Types of Seismic Waves

Why This Matters

Seismic waves are your window into Earth's interior—and they're fundamental to understanding how we know what lies beneath our feet. You're being tested on more than just wave names and speeds; exams focus on wave propagation mechanics, material interactions, and how seismologists use wave behavior to infer Earth's structure. The fact that certain waves can't pass through liquids, for instance, is exactly how we discovered Earth's liquid outer core.

When you study seismic waves, think about the physics: compression vs. shear motion, solid vs. fluid transmission, and body vs. surface propagation. These distinctions show up constantly in problems about earthquake detection, damage patterns, and Earth's layered structure. Don't just memorize that P-waves are fastest—know why they arrive first and what their behavior tells us about the material they've traveled through.

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Body Waves: Traveling Through Earth's Interior

Body waves propagate through Earth's internal layers, and their behavior changes based on the material properties they encounter. The key distinction is between compressional motion (particles moving parallel to wave direction) and shear motion (particles moving perpendicular to wave direction).

P-Waves (Primary Waves)

• Fastest seismic waves at 5–8 km/s in the crust—this speed increases with depth as rock density and rigidity change
• Compressional (longitudinal) waves that push and pull material parallel to the direction of travel, like sound waves through air
• Travel through solids, liquids, and gases—this universal transmission is why P-waves are always first to arrive at seismograph stations

S-Waves (Secondary Waves)

• Slower than P-waves at 3–4.5 km/s in the crust—the speed difference creates the predictable arrival-time gap used to locate earthquakes
• Shear (transverse) waves that move material perpendicular to wave direction, requiring rigid bonds between particles
• Cannot travel through liquids or gases—this limitation creates the famous S-wave shadow zone that proved Earth's outer core is liquid

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Surface Waves: Maximum Destruction at the Boundary

Surface waves travel along Earth's outer boundary and arrive after body waves, but they carry the most destructive energy. Their amplitude is largest near the surface and decreases with depth, which is why shallow earthquakes cause more surface damage.

Love Waves

• Horizontal shear motion—ground moves side-to-side perpendicular to the wave's travel direction
• Faster than Rayleigh waves but slower than body waves, typically arriving as the first surface wave on seismograms
• Particularly destructive to building foundations—the horizontal shaking stresses structures not designed for lateral loads

Rayleigh Waves

• Elliptical particle motion combining vertical and horizontal movement, creating the rolling sensation like ocean swells
• Slowest of all seismic waves—but their long wavelengths allow them to travel great distances with less energy loss
• Cause ground roll and structural damage—the complex motion affects both foundations and upper floors differently

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Using Wave Behavior to Probe Earth's Structure

The differences in how seismic waves propagate aren't just physics trivia—they're the primary tool for understanding Earth's interior composition. Wave velocity, refraction, and shadow zones reveal density changes and phase boundaries we can't directly observe.

Body Wave Analysis

• Arrival time differences locate earthquake epicenters—the gap between P-wave and S-wave arrival (the S-P interval) correlates directly with distance from the source
• Velocity changes indicate layer boundaries—waves speed up or slow down when crossing from crust to mantle to core, revealing compositional changes
• Shadow zones map internal structure—the P-wave shadow zone (103°–142° from epicenter) and complete S-wave shadow beyond 103° define the core-mantle boundary

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Quick Reference Table

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Self-Check Questions

1. Which two wave types share shear motion as their propagation mechanism, and why can one travel through Earth's interior while the other cannot?

2. A seismograph station detects P-waves but no S-waves from a distant earthquake. What does this tell you about the wave path through Earth's interior?

3. Compare and contrast Love waves and Rayleigh waves in terms of particle motion, relative speed, and the type of structural damage each causes.

4. If you wanted to determine the distance to an earthquake's epicenter using a single seismograph, which wave property would you measure and why?

5. An FRQ asks you to explain how seismic waves provided evidence for Earth's layered internal structure. Which specific wave behaviors would you cite, and what do they reveal about each layer?