slide presentation pdf

Geology 112 - Earthquakes
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Objective
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Activity #3 Objective: Outcome #1
Measuring an Earthquake
Outline
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• Seismic Wave Properties (cont.)
• Velocities of Seismic Waves
• Surface Waves
• Wave Characteristics/Properties
• Measuring Earthquakes
• Traveltime Curves
• Next Time - Learning Outcome #3 -
Use Traveltime Curves to Locate
Earthquakes and Determine Magnitude
Body Wave Velocity
Velocity propagation of disturbance, displacement or energy
Compressional Waves
“P Waves”
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ShearWaves
“S Waves”
• Velocity is a function of material properties (elastic moduli)
• Shear modulus and Modulus of incompressibility (more rigid=faster)
• P wave velocity is approx. 1.7 times S wave velocity
• S wave velocity is about 60% of P wave velocity
Recap Body Waves
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Particle Motion and Speed
P Wave
S Wave
Seismic Waves
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http://www.seismolab.caltech.edu/gen_eq_info.html
Surface Waves
http://www.tjhsst.edu/~jlafever/wanimate/Wave_Properties2.html
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Surface Waves => Rayleigh Waves
Daniel A. Russell, Graduate Program in Acoustics, The Pennsylvania State University
http://www.acs.psu.edu/drussell/demos/waves/wavemotion.html
Surface Waves
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Damage from Surface Waves14
• Rayleigh surface waves cause
the most damage
• Velocities slower than S
waves, and arrive later,
• Much greater amplitudes and
therefore energy
• Most easily felt during an
earthquake
• Both up-down and side-to-
side motion.
http://www.seismolab.caltech.edu/gen_eq_info.html
http://www.seismolab.caltech.edu/gen_eq_info.html
Recap - Seismic Waves
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Wave Characteristics
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•Period - time for one wavelength to pass a
fixed location
http://www.seismolab.caltech.edu/gen_eq_info.html
http://www.geologycafe.com/class/chapter12.html
Seismic Waves are
More Complicated
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Wavefront & Raypath Down into Earth
Ground Surface
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From Tom Boyd’s WWW Site - http://talus.mines.edu/fs_home/tboyd/GP311/introgp.shtml
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How do scientists acquire data on earthquakes?
Vertical Seismograph 20
When an earthquake occurs the seismic waves travel through the Earth to the
seismic station where the information is transmitted to distant computers.
A seismometer detects
motion and then recorded
on seismograph.
A seismogram is the EQ record.
http://www.youtube.com/watch?v=DX5VXGmdnAg
Incorporated Research Institutions for Seismology
Incorporated Research Institutions for Seismology
Horizontal Seismograph
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Drum Seismograph
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http://www.youtube.com/watch?v=Q1vC4jRR0uQ
http://www.youtube.com/watch?v=83GOKn7kWXM
Incorporated Research Institutions for Seismology
Incorporated Research Institutions for Seismology
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Pick Arrival Times of P, S and L
(Surface) Waves
Earthquake, Seismic Waves, Shaking &
Seismogram
Time of Day - Increasing
http://youtu.be/ZESO9bNgZ-M
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Seismometer - 3 Component 25
Seismic Shaking - 4 Stations 26
http://www.youtube.com/watch?v=ZESO9bNgZ-M
http://youtu.be/4nS10xNBfVE
Incorporated Research Institutions for Seismology
What do you want to know?
Where - Traveltime Curves 28
• When?
• Where?
• How big?
• Depth?
• Type?
http://earthquake.usgs.gov/earthquakes/map/
http://youtu.be/4nS10xNBfVE
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Travel-time Curve
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Location = Epicenter & Hypocenter
Epicenter—Location directly above EQ on Earth’s surface.
Hypocenter or Focus; point within Earth where the EQ occurred.
Incorporated Research Institutions for Seismology
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Calculate Distance away from seismic station using S-arrival
time minus P-arrival time plotted this graph of travel-time
curves.
http://youtu.be/4nS10xNBfVE
Faulting & Earthquake
Plot S-P on Travel Time Curve
Summary
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Learning Outcome #1
• Seismic Wave Properties (cont.)
• Velocities of Seismic Waves
• Surface Waves
• Wave Characteristics/Properties
• Measuring Earthquakes
• Traveltime Curves
• Next Time - Learning Outcome #3 -
Use Traveltime Curves to Locate
Earthquakes and Determine Magnitude