7.1 Rocks Move Along Faults

A fault is a break or fracture in the lithosphere, where blocks of rock move past each other.

Along parts of a fault, rocks may stick or lock together and bend as stress is put on them.

Stress is the force exerted when an object presses on, pulls on, or pushes against another object.

As stress increases, the rocks break free.

The sudden release of stress in the lithosphere is called an earthquake.

An earthquake is a shaking of the ground caused by the sudden movement of large blocks along a fault.

*Most earthquakes occur along plate boundariesbecause large amounts of stress build up as plates move.

• Compression: the force that occurs

when two tectonic plates collide

Most earthquakes occur in the lithosphere because the tectonic plates are rigid and brittle, can break easily, and can move suddenly.

7.2 Earthquakes Release Energy

All earthquakes start beneath Earth’s surface.

A focus is the point underground where rocks first begin to move. (where the earthquake begins)

The epicenter is the point on Earth’s surfacedirectly above the focus.

As energy is released, it travels as seismic waves, outward in all directions.

Seismic waves are the vibrations caused by earthquakes.

A WAVE IS A DISTURBANCE IN A MEDIUM. Examples are a rock thrown in a pond, a sound, or light.

Shallow vs. Deep Focus• In general, if two earthquakes with equal

strength have the same epicenter, the SHALLOW FOCUS will cause MORE DAMAGE, as seismic waves from the deep-focus earthquake lose more energy as they travel up to the surface.

Questions: 1.Why do the waves lose energy as they move to the surface?2. If the plates are pulling apart in the picture on this page, what sort of fault is this?3. Is this type of fault likely to have deep earthquakes?

There are 3 types of Seismic Waves THAT WE’LL DISCUSS:

1. Primary Waves (P waves)

2. Secondary Waves (S waves)

3. Surface Waves (There are Love and Rayleigh surface waves, but we will simply

call them surface waves.)

P and S waves are known as BODY WAVES since they go through the BODY of the Earth, instead of along its surface.

As these waves travel through Earth, they reflect (bounce off of) boundaries between different layers.

The waves can also refract (bend) as they enter a new layer.

Whether the waves reflect or refract depends on the angle that they hit the new layer(s) and the density of the new layer(s). (Same is true for light and sound waves!)

Properties of Primary Waves (P waves):

*Fastest and 1st to arrive (we use 5km/sec)

*Can move through Solids, Liquids, and Gases

*Particles are slightly pushed together or pulled

apart

in the direction of the waves’ travel.

*Buildings may experience push/pull in response

*One of the two types of body waves (with S

waves)

Properties of Secondary Waves (S waves):

*Second to arrive

*Travel ½ the speed a primary waves (that’s 2.5

km/sec)

*Particles move perpendicular (right angle) to

the

direction of the wave, causing small buildings to

rock

back and forth

*ONLY TRAVEL THROUGH SOLIDS! (This is how

scientists realized our outer core is NOT solid!)

*One of the two types of body waves (with P waves)

Properties of Surface Waves:

*Move along the SURFACE of the Earth

*As depth increases, motion from surface waves

decreases, then stops

*Make ground roll up and down or shake side to side

*SLOWEST!

*BIGGEST GROUND MOVEMENT/MOST

DAMAGE!!

Seismic Stations are placed all over the world to detect and measure ground movements.

Seismographs are instruments that constantly record ground movements, both up and down and side to side.

Recordings produced by the seismograph are known as seismograms.

*Scientists use these seismograms to determinelocations and strengths of earthquakes. They need at least 3 to locate an EXACT location.

There are 2 types of seismographs, one to measure up and down movement and one to measure side to side movement of the ground.

7.3 Earthquake Damage Can Be Reduced

There are 2 scales that measure earthquake strength:*The Richter Scale*The Moment Magnitude Scale

The Richter Scale is based on how fast the ground moves (amount of shaking) at the seismic station. Has accuracy issues since it was developed for the ground in southern California.

The Moment Magnitude Scale is based on the total amount of energy that is released by earthquakes.

Both scales show a top value of 10, but neither has a maximum value. Most scientists today prefer the Moment Magnitude Scale.

Moment Magnitude Effects Near Epicenter

0-3.9 Very Minor to Minor rarely noticed

4.0-4.9 Light slight damage

5.0-5.9 Moderate some structures damaged

6.0-6.9 Strong major damage to

structures

7.0-7.9 Major some well-built structures destroyed

8.0 and above Great major to total destruction

An increase in one whole number indicates an increase of 32 times more energy

*GREATER MAGNITUDE = GREATER DAMAGE*

Damage from EarthquakesAftershock: a smaller earthquake that follows a more powerful earthquake. Liquefaction: process where shaking of the ground causes certain soils to act like a liquid, causing buildings to topple or just sink into the ground. SEATTLE may look like the photo on the right some day.

Tsunami: large water wave caused by an earthquake.

• Japan’s recent tsunami killed thousands and destroyed a nuclear reactor, spreading contamination worldwide.

• A tsunami in Indonesia

Earthquakes Cause Damage, cont’d

A lot of damage from earthquakes is caused as the earth is ripped apart and fires start due to broken natural-gas lines, broken electrical power lines, or overturned stoves.

Most injuries and deaths are not caused by direct movement of the ground hurting people, but by collapsing buildings and other structures, fires, and the occasional tsunami.

Structures can be designed to resist earthquake damage

Many structures are shaken off of their foundations during an earthquake.

Building engineers use base isolators (left) to absorb ground motion and cross braces (right) for support and structure to buildings in locations of fault zones.