Tsunami notes

 

 

 

Tsunami notes

 

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Tsunami notes

 

Notes on tsunami

 

Tsunami are hazardous ocean waves characterized by an enormous wavelength and high velocity that are triggered by a sudden disturbance.  The most common disturbance is a large earthquake beneath the seafloor, but tsunami may also be triggered by volcanic eruptions, landslides, or meteor impacts.

 

Tsunami are not related to tides, so the term “tidal wave” should be avoided.

 

 

 

Earthquakes that generate tsunami: (a) are very large, generally M>7.3; (b) occur beneath the seafloor; and (c) displace the seafloor a large distance upward.  The sudden vertical motion pushes up a bulge on the sea surface that radiates outward as a tsunami wave. 

 

Most tsunami-generating earthquakes occur above subduction zones because this is where the largest earthquakes occur, and because subduction zones are characterized by reverse faults or thrust faults which have a large vertical component.

 

 

 

The intensity of tsunami is measured by the runup, the maximum water height of the wave on land.  Runup is related to the magnitude of the generating earthquake, and may exceed 30 meters (100 feet) for the largest tsunami. 

 

The largest runup ever recorded from a tsunami was at Lituya Bay, Alaska, where a landslide caused water to slosh up over 500m (1,700 feet) on the opposite side.  The great 2004 Indian Ocean tsunami had a maximum runup of about 30 meters.

 

Water waves can be characterized as either shallow-water or deep-water waves. Waves cause circular orbital motion of the water beneath them, with a radius that decreases with depth.  At a depth = ½ × the wavelength the radius = 0.

 

Waves are shallow-water waves if the depth is less than ½ of their wavelength.  The circular orbital motion is squashed into ovals because of friction with the bottom. 

 

The velocity of a shallow-water wave depends only on the depth and is given by:  where g is the gravitational acceleration ~ 10 m/s2. 

 

In comparison with normal, wind-driven waves, tsunami: (a) are caused by a disturbance, not by wind; (b) have a small amplitude, generally < 1 meter, in the open ocean; (c) have an enormous wavelength, on the order of 100 km compared to 10 m for wind-driven waves.

 

The enormous wavelength of tsunami ensures they are always shallow-water waves, because the oceans are nowhere deeper than 11 km. 

 

Because they are shallow-water waves, the velocity of tsunami is given by , and is on the order of 700 kph in the deep ocean, about 70 × faster than wind-driven waves.

 

The enormous wavelength also causes tsunami waves to contain much more energy content than wind-driven waves, which is why they cause so much damage.

 

Since the attenuation of waves is inversely related to their wavelength, the enormous wavelength of tsunami waves means they suffer little attenuation.  Tsunami waves may cross the Pacific Ocean and still retain enough energy to cause significant damage.

 

Tsunami waves slow down as they approach the shore because the depth decreases.

 

The energy of a wave is proportional to A2 × v (A = amplitude, v = velocity).  As the velocity decreases and the energy remains approximately constant, the amplitude of the wave must increase.  As they approach the shore tsunami waves may reach a height of 10’s of meters.

 

Damage from tsunami is caused by: (a) the force of  the water; (b) impact from floating debris; (c) residual water damage; (d) salt water, which damages fresh-water plants.

 

Tsunami usually occur as a series of 5-10 waves separated by 5-300 minutes.  The first is sometimes preceded by a severe decrease in water level, and is rarely the largest wave.  Many people die because they do not realize the tsunami threat remains after the first wave has receded.

 

Because they lose little energy, tsunami may cross the ocean and still cause damage.  The 1960 Chile earthquake produced a tsunami that caused damage to Hawaii (especially Hilo) and Japan, over 10,000 miles away.

 

Tsunami caused by objects falling into the ocean (landslides, meteorites) or exploding out (volcanic eruptions) have wavelengths similar in size to the object or explosion that triggers tham, on the order of 10’s of km.  These wavelengths are smaller than earthquake-generated tsunami.  As a result, non-earthquake tsunami attenuate faster and do not cause as much damage far from their source.

