Tsunamis
Megathrust Genesis · Shallow-Water Wave Physics · Shoaling & Drawback · DART & INCOIS Warning (28 Micro-Realms)
From Abyssal Seabed Piston to Coastal Wall of Water
A Tsunami (Japanese for "Harbour Wave") is a series of long-wavelength seismic sea waves generated when a vertical submarine displacement—most commonly a megathrust subduction earthquake—lifts the entire overlying water column. Racing across the deep ocean at jet-plane speeds ($700\text{–}800\text{ km/h}$) with an imperceptible height under $1\text{ m}$, the wave undergoes dramatic Shoaling over shallow continental shelves, compressing its wavelength and surging upward into a devastating coastal bore.
I. Tsunami Genesis & Submarine Displacement Triggers
"Tsu-nami" (Harbour Wave)
- Japanese: Tsu (harbour) + Nami (wave)
- Long-period gravity wave train
- Unrelated to lunar/solar tides!
Megathrust Dip-Slip Quakes
- #1 cause (~80% of all tsunamis)
- Convergent subduction trenches
- Requires shallow focus + $M_w > 7.0$
Strike-Slip Non-Tsunami Rule
- Transform faults slide sideways
- Zero vertical seafloor uplift
- 2012 Sumatra M8.6 caused no tsunami
Water Column Piston Lift
- Locked upper plate snaps upward
- Displaces billions of tons of water
- Gravity collapses dome outward
Submarine Landslides
- Continental slope turbidite slumps
- Storegga Slide & Grand Banks (1929)
- Lituya Bay 1958 (524-m fjord run-up)
Volcanic Tsunamis
- Caldera collapse & pyroclastic surge
- Krakatoa (1883) & Anak Krakatau (2018)
- Hunga Tonga (2022) pressure-wave
Cosmogenic & Meteotsunamis
- Asteroid/bolide ocean impacts
- Chicxulub (66 Ma) km-high waves
- Meteotsunami: Barometric squall line
II. Deep-Ocean Wave Physics vs. Shallow-Water Shoaling
Shallow-Water Wave Law
- Wavelength $\gg 20 \times$ ocean depth
- Behaves as a "shallow-water wave"
- Speed depends ONLY on depth ($d$)
700–800 km/h Jet Speed
- At $4,000\text{ m}$ abyssal depth:
- $c = \sqrt{9.8 \times 4000} \approx 198\text{ m/s}$
- Equals $\sim 713\text{ km/h}$ (jet airliner)
200+ km Wavelength ($\lambda$)
- Crest-to-crest $\lambda = 100\text{–}500\text{ km}$
- Wave period $T = 10\text{ to }60\text{ minutes}$
- Minimal energy loss across oceans
<1 m Open-Ocean Stealth
- Deep-sea height only $0.3\text{ to }1.0\text{ m}$
- Spread over a 200-km base
- Ships at sea cannot feel it pass!
Full-Column Water Motion
DEPTH
- Wind waves stir top $100\text{ m}$ only
- Tsunami moves ENTIRE water column
- From surface down to $4,000\text{ m}$ floor
The Shoaling Brake
- Enters shallow continental shelf
- Depth ($d$) drops $\to$ speed ($c$) drops
- Slows to $30\text{–}50\text{ km/h}$ near shore
Amplitude Surge (Green's Law)
- Kinetic energy converts to potential
- Wavelength compresses ($500 \to 20\text{ km}$)
- Wave height ($H \propto d^{-1/4}$) skyrockets!
III. Coastal Arrival, Shoreline Drawback & Destructive Dynamics
Shoreline Drawback Warning
- When wave trough arrives first
- Sea recedes hundreds of metres
- Exposes fish/seabed 5–10 min before hit
Multi-Crest Wave Train
- Not a single solitary wave
- Series of crests 15–60 min apart
- 2nd or 3rd wave is often largest!
Harbour Funneling Effect
- V-shaped bays & narrow inlets
- Concentrate wave energy laterally
- Amplifies bore height (Sanriku coast)
Run-up vs. Inundation
- Run-up: Max vertical elevation above sea
- Inundation: Horizontal inland distance
- Can flood 5–10 km inland on flat plains
Turbulent Bore & Backwash
- Advances as a churning fast-rising flood
- Loaded with cars, boats, masonry
- Receding backwash sweeps victims out
Island Wave Diffraction
- Waves bend around island flanks
- Wraps into leeward "shadow" side
- Hit western Sri Lanka & Kerala in 2004
Mangrove & Coral Buffers
- Coral reefs break offshore energy
- Dense mangrove roots absorb drag
- Pichavaram (TN) shielded villages in 2004
IV. Historic Mega-Tsunamis, DART Buoys & Indian Warning Architecture
2004 Indian Ocean Tsunami
SUMATRA–ANDAMAN
- 1,300-km Sunda megathrust rupture
- 30-m waves in Banda Aceh
- Hit Tamil Nadu, AP, Kerala & Andaman
Indira Point Submergence
- India's southernmost point ($6^\circ 45'\text{N}$)
- Great Nicobar subsided by $4.25\text{ m}$
- Base of lighthouse submerged in sea
2011 Tōhoku Tsunami
- Japan Trench plate-boundary slip
- Overtopped seawalls across Sendai
- Triggered Fukushima nuclear crisis
Makran Zone (West Coast)
- Arabian plate subducts under Eurasia
- 1945 Makran $M_w\ 8.1$ tsunami hit Mumbai/Kutch
- Dual East + West coast Indian threat
DART Buoy Architecture
- Deep-ocean Assessment & Reporting
- Seabed Bottom Pressure Recorder (BPR)
- Acoustic link to satellite surface buoy
ITEWC (INCOIS Hyderabad)
10-MIN BULLETIN
- Established Oct 2007 under MoES
- 24/7 monitoring of Indian Ocean
- UNESCO Regional Tsunami Provider
UNESCO "Tsunami Ready"
NOLIASAHI (ODISHA)
- Venkatraipur & Noliasahi (Odisha)
- 1st Indian Ocean "Tsunami Ready" villages
- NDMA coastal bio-shields & sirens
Core Distinctions
• In the Deep Ocean ($4,000\text{ m}$): Extremely long wavelength ($100\text{–}500\text{ km}$), extreme speed ($700\text{–}800\text{ km/h}$), and very small wave height ($<1\text{ m}$) — making it virtually undetectable to ships.
• Near the Shallow Coast (Shoaling): Friction slows the wave drastically ($<40\text{ km/h}$), compressing the wavelength ($<20\text{ km}$) and forcing the water upward into a $10\text{–}30\text{ m+}$ destructive wall while wave period ($T$) remains constant.
Wind Waves have short wavelengths ($10\text{–}200\text{ m}$), short periods ($5\text{–}20\text{ sec}$), and disturb only the upper $100\text{ m}$ of the sea. A Tsunami has a wavelength of数百 km, a period of $10\text{–}60\text{ minutes}$, and sets the entire water column in motion from surface to abyssal floor.
3. India's Two Tsunamigenic Source Zones1) Eastern Margin: The Andaman–Nicobar–Sumatra (Sunda) Subduction Trench in the Bay of Bengal (2004 event). 2) Western Margin: The Makran Subduction Zone off Balochistan/Iran in the northern Arabian Sea (threatens Gujarat, Maharashtra, and western coast).
World E Complete Synthesis
Tsunami Transformation & Warning Matrix