EARTHQUAKES When good rock goes bad!.

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Presentation transcript:

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 Shaking of the ground caused by sudden release of energy stored in rocks.

A force that acts upon a rock to change its shape or volume STRESS! A force that acts upon a rock to change its shape or volume

Stress Types Compression - pushing together Add compression…

Stress Types Tension - pulling apart Add tension…

Stress Types Shearing – pushing in opposite directions 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 (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-

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

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

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

How are earthquakes detected? Seismographs

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!) To calculate lag time, simply subtract arrival time of S-wave from arrival time of P-wave. 7:14.2 7:17.4 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 16:00 17:00 18:00 19:00 20:00 21:00

P wave arrival = Lag time = S wave arrival = L wave arrival = 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

Charleston, SC August 31, 1886 ~ 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

Fissures

Offset Rail Road Tracks

Broad Street

East Bay Street

College of Charleston

Destruction was random

Approximately 6 minutes 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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