Mechanical Waves
4. Seismic Waves
Learning outcomes
- I can distinguish between P-waves and S-waves.
- I can explain how earthquakes produce seismic waves.
- I can identify which seismic waves travel through solids and liquids.
- I can describe how seismic waves help scientists study Earth's interior.
- I can compare different seismic wave types.
Seismic Waves
Seismic waves are waves of energy that travel through Earth or along its surface. They are produced when energy stored in rocks is suddenly released, most commonly during an earthquake.
Earthquakes can generate several types of seismic waves. The two most important waves that travel through Earth's interior are:
- P-waves — Primary waves
- S-waves — Secondary waves
These waves behave differently because they involve different types of particle motion.
Most importantly:
P-waves can travel through solids and liquids.
S-waves can travel through solids but not through liquids.
This difference has provided scientists with some of the strongest evidence about the structure of Earth's interior.
How Earthquakes Begin
Earth's crust is divided into large sections called tectonic plates.
These plates move slowly because of processes occurring within Earth.
Where plates interact, rocks can become:
- compressed
- stretched
- bent
- sheared
Friction can prevent rocks on opposite sides of a fault from moving smoothly.
As plate movement continues, stress builds in the rocks.
Eventually, the stress may become greater than the strength of the rocks or the friction holding them in place.
The rocks suddenly move.
This produces an earthquake.
Elastic Energy in Rocks
Rocks can deform slightly when forces act on them.
As stress builds, elastic potential energy can be stored in the rocks.
When the rocks suddenly fracture or slip along a fault, some of this stored energy is released.
The energy travels away from the earthquake source as:
seismic waves
This process is often described using the idea of elastic rebound.
The Focus
The point inside Earth where an earthquake begins is called the focus or hypocentre.
This is where the sudden movement and energy release originate.
Seismic waves spread outward from the focus in many directions.
The focus may be:
- shallow
- intermediate in depth
- deep
depending on the tectonic setting.
The Epicentre
The epicentre is the point on Earth's surface directly above the focus.
Therefore:
focus → inside Earth
epicentre → Earth's surface
These terms should not be confused.
Types of Seismic Waves
Seismic waves can be divided into two broad groups:
Body waves
These travel through Earth's interior.
They include:
- P-waves
- S-waves
Surface waves
These travel mainly along Earth's surface.
Important surface waves include:
- Love waves
- Rayleigh waves
P-waves and S-waves are particularly important for studying Earth's internal structure.
P-Waves
P-wave stands for Primary wave.
They are called primary waves because they are the fastest major seismic waves and therefore normally arrive first at a seismic station after an earthquake.
P-waves are longitudinal waves.
Particle motion occurs approximately parallel to the direction in which the wave travels.
If the P-wave travels:
→
particles vibrate:
↔
This produces alternating regions of compression and expansion.
Particle Motion in a P-Wave
Imagine pushing and pulling one end of a spring.
A region of compressed coils travels along the spring.
The coils themselves move backward and forward.
This provides a useful model of a P-wave.
In Earth:
- particles are pushed together
- particles move apart
- the compression disturbance travels forward
P-waves are therefore sometimes called compressional waves.
P-Waves Through Matter
P-waves can travel through:
- solids
- liquids
- gases
They can do this because compression and expansion can be transmitted through all of these states of matter.
For studying Earth, the important fact is:
P-waves can travel through both the solid mantle and the liquid outer core.
Their speed and direction change as they enter materials with different physical properties.
S-Waves
S-wave stands for Secondary wave.
S-waves travel more slowly than P-waves, so they normally arrive later at a seismic station.
S-waves are transverse waves.
Particle motion is perpendicular to the direction of wave travel.
If the S-wave travels:
→
particles might move:
↑ ↓
S-waves are also called shear waves.
Particle Motion in an S-Wave
Imagine shaking one end of a rope vertically.
The disturbance travels horizontally while pieces of rope move vertically.
This resembles the basic motion of an S-wave.
The material is displaced sideways relative to the direction in which the wave travels.
Therefore:
particle motion ⟂ wave direction
S-Waves Through Matter
S-waves can travel through:
solids
but they cannot travel through:
liquids
A liquid cannot support the type of shear deformation required to transmit an S-wave through its bulk.
