# The case of the trembling table: this month, learn about detecting earthquakes.

SCIENCE CONTENT STANDARDS

* Abilities necessary to do scientific inquiry

* Properties of Earth materials

* Abilities necessary to do scientific inquiry

* Natural hazards

ESTIMATED TIME: 60 minutes

Set a Purpose

Background

* A seismometer, also known as a seismograph, is an instrument that measures the vibrations of an earthquake. The record made by the seismometer is called a seismogram.

* The point in Earth's crust where an earthquake starts is called the focus. The point on Earth's surface that is directly above the focus is called the epicenter.

* The energy of an earthquake moves away from the focus in vibrating energy waves. There are two types of waves: primary waves, or P waves, and secondary waves, or S waves. P waves travel faster than S waves, but S waves are significantly larger than P waves.

* A seismometer will record the vibrations from both P waves and S waves. Scientists use the difference in the arrival time of the waves to help determine how far away the earthquake occurred. It is similar to how the time difference between thunder and lightning tells you how far away a storm is located. If you are farther away, there will be a greater difference in the arrival times.

Discussion Questions

* What is a science fair? (Possible answer: a competition where students create and present a project on a scientific concept.)

* What happens to the ground during an earthquake? (Possible answer: It shakes and vibrates.)

Discussion Questions

* What is a seismometer? (Possible answers: a device the measures vibrations; a device that scientists use to detect the vibrating energy waves of earthquakes.)

* What type of line would be created on a seismometer if an earthquake occurred very near the seismometer? (Possible answer: a very jagged line.)

RESOURCES

* http://earthquake.usgs.gov/learning/kids/There are lots of activities about earthquakes at this site.

* http://www.pbs.org/wnet/savageearth/index.html This site on the PBS program The Savage Earth includes teaching materials related to earthquakes.

It was the day of the school's science fair. Todd, Erika, Ali, and David were setting up their science projects on a long table.

Todd was at one end of the table. His model of the solar system was only half done. "I guess I won't win," he said. "I didn't finish my project."

Next to him, Erika was placing plants on the table. "I might win," she said proudly. "My experiment testing plant foods worked really well."

Todd noticed Ali setting up on the other side of Erika. A cone-shaped structure with a hole in the top was on the table. "Your project looks really cool, Ali," he said. "What is it?"

"I made a volcano," Ali replied. "It has baking soda inside it. When I add vinegar to it, it erupts!"

"I'll bet you'll win," said Todd.

SHAKE AND QUAKE

David was next to Ali. He put a cardboard box on its side on the table. A cup with a marker stuck through the bottom of it was hanging inside the box.

"It's a seismometer," said David. "It's a device that measures vibrations. Scientists use seismometers to detect the vibrating energy waves that travel through the ground during an earthquake."

[ILLUSTRATION OMITTED]

"How does it work?" Ali asked.

"I pull a piece of paper through the seismometer," explained David. "If there's a vibration, the marker shakes and makes a jagged line on the paper."

Ali noticed some pieces of paper on the table. Some of the pieces had very jagged lines on them. Others had relatively smooth lines. "Why are the lines different?" she asked.

"It depends on where the vibration occurs," said David. "If the vibration occurs nearby, it will create a jagged line. If it occurs far away, a smoother line will form. The lines made by seismometers help scientists determine how far away an earthquake was."

ALL SHOOK UP

"The judges will be at our table soon," said David. "I am going to test my seismometer."

"I can't test my experiment. I only have enough vinegar for one eruption," said Ali. She placed a small cup of vinegar on the table.

Just then, Erika bumped into the table as she was arranging her plants. Ali grabbed her vinegar-filled cup before it tipped over.

"Watch out, Erika!" Ali said. "If my vinegar spills, I won't be able to show my experiment."

Ali placed the cup back on the table. A second later, the table shook again. A little vinegar sloshed out of the cup and spilled onto the table. "Erika!" said Ali.

"Todd shook the table that time" said Erika. "He's still putting together his model."

[ILLUSTRATION OMITTED]

"Well, be careful," said Ali. "I am going to get a towel to clean the table before the judges come over." Ali walked away.

TIPPED OVER

Ali returned a minute later. "Oh no!" she exclaimed. Her cup of vinegar was lying on its side. Vinegar covered the table.

"I told you to be careful, Erika," Ali said. "You bumped the table and knocked over my vinegar!"

"It was Todd!" said Erika.

"I didn't do it," said Todd.

Ali stared at them. Erika was right next to Ali's volcano. Todd was a little way down the table.

"Yes," said David.

Ali looked at the piece of paper inside his seismometer. She saw a very jagged line on the paper.

"I know who did it!" she said.

solve, the mystery

Who knocked over Ali's vinegar?

To solve the mystery, grab these materials:

* Paper cup

* Scissors

* Marker

* Piece of string, 45 centimeters (16 inches) long

* Large cardboard box, empty with the top flaps cut off

* Yard stick

* Pinto beans

* Long table

* Paper

[ILLUSTRATION OMITTED]

1 Build a seismometer (see diagram). Have an adult poke three holes in a paper cup: One in the center of the cup's bottom and two holes opposite each other along the cup's rim. Put a marker through the hole in the bottom of the cup. The writing end should stick out of the bottom. Thread a piece of string through the two holes in the rim of the cup. Place an empty box on a table so that the open end faces you. Have an adult poke two holes one inch apart in the top of the box. Thread the ends of the string from the cup through the holes in the box. Position the string so that the marker in the cup is just touching the bottom of the box. Tie a knot in the string. Fill the cup half-full with pinto beans to weigh it down.

2 Use your seismometer to find out who knocked over Ali's cup. Place the seismometer on one end of a long table. Place a sheet of paper in the box so that the marker is touching the paper. Have a partner pull the paper slowly out of the box while you bump the table at a point two feet away from the seismometer. Note the line created on the paper. Place a clean sheet of paper in the bottom of the seismometer. While your partner pulls the paper, bump the table with the same amount of force as before. This time, bump the table six feet away from the seismometer. Note the line created. The vibration that knocked over Ali's cup made a very jagged line in David's seismometer. Who would have created that line?