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Tsunamis
Dan—For text on screen, use black unless otherwise indicated, but please feel free to use white instead of black if it shows up better on a
given graphic. Please use Times New Roman for fonts displayed on screen.
Narration
Visuals
Instructions
3.0 Initiation
s01p0001.swf
[1]When a great deal of seawater is suddenly
displaced, tsunamis result. Let’s look at the
mechanics. Thrust faults tend to make the
biggest tsunamisl. [2]If the rupture happens
quickly—typically within a few seconds to
minutes —the water rides up with the seabed
and then, propelled by gravity, slides off as
expanding waves.
Audio 3.0.wav
[1] Display
[1]
Thrust_fault_initiation.jpg
[2] Replace, zoomed in on waves forming
at ocean surface
[2] thrust_fault_tsunami.swf
3.1 Rock-to-Water Energy Transfer
[1]The mechanism of energy transfer from rock
[1]
Audio 3.1.wav
to water is the potential energy change that
results from lifting up or dropping down water as
the seabed deforms and pushes the sea surface
out of equilibrium. Waves form as the displaced
water tries to regain equilibrium with the help of
gravity. The size of the seafloor deformation is
the most important factor in how big a tsunami
will be.
[1] Display
Comet_ocean_potentialenergy.jpg
3.2 Vertical Displacement
[1]How much the ocean floor rises directly
determines the height of resulting waves. In the
early stages of wave initiation, the water at the
ocean surface copies movement on the ocean
bottom, either up or down. The more movement
in either direction, the bigger the waves.
Audio 3.2.wav
[1] Display
[1]
Comet_vertical_displacement.jpg
Delete middle image; change labels to Before and After
3.3 Rupture Length
Lead-in question:What do you think?
In general, a [Longer/Shorter] rupture means more seafloor will
[1]In general, longer ruptures mean more
seafloor will be displaced and that any resulting
tsunamis will be bigger. Both the 2004 Indian
Ocean tsunami and the 1960 Chilean tsunami
(produced by the largest earthquake on record)
ruptured over 1,000 km (600 mi) of seabed. [2]In
addition, the longer the rupture, the more
directional the propagated tsunami energy. That
is, more energy will be focused perpendicular to
the fault.
be displaced.
Feedback: Longer is correct.
[1]
Audio File 3.3.wav
[1] Display
wiki_IOT_3Dwave.jpg
[2] Display
[2]
wcatwc_length_comparison.jpg
3.4 Duration of Quake
[1] The longer an offshore earthquake lasts, the
more likely it will produce a strong tsunami. The
length of the shaking is related to the length of
the rupture; the longer the shaking, the longer
the rupture. A magnitude 7 earthquake might
rumble for 20-40 seconds. In a magnitude 9
Lead-in question: What do you think?
[Longer/Shorter] duration earthquakes tend to produce bigger
tsunamis.
Answer: Longer.
[1] usgs_IOT_seafloormodel.mov
Audio file 3.4.wav
[1]Display
earthquake, the shaking may last 300 seconds —
10 times longer.
3.5 Water Depth
[1] The quantity of water above an earthquake
also affects tsunami energy. Think of it this way:
the deeper the seabed in which the earthquake
occurs, the more water is lifted and the more
potential energy is created. An earthquake in an
ocean basin 3 km (2 mi) deep uplifts more water
and generates far more potential energy than the
same sized earthquake on the continental shelf
in water 200 m (660 ft) deep. A one-meter uplift
in the deep ocean will generate a larger tsunami
at the shore than a one-meter uplift in shallow
water.
Audio file 3.5.wav
[1] Display
[1]
comet_water_depth.jpg
[2]
[2]A vivid illustration of this effect is the Nias
Island Tsunami in 2005 in the same subduction
zone that produced the 2004 Indian Ocean
tsunami. Though the quake was a strong
magnitude 8.6, it produced a tsunami that was
significant only near the source. The temblor
happened in shallow water in a region of barrier
islands to the south of the previous quake.
Comet_map_nias.jpg
3.6 Directionality of Wave Form
What do you think? True or False: Tsunamis are always preceded
by a retreat of the sea prior to an inundation.
Feedback: This is not true. The initial appearance of a tsunami is
determined by the wave form created by the water displacement.
[2] Replace with graphic
[1]Some tsunamis arrive as a drop in sea level -- a
classic leading-edge recession where the water is
sucked offshore before the first crest arrives. In
others, the tsunami arrives as a rapid rise in sea
level and flows in without natural warning unless
the earthquake was felt first. Whether the sea
level rises or falls is determined by which side of
a fault a coast lies on. Where the seafloor drops,
the water above drops, creating a trough; where
the seafloor rises, the water rises as a crest. You
can see the effect in this image showing an ideal
ocean responding to earthquakes on either a
reverse or normal fault.
Audio file 3.6.wav
[1] Display
[1]
wcatwc_okada.jpg
[2]
usgs_sum2TNW_small.mov
[3] [Downward pointing arrows. Thin, rust-orange, pointed]
[2] Replace
[3] Add arrows that follow leading crest on
left side and leading trough on right side
of animation.
[4]
Oceanmodeling_iot_marigram.jpg
[4] Replace
[1] Comet_Landslide_tsunami.swf NOTE: Move this animation to
Section 2 on Submarine landslides.
