Seismic Waves — Explain seismic waves (P/S) and how they reveal internal structure (conceptual) - Earth Science

SECTION 1

Historical Context & Motivation

How do we know what's inside the Earth? Nobody has ever dug a hole deep enough to reach the core. The deepest borehole ever drilled, the Kola Superdeep Borehole in Russia, only reached about 12 kilometers — barely scratching the surface of a planet with a radius of roughly 6,371 km. Instead of digging, scientists discovered they could use earthquake vibrations to peek inside the Earth, much like a doctor uses ultrasound to see inside your body.

1889 • First Distant Earthquake Recorded

Ernst von Rebeur-Paschwitz detected seismic waves in Germany from an earthquake in Japan, proving that vibrations travel through Earth's interior across great distances.

1906 • Discovery of Earth's Core

Richard Dixon Oldham analyzed earthquake records and noticed that certain waves disappeared on the far side of the planet. He concluded that Earth must have a distinct core that blocks or bends those waves.

1909 • The Moho Boundary

Andrija Mohorovičić discovered a sharp boundary between Earth's crust and mantle, now called the Mohorovičić discontinuity (or Moho), by observing changes in wave speed.

1936 • Inner Core Identified

Danish seismologist Inge Lehmann studied waves that appeared in places they shouldn't and proposed that Earth has a solid inner core inside the liquid outer core.

These discoveries all came from studying seismic waves — vibrations that travel through and across the Earth after an earthquake. The big question that drove all of this research was simple: What is Earth made of on the inside, and how can we figure that out without going there?

SECTION 2

Core Principles & Definitions

When an earthquake occurs, energy is released from a point underground called the focus (or hypocenter). The spot on the surface directly above the focus is the epicenter. This energy radiates outward in all directions as seismic waves. There are two main categories: body waves that travel through Earth's interior, and surface waves that travel along the surface. In this lesson, we focus on body waves — specifically P-waves and S-waves — because they reveal Earth's internal structure.

P-Waves (Primary Waves)

Compressional waves that squeeze and stretch rock in the same direction the wave travels. They are the fastest seismic waves, so they arrive at seismograph stations first. P-waves can travel through solids, liquids, and gases.

S-Waves (Secondary Waves)

Shear waves that shake rock side-to-side (perpendicular to the direction of travel). They are slower than P-waves and arrive second. Crucially, S-waves cannot travel through liquids.

Shadow Zones

Regions on Earth's surface where seismic waves from a given earthquake are not detected. The S-wave shadow zone covers the entire far side of the planet, proving the outer core is liquid. The P-wave shadow zone is a ring where P-waves are bent away by the core.

Wave Speed Changes

Seismic waves speed up, slow down, or bend (refract) when they pass from one material to another. These speed changes tell scientists where one layer ends and another begins.

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KEY TAKEAWAY

Think of P-waves and S-waves like two friends running through a building. Friend P (P-wave) can run through every room — even the swimming pool. Friend S (S-wave) refuses to go in the water and stops at the pool edge. If you're standing on the other side and only Friend P arrives, you know there's a pool (liquid layer) in between. That's exactly how scientists figured out Earth's outer core is liquid: S-waves never make it through.

SECTION 3

How P-Waves and S-Waves Move Through Rock

The difference between P-waves and S-waves comes down to how they move the rock particles around them. The diagram below shows both wave types traveling from left to right. Notice how the particles (small dots) move in different directions relative to the wave's travel direction.

Top: A P-wave compresses and stretches rock particles along the direction of travel, like pushing and pulling a slinky. Bottom: An S-wave shakes particles up and down (or side to side), perpendicular to the direction of travel, like shaking a rope.

In the P-wave section, you can see regions where dots cluster tightly together — these are compressions. Between them, dots are spread apart in rarefactions. It works just like a slinky toy when you push one end. For S-waves, notice how the dots move up and down while the wave itself travels left to right. This is the same motion you see when you flick a rope. Because liquids can be compressed but cannot resist being sheared (pulled sideways), S-waves simply cannot pass through liquid layers.

