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

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

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)

1990-2000

26 December 2004: ¼ million fatalities

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

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 and energies

Velocity = 3.132 x (water depth)½

where water depth is in meters and velocity is in meters/second (1 m/s = 3.6 km/hr)

Wave energy h2 (approximately)

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

Arrival of a tsunami on a coast

The wave will break when its height exceeds ~one seventh (1/7) of its wavelength…

…so some very long waves actually may not break

initially, there may be a rise or fall (drawdown) in sea level (which may attract people, to their great misfortune)

Long wavelengths and the coast

Due to its long wavelength, it may take a long time for a tsunami wave to crest

The wave then may remain high for several minutes

And it may take a while (hours) for the crests of successive waves to reach the shore…so don’t go surfing !

Wave runup - complicated

This depends on several factors:

water depth

sea floor profile

shape of coastline (focussing of energy, tsunamis travelling up rivers

An example of wave focussing at Krakatau, 1883

Causes of tsunamis - all involve displacement of water

Earthquakes

Volcanic activity

Landslides

Meteorite impacts

Earthquakes

Mainly vertical crustal movements…

…so strike-slip faults perhaps less hazardous…

...although these too can trigger mass movements such as landslides

Types of faults

Earthquakes

In general, the larger the quake, the larger the tsunami…but not a perfect correlation

Some anomalously large tsunamis generated from small quakes…

...energy released at longer periods than can be registered on normal seismometers ?

Shallow quakes

Quake energy seismic moment = slip x fault area x rigidity of rocks

For a given quake magnitude, if displacement is large, then rigidity may be low

This may indicate that the shallow parts of subduction zones are frictionally weak (unconsolidated sediments, fractures, fluids, etc.)

Submarine landslides

Another contributing factor to large tsunamis may be submarine landslides:

-generated by shaking associated with the earthquake

-cause additional displacement of water, thus a larger and more complicated tsunami event

Subduction association

Tsunamis typically are associated with earthquakes generated at subduction zones

Rupture of sea floor surface

Sediment slumps into subduction trench

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)

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.)

Landslides

Enormous submarine landslides can occur on the flanks of ocean islands (e.g., Hawaii, Canaries)

The wave washup can approach 400 meters in some cases

Canary Islands

Meteorite impacts

Too terrible to contemplate !!!

Hundreds to thousands of meters in height ?

Terminal Cretaceous event

Read and find out !

4 case histories

Alaska 1964 (earthquake-generated)

Krakatau 1883 (caldera-generated)

Unzen 1792 (landslide-generated)

Grand Banks 1929 (submarine landslide-generated

1964 Alaska quake and tsunami

Prince William Sound

epicenter

Old Valdez

1964 events

27 March 1964, 5:36 PM local time (early evening, people in their homes)

Magnitude 9.2 quake…largest ever recorded in North America…second largest ever

Shaking lasted 4-5 minutes (to compare, the 1906 San Francisco event lasted 45-60 seconds

Tectonic setting

Subduction in the Aleutian region results in very large quakes

Between 1899-1965:

7 quakes with M 8

60 quakes with M 7

Tsunami generation

In this region, tsunamis are generated by two mechanisms:

1) large vertical movements of the sea floor along faults (local and distant tsunamis)

2) slumping of material, both underwater and from land to water, by ground shaking

Nature of the 1964 tsunami

106 people were killed by the wave, 114 people total (consider the small coastal population of the area)

The extensive ground deformation caused by the quake triggered tsunamis

Destructive force of the wave

Avalanches and landslides were generated

Some of these generated locally damaging tsunamis

The force of such a wave can be seen in this picture

Boat runups

Carried inland by tsunami waves, boats acted as battering rams, efficiently destroying buildings

