Transcript Chapter 8

Chapter 8
Tsunami Versus Wind-Caused Waves
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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 IndianAustralian plate under Asian plate
• Movements on fault of up to 20 m
• Second earthquake 28 March 2005, magnitude 8.7,
south of first rupture
Indian Ocean Tsunami,
26 December 2004
Figure 8.1
Indian Ocean Tsunami,
26 December 2004
Figure 8.3
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 8.10
Tsunami vs. Wind-Caused Waves
Earthquake 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 Waves
Tsunami 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 8.11
Figure 8.12
Wavelength and Period versus Height
• Destructive power of tsunami is not due to height,
but due to momentum of large mass, with ultralong 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
Earthquake-Caused Tsunami
Alaska, 1 April 1946
• Two large subduction earthquakes in Aleutian
islands, shook Scotch Gap lighthouse (steelreinforced 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
• Rushed ashore and killed 159 people in Hilo,
despite warnings (April Fool’s Day)
Earthquake-Caused Tsunami
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
Earthquake-Caused Tsunami
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
Earthquake-Caused Tsunami
Alaska, 27 March 1964
Figure 8.16
Figure 8.17
Earthquake-Caused Tsunami
British Columbia, Washington, and Oregon – upcoming
• 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
Figure 8.18
Insert revised
Figure 5.12 here
Volcano-Caused Tsunami
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
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
• 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 8.21
Figure 8.20
Landslide-Caused Tsunami
• 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 8.23
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
Landslide-Caused Tsunami
• 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
Landslide-Caused Tsunami
• 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
faults 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
Tsunami and You
• 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
Insert Table 8.5
Tsunami and You
• Simuele Island, Indonesia, 26 December 2004
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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 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 8