Lecture Outlines Natural Disasters, 7 th edition

Lecture OutlinesNatural Disasters, 7th edition

Patrick L. Abbott

TsunamiNatural Disasters, 7th edition, Chapter 5

Indian Ocean Tsunami, 26 December 2004

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

• Estimated 245,000 deaths (probably higher)• Seafloor west of Sumatra ruptured northward for

1,200 km over 7 minutes• Caused by second largest earthquake (magnitude

9.2) 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 8.7,

south of first rupture


Figure 5.1


Figure 5.3

Tsunami• 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• 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

Tsunami vs. Wind-Caused Waves• 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: v = (g x D) ½

• 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

Tsunami vs. Wind-Caused Waves• 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

Tsunami vs. Wind-Caused Waves

Figure 5.4

Tsunami vs. Wind-Caused WavesEarthquake and Tsunami in Chile, 8 August 1868• Large earthquake shook Arica in Bolivia, where

several ships were moored in harbor• Eyewitness accounts of rising of sea, second

earthquake followed by falling of sea, then massive second (phosphorescent) wave hours later which carried ship two miles inland

Tsunami vs. Wind-Caused WavesTsunami at Hilo, Hawaii, 1 April 1946• Large earthquake in Aleutian Islands of Alaska

created tsunami across Pacific

• Eyewitness accounts of loud hissing sound, with advancing and retreating waves for several minutes

Tsunami at Oahu, Hawaii, 9 March 1957• Advancing sheet of water

Figure 5.5 Figure 5.6

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 rushes 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 protected shores, as with wind waves

Causes of Tsunami• Water mass is hit with massive jolt of energy, such

as earthquakes, volcanoes, mass movements, impacts

• Biggest tsunami caused by rarest events – impacts of asteroids and comets

Earthquake-Caused Tsunami

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

• Tsunami Warnings– Feel the earthquake

– See sea level draw down significantly

– Hear wave coming

– Seek high ground immediately

– Go upstairs in well-built building

– Warning system

• First sensors activated in 2003

• Tsunami warning center in Honolulu for Pacific Ocean

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 485 mph, slowing to 30 mph near Hilo

• 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

• Adequate warning was given in Hawaii but 61 people killed

• 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 in 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


Alaska, 27 March 1964


Figure 5.10Figure 5.11

British Columbia, Washington, and Oregon – upcoming


Figure 5.12

• Most killer tsunami generated at subduction zones

• All oceans have at least some short subduction zones (Atlantic Ocean’s Puerto Rico trench had magnitude 7.3 earthquake on 11 October 1918, causing submarine landslide and 6 m tsunami hitting Puerto Rico coast)

• British Columbia, Washington and Oregon coastlines slipped in magnitude 9 earthquake on 26 January 1700, generated massive tsunami recorded in Japan

• Next event will be deadly

Insert revisedFigure 5.12 here

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 collapse of mountain into partially emptied magma chamber, creating tsunami 40 m high

• More than 36,000 people killed

Volcano-Caused Tsunami

Landslide-Caused Tsunami• Volcano Collapses

– Hawaii in the Pacific Ocean

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

• Huge tsunami when chunk of island collapses into ocean

Figure 5.14

• Coastal area southeast of Kilauea (active volcano on Big Island of Hawaii) slides at up to 25 cm/yr into ocean, would create tsunami up to 30 m high, directed to southeast

Figure 5.15

• 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 tsunami across Atlantic Ocean

• Flank collapses occur globally about every 10,000 years

Figure 5.17

Landslide-Caused Tsunami

• Earthquake-Triggered Movements– Newfoundland, Canada, 18 November 1929

• Magnitude 7.2 earthquake offshore, triggering submarine mass movement, which set off tsunami

• Waves arrived at coast of Newfoundland 2.5 hours later, in three pulses over 30 minutes

– Papua New Guinea, 17 July 1998

• Magnitude 7.1 earthquake 20 km offshore, triggered underwater landslide that caused tsunami

• Hit coastline of Papua New Guinea about 5 minutes later, washing four villages on barrier beaches into lagoons

• Rethinking tsunami threat – not caused just by large earthquakes, also by landslides from moderate earthquakes


• In Bays and Lakes– Lituya Bay, Alaska, 9 July 1958

• Largest historic wave run-up

• Magnitude 8 earthquake on Fairweather fault, causing collapse of more than 900 m of rock and ice 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


• In Bays and Lakes– Lake Tahoe, California and Nevada

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

• 4% probability of magnitude 7 earthquake on lake-bounding lakes in next 50 years (low frequency)

• Would drop lake bottom about 4 m, generate 10 m waves across lake


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 6.3 and 7.5 earthquakes

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

• If You Feel the Earthquake– 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

Tsunami and You

Tsunami and You

Insert table 5.4

• Simuele Island, Indonesia, 26 December 2004– Closest inhabited land to epicenter of magnitude 9.2 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

Tsunami and You

Tsunami and You• Tsunami Warnings

– Coastal Maps

• Tsunami-hazard map of Hawaii Big Island, based on local topography

• Coastline mapping of Indonesia, India and Sri Lanka after 2004 tsunami indicated where human activities increased damage and loss of life

– Removal of vegetation, reefs increased impact of waves

– Buoys and Pressure Sensors

• Before 2004, NOAO operated six-buoy warning system in northern Pacific Ocean

• Funding since provided for 32 buoys around world, transmitting information to scientists

• New tide-gauge stations and seismometers along coastlines

End of Chapter 5

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Lecture Outlines Natural Disasters, 7 th edition