Earthquake

1. Introduction

An earthquake is the shaking or trembling of the Earth’s surface caused by a sudden release of energy inside the crust or upper mantle. The energy travels outward in all directions as seismic waves.

The scientific study of earthquakes is called Seismology.
Instrument that records seismic waves: Seismograph (also called seismometer). The record it produces is a seismogram.
Earthquakes are a natural hazard: high-magnitude tremors damage life and property.

Key Terms

Term	Meaning
Focus (Hypocentre)	The point inside the Earth where the energy is released and the earthquake starts
Epicentre	The point on the surface directly above the focus; nearest to the focus, so it is the first place to experience the waves and usually suffers the maximum damage
Fault	A sharp break or fracture in crustal rocks along which blocks move
Foreshock	Smaller tremor before the main shock
Mainshock	The largest event of an earthquake sequence
Aftershock	Smaller tremors after the main shock, as the crust readjusts
Isoseismal line	Line joining places that experienced equal intensity of shaking
Homoseismal line	Line joining places where the waves arrived at the same time

2. Causes and Types of Earthquakes

2.1 Fault Zones (Faulting)

Energy is released along a fault.
Rocks on the two sides of a fault tend to move in opposite directions, but friction locks them together as the overlying rock strata press down.
Eventually the tendency to move apart overcomes the friction. The blocks, already deformed, slide past each other abruptly.
This sudden slip releases energy, and waves travel in all directions.

Types of faults

Fault type	Stress	Movement	Associated boundary
Normal fault	Extension (tension)	Hanging block moves down relative to the footwall	Divergent
Reverse / Thrust fault	Compression	Hanging block moves up; a low-angle reverse fault is called a thrust	Convergent
Strike-slip fault	Shear	Blocks slide horizontally past each other	Transform (e.g., San Andreas Fault)

The two blocks in dip-slip faults are called the upthrown block and the downthrown block.

2.2 Tectonic Earthquakes

The most common type and the most destructive.
The Earth’s interior has layers: a solid crust, a hot, nearly solid mantle, a liquid outer core and a solid inner core.
Lithospheric (tectonic) plates drift slowly on the viscous, slowly flowing asthenosphere (upper mantle) below.
This non-stop movement builds stress in the crust. When stress gets too large, it cracks the crust into faults.
Slipping of land along the fault line at convergent, divergent and transform plate boundaries causes earthquakes.

2.3 Volcanic Earthquakes

A special class of tectonic earthquake, confined to areas of active volcanoes.
Caused by stress changes in solid rock due to injection or withdrawal of magma (molten rock).
They can cause land subsidence and large ground cracks, and can occur as rock moves to fill spaces where magma is no longer present.
Volcano-tectonic earthquakes do not necessarily indicate that an eruption is coming; they can occur at any time.

2.4 Human-Induced (Anthropogenic) Earthquakes

Type	Cause
Collapse earthquakes	Roofs of underground mines collapse in areas of intense mining, causing minor tremors
Explosion earthquakes	Ground shaking from chemical or nuclear explosions
Reservoir-induced earthquakes	Occur in areas of large reservoirs (the weight and seepage of water alter stress on rocks). Example: Koyna (Maharashtra), 1967

3. Classification by Depth of Focus

The focus can lie anywhere between 0 and 700 km below the surface.

Class	Depth of focus
Shallow	0 – 70 km
Intermediate	70 – 300 km
Deep	300 – 700 km

Shallow earthquakes are the most common and the most damaging, because the focus is close to the surface.
No earthquake focus is found deeper than about 700 km, because rocks at greater depth are too hot and plastic to fracture suddenly.

Wadati–Benioff Zone

A zone of subduction, where a downgoing oceanic plate sinks beneath a continental (or another oceanic) plate.
Earthquakes here are very common and some of the most powerful earthquakes occur along it. The foci get progressively deeper as one moves away from the trench, tracing the sinking slab; deep-focus earthquakes (300–700 km) occur only in such zones.
Earthquakes arise from slip along the subduction thrust fault or slip on faults within the downgoing plate as it is pulled into the mantle.
Named after Kiyoo Wadati (Japan) and Hugo Benioff (USA).

