earthquakes
DESCRIPTION
EARTHQUAKES. When good rock goes bad!. An Earthquake begins at the…. Focus: The point where the energy is released after elastic limit is reached. Epicenter: The point on the surface of the Earth directly above the focus. EARTHQUAKES. - PowerPoint PPT PresentationTRANSCRIPT
EARTHQUAKESWhen good rock goes bad!
An Earthquake begins at the…• Focus: The point where the
energy is released after elastic limit is reached.
• Epicenter: The point on the surface of the Earth directly above the focus.
EARTHQUAKES
Shaking of the ground caused by sudden release of energy stored in
rocks.
STRESS!A force that acts upon a rock to
change its shape or volume
Compression - pushing together
Stress Types
Add compression…
Tension - pulling apart
Stress Types
Add tension…
Shearing – pushing in opposite directions
Stress Types
Add shearing…
Fault Terminology
Types of FaultsNormal Fault – results from tensional stress, hanging wall moves down relative to foot wall
Types of FaultsReverse Fault – results from compressional stress, hanging wall moves up relative to foot wall
Types of FaultsStrike-Slip Fault – results from shearing stress, rocks on either side of fault slip past each other sideways with little motion up or down
Offset produced by 1906 San Francisco quake
Focus and Epicenter
Types of seismic waves:
Primary Waves (P-Waves)
Secondary Waves (S-Waves)
Surface Waves (Love and Rayleigh)
Primary Waves (P-Waves)
• The fastest wave, they arrive 1st
• Compressional motion in the wave (push-pull)
• Vibration is parallel to the direction of wave propagation
Primary (P) Waves:• Move out from the earthquake focus.• Travel the fastest of the 3 waves.• Travel twice as fast as secondary waves.• Move by causing particles in rocks to move
back and forth in the same direction that the wave is traveling.
• Example: slinky• Are bent and slowed when they hit the outer core.• Longitudinal wave Blue-
• Shear waves (side-side)• Vibration is perpendicular to the direction of wave propagation
Secondary Waves (S-Waves)
Slowest and most destructive– Rayleigh Waves: elliptical motion– Love Waves: horizontal motion (perpendicular to travel)
Surface Waves
Secondary (S) Waves:• Move out from the earthquake focus.• Move slower than primary waves.• Move by causing particles in rocks to move at right
angles to the direction of wave travel.• Example: rope• Cannot travel through liquids, so they are stopped by the outer core.• Transverse Wave Red-
Surface Waves:• Form when P and S waves reach the
surface.• Slowest Waves, Most destructive • Can cause the ground to shake making
rock roll and sway from side to side.• Only travel through crust
Seismographs
How are earthquakes detected?
Seismograph:
• instrument used to record the energy released by an earthquake. Recording time of wave arrival.
• Produces paper sheet called a seismogram
• A stationary pen traces a record of vibrations
Seismograph Stations• P waves arrive first• S waves arrive second• Surface waves arrive last (slowest)• 3 or more seismograph stations are
needed to determine the location of the epicenter.
• When an epicenter is far from a location, the p wave has more time to put distance between it and the s and surface waves.
Reading a Seismogram
Calculating lag time (oh no, more math!)
7:14.2 7:17.4To calculate lag time,simply subtract arrivaltime of S-wave from arrival time of P-wave.
P-wave arrival time
S-wave arrival time
S - P = 7:17.4 – 7:14.2 = 3.2 minutes
3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00
1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00
9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00
5:00 6:00 7:00 8:00 9:00
P wave arrival =
S wave arrival =
L wave arrival =
Lag time =
Finding the Epicenter
Shadow Zone Animation
• http://www.earthquake.gov/learning/animations/animation.php?flash_title=Shadow+Zone+Flash+Animation&flash_file=shadowzone&flash_width=220&flash_height=300
How damage occurs in an earthquake SHAKING some areas shake more than others
• unconsolidated sediments
• landfill
• wetlands
LIQUIFACTION
water rises to
Surface of sediments
August 31, 1886 Charleston, SC
• ~ 6.8 - 7.2 - 7.6 magnitude• over 60 people died• felt from NEW YORK to CUBA; from BERMUDA to
MISSISSIPPI RIVER• Wooden houses did better than brick - why?• Damage greatest on ‘made ground’ - why?• Sand/mud volcanoes common; some fissures
Charleston, SC August 31, 1886
Fissures
Offset Rail Road Tracks
Broad Street
East Bay Street
College of Charleston
Destruction was random
S – P = approx. 6 minutesFind that lag time difference between the S & P arrival time from seismic velocity graph, then come straight down to find the distance that station was to the earthquake
Approximately 6 minutes
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