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