NDA Geography · Earth's Structure, Landforms and Geological Time

Earthquakes and Seismic Waves

Sudden slip along a fault releases energy as seismic waves; P-waves race ahead through everything, S-waves lag and refuse to cross liquid, and the gaps they leave (shadow zones) reveal the liquid outer core.

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Why this matters

9 PYQs, many of them HARD, but they cluster on a small set of ideas: focus vs epicentre, the order and nature of P/S/L waves, and the shadow zones. The single most powerful fact — S-waves cannot pass through liquid — is how we know the outer core is liquid, and it is tested again and again.

Concept 1 of 3: Focus, hypocentre and epicentre

An earthquake starts at a point underground where rock finally slips — that point is the FOCUS (also called the hypocentre). The spot on the surface directly above it is the EPICENTRE, where shaking is usually strongest. Shallow quakes do more surface damage than deep ones because the energy has less distance to spread before reaching us.

Definition

Focus (hypocentre) — the point INSIDE the Earth where the rupture begins and energy is released. It can be many kilometres deep.
Epicentre — the point on the SURFACE directly above the focus.
Both P-waves and S-waves radiate outward from the focus.
Shallow-focus quakes are more damaging at the surface than deep-focus ones (energy reaches the surface less weakened).

Worked example

An earthquake ruptures 30 km below the ground. What do we call that rupture point, and what do we call the point on the surface right above it?

Practice this conceptself-check · 3 quick reps

The same idea in a real exam question:

NDA · Apr 2026 · Q114Easy

Example 1 · Earth's Structure, Landforms and Geological Time · Earthquakes and Seismic Waves

Consider the following statements regarding hypocentre: I. It is the point where the energy is released and is called the focus of an earthquake. II. It is the point where the earthquake starts and may be many kilometres deep within the Earth. Which are correct?

Hypocentre is NOT on the surface

A trap defines the hypocentre as 'the point on the surface nearest the focus'. Wrong — the hypocentre IS the focus, underground. The SURFACE point is the epicentre.

Drill 2 more on focus, hypocentre and epicentre

Concept 2 of 3: P, S and L waves

Three wave families leave the focus. P-waves (primary) are push-pull waves — fastest, first to arrive, and they travel through anything. S-waves (secondary) shake side-to-side — slower, and they cannot pass through liquids. L-waves (surface) crawl along the surface — slowest, but the most destructive. Body waves (P and S) travel through the interior; when they reach the surface they generate the surface (L) waves. All seismic waves speed up in denser material.

Definition

P-wave (Primary) — longitudinal (particles vibrate along the direction of travel). FASTEST, recorded FIRST. Travels through solid, liquid AND gas.
S-wave (Secondary) — transverse (particles vibrate at right angles to travel). Slower; CANNOT travel through liquid.
L-wave / surface wave — travels along the surface, slowest, MOST destructive; follows the Earth's circumference; moves at a roughly constant rate.
Body waves = P + S (travel through the interior); surface waves = L (generated when body waves reach surface rocks).
Wave speed is HIGHER in denser materials.

Wave	Motion	Speed / arrival	Travels through
P (Primary)	Longitudinal (push-pull)	Fastest · arrives first	Solid, liquid, gas NDA 2026 — P arrives before S; P is longitudinal, S is transverse.
S (Secondary)	Transverse (side-to-side)	Slower · arrives second	Solid only — NOT liquid
L (surface)	Along the surface	Slowest · most destructive	Surface rocks only NDA 2025 — L-waves follow Earth's circumference (but NOT at exactly constant speed).

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

NDA · Apr 2026 · Q113Easy

Example 2 · Earth's Structure, Landforms and Geological Time · Earthquakes and Seismic Waves

Consider the following statements: I. In a seismograph, P waves are recorded earlier than S waves. II. In P waves, particles vibrate to and fro in the direction of wave propagation; in S waves, they vibrate at right angles to propagation. Which are correct?

Fastest ≠ most destructive

P-waves are the fastest but the LEAST destructive. The slow L (surface) waves do the most damage. Don't conflate speed with destructiveness.

L-waves: circumference yes, constant speed no

L-waves do follow the Earth's circumference, but they do NOT travel at a strictly constant rate. A 'both correct' option on those two claims is the trap — only the circumference claim holds.

Drill 4 more on p, s and l waves

Concept 3 of 3: Shadow zones and the liquid outer core

After a quake, there are belts of the Earth's surface where certain waves never arrive — the shadow zones. They exist because the liquid outer core BLOCKS S-waves entirely and BENDS (refracts) P-waves. The pattern of who is missing where is direct proof that the outer core is liquid: S-waves vanish beyond a certain angle and never come back.

Definition

The liquid outer core stops S-waves completely (a liquid cannot carry a transverse wave) and refracts P-waves, deflecting them.
P-wave shadow zone: roughly 105°–145° from the epicentre. Beyond 145°, refracted P-waves reappear.
S-wave shadow zone: everything BEYOND ~105° (S-waves are absent over a much larger area than P-waves).
So the S-wave shadow zone is larger than the P-wave shadow zone — and the very existence of the S-shadow proves the outer core is liquid.

Worked example

Why does the existence of an S-wave shadow zone tell us the outer core is liquid?

Practice this conceptself-check · 3 quick reps

The same idea in a real exam question:

NDA · Sep 2025 · Q115Hard

Example 3 · Earth's Structure, Landforms and Geological Time · Earthquakes and Seismic Waves

Which of the following statements with reference to shadow zones in an event of an earthquake is/are correct ? 1. Zone between 105 and 145 degrees from epicentre was identified as the shadow zone for both P-waves and S-waves 2. The shadow zone of P-waves is much larger than that of the S-waves Select the answer using the code given below :

The S-shadow is the BIGGER one

It feels natural to say the P-wave shadow is larger, but it is the reverse: S-waves are absent over a much wider belt (everything past ~105°), so the S-wave shadow zone is larger.

Drill 1 more on shadow zones and the liquid outer core

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (1)

P, S and L waves3 rows

Wave	Motion	Speed / arrival	Travels through
P (Primary)	Longitudinal (push-pull)	Fastest · arrives first	Solid, liquid, gas NDA 2026 — P arrives before S; P is longitudinal, S is transverse.
S (Secondary)	Transverse (side-to-side)	Slower · arrives second	Solid only — NOT liquid
L (surface)	Along the surface	Slowest · most destructive	Surface rocks only NDA 2025 — L-waves follow Earth's circumference (but NOT at exactly constant speed).

Watch out for (4)

Hypocentre is NOT on the surface
→ Focus, hypocentre and epicentre
Fastest ≠ most destructive
→ P, S and L waves
L-waves: circumference yes, constant speed no
→ P, S and L waves
The S-shadow is the BIGGER one
→ Shadow zones and the liquid outer core

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