SECTION 4

How Seismic Waves Reveal Earth's Interior

Seismic waves change behavior when they encounter different materials. Two key principles make this work. First, wave speed depends on the material. Waves travel faster through denser, more rigid rock and slower through softer or molten material. Second, when a wave hits a boundary between two different materials, it can refract (bend) or reflect (bounce back), just like light bending when it enters water.

SEISMIC WAVE SPEED

v = d / t

Where v = wave speed (km/s), d = distance traveled (km), and t = travel time (s). By measuring the time between an earthquake and when waves arrive at a station, scientists calculate how fast the waves traveled — and therefore what materials they passed through.

Here is the clever part. Scientists set up seismograph stations all around the world. After an earthquake, each station records which types of waves arrive and how long they take. Stations close to the earthquake detect both P-waves and S-waves. But stations on the opposite side of Earth from the earthquake often detect only P-waves — no S-waves at all. This creates a large S-wave shadow zone on the far side of the planet.

Why? Because S-waves cannot pass through liquid. The existence of this shadow zone is powerful evidence that Earth's outer core is liquid. Meanwhile, P-waves do pass through the liquid outer core, but they get bent (refracted) as they enter and exit. This bending creates a P-wave shadow zone — a ring-shaped area between about 104° and 140° from the epicenter where neither direct P-waves nor S-waves are detected.

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Why Does Bending Happen?

When a wave passes from a slower material into a faster one, it bends away from the boundary. When it passes from faster to slower, it bends toward the boundary. This is the same reason a straw looks "broken" when you put it in a glass of water. In Earth, waves curve gradually because density and rigidity increase with depth, causing continuous refraction.

SECTION 5

Shadow Zones and Earth's Layers

The diagram below shows a cross-section of Earth with an earthquake occurring at the top. Seismic wave paths curve through the interior because of changing material properties. The diagram highlights the P-wave shadow zone and the S-wave shadow zone, which together reveal the boundaries between crust, mantle, outer core, and inner core.

Cross-section of Earth showing seismic wave paths. P-waves (cyan) curve through the mantle and can pass through the core, but are refracted, creating a P-wave shadow zone between 104° and 140° from the epicenter. S-waves (violet) are blocked entirely by the liquid outer core, creating a massive shadow zone on the far side of the planet beyond 104°.

Earth Layer	Approximate Depth (km)	State	Seismic Clue
Crust	0 – 5 to 70	Solid	Waves speed up sharply at the Moho boundary
Mantle	70 – 2,900	Mostly solid (some partially molten zones)	Both P- and S-waves travel here; speed increases with depth
Outer Core	2,900 – 5,150	Liquid	S-waves vanish; P-waves slow dramatically and bend
Inner Core	5,150 – 6,371	Solid	Faint P-waves detected in the shadow zone (Lehmann's discovery)

Earth's major layers as revealed by seismic wave behavior

SECTION 6

Worked Example — Reading Seismic Data

Let's work through a scenario where you use seismic wave arrival times to figure out how far away an earthquake occurred and what the waves passed through.

Determining Earthquake Distance from Wave Arrival Times

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Step 1 — Read the Seismogram

A seismograph station records that the P-wave arrived at 2:00:10 PM and the S-wave arrived at 2:01:30 PM. Calculate the time difference between the two arrivals.

S − P time difference = 1 minute 20 seconds = 80 seconds
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Step 2 — Understand What the Time Gap Means

P-waves travel faster than S-waves. The farther the earthquake is from the station, the bigger the gap between the P-wave arrival and the S-wave arrival. Think of it like two runners: a fast one and a slow one starting at the same time. Over a short distance, they finish almost together. Over a long distance, the gap grows.
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Step 3 — Use a Travel-Time Chart

Scientists use pre-calculated travel-time curves (graphs that show how long P- and S-waves take to travel various distances). Looking up an S − P lag of 80 seconds on a standard travel-time chart, we find the earthquake was approximately 700 km from the station.

