Earthquakes

Earthquakes

Middle School

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Learning Objectives

• Define an earthquake and explain its primary causes using the elastic rebound theory.

• Distinguish between an earthquake’s focus and epicenter, and classify them by depth.

• Describe the three types of plate boundaries and the earthquakes they cause.

• Compare seismic waves and explain how scales measure earthquake magnitude and intensity.

Key Vocabulary

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Earthquake

A sudden ground movement or vibration caused by the release of energy stored in rocks.

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Fault

A break in Earth's lithosphere along which blocks of rock can move past each other.

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Epicenter

The specific point on the Earth's land surface that is directly above an earthquake's focus.

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Seismic Waves

Waves of energy that travel through the Earth and are the result of an earthquake.

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Richter Scale

A scale that measures the magnitude of an earthquake based on the amount of ground motion.

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Seismometer

An instrument used by scientists to measure and record the ground motion from an earthquake.

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What Causes Earthquakes?

• Earthquakes are sudden ground vibrations from energy released in rocks.

• ​This happens along faults, which are breaks in Earth's lithosphere.

• Tectonic plate forces build up stress, bending rocks along a fault.

• The rocks suddenly snap back, releasing energy and causing an earthquake.

• This theory is known as Elastic rebound theory, proposed by the American geologist Harry Fielding Reid.

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Multiple Choice

According to the Elastic Rebound Theory, what is the direct cause of an earthquake?

• The transmission of seismic waves through the Earth's core.

• The sudden release of energy when rocks snap back to their original shape.

• The formation of new faults in the Earth's lithosphere.

• The slow movement of tectonic plates.

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Locating an Earthquake

• An earthquake's focus is where rock ruptures inside Earth; the epicenter is above.

• ​A shallow earthquake has a focus less than 70 kilometers deep and causes most damage.

• Intermediate-focus quakes are 70–300 km deep; deep-focus quakes are over 300 km deep.

• About 75% of earthquakes have a focus in the top 10 to 15 kilometers of the crust.

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Multiple Choice

Why do shallow earthquakes typically cause the most damage?

• Because they are the most common type of earthquake.

• Because they release more energy than deep-focus earthquakes.

• Because their focus is near the Earth's surface, concentrating the energy.

• Because they only occur in highly populated areas.

Plate Boundaries and Earthquakes

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Divergent Boundary

• Tectonic plates are pulling apart from each other here.

• This movement causes the Earth's crust to stretch thin.

• Shallow earthquakes occur at less than 20 km deep.

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Convergent Boundary

• Tectonic plates are colliding with one another at this boundary.

• Some of the world's strongest earthquakes happen right here.

• These earthquakes can be very deep, up to 700 km.

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Transform Boundary

• Here, the tectonic plates slide horizontally past each other.

• This sliding motion results in many shallow earthquakes.

• They happen in the upper 50 km of the crust.

Multiple Choice

At which type of plate boundary would you expect to find the strongest and deepest earthquakes?

• Divergent Boundary

• All boundaries produce equally strong earthquakes.

• Convergent Boundary

• Transform Boundary

Seismic Waves

• P-waves are the fastest seismic waves and have a push-pull motion.

• S-waves are slower and have an up-and-down motion.

• Surface waves are the slowest and cause a rolling motion on the surface.

Multiple Choice

What type of seismic waves are responsible for the majority of the damage during an earthquake due to their large amplitude and motion along the Earth's surface?

• P waves

• S waves

• Body waves

• Surface waves 

Measuring Earthquakes

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Seismometer

• Scientists who study earthquakes are known as seismologists.

• They use a seismometer to measure and record ground motion.

• This produces a visual illustration of the earthquake called a seismogram.

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Richter Scale

• This scale measures the magnitude of an earthquake from its source.

• It's based on ground motion recorded by the seismometer.

• Each unit increase represents 10 times the amount of ground motion.

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Mercalli Scale

• This scale measures the earthquake's intensity from its observed effects.

• Effects are based on what people feel and structural damage.

• It ranges from I (not felt) to XII (destroys everything).

Multiple Choice

What is the primary difference in what the Richter scale and the Modified Mercalli scale measure?

• The Richter scale measures the depth of the focus, while the Mercalli scale measures the epicenter's location.

• The Richter scale is used for shallow earthquakes, while the Mercalli scale is used for deep earthquakes.

• The Richter scale measures ground motion, while the Mercalli scale measures the earthquake's effects on people and structures.

• The Richter scale measures damage, while the Mercalli scale measures ground motion.

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Earthquake Damage and Risk

Earthquake Damage

• The shaking from an earthquake causes the greatest amount of damage to structures on the surface.

• This can result in building collapses, fires from broken gas lines, and floods from dam failures.

• An area’s local geology, such as soft soil, can amplify the shaking and increase damage.

Earthquake Risk

• Seismologists assess risk by studying past earthquakes, geology, population, and local building designs.

• Most earthquakes in the United States occur near transform and convergent plate boundaries.

• These high-risk areas include states like California, Oregon, Washington, and Alaska.

Multiple Choice

Which of the following is NOT a factor seismologists use to assess earthquake risk for a region?

• The current weather patterns of the region.

• The underlying geology of the area.

• The population density and building design.

• The region's past earthquake activity.

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Earthquake Myths vs. Facts

Misconception	Correction
Earthquakes are random and cannot be predicted.	Scientists can assess risk to identify high-risk zones using past activity.
The ground opens and swallows people.	The ground shakes violently, causing buildings to collapse and debris to fall.
A bigger magnitude always means more destruction.	A smaller, shallow quake in a populated area can cause more damage.
P-waves and S-waves are the most destructive.	Surface waves cause the rolling motion that is responsible for the most damage.

Multiple Choice

How does the movement of tectonic plates at a transform boundary lead to an earthquake?

• Plates slide past each other horizontally, building up stress that is suddenly released.

• Plates collide, squeezing the rock until it fractures.

• Plates cause volcanoes to erupt, which shakes the ground.

• Plates pull apart, stretching the crust and causing it to break.

Multiple Choice

Why is an earthquake with a magnitude of 7 on the Richter scale significantly more powerful than one with a magnitude of 6?

• It is felt over an area 10 times larger.

• It lasts 10 times longer.

• It causes 10 times more ground motion.

• It causes 100 times more ground motion.

Multiple Choice

A city is built on soft, loose sediment near a fault line. How would this geology likely affect the damage caused by an earthquake compared to a city on solid bedrock?

• The damage would be roughly the same in both scenarios.

• The soft sediment can amplify the shaking from seismic waves, increasing the damage.

• The bedrock would crack and cause more damage than the soft sediment.

• The soft sediment will absorb the seismic waves, reducing the damage.

Multiple Choice

Why are shallow-focus earthquakes generally more harmful than deep-focus earthquakes of the same magnitude?

• Deep earthquakes generate more S-waves which are less destructive than P-waves.

• Seismic energy from deep earthquakes is more absorbed by the Earth's interior before reaching the surface.

• Shallow earthquakes occur in areas with higher population density.

• Deep earthquakes only occur at convergent plate boundaries which are less populated.

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Summary

• Earthquakes result from a sudden release of energy along faults.

• The focus is the underground origin; the epicenter is the surface point directly above.

• Earthquake traits differ based on the type of plate boundary.

• Energy travels in seismic waves: P-waves, S-waves, and surface waves.

• The Richter scale measures magnitude; the Mercalli scale measures observable impact.

• Damage is affected by shaking intensity, local geology, and building design.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

• 1

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• 4