Warm-Up

• Write a paragraph describing what it is like to be in an earthquake.

• If you have never been in one, MAKE IT UP! Write about what you THINK it would be like.

• Use proper paragraph structure (from English) and sensory details to make the experience vivid.

Warm-Up

• Waves carry energy. List some examples of energy carried by waves.

Earthquakes

Chapter 19

What is an earthquake?

• An earthquake is a natural vibration of the ground.

• Earthquakes are caused by:• Movement along fractures

(cracks) in the earth’s crust• Volcanic activity

What forces affect earthquakes?

• Earthquakes are caused by fractures.

• Fractures are caused by stress.

• Stress is the amount of force per area acting on a material.

• When stress exceeds the strength of the rock, an earthquake happens.

• There are three types of stress:

• Compression—decreases the volume of a material

• Tension—pulls material apart• Shear—twists a material

• Strain is the deformation of a material because of stress.

What is a fault?

• A fault is a fracture, or system of fractures, along which movement occurs.

• Faults form when stress is too great or applied too quickly.

What are the three types of faults?

• Normal fault• Caused by horizontal tension of

rocks being pulled apart.• Movement is partly horizontal,

partly vertical.• Movement extends the crust.

• Reverse fault• Caused by horizontal compression

of rocks being pushed together.• Movement is partly horizontal,

partly vertical.• Movement shortens the crust.

• Strike-slip fault• Caused by horizontal shear of

rocks sliding past each other.• Movement is horizontal.• Ex. San Andreas fault in California

What happens in the ground during an earthquake?

• During an earthquake, there are vibrations in the ground called seismic waves.

• The place where seismic waves start is called the focus of the earthquake.

• The place above the focus, on the surface of the earth, is called the epicenter.

• There are three types of seismic waves.

What are the types of seismic waves?

• Primary waves (P-waves)• Squeeze and pull (compress and

stretch) rocks.• Occur inside the earth’s interior.

• Secondary waves (S-waves)• Move rocks up and down.• Occur inside the earth’s interior.

• Surface waves• Move rocks up and down as well

as side to side.• Occur on the surface of the earth.

What do seismic waves tell us about the earth’s interior?

• Seismic waves travel differently through different materials.

• P-waves refract (bend) when they reach the core.

• S-waves cannot go through the core because they cannot travel through liquid.

• This shows us that the layers of the earth have different densities.

• The time it takes seismic waves to travel through the earth tells us how thick each layer is.

How do we detect seismic waves?

• A seismometer is a device used to measure vibrations caused by earthquakes.

• Seismometers measure:• Intensity of P-waves• Intensity of S-waves• Intensity of surface waves• Time

Seismometer Demo

• In groups of three, get out one piece of paper and a pencil to share.

• Have one person lightly hold the pencil so that it is barely touching the paper.

• Have another person gently pull the paper so the pencil is making a straight line.

• Have the third person shake the table back and forth.

• It takes three seismometers to find the epicenter of an earthquake.

• The place where the three circles overlap is the epicenter.

• The time of an earthquake can be measured using a time-travel graph.

• P-waves travel faster and arrive first.

• S-waves travel slower and arrive last.

• The travel time is subtracted from the arrival time to find the time the earthquake occurred.

• Earthquake magnitude (intensity) is measured on the:

• Richter scale – based on the size of the largest waves generated by the quake.

– Increases by factors of 10

» Ex: A magnitude 7 earthquake is 10 times stronger than a magnitude 6.

• Moment magnitude scale – accounts for the size of the fault rupture, amount of movement, and rock stiffness

• Modified Mercalli scale – rates the type of damage caused by the earthquake.

What are the main earthquake hazards?

• Structural failure

• Land and soil failure• Liquefaction is when solid earth is

shaken until it acts like a liquid.• Can cause landslides and sinking

of structures.

• Fault scarps—vertical offset in the land

• Tsunami—large ocean waves caused by earthquakes