GeographyCBSE Class 11 · Interior of the Earth

Earthquake Waves, Shadow Zones, Types and Measurement (Class 11 Geography, Fundamentals of Physical Geography, Chapter 3)

Updated 17 October 2026 · Fact-checked

An earthquake releases energy at the focus; the epicentre lies directly above it. P-waves travel through solids and liquids, S-waves only through solids, and surface waves are the most destructive. Shadow zones (P-waves absent between 105 and 145 degrees, S-waves beyond 105 degrees) reveal a liquid outer core. Richter measures magnitude (0-10) and Mercalli intensity (1-12).

Understand Earthquakes: Waves, Shadow Zones, Types and Measurement

Why this topic matters. Questions ask you to compare P and S waves and explain the shadow zone.

1. Why study it. Nobody can reach the centre: the radius is about 6,378 km, mines reach 3-4 km (South Africa) and the deepest drill (Kola) 12 km. So we infer.

2. Sources. Direct: surface and mine rocks, deep drilling projects and volcanic magma. Indirect: temperature, pressure and density rising with depth, meteors, gravitation and gravity anomalies, magnetic surveys and above all seismic activity.

3. Earthquake. A fault is a sharp break in crustal rocks. Friction locks the blocks until the tendency to move overcomes it and they slide abruptly, releasing energy. The focus (hypocentre) is the point of release; the epicentre is the point on the surface nearest it, which first experiences the waves. Natural earthquakes occur in the lithosphere, up to 200 km.

4. Waves. Body waves travel through the body of the earth; surface waves form when they reach the surface.

Wave	Speed and arrival	Medium	Vibration
P (primary)	Fastest, first	Solid, liquid, gas	Parallel to direction of travel (squeeze and stretch)
S (secondary)	After a lag	Only solids	Perpendicular to direction
Surface	Last	Along surface	Most destructive

Velocity increases with density; waves reflect or refract at boundaries. S-waves' absence beyond a distance revealed a liquid outer core.

5. Shadow zones. A shadow zone is where seismographs record no waves. Both P and S-waves are recorded within 105 degrees of the epicentre; beyond 145 degrees only P-waves; between 105 and 145 degrees neither. S-waves are absent beyond 105 degrees, a zone a little over 40 per cent of the earth's surface. P-waves are refracted at the core-mantle boundary; S-waves cannot pass the liquid core.

6. Types. Tectonic (most common), volcanic, collapse (mines), explosion (chemical or nuclear) and reservoir-induced.

7. Scales. Richter measures magnitude (energy released), 0-10. Mercalli measures intensity (visible damage), 1-12. Shaking is devastating if magnitude is above 5.

8. Effects. Twelve: ground shaking, differential settlement, landslides, soil liquefaction, ground lurching, avalanches, ground displacement, floods from dam failures, fires, structural collapse, falling objects and tsunami. A tsunami needs an epicentre under ocean water and sufficient magnitude; it is a wave generated by the quake, not an earthquake.

9. Frequency. Earthquakes of magnitude 8 or more are rare (about once in 1-2 years); tiny quakes occur almost every minute.

Earthquakes: Waves, Shadow Zones, Types and Measurement: key facts

Waves compared: P: fastest, solids, liquids and gases, parallel vibration
S: slower, solids only, perpendicular vibration
Surface: last, most destructive; Table to learn.
Shadow zone angles: Both waves within 105 degrees
P absent between 105 and 145 degrees
S absent beyond 105 degrees
S zone over 40 per cent of earth surface; Numbers.
Scales: $R i c h t er : ma g ni t u d e, 0 - 10, e n er g y ∣ M er c a l l i : in t e n s i t y, 1 - 12, d ama g e$; Do not mix.
Types of earthquakes: $T ec t o ni c ∣ V o l c ani c ∣ C o l l a p se ∣ E x pl os i o n ∣ R eser v o i r - in d u ce d$; Five types.

