ch 08 tsunami versus wind-caused wavestsunami (1 of 2) • japanese word: tsu=harbor, nami=waves •...

8-1

Natural DisastersTenth Edition

Chapter 8Tsunami Versus

Wind-Caused Waves

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 8-2

Indian Ocean Tsunami, 26 December 2004

• Tsunami swept through Indian Ocean, hitting Asian and African shorelines

• Estimated 245,000 deaths

• Seafloor west of Sumatra ruptured northward for 1,200 km over 7 minutes

• Caused by third largest earthquake (magnitude 9.1) of last 100 years, on subduction zone of Indian-Australian plate under Asian plate

• Movements on fault of up to 20 m

• Second earthquake 28 March 2005, magnitude 8.5, south of first rupture


Figure 8.1. Indian Ocean Tsunami, 26 December 2004


Japanese Tsunami, 11 March 2011

• 9.0M earthquake off Japan’s northeast shore

• The Japanese know tsunami and were prepared

• However, tsunami were worse than expected and entire towns were obliterated along a 670 km stretch of coastline

• Tsunami was world’s most expensive natural disaster and killed over 19,000 people


Figure 8.2. Tsunami Damage in Otsuchi, Japan, 18 March 2011


Figure 8.3 Tsunami Travel Through the Pacific Ocean


Tsunami (1 of 2)

• Japanese word: tsu=harbor, nami=waves

• Tsunami reach greater height when they enter harbor or other narrow space

– 8 m wave on open coastline 30 m wave in

narrow harbor

• Japan, 1896

– Offshore earthquake shifted seafloor, causing tsunami to hit coastline 20 minutes later

– Highest waves (29 m) in narrow inlets

– 27,000 killed

• ‘Tidal wave’ inappropriate as not related to tides


Tsunami (2 of 2)

• Created most often by earthquakes

– Vertical shift of ocean floor that offsets water mass, transmitted throughout ocean in tsunami

– Usually vertical fault motions at subduction zones, mostly in Pacific Ocean

• 70,000 people killed by 141 tsunami in 20th

century

• Single tsunami on 26 December 2004 killed about 245,000 people in 13 countries


Figure 8.4 Cascadia Subduction Zone Event Created a 9M Earthquake and Huge Tsunami in

January 1700


Tsunami vs. Wind-Caused Waves (1 of 2)

• Wind waves

– Single wave is entire water mass

– Velocity depends on period of wave

▪ 17 mph for 5-second wave; 70 mph for 20-second wave

• Tsunami

– Huge mass of water with tremendous momentum

– Velocity:

▪ g – acceleration due to gravity; D – depth of water

▪ For average D = 5,500 m, v = 232 m/sec (518 mph)

▪ Actual observations of tsunami speed peak at 420 to 480 mph

▪ Wave will slow as approaches shore, but still fast

D*g1/2

v


Tsunami vs. Wind-Caused Waves (2 of 2)

• Tsunami

– Height: ~1 m in open ocean, 6 to 15 m in shallow water, higher in narrow topography

– Wave height is leading edge of sheet of water that flows on land for minutes

– Usually series of waves separated by 10 to 60 minutes

• Tsunami at the shoreline

– Not a gigantic version of breaking wave

– Very rapidly rising tide, rushing inland

– Sometimes water retreats first


Figure 8.11 Wind vs. Tsunami-Caused Waves [Wind images at top; Tsunami at

bottom] (1 of 2)


Figure 8.11 Wind vs. Tsunami-Caused Waves [Wind images at top; Tsunami at

bottom] (2 of 2)


Wavelength and Period Versus Height

• Destructive power of tsunami is not due to height, but due to momentum of large mass, with ultra-long wavelength and period

• Tsunami can rush inland for 30 minutes before water pulls back to form next wave

• Long wavelengths and periods mean waves can bend around islands and hit all shores – no shores are protected, as they are with wind waves


Earthquake-Caused Tsunami (1 of 4)

• Fault movements of sea floor – must be vertical movement, result in uplifting or downdropping seabed, earthquake of at least magnitude 7.5

• In subduction zone seaward edge of plate is dragged downward as landward edge bulges upward

• When stuck area ruptures leading edge of overriding plate breaks free and springs seaward and upward, causing a tsunami



Alaska, 1 April 1946

• Two large subduction earthquakes in Aleutian Islands, shook Scotch Gap lighthouse (steel-reinforced concrete, 14 m above low-water level)

• Twenty minutes after second earthquake, 30 m tsunami swept lighthouse away (first wave was biggest)

• Tsunami traveled across Pacific at about 485 mph, slowing to about 35 mph near Hilo, Hawaii

• Rushed ashore and killed 159 people in Hilo, despite warnings (April Fool’s Day)



Chile, 22 May 1960

• Magnitude 9.5 subduction event was most powerful earthquake ever recorded, created large tsunami

• Three waves, each successively larger, hit Chilean coast, killing 1,000 Chileans