Estimated distance ≈ 700 km
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Step 4 — Determine What the Waves Passed Through

Since both P-waves and S-waves were detected at this station, we know the waves did not pass through any liquid layer. At 700 km distance, the waves traveled through the crust and upper mantle — both solid. If only P-waves had been detected (and no S-waves), we would know the wave path included a liquid region, likely Earth's outer core.

Both wave types detected → path through solid material only (crust + upper mantle)

SECTION 7

P-Waves vs S-Waves — A Detailed Comparison

Property	P-Waves	S-Waves
Full Name	Primary waves (also called compressional or longitudinal)	Secondary waves (also called shear or transverse)
Particle Motion	Back and forth, parallel to wave direction (like a slinky)	Side to side, perpendicular to wave direction (like a rope)
Speed	≈ 6–14 km/s (fastest seismic body wave)	≈ 3.5–7.5 km/s (about 60% of P-wave speed)
Travel Through Liquids?	Yes	No
Travel Through Solids?	Yes	Yes
Arrival Order	First to arrive at a seismograph	Second to arrive
Key Discovery	Bending through the core reveals the core-mantle boundary and inner core	Absence beyond 104° proves the outer core is liquid

Side-by-side comparison of P-waves and S-waves

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KEY TAKEAWAY

Imagine you clap your hands in a gymnasium. The sound (a compressional wave, like a P-wave) bounces off every surface — walls, ceiling, floor, even the pool if there is one. Now imagine instead you tried to send a "wave" through a line of students by having each person push the next one sideways. That chain would break the moment it reached someone standing in water, because liquid doesn't hold its shape against sideways pushes. That is why S-waves cannot pass through Earth's liquid outer core.

SECTION 8

Beyond P and S — Connecting to Modern Seismology

P-waves and S-waves are the foundation, but modern seismology uses many more tools to map Earth's interior in detail. As you advance in Earth science, you'll encounter additional concepts that build on what you've learned here.

What You Learned Here	Where It Leads (Advanced)
P- and S-waves travel at different speeds	Seismic tomography — using thousands of wave arrival times to create 3D maps of Earth's interior, similar to how a medical CT scan builds images of your body
S-waves are blocked by liquids	Scientists study partially molten zones (low-velocity zones) in the upper mantle where S-waves slow down but don't vanish — evidence of partial melting
Waves refract at layer boundaries	Snell's Law (from physics) is used to precisely calculate how much waves bend at each boundary, allowing detailed layer mapping
Shadow zones reveal the liquid outer core	Analysis of very faint waves in shadow zones (called PKiKP waves) helped confirm the solid inner core and even its rotation rate

Connecting this lesson's concepts to advanced seismology

Scientists also study surface waves (Love waves and Rayleigh waves), which travel along Earth's surface and cause the most damage during earthquakes. While surface waves are less useful for probing the deep interior, they help map the structure of the crust and upper mantle. The combination of body waves and surface waves gives geologists a remarkably detailed picture of what lies beneath our feet — all without drilling a single hole.

SECTION 9

Practice Problems

Question 1: CONCEPTUAL

A seismograph station detects P-waves from a distant earthquake but does not detect any S-waves. What does this tell you about the path the waves traveled? What layer must they have passed through?

Show answer

If P-waves arrive but S-waves do not, the wave path must have passed through a liquid layer. Since S-waves (shear waves) cannot travel through liquids, the absence of S-waves means the waves crossed Earth's liquid outer core. The station must be located more than 104° from the earthquake's epicenter, placing it in the S-wave shadow zone. This observation is one of the strongest pieces of evidence that Earth's outer core is made of liquid iron and nickel.

