Tsunamis and the Louisiana Coast

Question

Scientists recently discovered evidence for an underwater landslide that occurred in the Mississippi Canyon about 7,000 years ago. The undersea canyon is located on the seafloor of the Gulf of Mexico just south of Louisiana. A scar in the canyon suggests that the landslide happened about 50 miles southwest of the Mississippi River outlet. The Mississippi Canyon landslide zone is shown in Map 1.

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During the landslide, Earth materials from the canyon's upper slope likely broke free and slid down the slope of the canyon until the Earth materials spread out and slowed down. The movement of this material on the bottom of the Gulf of Mexico resulted in a tsunami that eventually reached the Louisiana coast.

A tsunami usually forms when an event such as a sudden shift in the seafloor, a landslide, or volcanic activity releases a large amount of energy underwater. This causes a large wave to form on the surface. The wave is pulled back down by Earth's gravity and then splits into smaller waves that move outward in all directions. These waves can build to heights of 20 meters before reaching the shoreline. Figure 1 shows how a typical tsunami forms.

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The National Tsunami Warning System has a network of sensors to detect underwater events such as landslides and earthquakes. More recently, the Deep-ocean Assessment and Reporting of Tsunamis (DART®) system was developed, as shown in Figure 2. In this system, each sensor has a sensitive pressure recorder on the ocean floor to monitor small changes in water pressure. Any changes in pressure are sent to an anchored flotation device known as a buoy. The data is then sent to a satellite.

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Data from these events is used to make predictions about tsunamis and provide warnings in coastal areas. Tsunami warning maps can be generated within minutes of an underwater event using the DART system and computer modeling. Graph 1 shows how scientists can use DART data in models to better predict tsunami-related events.

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1
(a)
Part A
Based on the information in Figure 1, which statement best describes the differences in wave height and wavelength between tsunami waves in shallow water and tsunami waves in deep water?​
A
Tsunami waves in deep water have very large wave heights and long wavelengths, while tsunami waves in shallow water have very large wave heights and short wavelengths.
B
Tsunami waves in deep water have very small wave heights and short wavelengths, while tsunami waves in shallow water have very large wave heights and long wavelengths.
C
Tsunami waves in deep water have very small wave heights and long wavelengths, while tsunami waves in shallow water have very large wave heights and short wavelengths.
D
Tsunami waves in deep water have very large wave heights and short wavelengths, while tsunami waves in shallow water have very small wave heights and long wavelengths.
(b)

Part B
Which statements best explain the causes of the differences selected in Part A?
​
Select all that apply. ​
A
In deep water, there is a larger volume of water, resulting in waves with larger heights.
B
In shallow water, the rising seafloor drags on the bottom of the wave, resulting in a decreasing wavelength.
C
In deep water, there is more space for the energy of the wave to spread out under the water.
D
In shallow water, there is a smaller volume of water, resulting in waves with longer wavelengths.
E
In shallow water, there is less space for the energy of the wave under the water, so the height of the wave increases.
F
In deep water, the energy of the wave that forms decreases dramatically as the wave approaches the shoreline.

Answer 1

Part A

Answer: C
Tsunami waves in deep water have very small wave heights and long wavelengths, while tsunami waves in shallow water have very large wave heights and short wavelengths.

Part B

Select all that apply:

• B In shallow water, the rising seafloor drags on the bottom of the wave, resulting in a decreasing wavelength.
• C In deep water, there is more space for the energy of the wave to spread out under the water.
• E In shallow water, there is less space for the energy of the wave under the water, so the height of the wave increases.

These statements accurately describe the physical changes that occur to tsunami waves as they transition from deep water to shallow water, accounting for the differences in wave height and wavelength.