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.

EARTHQUAKESEARTHQUAKES

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

FissuresFissures

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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