Why the earth shakes: faults, focus and epicentre

Detail

What an earthquake is

• An earthquake is the shaking of the earth. It is a natural event.
• It happens when energy is released suddenly inside the earth.
• The released energy makes waves. These waves travel out in all directions.
• Most of this energy is released along faults.

The fault: where the energy is stored

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

• friction along the break

• So the blocks cannot move freely. They stay locked, and stress builds up in the rocks year after year.

How the quake happens, step by step

• Cause: the blocks keep trying to move, but friction locks them.
• Process:
• 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: the stored energy is set free as seismic waves (waves of energy moving through the earth), and the ground shakes.

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

Common fault movements (with real examples)

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 on the boundary 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: strike-slip faulting at shallow depth on the East Anatolian fault zone [9]

Focus and epicentre

• Focus (also called the 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 point on the surface nearest to the focus.
• It is the first place to feel the waves.

• It is usually the worst hit.

• Exam trap: the focus is underground. The epicentre is on the surface. A map can show the epicentre as a pair of latitude and longitude values. The depth of the focus is given separately, in kilometres.

Where earthquakes happen: depth

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

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

• Most of the energy comes from shallow quakes. That is why shallow quakes cause most of the damage.

Where earthquakes happen: pattern on the world map

• Mid-oceanic ridges (long undersea mountain chains where two plates move apart, e.g. the Mid-Atlantic Ridge): the foci are shallow.
• Alpine-Himalayan belt and the rim of the Pacific (the "Ring of Fire"): the foci are deep-seated as well as shallow.
• Why this mattered (a clue to subduction):
• Subduction: one plate sinks under another into the mantle.
• Deep-focus quakes line up in Benioff zones. These are belts of foci that slope down into the earth away from the ocean trench [3].

• The sloping line of foci traces the sinking slab of the plate. This was early proof for plate tectonics (Class 11, Distribution of Oceans and Continents).

• Note: even at a subduction zone, the giant quakes on the plate boundary can be shallow. Tohoku 2011 had its focus at only 29 km [7].

Three case studies

1. 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, Kachchh district [5].
• Focus depth: about 16 km. This is a shallow-focus quake [5].
• Type: intraplate (inside a plate, not on a plate edge). It was oblique reverse faulting inside the Indian plate, near the India–Eurasia boundary [5].
• Many M 5 aftershocks followed. Four of them came within 4 hours of the main shock [5].
• Kachchh lies in seismic Zone V, the highest-risk zone [8].

2. Tohoku, Japan: 11 March 2011

• Magnitude Mw 9.1. Focus depth: 29 km [7][10].
• Location: off the north-east coast of Honshu, on the Japan Trench. Cause: thrust faulting on the boundary between the Pacific and North America plates [7][10].
• The tsunami reached land within 30 minutes. It hit about 2,000 km of Japan's Pacific coast. Waves were almost 40 m high in Iwate Prefecture [10].
• Toll: 15,901 dead and 2,519 missing, presumed dead (figures as of March 2026) [10].

3. Kahramanmaraş (Turkey–Syria): 6 February 2023

• An M 7.8 quake in southern Turkey, near the northern border of Syria [9].
• Cause: strike-slip faulting at shallow depth, on the East Anatolian fault zone (or possibly the Dead Sea transform) [9].
• It was followed the same day by an M 7.5 quake. USGS calls the two together the "M7.8 and M7.5 Kahramanmaraş earthquake sequence near Nurdağı" [9].

Earthquake hazard in India

• The Bureau of Indian Standards (BIS) seismic zoning map puts India in four zones, II to V. Zone V has the highest risk; the Himalaya fall in it [8].
• Share of land area in each zone [8]:
• Zone V: about 11%
• Zone IV: about 18%
• Zone III: about 30%
• Zone II: the rest

Prelims Hooks

• Focus = hypocentre (inside the earth). Epicentre = the surface point directly above it. 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.
• Mid-ocean ridges → shallow foci. Pacific rim and Alpine-Himalayan belt → deep foci as well. Trap: deep-focus quakes do not happen at mid-ocean ridges.
• Benioff zone: a sloping belt of quake foci that marks a subducting slab.
• Elastic rebound theory: H.F. Reid, 1910, after the 1906 San Francisco quake.
• Matching pairs:
• Bhuj 2001: intraplate, oblique reverse fault, Kachchh (Zone V)
• Tohoku 2011: subduction thrust, Japan Trench, Mw 9.1

• Turkey 2023: strike-slip, East Anatolian fault

• Bhuj 2001 was shallow-focus (~16 km), and so was Tohoku 2011 (29 km). Trap: a subduction zone does not always mean a deep focus.

• Share of India's area in Zone V is about 11%. India has four zones (II–V); there is no Zone I.

Mains Points

• Why focus depth matters for disaster risk: most earthquake energy comes from shallow quakes. Bhuj (~16 km), Tohoku (29 km) and Turkey 2023 (shallow) all had shallow foci, so the strong shaking reached the surface close to the epicentre and caused heavy damage (GS-I physical geography; GS-III disaster management).
• Intraplate risk: Bhuj 2001 struck inside the Indian plate, away from the Himalayan plate edge. Peninsular India is not quake-free. Building codes and zoning must also cover old faults like those in Kachchh.
• Distribution pattern as a clue to how the earth works: shallow foci at ridges and sloping (Benioff) belts of deep foci at trenches together supported sea-floor spreading and subduction. This explains why India's Himalaya and the North-East fall in Zone V.
• Linked hazards: a thrust quake under the sea can lift the sea floor and start a tsunami (Tohoku: ~40 m waves, coast hit within 30 minutes). Hence the need for warning systems such as INCOIS's tsunami early warning centre for the Indian Ocean, and for coastal preparedness.