight.
• The pen stays in
place, the drum
moves during an
earthquake.
Seismograph
• More than 10,000 Stations
• Use Data to :
– Determine an epicenter
• Need 3 Stations
– Earthquake Magnitude
Seismogram
• The record sheet the pen records on
as the drum rotates.
– No movement=
– Movement =
Locating an Epicenter
• Need 3 Seismograph stations to determine
their distance from the epicenter.
• Determine distance by:
– Dist = S-wave arrival time - P-wave arrival time.
Locating an Epicenter
• Each station draws a circle.
– Where all three intersect is the epicenter.
Magnitude
• The measure of the amount of energy
released in an earthquake.
– Three types
• Mercalli
• Richter Scale Magnitude – most commonly used
• Moment Magnigude
Layers
of
Earth
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Mercalli Scale
• Rates earthquakes according to intensity.
(the strength of the ground movement)
• Not a precise measurement
• 12 steps describe how earthquakes effect
people, buildings, and the surface.
Layers
of
Earth
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Richter Scale
• Developed in the 1930’s
• Rates the size of the seismic waves.
• Measures the intensity of ground movements
– Provides accurate measurements for small,
nearby earthquakes, but does not work well for
large or distant.
Layers
of
Earth
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Richter Scale
Layers
of
Earth
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Moment Magnitude
• Indicates the total energy released during
earthquake’s more accurately.
• Used today.
• can be used for any size earthquake, near or
far.
Layers
of
Earth
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Moment Magnitude
• Combines the following to determine the
moment magnitude:
– The type of seismic waves produced and their
strength.
– How much movement occurred on the fault.
– Strength of the rocks that broke.
• 5.0 Little damage
• Above 5.0 major damage
Layers
of
Earth
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The point under the surface directly where
the Earthquake originates.
A. Fault
B. Seismic Waves
C. Earth’s Crust
D. Focus
E. Epicenter
Layers
of
Earth
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This scale measures total energy
released.
A. Richter Scale
B. Mecallie Scale
C. Bathroom Scale
D. Moment Magnitude
E. Magnitude
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of
Earth
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How many seismograph stations
are needed to locate an
earthquake?
A. 5
B. 3
C. 1
D. 6
E. 4
Layers
of
Earth
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The wave that arrives first
A. Secondary Wave
B. Seismic Waves
C. Sound Wave
D. Primary Wave
E. Tsunami Wave
Layers
of
Earth
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The point on the surface directly
above where the Earthquake
originates.
A. Fault
B. Seismic Waves
C. Earth’s Crust
D. Focus
E. Epicenter
Layers
of
Earth
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