Epicentre

Topic: Interior of the Earth: Earthquakes, Volcanoes and the Layered Earth · NCERT: Class 11, Ch 3 "Interior of the Earth"

Meaning

The epicentre is the point on the earth's surface that lies directly above the focus (the point inside the earth where an earthquake begins). It is the surface point nearest to the focus. So it usually feels the waves first and is usually the worst hit. Maps use the epicentre to show where an earthquake happened, so it matters for disaster response and for exam questions on location.

Explanation

How an earthquake reaches the epicentre (cause → process → result)

• Cause: stress builds up along a fault.
• A fault is a sharp break in the rocks of the crust. The rock blocks on each side try to move in opposite directions.
• Two things hold the blocks in place: the weight of the rock layers above them, and friction along the break.

• The blocks stay locked, and stress builds up year after year.

• Process: the rocks bend, then slip.

• The rocks first bend and store energy, like a stretched spring.
• When the push becomes stronger than the friction, the blocks suddenly slide past each other.

• USGS calls this "stick-slip": the fault sticks, then slips, and each slip sends out energy as seismic waves [4].

• Result: the waves reach the surface.

• The energy comes out at the focus (also called the hypocentre), the point inside the earth where the break in the rock starts [2][6].
• Seismic waves (waves of energy moving through the earth) spread out in all directions.

• The epicentre lies straight above the focus, so the waves have the shortest path to it. It feels them first and usually shakes the hardest.

• Elastic rebound theory (beyond NCERT): the bent rock snaps back to its unstrained shape and releases its stored energy. H.F. Reid gave this idea in 1910, after the 1906 San Francisco earthquake.

How an epicentre is located and reported

• An epicentre is a point on the surface, so it is written as a pair of latitude and longitude values.

• Example: the Bhuj 2001 epicentre was at 23.419°N, 70.232°E [5].

• The depth of the focus is given separately, in kilometres. The epicentre itself has no depth.

• Reports often describe the epicentre by its distance from a known town. Example: Bhuj 2001 was "17 km NW of Bhachau" [5].

Why focus depth decides how bad the shaking is at the epicentre

• All natural earthquakes happen in the lithosphere (the hard outer shell of the earth: the crust plus the top part of the mantle). NCERT gives its depth as up to 200 km.
• 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% [2]
Deep-focus	300–700 km	About 3% [2]

• A shallow focus means heavy damage at the epicentre.
• Shallow quakes release most of the energy.
• The strong shaking travels only a short distance before it reaches the surface at the epicentre.
• So shallow quakes cause most of the damage.

Where it occurs

• Bhuj (Kachchh), Gujarat, India: 26 January 2001
• Epicentre at 23.419°N, 70.232°E, about 17 km north-west of Bhachau, Kachchh district. Magnitude M 7.7 (another USGS study gives Mw 7.6) [5].
• The focus was about 16 km deep, so it was a shallow-focus quake [5].
• It was intraplate (inside a plate, not on a plate edge): oblique reverse faulting inside the Indian plate [5].

• Kachchh is in seismic Zone V, the highest-risk zone [8].

• Tohoku, Japan: 11 March 2011

• The epicentre was offshore, off the north-east coast of Honshu, on the Japan Trench. Magnitude Mw 9.1, focus depth 29 km [7][10].
• Cause: thrust faulting on the boundary between the Pacific and North America plates [7].

• Because the epicentre was under the sea, the quake set off a tsunami. The tsunami reached land within 30 minutes, hit about 2,000 km of coast, and had waves almost 40 m high in Iwate Prefecture [10].

• Kahramanmaraş, Turkey–Syria: 6 February 2023

• An M 7.8 quake in southern Turkey, near Syria's northern border, followed the same day by an M 7.5 quake [9].

• Cause: strike-slip faulting (the blocks slide sideways past each other) at shallow depth, on the East Anatolian fault zone [9].

• World pattern of epicentres

• Mid-oceanic ridges (e.g. the Mid-Atlantic Ridge): many epicentres, all with shallow foci.

• Rim of the Pacific ("Ring of Fire") and the Alpine-Himalayan belt: shallow foci, and also deep ones.

• India: the BIS seismic zoning map has four zones, II to V. Zone V (about 11% of India's land area) is the highest-risk zone and includes the Himalaya [8].

Don't confuse with

• Focus / hypocentre: the focus is inside the earth, where the rock first breaks. The epicentre is the surface point directly above it [2][6].
• Focus depth: the epicentre is given only as latitude and longitude. The depth belongs to the focus and is given separately, in kilometres.
• Fault: a fault is a break in the rocks that can run for a long distance. The epicentre is a single point on the surface above where the slip started.
• Benioff zone: a sloping belt of foci that goes down into the earth away from an ocean trench and marks a sinking plate [3]. It describes foci at depth, not epicentres on the surface.

Prelims Hooks

• Epicentre = the surface point directly above the focus (hypocentre). It is the nearest surface point to the focus, feels the waves first and is usually the worst hit.
• Depth classes of the focus: shallow 0–70 km, intermediate 70–300 km, deep 300–700 km [6]. Shallow quakes release most of the energy.
• Bhuj 2001: epicentre about 17 km NW of Bhachau, Kachchh, Gujarat. Shallow focus (~16 km), intraplate, oblique reverse fault [5]. Kachchh is in Zone V [8].
• Tohoku 2011: offshore epicentre on the Japan Trench, Mw 9.1, focus only 29 km deep [7]. Trap: a subduction zone does not always mean a deep focus.
• Turkey 2023: strike-slip, East Anatolian fault zone, M 7.8 followed by M 7.5 [9].
• Trap: mid-ocean ridges have only shallow foci. Deep-focus quakes occur along the Pacific rim and the Alpine-Himalayan belt.

Mains Points

• Why damage is worst near the epicentre:
• A shallow focus puts the strong shaking right under the epicentre.
• Bhuj (~16 km) [5], Tohoku (29 km) [7] and Turkey 2023 (shallow) [9] all had shallow foci, and all caused heavy damage.

• So zoning, building codes and relief planning should focus on the area around likely epicentres (GS-I physical geography; GS-III disaster management).

• Intraplate epicentres are a risk too:

• Bhuj 2001 had its epicentre inside the Indian plate, away from the Himalayan plate edge [5].

• This shows that Peninsular India is not quake-free. Old faults like those in Kachchh need to be covered by building codes and zoning (Kachchh: Zone V [8]).

• Offshore epicentres bring linked hazards:

• A thrust quake under the sea can lift the sea floor and start a tsunami. At Tohoku, ~40 m waves reached the coast within 30 minutes [10].
• This is why tsunami warning systems are needed, such as INCOIS's early warning centre for the Indian Ocean, along with coastal preparedness.

Related concepts

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

Read more

• Why the earth shakes: faults, focus and epicentre