natural hazards tsunami

TsunamiTsunami

Prof. David AlexanderGlobal Risk Forum Davos

Tsunami - seismic sea waves

tsu'nami - "port waves"

Tsunami - seismic sea waves

tsu'nami - "port waves"

The causes: tsunamigenic forces:-

• vertical movements of the sea bed caused by submarine earthquakes

• horizontal movements of the sea bed

• violent volcanic eruptions in the sea (e.g. Krakatau 1883)

• rapid undersea rockfalls and landslides.

Causes of tsunamisSource: NGDC Archives (1994)

MeteorologicalLandslide

LS/erupt/quakeLandslide/eruptLandslide/quake

Volcanic eruptionQuake/erupt/LSQuake/eruptionQuake/landslide

Earthquake

Generation of tsunamis:-

• a very limited area of origin tends to cause smooth, dispersive waves

• a source as wide as the water is deep tends to cause beat-type oscillations

• a tsunami generated over a large area of ocean floor may cause one great wave on which smaller waves ride.

Tsunamis generated by earthquakes:-

• magnitudo > 6.5

• epicentre under the sea or on coast

• shallow hypocentre (<25 km, mainly <10 km)

• not all undersea earthquake of M>6.5 are tsunamigenic

• a sudden displacement of the sea bed must occur: water is not a compressible medium.

Phases in the existence of a tsunami:-

• generation

• movement in deep water

• movement in shallow water

• arrival at the coast ("run-up").

Velocity:-

V = (gh)g = 9.81 m/sec²h = ocean depth in metres

Velocity:-

V = (gh)g = 9.81 m/sec²h = ocean depth in metres

...if...h = 5500 m,V = 230 m/sec = 830 km/hr (in deep ocean conditions).

...if...h = 5500 m,V = 230 m/sec = 830 km/hr (in deep ocean conditions).

Tsunamis in open ocean conditions:-

• wavelength 150-250 km; occasionally up to 1000 km

• amplitude in open ocean is only a few metres

• period 10-60 minutes

• speed: that of a criusing Boeing 747• therefore, in the open ocean tsunamis travel rapidly and are invisible.

The journey of a tsunami:-

• the waves travel as a result of the propagation of energy, non water

• the water moves in a shallow ellipse

• normal, wind-driven waves affect the top layer of the ocean; tsunamis affect the entire column of water down to the sea bed• the waves lose energy as they travel• the waves become small at great distances from the point of generation.

Simulation of atsunami generatedin Prince WilliamSound, Alaska:situation after1/2, 1, 2 and4 hours of travel(verticalexaggeration:x 3 million)

Arrival at the coast (run-up):-

• the biggest risk comes from 'near field' tsunamis (those generated locally)

• up to six large waves, of which the second or third may be the biggest

• at coasts, about a third of tsunamis begin with the drawing back of the sea

• creeks and inlets cause a "funnel effect" that increases wave amplitude.

The characteristics of run-up depend on:-

• the magnitude of the displacement of the water column at generation• distance from the origin (the waves disperse their energy as they travel)• the topography and bathymetry of the sea bed around the cost (neritic zone)• the presence of islands, creeks, etc.• the refraction of the waves in shallow water: speed reduced, wave height increased (up to 40 metres).

Refraction oftsunami wavesaround theisland of Hawaii

Altezza delle onde alla costa

2000: small tsunamiof volcanic origin on

island of Stromboli

The magnitude of the tsunamiis calculated with this formula:-

M = 3,32 log HorM = log2H

where H is the amplitude of the largest wave

Seiches: regular long-period oscillationsof a large, enclosed body of water(lakes, bays, straits, lagoons, etc.)

Effects of tsunamis whenthey arrive at the coast:-Effects of tsunamis whenthey arrive at the coast:-

• hydrostatic impacts: lifting and carrying objects away

• hydrodynamic impacts: tearing structures, eroding and scouring land

• shock impact effects: battering of objects.

Hydrostatic, hydrodynamic and shockeffects on Hawaii Island

Hydrostatic, hydrodynamic and shockeffects on Hawaii Island

Tsunami damage at Seward, Alaska, 1964

Tipical damage to coastal buildings,Indonesia, 1994

In the Philippines:

• from 1603 to 1975 27 tsunamis• from 1603 to 1975 27 tsunamis

• up to 5 metres wave height• up to 5 metres wave height

• Mindanao Island particularly vulnerable• Mindanao Island particularly vulnerable

• the 1976 tsunami: 3000 dead, 8000 injured e 75,000 homeless

• the 1976 tsunami: 3000 dead, 8000 injured e 75,000 homeless

• a tsunami would arrive in 16 hours from America, 3 hrs from Japan and 10 minutes from the nearby sea bed.

• a tsunami would arrive in 16 hours from America, 3 hrs from Japan and 10 minutes from the nearby sea bed.

Banda Aceh, Indonesia, before and after

Baia di Khao Lak, Thailand, before and after

Nonlinearrelationshipsbetween tsunamimagnitude, orreturn period,and meanannual mortality.

the increment in mortalitydiminishes with increasingtsunami magnitude

Max, wave height (m)

Recurrence interval (years)

An

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ality

Mean

an

nu

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pote

nti

al m

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Monitoring and alarm systems:-

• Pacific Tsunami Warning System (PTWS): 23 nations participate in a network that consists of 69 seismic measurement stations and 65 tide gauge stations

• Progetto THRUST - Tsunami Hazard Reduction Utilising Systems Technology: using satellite communcations connected to personal computers.

Travel times in the Pacific basin for atsunami generated in the Hawaiian Islands

Monitoring techniques:-

• seismographs and rapi determination of the characteristics of earthquakes

• analysis of tsunamigenic processes (earthquakes on the sea bed)

• analysis via satellite and computer in real time• continuous measurement of water level in seas and oceans• monitoring of barometric pressure on the sea bed.

• taut-wire buoy stations

• stations to measure the pressure trasduction of long-period waves

• physical and digital modelling of the impact of tsunami waves in coastal zones

• cartography of damage and flooding in run-up zones straight after tsunamis.

Monitoring techniques:-

Tsunami mitigation measures:-

• alarm and warning systems

• physical defences on coasts (sea walls)

• widen roads to facilitate evacuation of coastal populations to high ground

• remove urban development from areas of high tsunami risk.

Kowloon Tong,Hong Kong:barriersagainsttsunamis andstorm surgescaused bytyphoons

Kowloon Tong,Hong Kong:barriersagainsttsunamis andstorm surgescaused bytyphoons

Publictsunami

evacuationmap in

Padangwestern

Sumatra,Indonesia

• walls, belts of trees along the coast

• coastal buildings to be uninhabited and with low intensity of usage

• other buildings:- reinforced- with open spaces at ground level- built perpendicular to the shore.

Tsunami mitigation measures:-

Maps byStefano Tinti(Univ. of Bologna)of the tsunamirisk on Italiancoastlines.

Japan, March 2011

Japan, March 2011

Japan, March 2011

Japan, March 2011