Tsunamis
What is a tsunami ?
• A tsunami is a very long ocean wave
generated by sudden displacement of the sea
floor or of the oceanic mass
• The displacement of an equivalent volume of
water generates the tsunami
Tsunami Terminology
• The term “tsunami” is a Japanese word
meaning “harbour wave”
• It was so named because the wave is harmless
until it enters a harbour
• It is frequently called a “tidal wave”, but it has
nothing to do with tides
Structure of a wave
• Wavelength, , can exceed 200 km
• normal ocean waves have wavelengths of about 100
m
• trough; peak; wave height, h; amplitude
From Murck et al. (1996)
Velocities in deep water
• Tsunamis travel very quickly relative to normal ocean
waves
• This is particularly the case in open water, where
velocities increase with water depth
• Velocities can reach 1,000 km/hr in open ocean (normal
ocean wave: ~90 km/hr)
• Thus, velocities are about 10 times higher for tsunamis
Shallow water
• In shallow water, the tsunami waves pile up
• As a result, velocities and wavelengths
decrease...
• …but at the same time, amplitudes can
increase enormously...
Amplitudes
• In deep water, wave amplitudes are generally
less than 1 meter…
• …but in shallow water, amplitudes can reach
40 meters or more above normal sea level
Tsunamis take place when a huge earthquake occurs causing the plates below the
water to push up causing the water to create a huge wave.
In shallow water, the tsunami waves pile up
velocities and wavelengths decrease...
amplitudes can increase enormously...
Hazards and risks of tsunamis
• Tsunamis can hit with little or no warning
• 4,000 people have been killed between 1990
and 2000
• The most prone areas are those associated
with earthquakes and volcanoes (mainly
subduction zones)
How tsunamis are generated - all involve
displacement of water
• Earthquakes
• Volcanic activity
• Submarine landslides
• Meteorite impacts
Tsunami generation
1. Earthquakes
• Mainly vertical crustal movement caused by
near-shore or off shore earthquakes
• …so strike-slip faults perhaps less hazardous…
• Magnitude of the earthquake is directly
related to the magnitude of the tsunami
75% of tsunamis are produced in the pacific ocean, most others are in the
indian ocean
Tsunami generation
Subduction association
• Tsunamis typically are associated with
earthquakes generated at subduction zones
• Rupture of sea floor surface
• Sediment slumps into subduction trench
Tsunami Generation
1990-2000
Locally-generated tsunamis
• The subduction zone of
Cascadia has potential for
very large offshore quakes
(M  8)
• There is a great danger of
locally-generated tsunamis
here, since they travel so fast
• Many large cities are found
on the coast
The Boxing Day Tsunami
Banda Aceh 26-12-2004
Tsunami Wave
Tsunami waves approach Phuket, Thailand
two hours after the Banda Ace earthquake
Tsunami waves approach Phuket, Thailand
Tsunami wave strikes coastal city
Inundation
Devastation
Banda Aceh before the Tsunami
Banda Aceh after the Tsunami
Tsunami generation
2. Volcanic activity
• Displacement of rock
• Submarine caldera collapse (e.g., along faults)
(Krakatau 1883)
• Entrance of pyroclastic flows into water
(Krakatau 1883)
• Subaerial lateral collapse, generating debris
avalanches which enter water (Unzen 1792)
The eruption of Krakatau 1883
• Krakatau is a
volcano located
between Java
and Sumatra
• It is mainly a
submarine
volcano, with its
top sticking out
of the water
Caldera collapse
• The cataclysmic
eruption occurred on
26-27 August 1883
• A submarine caldera
was formed
• Displacement of
material during collapse
generated a series of
devastating tsunamis
Two views of the caldera margin on
Rakata, one soon after the eruption
and the other in 1979
This is Anak Krakatau,
which emerged through the
sea in 1928. It is within the
caldera
Tsunami
• 36,000 people were killed by the tsunami along the coasts
of Java and Sumatra
• At least 3 great waves occurred
• 165 coastal villages were destroyed by the waves
• The largest waves were recorded by tide gauges up to
7,000 km away on the Arabian Peninsula
Tsunami
• Coral blocks up to 600 tons were carried
inland…
• …these served efficiently as natural battering
rams
• Runup heights reached 40 meters
Maximum runup heights in meters (from Simkin and Fiske, 1983)
Telok Betong
Telok
Betong
From Simkin
and Fiske
(1983)
Before...
…and after
Shaded grey is
submerged
area
red=boat
yellow=buoy
blue=hill
buoy
hill
The District Hall in Telok Betong. The tsunami stopped
just before this building, sparing the people cowering
inside
The hill near Telok Betong. The lower part of the hill has
been cleansed of its vegetation by the tsunami
Boat runup…the Berouw...