Audio file 3.7.wav
[1] Display
[2] Add arrow that points at wave moving
same direction as landslide
[3] Add arrow that points at seawater
[2]Waves approaching a shore from the dropdown side [3]tend to be preceded by a trough, or
recession, while those on the uplift side tend to
see leading rises. For example, during the Indian
Ocean tsunami of 2004, the sea dropped before
it surged inland at Sumatra and Thailand, east of
the earthquake. In Sri Lanka, on the west side,
the sea rose first. Because the island was so far
from the earthquake, no shaking warned
inhabitants there, either.
[4]In this graph, or marigram, depicting tide
gauge readouts from locations surrounding the
Indian Ocean tsunami, the leading wave is visible
in the Sri Lankan tide gauges, while the leading
trough is evident on the Thailand side.
[2]
[3] Add arrow that points at seawater dropping
3.6 Summary Questions
1. Interaction: What factors can determine the strength of a
tsunami? Choose all that apply.<Work on this>
A. The amount of seafloor displacement
B. A length of the rupture zone.
C. The density of the seawater above the fault
D. The depth of the water above the seafloor displacement
E. The duration of the earthquake
Feedback: The correct answers is A, B, D, and E.
2. Quiz question: Why do tsunamis in deep water tend to make
stronger tsunamis? Select all that apply
A. More water is lifted or dropped
B. More potential energy is created
C. Earthquakes in deep water must be stronger to
overcome the water pressure
D. Colder, denser deep water produces more powerful
waves
Answer: A and B
3. What determines whether the sea initially advances or retreats
when a tsunami arrives?
A. Whether the coast is nearer to the center or edge of a
fault
B. Whether the earthquake takes place in deep or shallow
water
C. Whether the seafloor near the coast is uplifted or
dropped down
D. Whether the coast is in the northern or southern
hemisphere
dropping
Feedback: The correct answer is C. The movement of the seafloor
nearest to the coast determines whether wave advancement or
retreat is seen first.
4. Interaction: True/ False: Tsunamis generated by submarine
landslides are not directional and are never preceded by a
receding wave..
Feedback: False. Submarine landslides tend to move down slope
in a definite direction, so the waves they generate can be highly
directional, like earthquake-generated tsunamis. In addition,
because the seabed slumps downhill, the creates a trough at that
location, which can precede the crest at the shore, depending on
orientation.
3.7 Unit 3 Summary Interaction
Summary Interaction:
1.
1. Suppose two earthquakes struck the same subduction zone
just 3 months apart. Both ruptured the subduction zone 30 km
beneath the sea floor. Based only on this information, which of
the two do you think produced the bigger tsunami?
Choices: Choices: Earthquake A
Earthquake B
Neither/Cannot be Determined
Answer: Neither/Cannot be Determined
2. One earthquake displaces about 300,000,000 km2 (100,000
mi2) of seafloor. The other earthquake displaces about 50,000
km2(20,000 mi2). Based only on this data, which of the two do you
think produced the bigger tsunami?
Choices: Earthquake A
Earthquake B
Neither/Cannot be Determined
Answer: A
3.The maximum uplift and subsidence of Earthquake A was 5 m,
while for Earthquake B it was 1 m. Based only on this information,
which of the two do you think produced the bigger tsunami?
Choices: Same as 1
Answer: A
4. The ocean depth in the vicinity of Earthquake A is 500-1000
meters. The ocean depth in the vicinity of Earthquake B is 0-200
meters. Based only on this information, which of the two do you
think produced the bigger tsunami?
Choices: Same as 1
Answer: A
5. The magnitude of Earthquake A was 9.1. The magnitude of
Earthquake B was 8.7. Based only on this information, which
tsunami would you expect to be bigger?
Choices: A will be significantly bigger
B will be significantly bigger
No significant difference/Not enough information
Answer: A.
6. There were not many islands in the vicinity of Earthquake A.
there were more islands in the uplift zone of Earthquake B. Based
only on this information, which would you expect to produce the
bigger tsunami?
Choices: Same as 5.
Answer: Earthquake A. Feedback: Energy that goes into lifting
land does nothing to displace water.
7. Image: Sumatran_earthquakes_1.jpg
Here are the two earthquakes’ epicenters and approximate
rupture zones. Now, thinking about all the information you’ve
been given, which would you expect to generate the bigger
tsunami?
Choices: Same as 1.
Answer: Earthquake A.
[1] Sumatran_earthquakes_2.jpg
Feedback: That is correct. In fact, the two tsunamis you’ve just
analyzed were the [1]2004 Indian Ocean Tsunami (Earthquake A)
and the 2005 Nias Island tsunami (Earthquake B). When the
tremendous shaking of the 2005 quake was felt, many feared it
would generate another massive tsunami. In 2004 the IOT
produced runups of 24-30 m. The Nias event, on the other hand,
generated waves only 3 m high at most.
Optional: Write a short analysis here of why you think this was.
[Learner must type at least 25 words]. Now compare it to this
analysis:
The 2005 earthquake, while of a similar magnitude and depth,
occurred in shallower water and in the vicinity of many islans
whose surface uplift did nothing to generate waves. Further, the
rupture zone was much shorter and earthquake A uplifted or
downdropped the seabed much more than Earthquake B.