unamis? - Quia

unamis?

called seismic sea waves or, incorrectly, tidal waves, generally arecansed l _ earthquakes, less commonly by submarine landslides, infrequently bysnbmarine volcanic eruptions and very rarely by a large meteorite impact in the

uhmarine volcanic eruptions have the potential to produce truly awesomesunami waves. The Great Krakatau volcanic eruption of 1883 generated giantaves reaching heights of 125 feet above sea-level, killing thousands of people and

wiping out numerous coastal villages.

~he 1992 Nicaraguatsunami may have beenthe result of a "slow"earthquake comprised ofvery long-period movementoccurring beneath the seafloor. This earthquakegenerated a devastatingtsunami with localizeddamage to coastal

, ,~. communities in Nicaragua.

%ot all earthquakesgenerate tsunamis. Togenerate tsunamis,earthquakes must occur

underneath or near the ocean, be large and create movements in the sea floor. Alloceanic regions of the world can experience tsunamis, but in the Pacific Ocean thereis a much more frequent occurrence oflarge, destructive tsunamis because ofthemany large earthquakes along the margins of the Pacific Ocean.

Ring offire

@ About two-thirds ofthe Earth is covered by the waters ofthe four oceans. ThePacific Ocean is the world's largest, covering more than one third ofthe totalsurface area of our planet. The Pacific Ocean is surrounded by a series of mountainchains, deep ocean trenches and island ares, sometimes called-a-éei g of'fire." Thereat size of the Pacific Ocean and the lar e earth uakes ssociated ith the ''rIll

óf fire" combine to produce deadly tsunamis.

@In 1ess than a day, these tsunamis can travel from one side ofthe Pacific to theother. However, people living near areas where large earthquakes occur may findthat the tsunami waves will reach their shores within minutes of the earthquake.For these reasons, the tsunami threat to many areas (Alaska, the Philippines,Japan 01' the V.B. West Coast) can be immediate (for tsunamis from nearbyearthquakes taking only a few minutes to reach coastal areas) or 1ess urgent (fortsunamis from distant earthquakes taking from 3 to 22 hours to reach coastalareas).

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42

. Earth and earthquakes

Q The continents and sea floor that cover the Earth's surface are part of a worldwidesystem of plates that are in motion. These motions are very slow, only an inch ortwo per year. Earthquakes occur where the edges ofplates run into one another.Such edges are called fault lines 0J:f&tl:l . s..ometimes the forces along faults canbuild-up over long eriods ofti . he rocks finally break an---- . /earthguake occurs. Examples of fe trrres produced by forces released along plate;dge faults are the Andes Mountains in South America (on land) and the AleutianTrench near Alaska (under water). When powerful, rapid faulting occursunderneath or near the ocean, a large earthquake is produced and, possibly, atsunami. The deep ocean trenches off the coasts of Alaska, the Kuril Islands,Russia, and South America are well known for their violent underwaterearthquakes and as the source area for destructive Pacific-wide tsunamis.

(J) The tsunami generating process is more complicated than a sudden push against .the column of ocean water. The earthquake's magnitude and depth, water depth inthe region oftsunami generation, the amount ofvertical motion ofthe sea floor, thevelocity of such motion, whether there is coincident slumping of sediments and theefficiency with which energy is transferred from the earth's crust to ocean waterare all part of the generation mechanism.

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Source: What causes tsunamis? (2006, November 29). In NOAA's National Weather Center.Retrieved 17:13, November 29,2006, from:

http://www.nws.noaa.gov/om/brochures/tsnnami2.htm.

Exercises

Cause-effect relationships

1. Make a bar / pie chart representing the different causes oftsunamis, as they arepresented in paragraph

~4~Ú/l

S~ VV'-~IM.- la.m..á..~Lj¿k>

S-~¿t,(\>'V-- ".9lu~~ ~dA'>t~

t'h¿ita..o t.. re..-

43

~:!'e-~OIlEfrom rhe box below to fill in the blank spaces. Each expressiono ly onee.

Jseismic sea waves source

is the result of

..--are caused by

/earthquakes

t/landslides

the size of the

Pacific Ocean

a, Another correct name for tsunamis is S~.>rn.o::L 'S.e."'-. vJtÜ/·t,.S

3. The least frequent cause oftsunamis is M.Q.,~O ~l-t.~

Seismic sea waves (JV(Q. Ct'-L<-:..Ld. laJj earthquakes.

d. Large earthquakes c.~ c:~ destructive tsunamis in thePacific Ocean.

e. Two elements cause tsunamis in the Pacific Ocean: __ ..¿_~_- _~_-,,\\--(..V;?-.__ ~--=-)__ andi~..l:5l..:~J

f Violent water earthquakes and deep ocean trenches are the __ <;_o_t-_"'_~ _of deadly tsunamis.

3. What elements do the following signal words connect?

Signal Word Par./line

... are caused by ... par. 1, line 1-2.

... may have been the result ... par. 2, line 2-'~

... because of ... par. 3, line!>,?

par. 5, line! 4-.. .for these reasons ...

Elements connected

.fSU:II\~S 4- ~6~'1U¿JU-ltk 1992 ~{C~\~ ts~""'¿ .:É- slow e~,

eA.~t¡.•....¿t;¡.Vt. 'h/Á",-;r~¡ ~ '~~ (~..?¿"-f;tJ..~,

--hJCt 0í"hu..n ~ t-t~.,.í ~ P ~ (/~ r~Wt<

Comprehension questions

L What is the main cause oftsunamis according to the text? -e",.,{~\A.'''.,,{4\

2. What is the area of the world most affected by deadly tsunamis? ?~fv(. D (.R.~

3. What are the three conditions that an earthquake must have in order to generate/tsunamis?

/ 4. What causes earthquakes? ? /¿¡;ti21 f~M~ ¡•....'tú ~ e>~1"

Ó. What makes the Pacific Ocean be the most frequent source of tsunamis?.I:t-$ ~f'U.x 'Si2:-0.. ~J ~ L4e e•...A1.CJ~ Il-\.-twz., IQ;.~ .'f h:.N:-

;0 Tk L;:.r4'tk.~·~ ~s +:- DC-Ok'Y h<?'~-*k/~ ~ oa~- b e. LAI('1¿

- ~ I'\N'"\1 ~ br\- ~ ftotl e

Activities for this text were prepared by Pral Marta Trias.