Where earthquakes strike: global belts, the Himalaya and recent great earthquakes

Detail

1. Earthquakes are not random: they follow belts

• On a world map of earthquake epicentres, the dots fall in narrow lines. They do not spread evenly. (Epicentre: the point on the surface directly above the place where the quake starts.)
• These lines mostly follow plate boundaries. Plate boundaries are the edges where the Earth's rigid plates meet, pull apart or slide past each other.
• NCERT shows this pattern in two figures:
• Class 11, "Distribution of Earthquakes and Volcanoes", Figure 4.2

• Class 9, Fig. 2.4

• There are three main belts:

Belt	Where it runs	Plate setting	Key point
Mid-Atlantic–Indian Ocean line	Down the middle of the Atlantic Ocean, then into the Indian Ocean. South of India it splits into two: one branch goes to East Africa, and the other joins a line from Myanmar to New Guinea	Mostly plates moving apart along mid-ocean ridges	Shallow quakes along the ocean floor
Alpine-Himalayan belt	Alps → Turkey → Iran → Himalayas → Myanmar → Indonesia	The African, Arabian and Indian plates push into the Eurasian plate	Quakes are often deep-seated (they start far below the surface)
Circum-Pacific belt (rim of the Pacific)	A ring around the edges of the Pacific Ocean: the Andes, North America, Japan, the Philippines, New Zealand	Ocean plates sinking under other plates (subduction)	Releases most of the world's seismic energy. Also called the Ring of Fire (see Section 11)

2. The Alpine-Himalayan belt

• Alpine-Himalayan belt (definition): a long, east–west belt of young fold mountains and frequent earthquakes. It stretches from the Alps in Europe, through Turkey and Iran, along the Himalayas, and then turns south through Myanmar to Indonesia.
• How the belt forms:
• The African, Arabian and Indian plates move north.
• They push into the Eurasian plate.

• The crust is squeezed, folded and broken, so stress builds up on faults and is released as earthquakes.

• India's place in the belt: the Indian plate keeps pushing under Eurasia. This is why the Himalayas, the North-East and the Andaman–Nicobar arc are India's most quake-prone areas.

• Both the Turkey-Syria earthquake (2023) and the Myanmar earthquake (2025) happened inside this belt (see Part 4).

3. Himalayan seismicity and the seismic gap

3.1 Why the Himalaya shakes

• Cause → process → result:
• The Indian plate slides north, under the Eurasian plate.
• The main fault between them is the Main Himalayan Thrust (MHT). A thrust is a fault where one block of rock is pushed up and over another. Along much of the MHT the two sides stay "locked", so strain keeps building.

• When the stored strain becomes too great, the locked part suddenly slips, and a great earthquake follows.

• Studies of the 2015 aftershocks show that the MHT is not one simple surface. It has two north-dipping, almost flat fault planes joined by a set of stacked thrust faults. Geologists call this structure a duplex [9].

3.2 Great Himalayan earthquakes (west to east by place, oldest to newest by date)

Earthquake	Date	Location	Magnitude	Key facts
Kangra	1905	Himachal Pradesh (western Himalaya)	Great quake	Marks the western edge of the central seismic gap
Bihar-Nepal	1934	Nepal–Bihar border (eastern Nepal)	Great quake	Marks the eastern edge of the central seismic gap
Assam-Tibet	15 August 1950	Arunachal Pradesh (then Assam) – Tibet border, eastern Himalaya	about M 8.6	Strongest ever recorded on land
Gorkha	25 April 2015	Central Nepal	Mw 7.8	About 9,000 dead; thrust on the MHT

3.3 Assam-Tibet earthquake (1950)

• Assam-Tibet earthquake (1950): a great earthquake on 15 August 1950 (India's Independence Day) in the far eastern Himalaya, at about M 8.6.
• It is the largest earthquake ever recorded on land. The USGS calls it the largest instrumentally recorded Himalayan earthquake and places it in present-day Arunachal Pradesh [7].
• Chain of effects:
• Very strong shaking loosened mountain slopes, causing huge landslides.
• The landslides blocked rivers in Arunachal Pradesh and Assam, forming temporary lakes behind the debris.

