Science · Grade 7 · Form and Function of Structures · Term 4

Earthquakes and Seismic Waves

Understanding the causes of earthquakes, how they are measured, and the types of seismic waves.

• Ontario Curriculum Expectations
• MS-ESS2-2

About This Topic

Earthquakes occur when built-up stress along faults in Earth's crust releases suddenly, sending seismic waves through the planet. The focus marks the rupture point underground, and the epicenter lies directly above it on the surface. P-waves, or primary waves, compress and expand rock like a spring, traveling fastest through solids, liquids, and gases. S-waves, or secondary waves, move material perpendicular to their path, slower and limited to solids, causing more surface damage.

Students in Ontario Grade 7 science differentiate these waves and analyze seismograph records to locate epicenters by calculating arrival time differences and drawing circles from multiple stations. This builds data analysis skills and connects to plate tectonics, explaining why earthquakes cluster along boundaries.

Active learning benefits this topic greatly. Students physically model waves with ropes or slinkies to feel speed and motion differences, then collaborate on epicenter maps using real data sets. These experiences turn abstract underground events into observable phenomena, boosting engagement and conceptual grasp.

Key Questions

1. Explain what causes the ground to shake during an earthquake.
2. Differentiate between P-waves and S-waves and their behavior.
3. Analyze how seismographs are used to locate the epicenter of an earthquake.

Learning Objectives

• Explain the mechanism by which stress accumulation and release cause ground shaking during earthquakes.
• Compare and contrast the movement and properties of P-waves and S-waves.
• Calculate the distance to an earthquake's epicenter using seismograph data and arrival time differences.
• Analyze seismic wave data to determine the location of an earthquake's epicenter on a map.

Before You Start

Earth's Structure and Layers

Why: Understanding the Earth's crust and mantle is foundational to comprehending where faults are located and how seismic waves travel.

Forces and Motion

Why: Students need to understand concepts like stress, strain, and movement to grasp how earthquakes are initiated and how waves propagate.

Key Vocabulary

Fault	A fracture or zone of fractures between two blocks of rock where movement has occurred. Earthquakes are caused by sudden slips along faults.
Seismic Waves	Waves of energy that travel through the Earth's layers, originating from the focus of an earthquake. They include P-waves and S-waves.
P-wave (Primary Wave)	The fastest seismic wave, which travels through solids, liquids, and gases by compressing and expanding the material in the direction of its travel.
S-wave (Secondary Wave)	A slower seismic wave that travels through solids only, moving material perpendicular to the direction of its travel.
Epicenter	The point on the Earth's surface directly above the focus of an earthquake. It is the location where seismic waves are typically strongest.
Seismograph	An instrument used to detect and record the ground motion caused by seismic waves during an earthquake.

Watch Out for These Misconceptions

Common Misconception

Earthquakes can happen anywhere equally.

What to Teach Instead

Most earthquakes occur along tectonic plate boundaries due to stress buildup. Mapping activities with world earthquake data help students plot locations and identify patterns, correcting random distribution ideas through visual evidence.

Common Misconception

P-waves and S-waves travel at the same speed.

What to Teach Instead

P-waves move faster than S-waves, allowing epicenter location via time differences. Hands-on slinky or rope demos let students measure and compare speeds directly, reinforcing the distinction kinesthetically.

Common Misconception

The epicenter is the point of greatest shaking.

What to Teach Instead

The epicenter is the surface point above the focus; shaking intensity varies by distance and geology. Triangulation exercises show how wave data pinpoints epicenters, while intensity maps clarify damage distribution.

Active Learning Ideas

Slinky Demo: P- and S-Waves

Pairs stretch slinkies end-to-end. Create P-waves by pushing and pulling longitudinally to see compression; then shake side-to-side for S-waves to observe transverse motion. Time wave travel across the slinky and note which arrives first at the far end.

20 min·Pairs

DIY Seismograph: Station Build

Small groups assemble seismographs using a hanging weight, string, marker, and paper drum. Place on a shaken table to record simulated P- and S-waves. Compare tracings for wave patterns and measure arrival time gaps.

45 min·Small Groups

Epicenter Triangulation: Map Activity

Small groups receive three seismograph data sheets with P- and S-wave arrival times. Calculate time differences, draw circles on a map from each station using a scale, and find where circles intersect as the epicenter.

35 min·Small Groups

Jell-O Quake: Wave Observation

Pairs prepare trays of set Jell-O. Drop objects or shake trays to generate waves, observing how P-waves ripple through and S-waves cause shearing. Sketch wave paths and discuss solid-like behavior of Earth's crust.

25 min·Pairs

Real-World Connections

• Structural engineers in earthquake-prone regions like California use seismograph data to design buildings and bridges that can withstand seismic forces, incorporating base isolation or damping systems.
• Emergency management agencies, such as those in Japan, analyze earthquake data in real-time to issue tsunami warnings and coordinate evacuation efforts, saving lives and minimizing damage.
• Geologists at Natural Resources Canada monitor seismic activity across the country, mapping fault lines and assessing earthquake hazards to inform land-use planning and building codes in cities like Vancouver.

Assessment Ideas

Quick Check

Present students with a diagram showing a fault line and arrows indicating stress. Ask them to label the fault and write one sentence explaining what happens when the stress is released suddenly.

Exit Ticket

Provide students with a simplified seismogram showing the arrival times of P-waves and S-waves from one station. Ask them to calculate the time difference between the P-wave and S-wave arrival and state what this difference tells them about the earthquake's distance.

Discussion Prompt

Pose the question: 'If you were a scientist trying to locate an earthquake, why would you need data from at least three different seismograph stations?' Facilitate a discussion where students explain the triangulation method.

Frequently Asked Questions

How do earthquakes generate seismic waves?

Sudden movement along faults releases energy as vibrations that radiate as seismic waves from the focus. P-waves compress material first, followed by shearing S-waves. Classroom models with ropes demonstrate how rock deformation propagates these waves, helping students visualize energy transfer through Earth's layers over hundreds of kilometers.

What are the differences between P-waves and S-waves?

P-waves are longitudinal, pushing particles back and forth in the wave direction, and travel through all materials at higher speeds. S-waves are transverse, shaking particles side-to-side or up-down, but only pass through solids. Simple demos with springs clarify motion types and why S-waves cause more destruction, aligning with seismograph patterns students analyze.

How can teachers locate an earthquake epicenter in class?

Use arrival time differences from three or more seismographs: subtract S-wave from P-wave times, convert to distances using wave speed charts, and draw arcs on a map. The intersection gives the epicenter. This math-integrated activity builds precision and reveals how global networks determine locations accurately within tens of kilometers.

How does active learning support earthquake and seismic wave lessons?

Active approaches like building slinkies for wave types or mapping epicenters from data engage multiple senses, making invisible processes tangible. Groups collaborate on seismograph builds, discussing observations to dispel myths and solidify concepts. These methods increase retention by 20-30 percent over lectures, as students own discoveries through trial and shared reasoning.