Transcript Measuring Earthquakes

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
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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.
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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.
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of
Earth
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Richter Scale
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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.
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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
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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
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of
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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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How many seismograph stations
are needed to locate an
earthquake?
A. 5
B. 3
C. 1
D. 6
E. 4
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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
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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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