 

The 1883 eruption of Krakatau volcano in the straits between Java and Sumatra caused a tsunami that locally killed 34,000 people with a wave that exceeded 40 meters.

 

Most tsunami occur in the Pacific Ocean because it is ringed by subduction zones.  Tsunami in the Pacific Ocean have a RI of about 12 years.  The Indian Ocean is also exposed to subduction zones, and the RI for tsunami in the Indian Ocean is about 30 years.

 

Tsunami in the Caribbean basin are an underappreciated risk, and have killed more people than Pacific tsunami.  The Caribbean is exposed to volcanic islands, a subduction zone, and high relief subject to massive landslides.  Local tsunami in the Caribbean have occurred in the past 100 years, in particular the 1918 tsunami in Puerto Rico.

The parts of the US at most risk from tsunami are, in order: (a) Hawaii; (b) Alaska; (c) the Pacific coast, especially the Pacific Northwest; (d) the Atlantic coast.  Although tsunami in the Gulf of Mexico are possible (if triggered by a large landslide or earthquake, for example) the chances of this happening are exceedingly rare.

 

Hawaii faces tsunami risk from local landslides and from earthquakes along the Pacific rim.  Large debris fields in the seas around Hawaii were produced by huge slumps that produced local tsunami (though none in recent times).  The runup from distant earthquakes has exceeded 25 feet in parts of Hawaii, due to focusing of tsunami waves in embayments.

 

Tsunami in the Aleutian Islands of Alaska are fairly common, because this is a subduction zone.  The 1964 Alaska earthquake produced trans-Pacific effects that included damage to San Francisco and Japan, in addition to severe local damage.

 

The Cascadia subduction zone of the Pacific NW is a locked fault that is similar in size to the subduction zone that produced the great 2004 Sumatra earthquake and tsunami.  Earthquakes of M = 9 have occurred there before (most recently in 1700), and great tsunami are recorded in sediments several hundred years old.  The RI for great earthquakes along this subduction zone is approximately 500 years.

 

Tsunamis in the Atlantic are rare because it is not exposed to subduction zones, and there are few large islands capable of producing tsunami via landslides or slumps.  Tsunami have occurred in the Atlantic due to: (a) landslides off of Cumbre Vieja volcano in the Canary Islands, in the eastern Atlantic off the coast of Africa; (b) the great Lisbon earthquake of 1755 of M=8.5, which may have been related to incipient subduction off the coast of Portugal; (c) landslides off the eastern seaboard, in particular in Nova Scotia.  The risk to the eastern US posed by Cumbre Vieja is likely much smaller than recent TV programs have led people to believe, due largely to the attenuation of locally-induced tsunami across the ocean.

 

There are two approaches to tsunami mitigation: (a) zoning; (b) early warning.

 

Zoning restricts the types of activities in tsunami-prone low-lying areas, for example in Hilo, Hawaii.

 

Early warning systems have three facets: (a) identification of earthquake potentially able to generate tsunami; (b) identification of the tsunami wave itself in the open ocean; and (c) communicating the danger and ensuring evacuation of coastal areas.

 

Earthquakes are quickly identified and located with the Global Seismographic Network, an array of seismographs distributed worldwide.

 

Identifying tsunami waves in the open ocean is difficult because of their huge wavelength and small amplitude.  Specialized DART (Deep Ocean Assessment and Reporting of Tsunami) buoys have been deployed primarily in the Pacific, but also in the Atlantic and Indian Oceans, and are capable of recording tsunami waves and communicating with operations centers on the mainland.

 

It may someday be possible to identify tsunami waves from space using satellites.

 

In many cases, the biggest obstacle to effective tsunami warning is communicating the risk, disseminating the warning, and organizing evacuation.  Communication and warning systems may be primitive or nonexistent in poor countries, and evacuation must be practiced to avoid chaos.   

 

Source : http://chuma.cas.usf.edu/~juster/GLY2030/notes-tsunami.doc

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