Maarg Manthan · Topic 1.16

Understanding the Earth’s Interior Through Seismic Waves

Geography › Earth's Origin, Interior and Motions · Topic 1.16

Nobody can see inside the Earth, but earthquake waves pass through it and carry information about the layers they cross. P waves travel through solids and liquids, while S waves travel only through solids. Waves speed up with depth in solid rock, bend where the material changes, and are reflected at sharp boundaries. By timing the waves at stations all over the world, seismologists found the Moho, the liquid outer core, the solid inner core and the zones where waves are missing, called shadow zones.

5 min read · UPSC Prelims · Geography · Earth's Origin, Interior and Motions

At a Glance

P waves Pass through solids and liquids; fastest
S waves Pass through solids only
Key clue Change of wave speed with depth
Liquid outer core Proved by absence of S waves
Modern tool Seismic tomography
Exam link UPSC Prelims, Geography: Earth’s Origin, Interior and Motions

Where Does It Fit?

This topic uses the properties of the seismic waves described in the previous topic to show how the layers of the Earth were discovered. It prepares for the topics on the internal structure and the seismic discontinuities that give the names and depths of those layers. Previous topic: Seismic Waves: Body Waves and Surface Waves. Next topic: Internal Structure of the Earth: Crust, Mantle, Outer Core, Inner Core.

How Does Wave Behaviour Reveal the Interior?

In short: a wave’s speed and path depend on the material, so a change in either marks a change in the Earth.

Speed: waves go faster in denser, more rigid rock. Inside a uniform layer, speed rises gradually with depth as pressure increases.
Sharp jump: where the material changes abruptly, speed jumps. These boundaries are the discontinuities.
Bending (refraction): when a wave enters a layer with a different speed, it changes direction. In the Earth, the paths are curved because the speed generally increases with depth.
Reflection: some of the energy bounces back from a sharp boundary and returns to the surface, which is how the Moho was found.
Stopping: S waves cannot move through a liquid, because a liquid has no rigidity. Their disappearance shows a liquid layer.

What Do P and S Wave Speeds Show?

Layer	P wave speed (km per second, approximate)	S wave speed	What it shows
Crust	5.5 to 7	About 3.5	Solid, rocky; speed differs between oceanic and continental crust
Upper mantle	About 8 to 9	About 4.5	Jump at the Moho; slow ‘low velocity zone’ in the asthenosphere
Lower mantle	About 11 to 13.7	About 6 to 7	Dense, rigid, solid rock
Outer core	About 8 to 10	Zero	P speed drops sharply; S waves vanish, so the layer is liquid
Inner core	About 11	About 3.5	S waves appear again, so the layer is solid

These figures are approximate and are taken from standard reference models of the Earth. The pattern matters more than the exact numbers.

How Did Shadow Zones Prove the Core?

A seismograph registers the waves of an earthquake in most places, but in some areas no direct waves are recorded. These are shadow zones.

P wave shadow zone: between about 105° and 145° from the epicentre, no direct P waves arrive, because they are bent sharply when they enter the slower liquid outer core. Beyond 145° they are recorded again.
S wave shadow zone: S waves are not received at all beyond about 105°, which covers a little over half the Earth. This shows that a liquid layer in the interior stops them.

The core was first inferred by R. D. Oldham in 1906. Beno Gutenberg fixed the depth of the core-mantle boundary at about 2,900 km in 1914, and the Danish seismologist Inge Lehmann found the solid inner core in 1936 from faint P waves reflected from it. These boundaries are described in Seismic Discontinuities (Conrad, Mohorovicic, Repetti, Gutenberg, Lehmann). The method of locating an earthquake from the shadow zones and time differences is given in Shadow Zone and Locating an Epicentre.

What Does Modern Seismology Add?

The earlier model of simple concentric layers has been refined using thousands of seismographs and computers. Seismic tomography uses the travel times of waves from many earthquakes to build three-dimensional pictures of the mantle, much like a medical CT scan. It shows cold, dense slabs of old ocean floor sinking into the mantle at subduction zones, and hot columns of rock rising from near the core, called mantle plumes. These findings link the study of the interior to plate tectonics, as in Plate Tectonics: Major and Minor Lithospheric Plates. As an analytical point, seismic waves give velocities, not materials, so the conclusions are checked against laboratory experiments, meteorites and the density of the Earth, as in Interior of the Earth: Sources of Information (Direct and Indirect Evidence).

Why Is This Topic Important?

Method Best source on the interior
Names Oldham, Gutenberg and Lehmann
Link Plates and earthquakes

What Came Next?

The next topic sets out the structure of the Earth in detail, layer by layer. Next topic: Internal Structure of the Earth: Crust, Mantle, Outer Core, Inner Core.

Key People and Terms

Refraction Bending of a wave at a boundary
Shadow zone Region where waves are not received
Seismograph Instrument that records waves
Tomography 3D imaging of the interior
Low velocity zone Slow zone in the upper mantle
Mantle plume Hot column of rising rock

Exam Corner

Points to Remember

S waves cannot pass through a liquid.
P shadow zone: about 105° to 145°.
S shadow zone: beyond about 105°.
Inge Lehmann found the inner core in 1936.
Core-mantle boundary: about 2,900 km.

Do Not Confuse With

P and S shadow zones A band versus the whole far side
Refraction and reflection Bending versus bouncing back
Oldham, Gutenberg and Lehmann Found the core, fixed its depth, found the inner core

Memory Hook

S stops in liquid, P bends and hides; Oldham, Gutenberg, Lehmann, the core abides.

Mains Angle

Questions ask how the interior can be known without direct access. These points add depth.

Method Wave speed and path
Evidence Shadow zones
Advance Tomography and 3D models

A Question You May Face

An original practice question, not a past paper question.

“How have seismic waves helped to establish the layered structure of the Earth, especially the nature of the core?”

How to Answer

Waves P and S properties.
Layers Speed changes and reflections.
Core Shadow zones and S waves.
Modern Tomography.

GS Relevance

Prelims: how seismic waves reveal the interior, shadow zones, discoverers of boundaries. GS Paper 1: interior of the Earth and earthquakes.

Frequently Asked Questions

How do seismic waves help us study the interior?

Seismic waves change speed and direction as they pass through materials of different density and rigidity. By recording their arrival times at many stations, scientists can find where the material changes, and thus map the crust, mantle and core, because no one can drill or travel deep into the Earth.

Why do S waves not pass through the outer core?

S waves are shear waves that need a rigid material to move through. A liquid cannot carry a shear wave, so S waves stop when they meet the liquid outer core. Their absence beyond about 105 degrees from the epicentre shows that the outer core is liquid.

What is the shadow zone of P waves?

The P wave shadow zone is the area between about 105 and 145 degrees from the epicentre where no direct P waves arrive. P waves entering the slower liquid outer core are refracted sharply, which leaves this ring-shaped area without direct waves.

Who discovered the inner core of the Earth?

The Danish seismologist Inge Lehmann discovered the solid inner core in 1936. She found that faint P waves recorded in the shadow zone must have been reflected from a boundary inside the liquid core, which showed that there is a solid centre.

What is seismic tomography?

Seismic tomography is a technique that uses the travel times of earthquake waves recorded at many stations to build three-dimensional images of the Earth's interior. It reveals features such as sinking cold slabs of ocean floor and rising hot plumes of mantle rock.

PYQ Practice — Statement Analysis

1 S waves can travel through the liquid outer core.

False

S waves cannot pass through liquids.

2 The solid inner core was discovered by Inge Lehmann.

True

She found it in 1936.

3 P waves are recorded everywhere on the Earth after an earthquake.