• This boat, named
the Berouw, was
carried 2.5 km
inland at Telok
Betong by the wave,
which reached 24 m
in height
…and inland emplacement of its
mooring buoy
• This is the Berouw’s
mooring buoy, also
carried inland
• It is now a visually
pleasing monument
overlooking Telok
Betong
Tsunami generation
3. Landslides
• Landslides often are generated by quakes or volcanoes
• also occur on subduction trench slopes (steep)
• also can occur in enclosed bodies of water (lakes, bays,
reservoirs, etc.) (rockfalls, slumps of unconsolidated
material, etc.)
• Enormous submarine landslides can occur on the flanks
of ocean islands (e.g., Hawaii, Canaries)
Tsunami generation
Tsunami generated from landsliding
-Phase 1 material slides,
-phase 2 water is sucked down,
-phase 3 water rebounds and
-phase 4 Tsunami is generated
Unzen volcano, Japan: 1792 collapse of
Mt. Mayuyama
• In addition to its
recent lava dome and
pyroclastic flow
activity (1990-1995),
the volcano also has
collapsed
catastrophically in
the past
Mt. Mayuyama
scar
Pyroclastic debris, 1991-1995
islands
The 21 May 1792 collapse
• A debris avalanche
occurred from Mt.
Mayuyama in 1792
about 1 month after
lava stopped flowing
from Fugen-dake (site
of recent activity)
Fugen-dake
• The avalanche was
triggered by two
quakes
Mt. Mayuyama
Tsunamis
• The debris avalanche
entered the Ariake
Sea, generating a
tsunami
• The wave killed
between 14,000 and
15,000 people in
coastal communities
Geological map, showing 1792 debris
avalanche deposit
The debris avalanche deposit
From Siebert et al. (1987)
• Extent of the 1792
debris avalanche
deposit and the scar on
Mt. Mayuyama
• Note the islands
An artist’s rendition of the 1792
events
scar
deposit
New islands
Before...
…and after
Tsunami generation
4. Meteroite Impact
Too terrible to contemplate !!!
Hundreds to thousands of meters
in height ?
Terminal Cretaceous event
Read and find out!
Tsunami hazards
• Extensive flooding
• Action of wave on coastal structures, both
natural and built
• The incredible force of the waves can remobilize
huge objects
The event may create drawdown
Effects of tsunami drawdown
• Release of dissolved gases (CH4, CO2, H2S)
previously contained in shallow sediments
• Potential ignition of gases by their rapid expulsion
• As a result, a wave of noxious and burning gases
may engulf people BEFORE the wall of water arrives
Mitigation efforts
Warning times
• Every ~750 km of travel distance is equal to
about 1 hour of warning time
• So, there is very little warning time for
tsunamis generated by local sources,
compared to those from distant sources
Quake-generated tsunamis
• In general, the size of the quake is an
approximate indication of the size of the
tsunami
• But this guide doesn’t always work
Hawaii is particularly vulnerable, being in the
middle of the Pacific
Warning systems
• Mainly based on earthquake data
• Pacific-wide warnings: require at least 1 hour
warning time
• More local networks require warning times
less than 1 hour…this is difficult
A proposed system of real-time
detectors
NOAA=National
Oceanic and
Atmospheric
Administration
-
Response to tsunamis
• Requires good emergency planning and
preparation…
• …an educated and trained public…
• …which has access to information…
• …so the dissemination of this info needs to be
efficient and reliable
Personal mitigation
• Run (don’t walk) to higher ground
• Tell your family and friends
• Never go to the beach to watch tsunamis
Sign in the lobby of a
Hawaiian hotel:
• IN CASE OF TSUNAMI:
–Remain calm
–Pay your bill
–Run like hell
Response to tsunami
• Requires good emergency planning and
preparation…
• …an educated and trained public…
• …which has access to information…
• …so the dissemination of this info needs to be
efficient and reliable
A warning to Indonesians:
Kerry Sieh’s poster and
efforts to educate people
beforehand
Tsunamis -reading
•
Billings, L.G., 1915. Some personal experiences with earthquakes. National
Geographic, v. 27, no. 1, January 1915, pp. 57-71.
•
González, F.J., 1999. Tsunami! Scientific American, May, 1999.
•
Niven, L., and J. Pournelle, 1983. Lucifer’s Hammer. New York, Fawcett Crest, 629
pp.
•
Simkin, T., and R.S. Fiske, eds, 1983. Krakatau 1883, the volcanic eruption and its
effects. Washington, D.C., Smithsonian Institution Press, pp. 69-81.
Tsunamis - web
• Canada:
•
http://atlas.nrcan.gc.ca/site/english/maps/environment/naturalhazards/naturalhazards
1999/tsunamis
•
http://www.pep.bc.ca/hazard_preparedness/Tsunami_Preparedness_Information.html
• U.S.:
•
http://www.ess.washington.edu/tsunami/index.html
•
http://www.tsunami.noaa.gov/
• U.K.:
•
http://www.nerc-bas.ac.uk/tsunami-risks/