Earthquakes for UPSC: Causes, Waves and Seismic Zones

By Dooit UPSC expert · 26 October 2026 · 11 min read

An earthquake is the sudden shaking of the ground caused by the release of energy stored in the Earth's crust, usually when rocks along a fault slip past one another. For UPSC, you need three things cold: why they happen, how the energy travels and is measured, and where India is most vulnerable. Get those right and the questions answer themselves.

In three decades of teaching Geography to aspirants, I have watched this topic trip up candidates who treat it as rote. They memorise "P-wave, S-wave, Richter scale" and then freeze when the question asks why the P-wave shadow zone exists, or which zone the Kutch region falls in. Earthquakes are not a list to be learnt; they are a small, elegant system. Once you see the logic that runs from plate movement to ground shaking to seismic zoning, you will never need to cram it again. Let us build that logic, piece by piece.

What exactly is an earthquake?

The Earth's outer shell is not one solid piece. It is broken into large plates that are always moving, grinding and colliding. As they move, stress accumulates in the rocks along the boundaries and along cracks called faults. Rocks can store this strain energy only up to a point. When that limit is crossed, the rock snaps and slips, and the stored energy is released in waves that radiate outward in all directions. That radiating energy, felt at the surface as shaking, is the earthquake.

Two terms are tested almost every year, so fix them firmly. The focus (also called the hypocentre) is the point inside the Earth where the rupture starts. The epicentre is the point on the surface directly above the focus. The shaking is generally strongest near the epicentre, because that is where the waves arrive first and with the least energy lost on the way up. A quake with a shallow focus does far more surface damage than a deep one of the same magnitude, which is exactly why shallow Himalayan tremors are so feared.

Why earthquakes happen: the causes

The overwhelming majority of earthquakes are tectonic — the direct result of plate movement and faulting. If you understand the three types of plate boundaries, you already understand most of the world's seismicity. This is why this topic sits right next to plate tectonics and continental drift in the syllabus; never study one without revising the other.

• Tectonic earthquakes — caused by the sudden slip of rocks along fault lines at plate boundaries. These are the most common and the most destructive. The Himalayan and Kutch earthquakes belong here.
• Volcanic earthquakes — triggered by the movement of magma beneath an active volcano before or during an eruption. They are usually local and limited in reach.
• Collapse earthquakes — minor tremors produced by the collapse of the roofs of underground mines or caverns, felt only in the immediate vicinity.
• Explosion earthquakes — caused by the detonation of chemical or nuclear devices, used in fact to monitor clandestine nuclear tests.
• Reservoir-induced earthquakes — triggered when the enormous weight of water in a large dam reservoir stresses the rocks below. The 1967 Koynanagar earthquake in Maharashtra is the classic Indian example and a favourite of examiners.

Notice the pattern in that list. The first type is global and catastrophic; the rest are increasingly local and human-influenced. UPSC loves the human-induced ones — reservoir-induced seismicity in particular — because they link physical geography to dams, development and environmental impact assessment. Keep Koyna on the tip of your tongue.

Seismic waves: how the energy travels

When the focus ruptures, energy leaves it as seismic waves. These waves are the single most important concept in the whole topic, because they reveal both the damage potential of a quake and the hidden structure of the Earth's interior. There are two broad families: body waves, which travel through the interior, and surface waves, which travel along the crust.

Three facts from this table are heavily tested. First, P-waves are the fastest and reach seismograph stations first, which is why they are called "primary". Second, S-waves cannot pass through liquids — a fact that turns out to be one of the most important clues we have about the planet's core. Third, it is the slow surface waves, not the fast body waves, that flatten buildings; speed and destructiveness run in opposite directions here, and candidates frequently get this backwards under exam pressure.

Shadow zones: how earthquakes reveal the Earth's interior

Here is the piece that separates a prepared candidate from a crammer. Because S-waves cannot travel through liquids, they are completely blocked by the Earth's liquid outer core. P-waves can pass through liquids, but they bend sharply when they cross from the solid mantle into the liquid core. The result is a set of shadow zones on the far side of the Earth where certain waves simply do not arrive.