This fact is extremely important.
Scientists discovered that S-waves produced by earthquakes do not travel through Earth's outer core.
This provided evidence that:
Earth's outer core is liquid.
Comparing P-Waves and S-Waves
| Property | P-Waves | S-Waves |
|---|---|---|
| Full name | Primary waves | Secondary waves |
| Wave type | Longitudinal | Transverse |
| Particle motion | Parallel to wave direction | Perpendicular to wave direction |
| Relative speed | Faster | Slower |
| Arrival | First | After P-waves |
| Travel through solids | Yes | Yes |
| Travel through liquids | Yes | No |
| Other name | Compressional wave | Shear wave |
| Importance | Reveals changes in Earth's materials | Strong evidence for liquid outer core |
A useful memory aid is:
P = Primary, Pressure, Passes through liquids
S = Secondary, Shear, Stops at liquids
Why P-Waves Are Faster
P-waves and S-waves deform materials differently.
P-waves involve compression and expansion.
S-waves involve shear deformation.
Under the same conditions, the properties of Earth materials allow compressional disturbances to travel faster than shear disturbances.
Therefore:
P-waves arrive before S-waves.
This difference in arrival time is useful for locating earthquakes.
Surface Waves
Not all seismic waves travel through Earth's interior.
Surface waves travel primarily along or near Earth's surface.
They are generally slower than P- and S-waves but can produce large ground motions.
Two important types are:
- Love waves
- Rayleigh waves
Surface waves can be particularly destructive during earthquakes.
Love Waves
Love waves move the ground from side to side.
Their particle motion is primarily horizontal and perpendicular to the direction of wave travel.
They can produce strong horizontal shaking.
This can place large stresses on:
- buildings
- bridges
- roads
- foundations
Rayleigh Waves
Rayleigh waves produce a rolling motion.
Particles move in approximately elliptical paths.
The motion can resemble waves rolling across the surface of water.
Rayleigh waves can produce both:
- vertical motion
- horizontal motion
This rolling ground motion can be particularly noticeable during large earthquakes.
Comparing the Major Seismic Waves
| Wave | Travels Through | Particle Motion | Relative Arrival |
|---|---|---|---|
| P-wave | Solids and liquids | Parallel | First |
| S-wave | Solids only | Perpendicular | Second |
| Love wave | Along surface | Horizontal sideways | Later |
| Rayleigh wave | Along surface | Rolling/elliptical | Later |
Different seismic waves therefore provide different information and produce different types of ground motion.
Detecting Seismic Waves
Seismic waves can be detected using instruments called seismometers.
A recording produced by seismic instruments is called a seismogram.
A seismogram can show the arrival of different types of seismic waves.
Because P-waves travel faster:
P-wave arrival occurs first
followed by:
S-wave arrival
and then often:
larger surface-wave motion
Reading a Seismogram
A simplified seismogram might show:
small early signal → P-wave
later signal → S-wave
larger later motion → surface waves
Scientists examine:
- arrival times
- wave amplitudes
- wave frequencies
- differences between P- and S-wave arrival times
These measurements provide information about the earthquake and Earth's interior.
The P–S Arrival-Time Difference
P-waves travel faster than S-waves.
Therefore, the farther a seismic station is from an earthquake, the greater the time difference between the arrival of the P-wave and the S-wave.
For example:
P-wave arrival: 10:15:20
S-wave arrival: 10:16:05
Difference:
45 seconds
This time difference can be compared with known seismic-wave travel information to estimate the station's distance from the earthquake.
Locating an Earthquake
One seismic station can estimate its distance from an earthquake, but it cannot usually determine the exact direction.
Data from several stations can be combined.
A simplified method uses triangulation:
- determine the earthquake distance from Station A
- draw a circle around Station A
- repeat for Stations B and C
- identify the region where the distances intersect
This helps determine the earthquake's epicentre.
Seismic Waves and Earth's Interior
We cannot directly travel to most of Earth's interior.
Earth's radius is approximately:
6370 km
Even the deepest drilling reaches only a tiny fraction of this distance.