• These natural dams later broke, and the water rushed downstream as floods in Assam.

• The shaking also caused sand blows (sand and water forced up through the ground) and ground cracks over a wide area [7].

3.4 Gorkha earthquake (2015)

• Gorkha earthquake (2015): struck central Nepal on 25 April 2015, at Mw 7.8.
• It killed about 9,000 people and injured more than 22,000 [8].

• It set off avalanches on Mount Everest.

• Fault type: thrust. One block of crust was pushed up over another along the Main Himalayan Thrust.

• How the rupture moved:
• It started north-west of Kathmandu, at the lower edge of the locked part of the MHT [8].
• From there it tore eastward for about 140 km, "unzipping" the bottom edge of the locked zone [8].
• Only part of the fault slipped. Much of the MHT is still locked and could slip in a future quake [8].

3.5 Seismic gap

• Seismic gap (definition): a stretch of an active fault that has had no big earthquake for a long time. Strain keeps building there, so a large quake is thought likely in the future.
• Standard example: the "central seismic gap"
• Where: between the 1905 Kangra rupture in the west and the 1934 Bihar-Nepal rupture in the east, covering Uttarakhand and western Nepal.
• Why it matters: no great quake has broken this stretch in recorded history, so it may hold enough strain for a very large quake.

• Who is at risk: hill towns, dams and pilgrimage routes in Uttarakhand, as well as the densely populated Ganga plain to the south.

• Link to Gorkha: the 2015 rupture filled only part of the central Himalaya, and the fault next to it is still locked [8].

4. Recent great earthquakes beyond the Himalaya

4.1 Subduction-zone megaquakes (thrust)

• Sumatra-Andaman earthquake: 26 December 2004, about Mw 9.1.
• The Indian plate slipped under the Burma microplate along a long stretch off Sumatra and the Andaman–Nicobar Islands.

• The seabed was pushed up suddenly, which caused the Indian Ocean tsunami. The tsunami struck Indonesia, Sri Lanka, India (Tamil Nadu, Andaman and Nicobar) and Thailand.

• Tohoku earthquake, Japan: 11 March 2011, Mw 9.0–9.1.

• Thrust quake: the Pacific plate is pushed under north-eastern Japan (circum-Pacific belt).
• Chain of effects:
  • The seabed rose, causing a huge tsunami.
  • The tsunami flooded the coast and knocked out power and cooling at the Fukushima nuclear plant.
  • The result was the Fukushima nuclear accident.

4.2 Continental strike-slip quakes

• Turkey-Syria earthquake (2023): 6 February 2023, Mw 7.8 on the East Anatolian Fault, followed hours later by a second quake of Mw 7.5.
• More than 50,000 people died in south-east Türkiye and northern Syria.
• Setting: the Arabian plate pushes north into Eurasia, squeezing the Anatolian block sideways (Alpine-Himalayan belt).

• India's response: Operation Dost ("dost" means friend):

  • Two NDRF teams were sent for search and rescue in Türkiye [5]. (NDRF: National Disaster Response Force.)
  • In total, 295 NDRF personnel, 46.5 tonnes of relief material and five sniffer dogs were flown in by C-130J, C-17 and IL-76 aircraft [4].
  • An Indian Army field hospital was set up at Iskenderun, Hatay Province [4].
  • More than 6 tonnes of emergency relief was flown to Damascus, Syria on an IAF C-130J [6].

• Myanmar earthquake (2025): 28 March 2025, Mw 7.7 on the Sagaing Fault near Mandalay.

