earthquakes when good rock goes bad!. an earthquake begins at the… focus: the point where the...
TRANSCRIPT
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.
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
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
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.
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
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
• ~ 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
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