Tsunami — Generation, Warning, and Impact

5.1 What is a Tsunami?

Tsunami (Japanese: "harbour wave") — a series of ocean waves with very long wavelengths (100–500 km) and periods (10–60 minutes) generated by a sudden large-scale disturbance of the ocean.

5.2 Causes of Tsunamis

1. Submarine earthquakes (~80% of all tsunamis) — vertical fault displacement raises or lowers ocean floor, displacing water column
2. Submarine landslides — large mass movements on ocean floor (e.g., Storegga Slide ~8,150 BP caused a mega-tsunami that swept Britain)
3. Volcanic eruptions — collapse of volcanic islands or submarine eruptions (Krakatoa 1883, Tonga 2022)
4. Asteroid impacts — rare; prehistoric mega-tsunamis from impacts

5.3 Tsunami Characteristics

• In open ocean: wave height ~1 m, but wavelength 100–500 km, speed 700–900 km/h (near jet aircraft speed)
• At shore: wave slows (energy compressed) → height amplifies dramatically to 10–30+ m
• Can travel across entire Pacific Ocean in 20–22 hours

5.4 The 2004 Indian Ocean Tsunami

Triggered by Sunda Megathrust rupture off northern Sumatra (Mw 9.1–9.3, 26 December 2004).

• 14 countries affected; 227,898 deaths (world's deadliest tsunami)
• Deaths by country: Indonesia (167,540), Sri Lanka (35,399), India (16,389 — mainly Tamil Nadu and Andaman), Thailand (8,212)
• Direct result: Indian Ocean Tsunami Warning System (IOTWS) established 2006 — covers 28 countries
• INCOIS (Indian National Centre for Ocean Information Services, Hyderabad) is India's tsunami warning centre

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