4. World Distribution of Earthquakes

Earthquakes can strike anywhere, but they follow the same general patterns year after year, mainly along three belts:

Belt	Location	Share of world’s large earthquakes	Notes
Circum-Pacific Belt (Ring of Fire)	Rim of the Pacific Ocean (Chile, Peru, Mexico, California, Alaska, Aleutians, Japan, Philippines, Indonesia, New Zealand)	About 81 %	World’s greatest earthquake belt; also holds most of the world’s active volcanoes. Plates of mostly oceanic crust subduct beneath other plates
Alpide Belt (Mid-Continental Belt)	From Java and Sumatra through the Himalayas, Iran, Turkey, the Mediterranean and out into the Atlantic	About 17 %	Includes some of the most destructive earthquakes. India lies on this belt
Mid-Atlantic Ridge Belt	Along the submerged Mid-Atlantic Ridge	Remaining small share	A divergent boundary where plates spread apart; mostly deep underwater and far from human settlement

5. Seismic Waves

Seismic waves are waves of energy produced by earthquakes (or explosions) that travel through the Earth and are recorded on seismographs. They are of two main types: body waves and surface waves.

5.1 Body Waves

Generated at the focus; travel through the interior (body) of the Earth in all directions.

Feature	P-waves (Primary)	S-waves (Secondary)
Arrival	First to arrive (fastest)	Arrive after P-waves (time lag)
Nature	Longitudinal / compressional (particles vibrate parallel to the direction of travel); similar to sound waves	Transverse / shear (particles vibrate at right angles to the direction of travel)
Medium	Pass through solids, liquids and gases	Pass through solids only
Damage	Less destructive	More destructive than P-waves

5.2 Surface Waves

Body waves interact with surface rocks and generate a new set of waves called surface waves (also L-waves, long waves).
They move along the surface, are the last to be recorded on a seismograph, and are the most destructive, causing displacement of rocks and hence collapse of structures.
Two kinds:
- Love waves – horizontal shearing (side-to-side) motion.
- Rayleigh waves – rolling, elliptical motion like ocean waves.

Order of arrival: P-waves → S-waves → Surface waves

5.3 Shadow Zones and the Earth’s Interior

The behaviour of seismic waves has helped scientists understand the structure of the Earth’s interior.

S-waves do not pass through the liquid outer core, which proves that the outer core is liquid.
P-waves are refracted (bent) at the core boundary.
As a result, regions where waves are not recorded form shadow zones:
- P-wave shadow zone: about 103° to 142° from the epicentre.
- S-wave shadow zone: the entire region beyond 103° (the far side of the Earth).

6. Measurement of Earthquakes

Scientists determine the time, location and intensity of an earthquake from the record of a seismograph, which also reveals information about the rocks the waves travelled through.

Earthquakes are measured either by magnitude or by intensity.

Feature	Magnitude (Richter scale)	Intensity (Mercalli scale)
Measures	Energy released at the focus	Visible damage / effects at a place
Developed by	Charles F. Richter (USA, 1935)	Giuseppe Mercalli (Italian seismologist); later modified as the Modified Mercalli Intensity (MMI) scale
Range	0 – 10 (open-ended in principle)	I – XII (1 – 12)
Nature	Logarithmic. Each whole-number step = 10× greater ground-motion amplitude and roughly 31.6× more energy	Qualitative, based on observed effects
Value	One value for an earthquake	Varies from place to place, highest near the epicentre

The Moment Magnitude Scale (Mw) is now preferred by seismologists, especially for large earthquakes.
Earthquakes of magnitude below 2 are generally not felt; above 7 are major, and 8 and above are great earthquakes.

7. Effects of Earthquakes

Immediate hazardous effects:

Ground shaking
Differential ground settlement
Land and mudslides
Fires (broken gas lines and electrical short circuits)
Ground lurching
Avalanches
Ground displacement
Floods from dam and levee failures
Structural collapse
Tsunami

Tsunami

A tsunami (Japanese: harbour wave) is a series of huge sea waves produced by an undersea earthquake, volcanic eruption or landslide.
In the deep ocean the waves are low but extremely fast; they build up height as they reach the shallow coast.
Most tsunamis occur in the Pacific Ocean because of the Ring of Fire.
Example: the 26 December 2004 Indian Ocean (Sumatra) earthquake and tsunami affected Indonesia, Sri Lanka, India (Tamil Nadu, Andhra Pradesh, Andaman & Nicobar Islands), Thailand and others.

8. Earthquakes in India

8.1 Why Is India Vulnerable?

India lies on the Indian Plate, which is moving northward and colliding with the Eurasian Plate.
This collision created the young, technically active fold mountains — the Himalaya, which are still rising and accumulate huge stress. The Himalayan region lies on the Alpide Belt.
Large parts of India are therefore highly earthquake-prone.

8.2 Seismic Zonation of India (BIS)

The Bureau of Indian Standards (BIS) has divided India into four seismic zones: II, III, IV and V, based on scientific inputs on seismicity, past earthquakes and the tectonic set-up. (The earlier Zone I was merged with Zone II; the present map follows the 2002 revision.)

