Last reviewed 4 Nov 2026 · 3 min read
1. Earthquakes
| Point | Fact |
|---|---|
| Cause | Sudden release of strain energy along faults, mostly at plate boundaries; India's plate pushing into Eurasia drives Himalayan earthquakes |
| Focus / epicentre | Point of origin underground / point on the surface directly above |
| Magnitude | Energy released — Richter or moment magnitude scale (one unit up ≈ 32 times more energy) |
| Intensity | Effects at a place — Modified Mercalli or MSK scale (I–XII) |
| Seismic zones (IS 1893 (Part 1): 2016 map) | II, III, IV, V — Zone V is the most severe (North-East, Kashmir, Himachal, Uttarakhand parts, Kutch, Andaman & Nicobar) |
| Effects | Building collapse, landslides, liquefaction, fires, tsunamis |
| Examples | Bhuj 2001 (Gujarat), Latur 1993, Uttarkashi 1991, Sikkim 2011, Nepal 2015 |
2. Tropical cyclones
- Form over warm oceans (sea-surface temperature above about 26.5–27 °C), with Coriolis force (not near the equator), moist air and low wind shear.
- Rotate anticlockwise in the Northern Hemisphere; calm eye at the centre, strongest winds in the eye wall.
- India's main seasons: pre-monsoon (April–May) and post-monsoon (October–December) — the Bay of Bengal sees more and stronger cyclones than the Arabian Sea.
- Hazards: strong winds, very heavy rain, and storm surge — the biggest killer on flat coasts.
- IMD categories (by maximum sustained wind): Depression → Deep Depression → Cyclonic Storm → Severe → Very Severe → Extremely Severe → Super Cyclonic Storm (≥ 222 km/h).
- Names for the north Indian Ocean come from a list contributed by member countries of the WMO/ESCAP Panel.
- Examples: Odisha super cyclone 1999, Phailin 2013, Hudhud 2014 (Visakhapatnam), Fani 2019, Amphan 2020.