Scientists therefore rely on indirect evidence.
Seismic waves provide one of the most important ways of investigating the internal structure of Earth.
They act somewhat like a natural probe travelling through the planet.
Earth's Main Internal Layers
Earth can be divided into major layers:
- crust
- mantle
- outer core
- inner core
These layers have different:
- compositions
- densities
- temperatures
- pressures
- physical states
Seismic waves behave differently as they encounter these materials.
Seismic Wave Speed Changes
The speed of a seismic wave depends on the properties of the material through which it travels.
Important factors include:
- density
- rigidity
- compressibility
- physical state
As seismic waves move through different parts of Earth, their speeds change.
Scientists can analyze these changes to identify boundaries between internal layers.
Refraction Inside Earth
When a seismic wave enters a region where its speed changes, its direction can also change.
This is called refraction.
The same general wave phenomenon occurs with:
- light
- water waves
- sound
- seismic waves
Because Earth's properties change with depth, seismic-wave paths through Earth are generally curved rather than perfectly straight.
Reflection Inside Earth
Some seismic-wave energy can also be reflected from boundaries inside Earth.
By studying reflected and refracted waves, scientists can infer:
- where boundaries occur
- how deep they are
- how material properties change
This is similar in principle to using echoes to investigate something that cannot be seen directly.
Evidence for the Liquid Outer Core
One of the most important discoveries from seismic waves concerns Earth's outer core.
Scientists observed that:
S-waves do not pass through the outer core.
Because S-waves cannot travel through liquids, this provides strong evidence that:
the outer core is liquid.
This is a powerful example of using wave behaviour to infer something that cannot be observed directly.
P-Waves and the Outer Core
P-waves can travel through liquids, so they do enter the outer core.
However, their speed changes dramatically at the mantle–outer-core boundary.
Their paths are strongly refracted.
This produces regions of Earth's surface where direct P-waves from a particular earthquake are not detected.
These regions contribute to the P-wave shadow zone.
The S-Wave Shadow Zone
Because S-waves cannot travel through the liquid outer core, there are large regions on the opposite side of Earth from an earthquake where direct S-waves are absent.
This is called the:
S-wave shadow zone
The existence of this shadow zone was crucial evidence for Earth's internal structure.
What Is a Shadow Zone?
A seismic shadow zone is a region of Earth's surface where particular direct seismic waves from an earthquake are not detected.
Shadow zones occur because seismic waves:
- change speed
- refract
- reflect
- may be unable to travel through certain materials
The pattern of these zones gives scientists information about Earth's internal layers.
Evidence for the Inner Core
P-wave behaviour provides evidence that Earth contains more than simply a mantle surrounding one uniform core.
Detailed seismic observations indicate that the centre of Earth contains a solid inner core surrounded by a liquid outer core.
The inner core remains solid despite its extremely high temperature because the pressure at Earth's centre is enormous.
Using Seismic Waves Like Medical Imaging
A useful analogy is medical imaging.
Doctors can investigate structures inside the body without opening it.
Similarly, geophysicists analyze waves travelling through Earth to infer structures they cannot directly see.
Modern seismic tomography uses seismic data from many earthquakes and stations to construct three-dimensional models of Earth's interior.
This can reveal regions where seismic waves travel slightly faster or slower.
What Seismic Tomography Can Show
Seismic tomography can provide evidence about:
- variations in mantle temperature
- subducting tectonic plates
- mantle structures
- boundaries between major layers
- regions with different physical properties
It is similar in concept to creating a three-dimensional image using many different wave paths.
Earthquake Magnitude and Wave Amplitude
Larger earthquakes generally release more energy and produce stronger seismic waves.
Seismometers record wave amplitude, which contributes to measurements used to characterize earthquakes.
However, the size of recorded waves at a station also depends on:
- distance from the earthquake
- local geology
- wave type
- depth of the earthquake
- direction of wave propagation
Therefore, a single observed amplitude should not be interpreted without context.
Seismic Waves and Earthquake Hazards
Different seismic waves can contribute to different types of ground motion.
