Earthquake

Also called: Bhukampa · Topic: Interior of the Earth: Earthquakes, Volcanoes and the Layered Earth · NCERT: Class 9, Ch 2 "Shaping of the Earth's Surface"; Class 11, Ch 6 "Natural Hazards and Disasters: Causes, Consequences and Management"; Class 11, Ch 3 "Interior of the Earth"

Meaning

An earthquake is the sudden shaking of the ground. It happens when energy stored in the rocks of the earth is suddenly released, usually along a fault (a break in the crust), and travels outward as waves. Earthquakes matter for the exam because they explain how the earth works inside, and because they are among the deadliest natural hazards in India, which has its own seismic zones.

Explanation

How an earthquake happens (cause → process → result)

• Fault: a sharp break in the rocks of the crust. The rock blocks on the two sides tend to move in opposite directions.
• Cause: the blocks are locked. Two things hold them in place:
• the weight of the rock layers (strata) lying above them
• friction along the break

• So the blocks cannot move freely, and stress builds up year after year.

• Process: bending, then a sudden slip.

• The rocks first bend (deform) and store energy, like a stretched spring.
• One day the push to move becomes stronger than the friction.

• The blocks slide past each other suddenly.

• Result: seismic waves. The stored energy is set free as seismic waves (waves of energy moving through the earth). These waves travel out in all directions, and the ground shakes.

• Elastic rebound theory (bent rock snaps back to its unstrained shape and gives up its stored energy): put forward by H.F. Reid in 1910, after the 1906 San Francisco earthquake (USA).
• USGS calls this "stick-slip": the fault sticks, then slips. Each slip sends out energy as seismic waves, and that is the earthquake [4].

Focus and epicentre

• Focus (hypocentre): the point inside the earth where the energy is first released, that is, where the break in the rock starts [2][6].
• Epicentre: the point on the surface directly above the focus.
• It is the surface point nearest to the focus.
• It is the first place to feel the waves.

• It is usually the worst hit.

• A map shows the epicentre as latitude and longitude. The depth of the focus is given separately, in kilometres.

Types of fault movement

Fault movement	How the blocks move	Example
Reverse / thrust	One block is pushed up and over the other (the crust is squeezed)	Tohoku 2011: thrust faulting where the Pacific plate goes under the North America plate [7]
Oblique reverse	Up-and-over movement mixed with some sideways movement	Bhuj 2001: shallow oblique reverse faulting inside the Indian plate [5]
Strike-slip	The blocks slide sideways past each other	Turkey-Syria 2023: shallow strike-slip faulting on the East Anatolian fault zone [9]

• Types by cause (NCERT): tectonic earthquakes (from fault movement) are the most common and widespread. Volcanic, collapse (in mining areas), explosion and reservoir-induced (near large dams) earthquakes are local and smaller.

Depth of the focus

• All natural earthquakes happen in the lithosphere (the hard outer shell of the earth: the crust plus the top part of the mantle). NCERT puts the lithosphere at up to 200 km deep.
• Depth classes [6]:

Class	Focus depth	Share of total earthquake energy
Shallow-focus	0–70 km	Most of the energy
Intermediate-focus	70–300 km	About 12% (Britannica counts this band from about 60 km) [2]
Deep-focus	300–700 km	About 3% [2]

• Shallow quakes release most of the energy, so they cause most of the damage.

Where it occurs

• Mid-oceanic ridges (long undersea mountain chains where two plates move apart, e.g. the Mid-Atlantic Ridge): only shallow foci.
• Rim of the Pacific ("Ring of Fire") and the Alpine-Himalayan belt: deep as well as shallow foci.
• Deep-focus quakes line up in Benioff zones (belts of foci that slope down into the earth away from the ocean trench) [3].

• The sloping line traces the sinking slab in subduction (one plate sinking under another into the mantle). This was early proof for plate tectonics.

• Bhuj (Kachchh), Gujarat, India: 26 January 2001

• Magnitude M 7.7 (another USGS study gives Mw 7.6) [5].
• Epicentre 23.419°N, 70.232°E, about 17 km north-west of Bhachau [5].
• Focus about 16 km deep (shallow). It was an intraplate quake (inside a plate, not at a plate edge), near the India–Eurasia boundary [5].

• Four M 5 aftershocks came within 4 hours [5]. Kachchh is in Zone V [8].

• Tohoku, Japan: 11 March 2011

• Mw 9.1, focus 29 km deep, off the north-east coast of Honshu on the Japan Trench [7][10].
• The tsunami reached land within 30 minutes. It hit about 2,000 km of coast, with waves almost 40 m high in Iwate Prefecture [10].

• 15,901 dead and 2,519 missing (as of March 2026) [10].

• Kahramanmaraş, Turkey–Syria: 6 February 2023

• M 7.8 in southern Turkey near the Syrian border. It was shallow strike-slip faulting on the East Anatolian fault zone (or possibly the Dead Sea transform) [9].

• An M 7.5 quake followed the same day [9].

• India's seismic zones (BIS map): four zones, II to V. The Himalaya fall in Zone V [8].

• Zone V about 11% of land area, Zone IV about 18%, Zone III about 30%, and Zone II the rest [8].

Don't confuse with

• Focus vs epicentre: the focus (hypocentre) is underground, where the rock breaks. The epicentre is the surface point directly above it.
• Magnitude vs intensity: magnitude (Richter scale) measures the energy released, one value per quake. Intensity (Mercalli scale) measures the damage felt at a place, so it changes from place to place.
• Interplate vs intraplate earthquake: an interplate quake happens on a plate boundary (Tohoku 2011). An intraplate quake happens inside a plate (Bhuj 2001) [5].
• Subduction zone ≠ deep focus: Benioff zones do hold deep foci, but the giant boundary quakes can be shallow. Tohoku 2011 was only 29 km deep [7].

Prelims Hooks

• Focus = hypocentre (inside the earth). The epicentre feels the waves first and is usually the worst hit.
• Depth classes: shallow 0–70 km, intermediate 70–300 km, deep 300–700 km [6]. Trap: mid-ocean ridges have no deep-focus quakes.
• Benioff zone: a sloping belt of quake foci that marks a subducting slab [3].
• Elastic rebound theory: H.F. Reid, 1910, after the 1906 San Francisco quake.
• Match the pairs: Bhuj 2001: intraplate, oblique reverse, Kachchh (Zone V). Tohoku 2011: subduction thrust, Japan Trench, Mw 9.1. Turkey 2023: strike-slip, East Anatolian fault.
• India has four zones (II–V) and no Zone I. Zone V covers about 11% of land area [8].

Mains Points

• Why focus depth matters for disaster risk (GS-I, GS-III):
• Most earthquake energy comes from shallow quakes.
• Bhuj (~16 km), Tohoku (29 km) and Turkey 2023 (shallow) all had shallow foci.

• So strong shaking reached the surface close to the epicentre and caused heavy damage.

• Intraplate risk for India: Bhuj 2001 struck inside the Indian plate, far from the Himalayan plate edge. Peninsular India is not quake-free, so building codes and zoning must also cover old faults like those in Kachchh. The Himalaya and the North-East fall in Zone V because of the India–Eurasia collision.

• Linked hazards and preparedness:
• A thrust quake under the sea can lift the sea floor and start a tsunami. At Tohoku, waves of about 40 m hit the coast within 30 minutes [10].
• This is why India needs warning systems such as INCOIS's tsunami early warning centre for the Indian Ocean, and coastal preparedness.

Related concepts

• Fault
• Focus
• Epicentre
• Shallow-focus and deep-focus earthquakes

Read more

• Why the earth shakes: faults, focus and epicentre