How to answer Earthquakes: Waves, Shadow Zones, Types and Measurement questions

Seismology answers need a comparison and a reason.

1Define focus and epicentre.
2Compare wave types in a table.
3State the angles of the shadow zone.
4Explain that the liquid outer core stops S-waves.
5Add scale and type.

Earthquakes: Waves, Shadow Zones, Types and Measurement in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Sound versus a shake

Suppose you tap one end of a long metal rail and a long spring, and compare the movements.

Which wave type matches each: squeezing along the length or side-to-side shaking?

Try it first, then show the solutionSolution

P-waves vibrate parallel to travel, squeezing and stretching.
S-waves vibrate perpendicular, with crests and troughs.

Answer: Squeezing along the length matches P-waves; sideways shaking matches S-waves.

Memory hook: P is push, S is side.

Intensity at a place

Suppose after a quake one village reports cracked walls while a village farther away only feels slight swaying.

Which scale measures such local effects, and which measures the energy released?

Try it first, then show the solutionSolution

Mercalli measures intensity (visible damage).
Richter measures magnitude (energy).

Answer: Mercalli for local effects; Richter for energy.

Memory hook: Intensity local, magnitude total.

Tricks to remember Earthquakes: Waves, Shadow Zones, Types and Measurement

Short memory aids. Say each one aloud once, then use it on a question the same day.

105-145
P shadow band
Use it when: Shadow zone.
P = Primary, Push; S = Secondary, Side
P and S
Use it when: Waves.
R-M
Richter = Real energy, Mercalli = Marks of damage
Use it when: Scales.

Earthquakes: Waves, Shadow Zones, Types and Measurement: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Epicentre · 1 mark · MCQBoard level

The epicentre of an earthquake is:
(A) the point where energy is released
(B) the point on the surface nearest the focus
(C) a type of wave
(D) a fault line

Try it first, then show the solutionSolution

The point nearest the focus, directly above it (1 mark).

Answer: (B)

Example 2: Why S-waves vanish · 2 marks · Very short answerBoard level

Explain why S-waves are not recorded beyond 105 degrees from the epicentre. (2 marks)

Try it first, then show the solutionSolution

S-waves cannot pass through liquid (1 mark).
The outer core is liquid, so they vanish beyond 105 degrees (1 mark).

Answer: S-waves travel only through solids; the outer core is liquid, so S-waves do not reach beyond 105 degrees.

Example 3: Compare P and S-waves · 3 marks · Short answerHarder board level

Distinguish between P-waves and S-waves on any three points. (3 marks)

Try it first, then show the solutionSolution

P arrive first, S later (1 mark).
P pass through solids, liquids and gases; S only solids (1 mark).
P vibrate parallel to the direction; S perpendicular (1 mark).

Answer: P-waves are faster and arrive first, while S-waves come after a time lag. P-waves pass through solid, liquid and gas, but S-waves only through solids. P-waves vibrate parallel to their direction of travel, S-waves perpendicular to it.

Example 4: Assertion and reason: surface waves · 1 mark · Assertion-reasonHarder board level

Assertion (A): Surface waves cause the greatest destruction.
Reason (R): They displace rocks and cause structures to collapse.
(A) Both true, R explains A
(B) Both true, R does not explain A
(C) A true, R false
(D) A false, R true

Try it first, then show the solutionSolution

Surface waves are the most destructive (1 mark).

Answer: (A)

Example 5: HOTS: P-wave shadow zone · 5 marks · Long answerHOTS / competency

Why does a P-wave shadow zone exist between 105 and 145 degrees although P-waves can pass through liquids? (5 marks)

Try it first, then show the solutionSolution

P-waves pass through liquids (1 mark).
At the core-mantle boundary density changes sharply (1 mark).
So P-waves are refracted (1 mark).
This bends them away from the 105-145 degree band (1 mark).
They reappear beyond 145 degrees (1 mark).

