Tsunamis What is a tsunami A tsunami is

Tsunamis

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

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

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)

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

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. . .

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

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. z. In shallow water, the tsunami waves pile up zvelocities and wavelengths decrease. . . zamplitudes can increase enormously. . .

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)

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

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

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

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

Tsunami Generation

1990 -2000

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

The Boxing Day Tsunami Banda Aceh 26 -12 -2004

Tsunami Wave

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

Tsunami waves approach Phuket, Thailand

Tsunami wave strikes coastal city

Inundation

Devastation

Banda Aceh before the Tsunami

Banda Aceh after the Tsunami

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)

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

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

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

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

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

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

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

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

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

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

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

…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

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)

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

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

Mt. Mayuyama scar Pyroclastic debris, 1991 -1995 islands

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) Fugen-dake • The avalanche was triggered by two quakes Mt. Mayuyama

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

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

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

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

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

Effects of tsunami drawdown • Release of dissolved gases (CH 4, CO 2, H 2 S) 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

Mitigation efforts

Warning times • Every ~750 km of travel distance is equal to about 1 hour of warning time • So, there is very little warning time for tsunamis generated by local sources, compared to those from distant sources

Quake-generated tsunamis • In general, the size of the quake is an approximate indication of the size of the tsunami • But this guide doesn’t always work

Hawaii is particularly vulnerable, being in the middle of the Pacific

Warning systems • Mainly based on earthquake data • Pacific-wide warnings: require at least 1 hour warning time • More local networks require warning times less than 1 hour…this is difficult

A proposed system of real-time detectors NOAA=National Oceanic and Atmospheric Administration -

Response to tsunamis • Requires good emergency planning and preparation… • …an educated and trained public… • …which has access to information… • …so the dissemination of this info needs to be efficient and reliable

Personal mitigation • Run (don’t walk) to higher ground • Tell your family and friends • Never go to the beach to watch tsunamis

Sign in the lobby of a Hawaiian hotel: • IN CASE OF TSUNAMI: – Remain calm – Pay your bill – Run like hell

Response to tsunami • Requires good emergency planning and preparation… • …an educated and trained public… • …which has access to information… • …so the dissemination of this info needs to be efficient and reliable

A warning to Indonesians: Kerry Sieh’s poster and efforts to educate people beforehand

Tsunamis -reading • Billings, L. G. , 1915. Some personal experiences with earthquakes. National Geographic, v. 27, no. 1, January 1915, pp. 57 -71. • González, F. J. , 1999. Tsunami! Scientific American, May, 1999. • Niven, L. , and J. Pournelle, 1983. Lucifer’s Hammer. New York, Fawcett Crest, 629 pp. • Simkin, T. , and R. S. Fiske, eds, 1983. Krakatau 1883, the volcanic eruption and its effects. Washington, D. C. , Smithsonian Institution Press, pp. 69 -81.

Tsunamis - web • Canada: • http: //atlas. nrcan. gc. ca/site/english/maps/environment/naturalhazards/naturalhazard s 1999/tsunamis • http: //www. pep. bc. ca/hazard_preparedness/Tsunami_Preparedness_Information. html • U. S. : • http: //www. ess. washington. edu/tsunami/index. html • http: //www. tsunami. noaa. gov/ • U. K. : • http: //www. nerc-bas. ac. uk/tsunami-risks/