• Third wave was the biggest

• Adequate warning was given in Hawaii but 61 people killed anyway

• Tsunami continued to Japan, killing 185 people

• Could continue to be measured in Pacific Ocean for a week



Alaska, 27 March 1964

• Magnitude 9.2 subduction earthquake killed 122 people along sparsely populated Alaskan coast

• Tsunami hit Vancouver Island, then California

• Series of waves, with fifth one largest

• Which wave in series will be largest is not predictable


Figure 8.14a


Figure 8.14b


Figure 8.14c


Figure 8.14d


Figure 8.16 Indian Ocean Tsunami Damage, 28 March 2005


Figure 8.17 Scotch Gap lighthouse, Alaska Before (top)and After (bottom) the 1 April 1946 earthquake


Figure 8.18



Figure 8.19



Volcano-Caused Tsunami: Krakatau

Krakatau, Indonesia, 26-27 August 1883

• Volcanic eruptions and explosions increased in frequency and strength, with volcanic masses flowing into sea and creating tsunami

• Culmination of eruption sequence was collapseof mountain into partially emptied magma chamber, creating tsunami 40 m high

• More than 36,000 people killed


Landslide-Caused Tsunami: Volcano Collapses (1 of 2)

• Volcano Collapses: Hawaii in Pacific Ocean

– Deposits of slumps and flank-collapses cover more than five times land area of islands

– Huge tsunami when island collapses into ocean

– Coastal area southeast of Kilauea (active volcano on Big Island of Hawaii) slides at up to 25 cm/yrinto ocean; a major movement would create tsunami up to 30 m high, directed to southeast


Landslide-Caused Tsunami: Volcano Collapses (2 of 2)

• Volcano Collapses:

– Canary Islands in the Atlantic Ocean

▪ Three of Canary Islands have had mega-collapses, last one 15,000 years ago

▪ Next mega-collapse could send powerful tsunami to coastlines of Africa, Europe, North and South America

▪ Models simulate 10 to 20 m high tsunami across Atlantic Ocean


Figure 8.21 Hawaiian Island failures


Figure 8.22 Massive block of rock sliding into the Pacific Ocean. A rapid, catastrophic movement

could cause huge tsunami.


Figure 8.24


Landslide-Caused Tsunami: In Bays and Lakes (1 of 2)

• Lituya Bay, Alaska, 9 July 1958

−Largest historic wave run-up

−Magnitude 8 earthquake on Fair weather fault, caused collapse of rock and ice down more than 900 m into Lituya Bay

−Three boats anchored in bay, hit by huge wall of water about 30 m high, faster than 100 mph

−Crews of two boats survived being lifted and dropped by wave

−Wave sent surge of water 525 m up side of bay


Landslide-Caused Tsunami: In Bays and Lakes (2 of 2)

• Lake Tahoe, California and Nevada

− High in Sierra Nevada, created by active normal faults dropping land between two faults (10th deepest lake in world)

− 4% probability of magnitude 7 earthquake on lake-bounding faults in next 50 years

− Would drop lake bottom about 4 m, generate 10 m tsunami across lake


Figure 8.27a Tsunami Effects in Lituya Bay, Alaska


Figure 8.28 Lake Tahoe basin formed between two down-dropping faults. Note landslide deposit on lake floor in upper

left that probably created a tsunami when it slid.


Seiches

• Oscillating waves in enclosed body of water –sea, bay, lake, swimming pool

• Energy from strong winds or earthquakes

• Hebgen Lake, Montana, 17 August 1959

– Two faults under lake shifted in 6.3M and 7.5M earthquakes

– Eyewitness accounts of water migrating from one end of lake to other, over 11.5 hours


Tsunami and You (1 of 2)

• If You Feel an Earthquake Think About the Possibility of Tsunami

– Mild shaking for more than 25 seconds: powerful, distant earthquake may have generated tsunami

– Sea may withdraw significantly, or may rise, before first big wave

– Water may change character, make unusual sounds

– Take action (if you wait until you see the tsunami you have waited too long)


Tsunami and You (2 of 2)

• Simeuele Island, Indonesia, 26 December 2004

– Closest inhabited land to epicenter of magnitude 9.1 earthquake

– After shaking stopped, residents fled uphill immediately

– Only 7 out of 75,000 inhabitants were killed

– Oral history reminded people: when ground shakes, run to hills before giant waves arrive

• Nicaragua, 1 September 1992

– Subduction earthquake shifted ground very slowly, creating little ground shaking, but transmitting energy into water very efficiently, generating large tsunami


Figure 8.29. Tsunami at Hilo, Hawaii, 1 April 1946


Table 8.5

Surviving a Tsunami

Abandon your belongings.

Many lives are lost while trying to save possessions.

Head for high ground—and stay there.

If there is no high ground nearby, then

Climb to an upper floor or roof of a strong building.

If there is no sturdy building, then

Climb a tree.

If there are no climbable trees, then

Grab onto something that floats.

Look for something to use as a raft.


Figure 8.31

ch 08 tsunami versus wind-caused wavestsunami (1 of 2) • japanese word: tsu=harbor, nami=waves •...

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