The S-wave shadow zone is the entire area beyond about 105° from the epicentre — no S-waves are recorded there at all, proving the outer core is liquid. The P-wave shadow zone is a narrower band, roughly between 105° and 145° from the epicentre, created by the bending of P-waves at the core boundary. This is not trivia; it is the very evidence that told geophysicists the Earth has a liquid outer core and a solid inner core without anyone ever drilling down to see it. If a Prelims question asks what the existence of the S-wave shadow zone proves, the answer is a liquid outer core.

Measuring earthquakes: magnitude versus intensity

Aspirants routinely confuse these two, and a single sloppy sentence in a Mains answer can cost marks. They measure different things.

The Richter scale, devised in 1935, measures magnitude on a logarithmic scale, so each whole number is about ten times greater in amplitude and roughly 32 times more energy than the one below. A magnitude 7 quake is not slightly worse than a 6; it releases about 32 times more energy. For large earthquakes, seismologists today prefer the moment magnitude scale (Mw), which is more reliable for very powerful events because it is based on the actual seismic moment — the rigidity of the rock, the area of the fault that slipped, and how far it slipped. Intensity scales like the Modified Mercalli or the MSK scale, by contrast, describe the felt severity in Roman numerals from I to XII, and the same earthquake will register different intensities in different towns depending on distance, soil and construction quality.

Seismic zones of India

This is the part of the topic most tied to current affairs and disaster management, so it rewards precise knowledge. India is one of the most earthquake-prone countries in the world. According to the National Disaster Management Authority, about 59 per cent of India's land area is vulnerable to moderate or severe earthquakes. The Bureau of Indian Standards, through the code IS 1893, divides the country into four seismic zones based on expected intensity.

Zone V is the danger zone, and the logic behind it is pure plate tectonics. The Himalayas and the Northeast lie along the active collision boundary where the Indian plate drives relentlessly into the Eurasian plate; the strain released there produces the country's most frequent and most violent quakes. Kutch is the odd one out — it is far from the plate boundary yet sits in Zone V because of a major intra-plate fault system, as the devastating Bhuj earthquake of 2001 reminded the nation. Peninsular India, old and stable, mostly sits in the calmer Zones II and III.

Effects and secondary hazards

An earthquake rarely does its damage alone. For Mains, you must be able to list the chain of secondary hazards it can trigger, because the disaster-management questions almost always ask about cascading effects rather than the shaking itself:

• Ground shaking and building collapse — the direct and biggest killer, worsened by poor construction.
• Tsunamis — when the focus lies under the ocean floor, as in the 2004 Indian Ocean tsunami that struck the Indian coast and the Andaman Islands.
• Liquefaction — water-saturated sandy soil temporarily behaves like a liquid, and buildings sink or tilt. The Indo-Gangetic plains are especially prone to this.
• Landslides and avalanches — common in the steep, unstable Himalayan terrain.
• Fires, dam failures and disease — ruptured gas lines, cracked dams and contaminated water in the aftermath.

Earthquake preparedness and mitigation

You cannot predict an earthquake reliably, and no serious examiner expects you to claim otherwise. What you can do is reduce the damage, and this is where the governance angle enters. The practical mitigation measures worth writing in a Mains answer are concrete, not vague: enforcing earthquake-resistant building codes, especially in Zone IV and V cities; retrofitting old public buildings such as schools and hospitals; seismic micro-zonation of large cities to map street-level risk; community awareness and mock drills; and strengthening early-warning and rapid-response systems. India's institutional backbone here is the National Disaster Management Authority at the apex and the National Center for Seismology, which monitors earthquakes across the country.

If you want to see how this physical topic connects upward into policy and response, pair it with our guide on the disaster management strategy for GS Paper 3. The physical geography of earthquakes is the foundation; disaster management is the storey built on top of it, and the best answers move smoothly between the two.

How earthquakes are asked in UPSC

In Prelims, expect crisp factual questions: the type of wave that cannot pass through the liquid outer core, what a shadow zone proves, which zone a named region falls in, or matching a famous earthquake to its cause. These are pure recall, and you cannot afford to lose them. In Mains GS Paper 1, the questions turn conceptual — the distribution of earthquakes and their relationship to plate boundaries — while GS Paper 3 frames the same topic through disaster management and mitigation. The candidate who has understood the single thread running from plate movement to seismic zoning can write confidently across all three.