Question 2: BASIC CALCULATION

A P-wave travels through the upper mantle at an average speed of 8 km/s. If a seismograph station is 480 km from the earthquake's epicenter, how many seconds will it take the P-wave to arrive? (Use the formula v = d / t.)

Show answer

Rearrange the formula v = d / t to solve for time: t = d / v. Substituting, t = 480 km ÷ 8 km/s = 60 seconds. The P-wave would arrive approximately 60 seconds (1 minute) after the earthquake. In reality, the path might curve slightly, making the actual travel time a bit longer, but this gives a good estimate for a wave traveling through uniform material.

Question 3: INTERMEDIATE

At a seismograph station, the P-wave arrives 45 seconds before the S-wave. At a second station farther away, the P-wave arrives 120 seconds before the S-wave. Which station is closer to the earthquake, and why does the time gap increase with distance?

Show answer

The first station (45-second gap) is closer to the earthquake. The S − P time gap increases with distance because P-waves travel faster than S-waves. Over short distances, both waves arrive almost simultaneously. Over longer distances, the faster P-wave pulls further and further ahead of the slower S-wave, increasing the gap. This is like two cars driving at different speeds — a slow car and a fast car starting at the same point will be almost side-by-side after one block, but after 100 miles, the fast car will be far ahead. Scientists use this principle (the S − P lag) with data from at least three stations to triangulate the earthquake's exact epicenter.

Question 4: APPLIED

Imagine scientists discover a new planet and set up seismograph stations around its surface. After a quake, every station on the planet detects both P-waves and S-waves — there is no S-wave shadow zone at all. What could you conclude about the interior of this planet?

Show answer

If there is no S-wave shadow zone, it means S-waves can travel through the entire interior of the planet without being blocked. Since S-waves cannot pass through liquid, this tells us the planet likely has no large liquid layer in its interior. The planet could be solid all the way through — no liquid outer core like Earth has. Alternatively, any liquid layers would need to be very thin and not positioned in a way that creates a detectable shadow zone. This finding would be very different from Earth, where the liquid outer core creates a huge S-wave shadow zone covering the entire far side of the planet beyond 104° from the epicenter.

Question 5: CRITICAL THINKING

Inge Lehmann noticed that faint P-waves appeared in the P-wave shadow zone (between 104° and 140°) where no direct P-waves should have reached. How did she use this observation to propose the existence of a solid inner core? Explain the reasoning in your own words.

Show answer

Lehmann's reasoning was brilliantly logical. The P-wave shadow zone exists because the liquid outer core bends (refracts) P-waves away from certain angles. If the outer core were liquid all the way to the center, no P-waves should reach the shadow zone. But Lehmann detected faint P-waves there. She realized that if there were a solid inner core inside the liquid outer core, P-waves entering the core could refract again when they hit the solid inner core boundary. This second refraction could redirect some waves back into the shadow zone, like light bouncing off a mirror inside a room. The only way to explain those unexpected arrivals was to add another boundary — between a liquid outer core and a solid inner core. This brilliant deduction, confirmed over decades of additional data, showed that Earth's core has two distinct parts, not one.

SUMMARY

Lesson Summary

Summary

Earthquakes generate seismic waves that travel through Earth's interior and reveal its hidden structure. P-waves (primary waves) are compressional waves that travel fastest and can pass through solids, liquids, and gases. S-waves (secondary waves) are shear waves that move rock sideways and cannot pass through liquids. Both wave types change speed and direction when they cross boundaries between different materials, a process called refraction.

The fact that S-waves vanish beyond 104° from an earthquake proves that Earth's outer core is liquid, creating a massive S-wave shadow zone. P-waves bend as they enter and exit the core, creating a P-wave shadow zone between 104° and 140°. Inge Lehmann's detection of faint P-waves within this shadow zone led to the discovery of the solid inner core. Together, these observations reveal Earth's four main layers: crust, mantle, outer core, and inner core — all mapped using earthquake vibrations as natural probes.