Which of the following statements with reference to L-Wave or Long Wave generated by an earthquake is/are correct? 1. They follow the Earth's circumference 2. They travel at more or less at a constant rate Select the answer using the code given below:

Q: What is the answer to NDA (I) 2025 GAT Q103?

C. Correct — C, Both 1 and 2. L-waves (long or surface waves) are the last group of seismic waves recorded after an earthquake. Unlike the body waves (P and S) that pass through the Earth's interior, L-waves travel along the outer surface, so their path effectively follows the Earth's circumference (statement 1). Because they stay within the relatively uniform surface layers rather than diving through zones of changing density, they move at a more or less constant rate (statement 2). Both statements are therefore correct. (Provisional — pending the official UPSC key.)

(a)1 only
(b)2 only
(c)Both 1 and 2
(d)Neither 1 nor 2

Answer

Why

Correct — C, Both 1 and 2. L-waves (long or surface waves) are the last group of seismic waves recorded after an earthquake. Unlike the body waves (P and S) that pass through the Earth's interior, L-waves travel along the outer surface, so their path effectively follows the Earth's circumference (statement 1). Because they stay within the relatively uniform surface layers rather than diving through zones of changing density, they move at a more or less constant rate (statement 2). Both statements are therefore correct.

Why the others are wrong

(a)
1 only — Incomplete. Statement 1 is right, but statement 2 is also correct — surface L-waves keep a roughly constant speed because they stay in the homogeneous surface layer, so limiting the answer to statement 1 alone is wrong.
(b)
2 only — Incomplete. Statement 2 is true, but statement 1 is also true — L-waves do travel along the surface, i.e. following the Earth's circumference, so leaving out statement 1 is wrong.
(d)
Neither 1 nor 2 — Both statements are actually correct, so 'neither' is wrong. L-waves both follow the surface and move at a nearly constant rate.

Concept

An earthquake radiates three main kinds of waves. Body waves travel through the Earth's interior: P-waves (primary, fastest, push-pull, pass through solids and liquids) arrive first, then S-waves (secondary, slower, side-to-side, cannot pass through liquids). Surface waves, also called L-waves or long waves, travel along the ground surface, arrive last, move slowest and cause the most damage.

The key contrast is interior vs surface travel. P and S waves cut through the deep Earth, where density changes bend and speed them up or down. L-waves instead run along the crust, so their route hugs the surface (the Earth's circumference) and their speed stays fairly steady. Recognising L-waves as the surface group makes both statements fall out naturally.

Key facts

Order of arrival at a seismograph: P-waves first, then S-waves, then L-waves last.
L-waves (surface / long waves) travel along the Earth's surface and are the most destructive.
P-waves pass through solids and liquids; S-waves travel only through solids.
The instrument that records seismic waves is a seismograph; magnitude is read on the Richter scale.

Study next

P-waves vs S-waves — speed, medium and shadow zonesHow the S-wave shadow zone reveals the liquid outer coreFocus (hypocentre) and epicentre of an earthquakeRichter and Mercalli scales — magnitude vs intensitySeismograph and how earthquake location is triangulated

Common traps

Assuming L-waves travel through the Earth's interior — they are surface waves.
Thinking L-waves arrive first because they do the most damage — they actually arrive last and slowest.

Seismic waves are asked by property — order of arrival, medium they pass through, or which reveals the core — so learn P/S/L features side by side.

Related PYQs

NDA_GAT_2024_I_Q1092024Open & attempt →

Which of the following statements regarding earthquakes is/are correct? 1. The point on the Earth’s surface directly above the focus is called the epicentre of the earthquake. 2. Earthquakes generate Primary and Secondary waves that radiate outward from the earthquake focus. 3. Deep-focus earthquakes are likely to cause more damage than shallow-focus earthquakes. Select the answer using the code given below:

(a) 1 only
(b) 1 and 2 only
(c) 2 and 3 only
(d) 1, 2 and 3

Answer(b) 1 and 2 only

Same concept — earthquakes and the waves they generate. That NDA item tests the focus/epicentre and the P and S body waves; this one tests the L (surface) wave, completing the P-S-L set.

Practice

practice — not a real PYQ
Which seismic waves cannot pass through the liquid outer core, producing a shadow zone?
- (a)P-waves
- (b)S-waves
- (c)L-waves
- (d)All of them
Answer(b) S-waves — they travel only through solids, so their shadow zone shows the outer core is liquid.
practice — not a real PYQ
At a seismograph, which earthquake wave is recorded first?
- (a)L-wave
- (b)S-wave
- (c)P-wave
- (d)Surface wave
Answer(c) P-wave — the primary wave is the fastest and arrives first.