EARTHQUAKES When good rock goes bad!. EARTHQUAKES Shaking of the ground caused by sudden release of energy stored in rocks.

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Presentation transcript:

EARTHQUAKES When good rock goes bad!

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 Faults Normal Fault – results from tensional stress, hanging wall moves down relative to foot wall

Types of Faults Reverse Fault – results from compressional stress, hanging wall moves up relative to foot wall

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

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 Waves (P-Waves)

Shear waves (side-side) Vibration is perpendicular to the direction of wave propagation Secondary Waves (S-Waves)

Secondary Waves (S-Waves)

Slowest and most destructive – Rayleigh Waves: elliptical motion – Love Waves: horizontal motion (perpendicular to travel) Surface Waves

(Love and Rayleigh) RayleighWaves LoveWaves

Seismographs How are earthquakes detected?

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.

Reading a Seismogram

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

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

5:006:007:008:009:00 P wave arrival = S wave arrival = L wave arrival = Lag time =

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!

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

~ 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 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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