• Sagaing Fault: a strike-slip fault. The two blocks slide past each other sideways, not up or down. It runs north–south through central Myanmar and takes up the sideways movement between the Indian and Eurasian (Burma) plates.
• India's response: Operation Brahma:
  • The Indian Army sent a 118-member team from the Shatrujeet Brigade Medical Responders and set up a 60-bed Medical Treatment Centre [2].
  • Navy ships INS Satpura and INS Savitri (Eastern Naval Command) sailed for Yangon on 29 March 2025 [3].
  • Overall, the field hospital treated over 2,500 injured people within two weeks. Six aircraft and five naval ships delivered 750 MT of relief material [4].

5. Using these quakes to learn fault types

Fault type	How the blocks move	Typical setting	Examples
Thrust (reverse) fault	One block is pushed up and over the other because the crust is squeezed	Plates colliding; subduction zones	Gorkha 2015 (Main Himalayan Thrust), Tohoku 2011, Sumatra-Andaman 2004
Strike-slip fault	Blocks slide sideways past each other, with little up–down movement	Transform-type boundaries inside collision zones	Turkey-Syria 2023 (East Anatolian Fault), Myanmar 2025 (Sagaing Fault)

• Tsunami link: a thrust quake under the sea lifts the seabed and the water above it, so it can cause a tsunami (2004, 2011). A strike-slip quake on land mainly causes strong shaking and building collapse (2023, 2025).

Prelims Hooks

• The mid-Atlantic earthquake line continues into the Indian Ocean and splits south of India: one branch goes to East Africa, the other joins the Myanmar–New Guinea line.
• Alpine-Himalayan belt order, west to east: Alps → Turkey → Iran → Himalayas → Myanmar → Indonesia. Plates pushing into Eurasia: African, Arabian, Indian.
• The circum-Pacific belt releases the most seismic energy. The Alpine-Himalayan belt comes second.
• Assam-Tibet earthquake: 15 August 1950, about M 8.6, the largest recorded on land. It was in present-day Arunachal Pradesh [7].
• Central seismic gap = between Kangra 1905 and Bihar-Nepal 1934 → Uttarakhand and western Nepal.
• Gorkha 2015 (Mw 7.8) = thrust on the Main Himalayan Thrust. The rupture moved about 140 km eastward from north-west of Kathmandu [8].
• Trap: Turkey 2023 and Myanmar 2025 were strike-slip, not thrust. Tohoku 2011 and Gorkha 2015 were thrust.
• Match: Operation Dost → Türkiye–Syria (2023); Operation Brahma → Myanmar (2025).
• Match: East Anatolian Fault → Türkiye; Sagaing Fault → Myanmar (near Mandalay).
• Tohoku 2011 → tsunami + Fukushima nuclear accident. Sumatra-Andaman 2004 (about Mw 9.1) → Indian Ocean tsunami.

Mains Points

• Distribution pattern (GS-I): earthquakes are concentrated along plate boundaries in three belts: mid-ocean ridges, the Alpine-Himalayan collision belt and the circum-Pacific subduction belt. India is exposed because the Indian plate keeps pushing into Eurasia, which makes the Himalayas, the North-East and the Andaman–Nicobar arc high-risk zones.
• Himalayan hazard and the seismic gap (GS-I/III): the Main Himalayan Thrust stays locked and stores strain. The 2015 Gorkha quake released only part of it [8], and the central seismic gap (Uttarakhand–western Nepal) has had no great quake since before 1905. Fast-growing hill towns, hydropower dams and dense Ganga-plain cities therefore need strict building codes, careful land-use zoning and good preparedness.
• Cascading disasters (GS-III): a single quake can set off a chain of further disasters. These include landslide dams that later flood the plains (Assam 1950), tsunamis (2004, 2011), damage to nuclear plants (Fukushima 2011) and the mass collapse of non-engineered buildings (Türkiye 2023). Disaster planning should treat these as linked risks, not separate ones.
• HADR diplomacy (GS-II/III): under Operation Dost (2023) and Operation Brahma (2025), India quickly deployed NDRF teams, Army field hospitals, IAF aircraft and Navy ships [2][4][5]. These operations show India acting as a first responder in its region under its "Neighbourhood First" policy. (HADR: Humanitarian Assistance and Disaster Relief.)