Potential earthquake hazards include:
- building collapse
- bridge damage
- landslides
- liquefaction
- fires
- infrastructure failure
- tsunamis
The severity of shaking depends on much more than earthquake magnitude alone.
Local Geology and Shaking
The material beneath a location can influence how strongly the ground shakes.
Soft sediments can sometimes amplify seismic motion compared with solid bedrock.
Therefore, two places at similar distances from an earthquake may experience different levels of shaking.
Engineers use seismic information when designing structures in earthquake-prone regions.
Seismic Waves and Building Design
Engineers consider earthquake motion when designing:
- skyscrapers
- bridges
- hospitals
- dams
- transportation systems
Earthquake-resistant designs may use:
- flexible structures
- reinforced frames
- dampers
- base isolation systems
Understanding the frequencies and motions produced by seismic waves helps engineers reduce earthquake risk.
A Complete Comparison Example
Suppose an earthquake occurs and a seismic station records two body waves.
Wave A:
- arrives first
- particles oscillate parallel to wave direction
- can pass through the outer core
Wave B:
- arrives later
- particles oscillate perpendicular to wave direction
- disappears when it reaches the liquid outer core
Therefore:
Wave A = P-wave
Wave B = S-wave
The observations also provide evidence that:
Earth's outer core is liquid.
Interpreting a Seismic-Wave Diagram
When looking at a diagram, ask:
- Which direction is the wave travelling?
- Which direction are particles moving?
- Is particle motion parallel or perpendicular?
- Is the wave passing through solid material?
- Is it passing through liquid?
- Does its path bend at a boundary?
- Is there a region where the wave is absent?
- What does this reveal about Earth's interior?
These questions help turn a diagram into physical evidence.
Identifying P- and S-Waves from Motion
If particles move:
parallel to wave travel
identify the wave as:
P-wave
If particles move:
perpendicular to wave travel
identify the wave as:
S-wave
Remember:
P-wave → longitudinal
S-wave → transverse
Identifying P- and S-Waves from Materials
If a seismic wave travels through:
solid mantle → liquid outer core
it must be capable of travelling through liquid.
This points to a:
P-wave
If a wave travels through the solid mantle but does not pass through the liquid outer core:
S-wave
This reasoning is often used when interpreting diagrams of Earth's interior.
A Reliable Strategy for Comparing Seismic Waves
When comparing seismic waves, consider:
Wave type
Longitudinal, transverse, or surface.
Particle motion
Parallel, perpendicular, horizontal, or rolling.
Speed
Which arrives first?
Materials
Can it travel through solids? Liquids?
Path
Does it refract or reflect?
Evidence
What does its behaviour reveal about Earth's interior?
Effects
How does it contribute to ground motion?
This produces a more complete comparison than simply memorizing wave names.
Common Mistakes
Thinking P means "powerful"
P means Primary, referring to its earlier arrival.
Thinking S means "surface"
S means Secondary. S-waves are body waves, not surface waves.
Thinking S-waves travel through liquids
They do not propagate through Earth's liquid outer core.
Thinking P-waves only travel through solids
P-waves can travel through both solids and liquids.
Confusing P-wave motion with S-wave motion
P-wave particle motion is parallel.
S-wave particle motion is perpendicular.
Confusing the focus and epicentre
The focus is inside Earth.
The epicentre is directly above it at the surface.
Thinking seismic waves carry rocks across Earth
Particles vibrate locally while energy propagates through the material.
Thinking all seismic waves travel at the same speed
Different seismic-wave types travel at different speeds.
Thinking wave paths through Earth are always straight
Changing material properties cause refraction and curved paths.
Thinking the outer core was discovered by direct observation
Its liquid state is inferred from multiple lines of evidence, including seismic-wave behaviour.
Thinking surface waves are the fastest
P-waves are the fastest major seismic waves.
Key Terms
Seismic wave — wave of energy travelling through Earth or along its surface.
Earthquake — sudden release of stored energy caused by movement or fracture of rock.
Fault — fracture or zone of fractures along which rock movement can occur.
Elastic rebound — release of stored elastic energy when stressed rocks suddenly move.
Focus (hypocentre) — point inside Earth where an earthquake begins.
Epicentre — point on Earth's surface directly above the focus.
