Earthquakes send waves of energy through the Earth in all directions. Seismologists study two main types: P-waves (longitudinal, travel through solids and liquids) and S-waves (transverse, travel only through solids). By tracking how these waves bend and where they disappear, scientists have mapped the Earth's internal structure without drilling there.
What are seismic waves?
When an earthquake occurs, energy is released at a point underground called the focus (or hypocentre) and travels outward in all directions as seismic waves — vibrations that pass through the solid rock and liquid layers of the Earth. The point on the Earth's surface directly above the focus is called the epicentre.
Seismic waves are detected by instruments called seismometers at monitoring stations around the world. Because different types of seismic wave travel at different speeds and respond differently to the materials they pass through, the data collected tells scientists what the inside of the Earth looks like — without anyone ever having dug there (the deepest borehole ever drilled, in Russia, reached only about 12 km into a crust that is up to 70 km thick).
What is the difference between P-waves and S-waves?
The two most important seismic wave types in KS3 physics are:
| Property | P-waves (Primary) | S-waves (Secondary) |
|---|---|---|
| Wave type | Longitudinal (compression wave) | Transverse (shear wave) |
| Direction of vibration | Parallel to direction of travel (push-pull) | Perpendicular to direction of travel |
| Materials they travel through | Solids and liquids | Solids only |
| Speed in rock | Faster (~6–8 km/s in crust) | Slower (~3.5 km/s in crust) |
| Which arrives first? | First (hence "primary") | Second (hence "secondary") |
Why can S-waves not travel through liquids? S-waves are transverse — they rely on the material shearing (layers sliding past each other). Liquids cannot sustain a shear force; they simply flow. Because S-waves cannot travel through liquids, they cannot pass through the Earth's outer core, which is liquid iron and nickel.
How have seismic waves revealed the structure of the Earth?
When seismic waves travel through the Earth, two things happen at boundaries between different materials:
- Refraction: the wave changes direction because it changes speed as it enters a different material
- Reflection: some of the wave bounces back at the boundary
By plotting the arrival times of waves at many seismometer stations around the world after a major earthquake, scientists noticed two important features:
-
S-waves disappear completely beyond about 103° from the epicentre (opposite the earthquake). S-waves cannot travel through liquid, so their absence proves that a large region inside the Earth is liquid — the outer core.
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P-waves are absent in a "shadow zone" between about 103° and 142° from the epicentre. They reappear beyond 142°, bent by refraction. This shadow zone results from P-waves refracting as they enter the denser liquid outer core, bending away from their straight path.
These shadow zones were the key evidence that:
- The outer core is liquid
- There is a solid inner core (P-waves that pass all the way through the centre and emerge beyond the P-wave shadow zone travel slightly faster than expected through a liquid, suggesting a solid inner region)
What are the layers of the Earth?
Scientists have identified four main layers:
| Layer | Depth | State | Composition |
|---|---|---|---|
| Crust | 0–5 km (oceanic) to 70 km (continental) | Solid | Silicate rocks (basalt under oceans; granite on continents) |
| Mantle | 35–2,900 km | Solid (but can flow slowly) | Dense silicate minerals; convection currents drive tectonic plates |
| Outer core | 2,900–5,100 km | Liquid | Iron and nickel; convecting liquid generates Earth's magnetic field |
| Inner core | 5,100–6,370 km | Solid | Iron and nickel; solid despite very high temperature because of extreme pressure |
The inner core is solid even at temperatures above 5,000 °C because the enormous pressure at the Earth's centre prevents the iron from becoming liquid — pressure raises the melting point.
How does a seismometer work?
A seismometer (or seismograph) detects ground motion by using the principle of inertia — a large mass tends to remain stationary when the ground shakes around it.
The basic design:
- A heavy mass is suspended from a frame anchored to the ground, either by a spring or a pendulum arrangement.
- When an earthquake makes the ground shake, the frame moves but the heavy mass resists moving (inertia keeps it approximately stationary).
- The relative motion between the frame and the mass is recorded — electronically in modern seismometers, or mechanically as a trace on a rotating drum in older designs.
Modern seismometers can detect ground movements of less than one nanometre (smaller than an atom's diameter). Multiple seismometers around the world allow the epicentre of an earthquake to be located by triangulation — comparing the arrival times of waves at three or more stations.
Frequently asked questions
Why do P-waves travel faster than S-waves?
P-waves are longitudinal compression waves — the rock is alternately compressed and rarefied in the direction of travel. This type of wave can be transmitted very efficiently through any material that resists compression (a bulk modulus), which includes both solids and liquids. S-waves are transverse shear waves that require the material to resist deformation perpendicular to the wave's direction. Shear resistance is generally weaker than bulk resistance in rock, so S-waves transfer energy more slowly. This is why P-waves always arrive first at a seismometer after an earthquake, regardless of distance.
How do scientists know the outer core is liquid if they cannot drill there?
The key evidence is the S-wave shadow zone. S-waves are transverse waves that can only travel through solid materials — they are blocked by any liquid region because liquids cannot sustain shear stress. After a major earthquake, seismometers on the far side of the Earth receive no S-waves in a large zone (beyond 103° from the epicentre). The only explanation is that a large liquid region inside the Earth is blocking their path. The size and position of this shadow zone matches what would be expected if the outer core (at 2,900 to 5,100 km depth) is liquid.
Why does the inner core remain solid even at such high temperatures?
The inner core is estimated to be at temperatures of around 5,000–6,000 °C — hotter than the surface of the Sun. At normal pressures, iron melts well below 1,500 °C. However, pressure raises the melting point of a material (more energy is needed to disrupt the structure when atoms are being squeezed closer together). At the centre of the Earth, the pressure is approximately 360 gigapascals — over 3.5 million times atmospheric pressure. This extreme pressure forces the iron and nickel atoms into a solid crystalline arrangement even at temperatures that would normally produce a liquid.
How is the epicentre of an earthquake located?
The difference in arrival times of P-waves and S-waves at a seismometer tells the seismologist how far the earthquake was from that station — the greater the time gap, the further away the epicentre. Using the arrival time difference at just one station gives only the distance (a circle on a map, not a point). To find the exact epicentre, data from at least three stations are needed: three circles drawn on a map, each centred on one station with the appropriate radius, all intersect at a single point — the epicentre. This method is called triangulation.
For predict-first KS3 physics with Professor Newton — using wave properties and observations to deduce what we cannot see directly — visit aitutors.me.