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Published byDorcas Clarissa Bates Modified over 8 years ago
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EARTHQUAKES When good rock goes bad!
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EARTHQUAKES Shaking of the ground caused by sudden release of energy stored in rocks.
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STRESS! A force that acts upon a rock to change its shape or volume
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Compression - pushing together Stress Types Add compression…
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Tension - pulling apart Stress Types Add tension…
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Shearing – pushing in opposite directions Stress Types Add shearing…
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Fault Terminology
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Types of Faults Normal Fault – results from tensional stress, hanging wall moves down relative to foot wall
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Types of Faults Reverse Fault – results from compressional stress, hanging wall moves up relative to foot wall
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Types of Faults Strike-Slip Fault – results from shearing stress, rocks on either side of fault slip past each other sideways with little motion up or down
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Offset produced by 1906 San Francisco quake
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Focus and Epicenter
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Types of seismic waves: Primary Waves (P-Waves) Secondary Waves (S-Waves) Surface Waves (Love and Rayleigh)
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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
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Primary Waves (P-Waves)
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Shear waves (side-side) Vibration is perpendicular to the direction of wave propagation Secondary Waves (S-Waves)
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Secondary Waves (S-Waves)
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Slowest and most destructive – Rayleigh Waves: elliptical motion – Love Waves: horizontal motion (perpendicular to travel) Surface Waves
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(Love and Rayleigh) RayleighWaves LoveWaves
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Seismographs How are earthquakes detected?
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Seismograph A sensitive instrument that detects earth movements. Seismogram Produced by a recording seismograph, a seismogram is a graphical record of the movement of the earth over time.
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Reading a Seismogram
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Calculating lag time (oh no, more math!) 7:14.27:17.4 To calculate lag time, simply subtract arrival time 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
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3:004:005:006:007:008:009:0010:0011:0012:0013:0014:0015:00 1:002:003:004:005:006:007:008:009:0010:0011:0012:0013:00 9:0010:0011:0012:0013:0014:0015:0016:0017:0018:0019:0020:0021:00
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5:006:007:008:009:00 P wave arrival = S wave arrival = L wave arrival = Lag time =
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Finding the Epicenter Scientists then use a method called triangulation to determine exactly where the earthquake was. It is called triangulation because a triangle has three sides, and it takes three seismographs to locate an earthquake. If you draw a circle on a map around three different seismographs where the radius of each is the distance from that station to the earthquake, the intersection of those three circles is the epicenter!
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How damage occurs in an earthquake SHAKING some areas shake more than others unconsolidated sediments landfill wetlands LIQUIFACTION water rises to Surface of sediments
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August 31, 1886 Charleston, SC
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~ 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
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Fissures
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Offset Rail Road Tracks
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Broad Street
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East Bay Street
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College of Charleston
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Destruction was random
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S – P = approx. 6 minutes Find 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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