Zone	Risk level	Intensity (MSK / MMI)	% of India’s area	Zone factor (Z)	Examples of areas
Zone V	Very high	IX and above	10.79 %	0.36	Entire North-East (most of it), Kashmir Valley, parts of Himachal Pradesh and Uttarakhand, Rann of Kutch (Gujarat), Andaman & Nicobar Islands
Zone IV	High	VIII	17.49 %	0.24	Delhi-NCR, parts of J&K, Himachal Pradesh, Uttarakhand, Sikkim, northern Bihar, Uttar Pradesh (Terai), parts of Gujarat and Maharashtra (Koyna)
Zone III	Moderate	VII	30.79 %	0.16	Kerala, Goa, Lakshadweep, parts of Gujarat, Punjab, Rajasthan, West Bengal, Maharashtra (Mumbai), Tamil Nadu (Chennai)
Zone II	Low	VI and below	40.93 %	0.10	Large parts of the Peninsular plateau and central India (e.g., Karnataka interior, Madhya Pradesh, Chhattisgarh, Rajasthan)

About 59 % of India’s land area (Zones III + IV + V) is liable to seismic hazard damage.
Zone V is the most vulnerable; Zone II the least.
The Peninsular plateau is an old, stable landmass, but is not entirely safe (e.g., Koyna 1967, Latur 1993).

8.3 Major Earthquakes in India

Year	Earthquake	Region	Approx. magnitude
1897	Shillong (Assam) earthquake	Meghalaya / Assam	8.1
1905	Kangra earthquake	Himachal Pradesh	7.8
1934	Bihar–Nepal earthquake	Bihar / Nepal	8.0
1950	Assam–Tibet earthquake	Arunachal Pradesh / Assam	8.6
1967	Koyna (reservoir-induced)	Maharashtra	6.5
1991	Uttarkashi	Uttarakhand	6.8
1993	Latur (Killari)	Maharashtra	6.2
1999	Chamoli	Uttarakhand	6.8
2001	Bhuj (Gujarat), on 26 January (Republic Day)	Gujarat	7.7
2004	Sumatra–Andaman earthquake and Indian Ocean tsunami	Andaman & Nicobar / Tamil Nadu coast	9.1
2005	Kashmir (Muzaffarabad)	J&K / Pakistan-administered Kashmir	7.6
2011	Sikkim	Sikkim	6.9
2015	Gorkha (Nepal) — strongly felt in Bihar, UP and the North-East	Nepal	7.8

8.4 Institutions

Institution	Role
National Center for Seismology (NCS)	Under the Ministry of Earth Sciences; the nodal agency for monitoring earthquakes in India
Bureau of Indian Standards (BIS)	Prepares the seismic zonation map and the earthquake-resistant design code (IS 1893)
National Disaster Management Authority (NDMA)	Apex body for disaster management; chaired by the Prime Minister (established under the Disaster Management Act, 2005)
National Disaster Response Force (NDRF)	Specialised force for rescue and relief

9. Earthquake Safety Measures

Before

Build earthquake-resistant structures and follow the BIS code; retrofit old buildings.
Avoid construction on loose soil, steep slopes and active fault lines.
Secure heavy furniture and keep an emergency kit.

During

Drop, Cover and Hold On – get under a sturdy table.
Stay away from windows, glass and heavy objects. Do not use lifts.
If outdoors, move to an open area, away from buildings, trees, electric poles and flyovers.
Near the coast, move to high ground after strong shaking (tsunami risk).

After

Expect aftershocks. Check for injuries, gas leaks and damaged structures.
Listen to official announcements; do not spread rumours.

PYQs

1. Which of the following seismic waves travels fastest and is the first to be recorded on a seismograph during an earthquake? (SSC CHSL 2025)

(A) Secondary waves (S-waves)

(B) Surface waves (L-waves)

(D) Rayleigh waves

Answer: (C) Primary waves (P-waves)

2. Which class of seismic waves is responsible for causing the most severe structural damage on the Earth’s surface? (SSC CGL 2024)

(A) Primary waves

(B) Secondary waves

(D) Surface waves

Answer: (D) Surface waves

3. What does the Mercalli Scale measure in the context of earthquakes? (SSC CGL 2025)

(A) Magnitude of total energy released

(B) Intensity based on visible damage and human observation

(D) Velocity of primary waves

Answer: (B) Intensity based on visible damage and human observation

4. According to the Bureau of Indian Standards (BIS), India is classified into how many Seismic Hazard Zones? ( SSC CHSL 2025)

(A) 3

(B) 4

(D) 6

Answer: (B) 4

5. Which theory best explains how strain energy gradually accumulates along a geological fault and is suddenly released to trigger an earthquake? (SSC MTS 2025)

(A) Continental Drift Theory

(B) Elastic Rebound Theory

(D) Isostasy Theory

Answer: (B) Elastic Rebound Theory