Here is a beached boat at Seward after the events

Submarine sliding at Valdez, Seward, and Whittier

These towns were built on unconsolidated sediments

Seismic shaking ruptured petroleum storage tanks in these towns, causing fires

The shaking also initiated submarine landslides, causing tsunami waves

Effects at Valdez

The landslides carried burning oil out into the bays…

…while the tsunamis returned the burning oil to the harbours and townsites, exacerbating the fires

Old and new Valdez

Unconsolidated sediments

Wave runup

This is Valdez Inlet after the main tsunami hit

Here the wave runup was the highest, reaching 67 meters

At Kodiak, tsunami effects were made worse by tectonic subsidence (faulting)

Wave runup

Valdez

It took 2-3 minutes to generate the tsunami from the landslide

30 people died

$ 15 million US in damage

Distant effects

As you can see, the wave affected the entire Pacific basin

The tsunami was hugely destructive along the west coast of Canada and the US (but only 16 dead)

Each colour band represents a 1-hour tsunami travel time increment

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

Krakatau

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

buoy

Shaded grey is submerged area

red=boat

yellow=buoy

blue=hill

Telok Betong

From Simkin and Fiske (1983)

Before...

…and after

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

Refraction diagram of the tsunami, showing transport times in minutes

From Simkin and Fiske (1983)

Krakatau

26 December 2004 earthquake and tsunami

From Brumbaugh (1999)

Magnitude 9.0-9.3

A warning to Indonesians:

Kerry Sieh’s poster and efforts to educate people beforehand

Plate tectonics of the eastern Indian Ocean region

Courtesy USGS

From Lay et al 2005, Science

Tectonics and previous great earthquakes

From Lay et al 2005, ScienceCumulative energy from global seismicity

From Liu et al 2005, Science

Tsunami runups (blue) and maximum tsunami heights (black) in Sri Lanka

Global propagation of the 26 December 2004 tsunami based on a model by Titov et al 2005 in

Science

Tsunami wave heights around the world (from Titov et al 2005 Science)

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

islands

Pyroclastic debris, 1991-1995

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

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

Extent of the 1792 debris avalanche deposit and the scar on Mt. Mayuyama

Note the islands

From Siebert et al. (1987)

An artist’s rendition of the 1792 events

Before...…and after

scar

deposit

New islands

18 November 1929 Grand Banks tsunami

This tsunami was caused by a M 7.2 quake on the Grand Banks

The quake triggered a submarine landslide which resulted in the tsunami

1: 1700 quake

3: M9.5 Chilean quake in 1960

4: M9.2 Alaskan quake in 1964

2: 1929 Grand Banks quake

1: 1700 quake

3: M9.5 Chilean quake in 1960

4: M9.2 Alaskan quake in 1964

2: 1929 Grand Banks quake

The 1929 landslide

The volume of the landslide was approximately 200 km3 (big !)

It flowed at speeds up to 70 km/hr

The flow cut 12 trans-Atlantic cables in 28 places

The 1929 tsunami

The height of the tsunami reached 5 meters in height

The wave struck the south coast of the Burin Peninsula on Newfoundland

Between 27 and 29 people drowned

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 (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

Mitigation efforts

Warning times

Every ~750 km of travel distance is equal to about 1 hour of warning time

So, as discussed above, there is very little warning time for tsunami 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

To determine the amount and orientation of crustal displacement at the surface, the moment magnitude is more useful than the Richter magnitude

Moment magnitudes

(fault slip) x (fault area) x (rigidity of rox)

The point is that we cannot always rely on quake magnitude to determine the magnitude of the tsunami

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

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

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

Hazard maps

As we have seen for earthquakes and volcanoes, hazard maps are critically useful pieces of information

Here are two examples, the first from Hawaii, and the second from Eureka, California

Note inundation areas and arrows for evacuation centres

Eureka

Eureka, Calif.

Eureka, California

Located in northwestern California, and is part of Cascadia

Hazards from tsunamis, liquefaction, ground shaking associated with liquefaction, etc.

But don’t forget...

Many areas and towns do not have such maps

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/naturalhazar

ds/naturalhazards1999/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/