Section 1: Earth’s Crust in MotionHow Do Stress Forces Affect

Rock?

The movement of earth’s plates creates powerful forces that squeeze or pull the rock in the crust – these forces are examples of stress

Stress – a force that acts on rock to change its shape or volume

An earthquake is the shaking and trembling that results from the movement of rock beneath earth’s surface

How Does Stress Effect the Earth’s Crust?

Deformation – any change in the volume or shape of earth’s crust

Three kinds of stress in the crust:

–Shearing – stress that pushes a mass

of rock in two opposite directions

–Tension – pulls on the crust, stretching

rock so that it becomes thinner in the middle like warm bubble gum

–Compression – squeezes rock

until it folds or breaks like a giant trash compactor

What Is a Fault?

Fault – a break

in earth’s crust where slabs of crust slip past each other; These usually occur at plate boundaries

What Kind of Faults Are There?

Three Kinds:

–Strike-slip faults

–Normal Faults

–Reverse Faults

What Are Strike-slip Faults?

Strike-slip faults

–Shearing forces cause

rocks to slip past each other

sideways with little up

and down Motion;

– Ex. San Andreas fault in California

What Are Normal Faults?

Normal faults

–Tension forces cause the rocks to

form the fault at an angle– One block is above the fault

–Hanging wall – the half of the

fault that lies above

–Footwall – the half of the fault that

lies below– Ex. Rio Grande rift valley

What Are Reverse Faults?

Reverse faults

–compression forces

cause the rocks to move

towards each other

– Same structure as normal fault but the blocks move in opposite direction; hanging wall move up

– Ex. Appalachian Mountains and Mount Gould in Glacier National Park

What type of fault?What type of fault?

A miner walks on the foot wall and looks up at the hanging wall!

A B

Normal Fault Reverse Fault

Hanging wall moves down

Hanging wall moves up

How Are Mountains Effected by These Forces?

Fault-block mountains – normal faults uplift a block of rock

Folding – bends in the rock that form when compression shortens and thickens part of the earth’s crust. Ex. Himalayas

How Are Mountains Effected by These Forces? (Continued)

Anticlines – a fold upward into an arch

Syncline – a fold downward into an arch

Plateaus – a large area of flat land elevated high above sea level

Section 2: Measuring QuakesHow Does the Energy of an

Earthquake Travel Through Earth?Earthquakes – most

begin in the lithosphere

Focus – the point beneath the earth’s surface where rock that is under stress breaks, triggering an earthquake

Epicenter – the point on the earth’s surface directly above the focus

What Are Seismic Waves?

Seismic Waves – vibrations that travel through Earth carrying the energy released during an earthquake – They move like ripples on a pond– They carry the energy of an earthquake

away from the focus, through Earth’s interior, and across the surface

– The energy is greatest at the

Epicenter

What Are the Different Kinds of Seismic Waves?

Three categories:

–P waves–S waves –Surface waves

P waves and S waves are sent out from the

focus; Surface waves develop when

the waves reach the surface

What Are P Waves?

P waves are primary waves

–The first waves to arrive

– Earthquake waves that

compress and expand the

ground like an accordion

– Cause buildings to contract and expand

What Are S Waves?

S waves are secondary waves– Earthquake waves that vibrate from side

to side as well as up and down

– These waves shake the ground

back and forth– Shake structures violently

–Cannot move through liquids

What Are Surface Waves?

When P waves and S waves reach the surface some are transformed into surface waves– Surface waves move more

slowly than P waves and S waves

–Produce the most severe

ground movements– Can make the ground roll like ocean

waves or shake buildings from side to side

How Do Scientists Detect Seismic Waves?

Seismograph – records

the ground movements caused by seismic waves as they move through the Earth

                  

How Do Scientists Measure Earthquakes?

There are at least 20 different measures for rating earthquakes, three are:

–Mercalli–Richter–Moment Magnitude

Magnitude – a measurement of earthquake strength based on seismic waves

What Is the Mercalli Scale?

Rated earthquakes according to

their intensity

–Intensity: strength of ground motion in a given place

Not a precise measurementDescribes how earthquakes

affect people, buildings, and the land surface

What Is the Richter Scale?

A rating of the size of

seismic waves as measured by a particular type of seismograph

Accurate measurements for small, nearby earthquakes not large, distant earthquakes

What Is the Moment Magnitude?

A rating system that

estimates the total energy released by an earthquake

Can be used to rate earthquakes

of all sizes, near or far

Below 5.0 – little damage

Above 5.0 – great destruction

How Do Scientists Locate the Epicenter?

Geologists use seismic waves

– P waves arrive first

– S waves arrive close behind

– Scientist measure the

difference in arrival times

• The farther away an earthquake is the greater the time between their arrival

– Scientists draw three circles using data from seismographs set at different stations to see where they intersect – the epicenter