Tsunamis

1. Tsunamis

2. What is a tsunami ? • A tsunami is a very long ocean wave generated by sudden displacement of the sea floor or of the oceanic mass • The displacement of an equivalent volume of water generates the tsunami

3. Tsunami Terminology • The term “tsunami” is a Japanese word meaning “harbour wave” • It was so named because the wave is harmless until it enters a harbour • It is frequently called a “tidal wave”, but it has nothing to do with tides

4. Structure of a wave • Wavelength, , can exceed 200 km • normal ocean waves have wavelengths of about 100 m • trough; peak; wave height, h; amplitude From Murck et al. (1996)

5. Velocities in deep water • Tsunamis travel very quickly relative to normal ocean waves • This is particularly the case in open water, where velocities increase with water depth • Velocities can reach 1,000 km/hr in open ocean (normal ocean wave: ~90 km/hr) • Thus, velocities are about 10 times higher for tsunamis

6. Shallow water • In shallow water, the tsunami waves pile up • As a result, velocities and wavelengths decrease... • …but at the same time, amplitudes can increase enormously...

7. Amplitudes • In deep water, wave amplitudes are generally less than 1 meter… • …but in shallow water, amplitudes can reach 40 meters or more above normal sea level

8. Tsunamis take place when a huge earthquake occurs causing the plates below the water to push up causing the water to create a huge wave. • In shallow water, the tsunami waves pile up • velocities and wavelengths decrease... • amplitudes can increase enormously...

9. Hazards and risks of tsunamis • Tsunamis can hit with little or no warning • 4,000 people have been killed between 1990 and 2000 • The most prone areas are those associated with earthquakes and volcanoes (mainly subduction zones)

10. How tsunamis are generated - all involve displacement of water • Earthquakes • Volcanic activity • Submarine landslides • Meteorite impacts

11. Tsunami generation 1. Earthquakes • Mainly vertical crustal movement caused by near-shore or off shore earthquakes • …so strike-slip faults perhaps less hazardous… • Magnitude of the earthquake is directly related to the magnitude of the tsunami

12. 75% of tsunamis are produced in the pacific ocean, most others are in the indian ocean

13. Tsunami generation Subduction association • Tsunamis typically are associated with earthquakes generated at subduction zones • Rupture of sea floor surface • Sediment slumps into subduction trench

14. Tsunami Generation

15. 1990-2000

16. Locally-generated tsunamis • The subduction zone of Cascadia has potential for very large offshore quakes (M  8) • There is a great danger of locally-generated tsunamis here, since they travel so fast • Many large cities are found on the coast

17. The Boxing Day Tsunami Banda Aceh 26-12-2004

18. Tsunami Wave

19. Tsunami waves approach Phuket, Thailand two hours after the Banda Ace earthquake

20. Tsunami waves approach Phuket, Thailand

21. Tsunami wave strikes coastal city

22. Inundation

23. Devastation

24. Banda Aceh before the Tsunami

25. Banda Aceh after the Tsunami

26. Tsunami generation 2. Volcanic activity • Displacement of rock • Submarine caldera collapse (e.g., along faults) (Krakatau 1883) • Entrance of pyroclastic flows into water (Krakatau 1883) • Subaerial lateral collapse, generating debris avalanches which enter water (Unzen 1792)

27. The eruption of Krakatau 1883 • Krakatau is a volcano located between Java and Sumatra • It is mainly a submarine volcano, with its top sticking out of the water

28. Caldera collapse • The cataclysmic eruption occurred on 26-27 August 1883 • A submarine caldera was formed • Displacement of material during collapse generated a series of devastating tsunamis

29. Two views of the caldera margin on Rakata, one soon after the eruption and the other in 1979

30. This is Anak Krakatau, which emerged through the sea in 1928. It is within the caldera

31. Tsunami • 36,000 people were killed by the tsunami along the coasts of Java and Sumatra • At least 3 great waves occurred • 165 coastal villages were destroyed by the waves • The largest waves were recorded by tide gauges up to 7,000 km away on the Arabian Peninsula

32. Tsunami • Coral blocks up to 600 tons were carried inland… • …these served efficiently as natural battering rams • Runup heights reached 40 meters

33. Maximum runup heights in meters (from Simkin and Fiske, 1983) Telok Betong

34. Telok Betong From Simkin and Fiske (1983) Before... …and after Shaded grey is submerged area red=boat yellow=buoy blue=hill buoy hill

35. The District Hall in Telok Betong. The tsunami stopped just before this building, sparing the people cowering inside

36. The hill near Telok Betong. The lower part of the hill has been cleansed of its vegetation by the tsunami

37. Boat runup…the Berouw... • This boat, named the Berouw, was carried 2.5 km inland at Telok Betong by the wave, which reached 24 m in height

38. …and inland emplacement of its mooring buoy • This is the Berouw’s mooring buoy, also carried inland • It is now a visually pleasing monument overlooking Telok Betong

39. Tsunami generation 3. Landslides • Landslides often are generated by quakes or volcanoes • also occur on subduction trench slopes (steep) • also can occur in enclosed bodies of water (lakes, bays, reservoirs, etc.) (rockfalls, slumps of unconsolidated material, etc.) • Enormous submarine landslides can occur on the flanks of ocean islands (e.g., Hawaii, Canaries)

40. Tsunami generation Tsunami generated from landsliding -Phase 1 material slides, -phase 2 water is sucked down, -phase 3 water rebounds and -phase 4 Tsunami is generated

41. Unzen volcano, Japan: 1792 collapse of Mt. Mayuyama • In addition to its recent lava dome and pyroclastic flow activity (1990-1995), the volcano also has collapsed catastrophically in the past

42. Mt. Mayuyama islands scar Pyroclastic debris, 1991-1995

43. The 21 May 1792 collapse • A debris avalanche occurred from Mt. Mayuyama in 1792 about 1 month after lava stopped flowing from Fugen-dake (site of recent activity) • The avalanche was triggered by two quakes Fugen-dake Mt. Mayuyama

44. Tsunamis Geological map, showing 1792 debris avalanche deposit • The debris avalanche entered the Ariake Sea, generating a tsunami • The wave killed between 14,000 and 15,000 people in coastal communities

45. The debris avalanche deposit • Extent of the 1792 debris avalanche deposit and the scar on Mt. Mayuyama • Note the islands From Siebert et al. (1987)

46. An artist’s rendition of the 1792 events scar deposit New islands Before... …and after

47. Tsunami generation 4. Meteroite Impact Too terrible to contemplate !!! Hundreds to thousands of meters in height ? Terminal Cretaceous event Read and find out!

48. Tsunami hazards • Extensive flooding • Action of wave on coastal structures, both natural and built • The incredible force of the waves can remobilize huge objects The event may create drawdown

49. Effects of tsunami drawdown • Release of dissolved gases (CH4, CO2, H2S) previously contained in shallow sediments • Potential ignition of gases by their rapid expulsion • As a result, a wave of noxious and burning gases may engulf people BEFORE the wall of water arrives

50. Mitigation efforts