Body wave — seismic wave that travels through Earth's interior.
Surface wave — seismic wave travelling mainly along Earth's surface.
P-wave — fast longitudinal seismic body wave capable of travelling through solids and liquids.
S-wave — transverse seismic body wave that travels through solids but not liquids.
Compression — region where particles are pushed closer together.
Shear — deformation in which material is displaced sideways.
Love wave — surface wave producing mainly horizontal side-to-side motion.
Rayleigh wave — surface wave producing rolling or elliptical particle motion.
Seismometer — instrument used to detect and measure seismic motion.
Seismogram — recorded trace of seismic-wave motion.
Refraction — change in wave direction caused by a change in speed.
Reflection — return of wave energy from a boundary.
Shadow zone — region where particular direct seismic waves from an earthquake are not detected.
Outer core — liquid layer of Earth surrounding the inner core.
Inner core — solid central region of Earth.
Seismic tomography — technique using many seismic-wave paths to investigate Earth's internal structure.
Key Takeaways
- Earthquakes release stored energy that travels through Earth as seismic waves.
- The earthquake begins at the focus or hypocentre.
- The epicentre is the point on Earth's surface directly above the focus.
- P-waves and S-waves are body waves that travel through Earth's interior.
- P-waves are longitudinal waves.
- P-wave particle motion is parallel to the direction of wave travel.
- P-waves are faster than S-waves and normally arrive first.
- P-waves can travel through both solids and liquids.
- S-waves are transverse waves.
- S-wave particle motion is perpendicular to wave travel.
- S-waves travel through solids but not through liquids.
- The absence of S-waves through Earth's outer core provides strong evidence that the outer core is liquid.
- P-waves refract strongly when they enter the outer core.
- Refraction and the inability of S-waves to cross the outer core produce seismic shadow zones.
- Seismic-wave speeds change when material properties change.
- These changes reveal boundaries between Earth's internal layers.
- Seismic evidence supports a layered Earth containing a crust, mantle, liquid outer core, and solid inner core.
- P- and S-wave arrival-time differences can help determine distance from an earthquake.
- Measurements from multiple seismic stations can help locate an earthquake.
- Love and Rayleigh waves travel mainly along Earth's surface.
- Surface waves can produce strong and damaging ground motion.
- Seismometers detect seismic waves and produce seismograms.
- Seismic tomography uses large numbers of seismic observations to create models of Earth's interior.
- Seismic waves allow scientists to investigate regions of Earth that cannot be directly observed.
- Comparing how different waves travel through different materials is one of the most powerful ways to understand Earth's internal structure.
Check Your Understanding
- Explain how an earthquake produces seismic waves.
- Distinguish between the focus and the epicentre of an earthquake.
- Describe the particle motion in a P-wave and identify whether it is longitudinal or transverse.
- Describe the particle motion in an S-wave and identify whether it is longitudinal or transverse.
- Which seismic body wave normally reaches a seismic station first? Explain why.
- Complete the comparison: Which of P-waves and S-waves can travel through solids? Which can travel through liquids?
- Explain why the absence of S-waves passing through Earth's outer core provides evidence that the outer core is liquid.
- A seismic wave passes through the mantle and continues into the liquid outer core. Is it more likely to be a P-wave or an S-wave? Explain.
- Compare P-waves and S-waves in terms of speed, particle motion, wave type, and the materials through which they can travel.
- Explain why seismic-wave paths through Earth may bend rather than travel in perfectly straight lines.
- A seismogram records a small signal first, another signal later, and large surface motion after that. Identify the likely P-wave, S-wave, and surface-wave arrivals.
- Explain how the difference between P-wave and S-wave arrival times can help scientists determine the distance to an earthquake.
- What is a seismic shadow zone, and what can shadow zones reveal about Earth's interior?
- Compare P-waves, S-waves, Love waves, and Rayleigh waves. Include where they travel and the type of particle motion associated with each.
- Scientists cannot directly travel through Earth's mantle and core. Explain how observations of P-waves, S-waves, changes in wave speed, refraction, and shadow zones provide evidence for a layered Earth containing a solid mantle, liquid outer core, and solid inner core.