Angela's Tools · Chapter 26 · Mass Wasting, Tsunamis, and Floods Worksheet 4 of 10

From Trench to Beach: Modeling a Tsunami and the Sensor That Catches It

Learning target: I can interpret a diagram of a tsunami's trip from its source to the shore, build a cross-section model showing how its speed, height and crest spacing change with depth, and trace how a deep-ocean sensor's data reach a warning center.

Diagrams and modelsAbout 45 minIndividual MS-ESS3-2 · practice visual modelingindependent practice
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Directions

Study Figure 1 from left to right: it follows one tsunami from the earthquake that starts it to the beach. Figure 2 follows the data from a deep-ocean sensor to the people who issue warnings. Answer Part A using the figures. Then draw your own cross-section model in the box and use it to answer Part B. Work in pencil so you can revise.

You will need: This worksheet, Pencil and eraser, Ruler, Colored pencils (optional)

Figure 1: One tsunami, five places

A tsunami's trip from the trench to the beach A tsunami's trip from the trench to the beach 1. Source: a subduction-zone earthquake The edge of the upper plate, stuck for centuries, suddenly springs upward while land nearer the coast drops. The seafloor shoves the entire column of water above it, from bottom to surface, over an area that can be hundreds of kilometers long. 2. Deep ocean, about 4,000 m deep Speed about 710 km/h. Height usually under 1 m. Crests often more than 100 km apart, so the sea surface rises and falls over many minutes. Ships barely notice it. 3. Top of the continental slope, about 200 m deep Speed about 160 km/h. The front of the wave reaches shallow water and slows first, so the back catches up, the crests bunch together and the height starts to grow. 4. Shallow water near shore, about 10 m deep Speed about 36 km/h, still faster than a person can run. Height can reach several meters, and more in narrow bays and harbors. 5. Shore The sea rises like a flood tide that keeps coming, pushes inland and then drains back out. More crests follow for hours, and the first is often not the largest. Sometimes the water pulls far back from the beach before a crest arrives. DART station (deep ocean) A pressure sensor on the seafloor feels the weight of the extra water as a crest passes overhead and the smaller weight as a trough passes.
Figure 1. Stages of one tsunami from source to shore. Speeds are calculated from ocean depth for long waves like tsunamis and rounded; heights and spacing are typical values, and real events vary.
How a deep-ocean reading reaches a warning center How a deep-ocean reading reaches a warning center Pressure sensor on the seafloor Acoustic modem Surface buoy Satellite Tsunami warning center pressure readings sound waves up through the water radio signal relayed to land
Figure 2. The path of a DART reading. Radio waves do not travel far through seawater, but sound does.

Part A: Read the figures

1.Using stage 1, explain what sets a tsunami in motion. How is that different from the way wind makes ordinary waves, which disturb only water near the surface?
2.Using stages 2 and 4, how many times faster does the tsunami travel in the deep ocean than in shallow water near shore? Show the division and round to the nearest whole number.
3.Describe how the height of the wave and the distance between crests change from stage 2 to stage 4. Use stage 3 to explain why slowing down makes the wave taller.
4.A sailor 150 km offshore, over water 4,000 m deep, says nothing unusual happened while the tsunami passed under her boat. Use Figure 1 to explain why she could be right.
5.Give two reasons the DART pressure sensor is placed on the deep seafloor instead of floating at the surface or sitting in shallow water near the coast.
6.Use Figure 2 to explain why the data travel by sound from the sensor to the buoy but by radio from the buoy to the satellite.

Part B: Build your own model

✎Draw a side view (cross-section) of the ocean from the earthquake source on the left to the beach on the right. Draw the seafloor dropping into a trench, then running across a deep ocean floor, then rising up a continental slope to a shallow shelf and the beach. Draw an arrow showing the seafloor moving at the source. Draw the same tsunami crest at three places: the deep ocean, the top of the continental slope where it meets the shelf, and the shallow shelf near the beach. Make the crest taller and the crests closer together as the water gets shallower, and label each place with its depth and approximate speed. Add a DART sensor on the deep seafloor and a buoy above it, and draw arrows showing the data path to a warning center on land.
7.On your drawing, circle the place where the tsunami is most dangerous to people. Explain your choice using both height and speed.
8.Where did you put the DART station: close to the earthquake source or close to the beach? Explain how that placement affects the warning time for the town on your beach.
9.Every model leaves things out. Name two things about a real tsunami that your drawing cannot show, and explain why each one matters for keeping people safe.

Draw the seafloor first

Start your drawing with the seafloor line only: a deep notch on the left for the trench, a long flat deep floor, a ramp up (the continental slope), then a short flat shelf and the beach. Write the depths under each part: about 4,000 m on the deep floor, about 200 m where the ramp meets the shelf, and about 10 m on the shelf near the beach. Then copy the three speeds from Figure 1 onto the matching parts. Only after that, draw the waves: a long, low bump over deep water, a shorter, taller bump at the shelf edge, and a short, tall bump on the shelf near the beach.

Why bays make it worse

Crescent City is known as one of the most tsunami-prone harbors on the U.S. West Coast, damaged in 1964, 2006 and 2011. Research how the shape of the seafloor offshore and the shape of a harbor can focus tsunami energy. Then add a narrow bay to your drawing (as a top view in the margin) and explain, using the idea of water being squeezed into a smaller space, why the same tsunami could be taller inside the bay than on a straight beach nearby.

Angela's Tools · Chapter 26 · Mass Wasting, Tsunamis, and FloodsWorksheet 4 of 10