Answer: P-waves can travel through liquid, but when they meet the core-mantle boundary the density changes, so they are refracted. The bending directs them away from the band between 105 and 145 degrees, so no P-waves are recorded there; beyond 145 degrees they are recorded again. S-waves, unlike P-waves, are stopped altogether by the liquid outer core.

Where students lose marks in Earthquakes: Waves, Shadow Zones, Types and Measurement

The slips examiners see most often, and what to write instead.

Mistake: Mixing the scales
Both measure quakes.
Wrong working
Richter measures intensity.
Right working
Richter: magnitude (energy), 0-10; Mercalli: intensity (damage), 1-12.
Do this: Pair scale, quantity, range.
Usually costs: 1 mark.
Mistake: Saying S-waves pass through liquids
P-waves do.
Wrong working
S-waves pass through all media.
Right working
S-waves pass only through solids.
Do this: Say "only solids".
Usually costs: 1 mark.
Mistake: Focus and epicentre swapped
Both are points.
Wrong working
Epicentre is where energy is released.
Right working
Focus is where energy is released; the epicentre is nearest on the surface.
Do this: Draw a vertical line.
Usually costs: 1 mark.

Exam tips

Draw the shadow zone diagram with angle marks.
Use the table for P and S-waves.
Give magnitude range and intensity range with scale names.

Practice questions on Earthquakes: Waves, Shadow Zones, Types and Measurement

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The Mercalli scale measures:
(A) magnitude
(B) intensity
(C) depth of focus
(D) wave speed
Hint
Damage.
Answer
(B) intensity.
Q2 · 2 marks · Very short answerBoard level
A seismograph station records no S-waves and no P-waves from an earthquake. State the possible range of its angular distance from the epicentre and explain why. (2 marks)
Hint
Think of where both waves are missing, and what stops each one.
Answer
The station lies between 105 and 145 degrees from the epicentre. S-waves are stopped by the liquid outer core, so none are recorded beyond 105 degrees. P-waves are refracted at the core-mantle boundary, so they are absent between 105 and 145 degrees.
Q3 · 3 marks · Short answerHarder board level
List the five types of earthquakes with one cause each. (3 marks)
Hint
Tectonic, volcanic, collapse, explosion, reservoir.
Answer
Tectonic: sliding along a fault plane; volcanic: active volcanoes; collapse: mine roofs; explosion: chemical or nuclear blasts; reservoir-induced: large reservoirs.
Q4 · 3 marks · Short answerHarder board level
Why are earthquakes called a source of indirect information about the interior? (3 marks)
Hint
Waves change with density.
Answer
Seismic wave velocity depends on density and waves reflect or refract at boundaries; records from seismographs thus reveal layers. Along with temperature, pressure, gravity and magnetic data, they give a complete picture of the layered interior.
Q5 · 4 marks · Short answerHOTS / competency
A tsunami followed an earthquake in the ocean. A student says a tsunami is a type of earthquake. Correct and explain. (4 marks)
Hint
Wave generated by the shake.
Answer
A tsunami is not an earthquake; it is a wave generated by the tremor. It occurs only if the epicentre is under ocean water and the magnitude is high enough. Earthquakes are classified by cause (tectonic, volcanic and so on), while a tsunami is one of the effects.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What are the two main groups of seismic waves?Show answer
Body waves and surface waves.
2.What is a gravity anomaly?Show answer
The difference between expected and observed gravity, revealing the mass distribution in the crust.
3.How deep is the Kola borehole?Show answer
12 km.
4.What is the maximum depth of natural earthquakes?Show answer
About 200 km, within the lithosphere.

Earthquakes: Waves, Shadow Zones, Types and Measurement: frequently asked questions

Which wave arrives first?

P-wave.

Are meteors from the earth's interior?

No, but they are solid bodies of similar material and structure.

What is the locating-epicentre activity?

It is outside the syllabus after rationalisation.