雅思阅读 107: Thunderquakes X-Ray the Earth

Adapted from 'How thunderquakes could help reveal hidden structures inside the Earth', Scientific American, August 2026.

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Reading Passage

When lightning strikes, it superheats the air and creates a shock wave we hear as thunder. But thunder does not only travel through the air. When that energy reaches the ground, some is converted into seismic waves that ripple through soil and rock beneath. The resulting vibrations are known as thunderquakes.

Until now, scientists had not understood these signals well enough to use them. New research shows thunderquakes can map the structure beneath the Earths subsurface, such as groundwater movement, contamination, sinkholes or building-site foundations, much like X-rays image the human body.

The discovery came from listening to thunder with an unusual instrument: fiber-optic cable. Using a technique called distributed acoustic sensing, researchers turned an ordinary fiber-optic cable, the kind used for internet service, into a string of thousands of vibration sensors.

A laser-pulsing computer called an interrogator sends light through the cable and measures tiny changes caused by vibrations. In the Penn State FORESEE project, the team used a cable over 2 miles long, yielding more than 2,100 sensors spaced a few feet apart. Over two years, they recorded 458 clear thunderquakes.

The most important seismic waves produced are air-coupled Rayleigh waves, measurable at the surface but capable of studying the subsurface down to about 300 feet. The trick is seismic dispersion: waves of different frequencies travel at different depths. By measuring how wave speeds changed with frequency, researchers reconstructed seismic wave speeds at different depths, producing an X-ray of the subsurface without drilling a single hole.

Imaging the subsurface usually requires truck-mounted vibration sources or sensor arrays, which are expensive. Thunderstorms offer a natural seismic source already spread across the landscape, and fiber-optic cables lie beneath cities worldwide. Instead of bringing a seismic source to the ground, we listen to storms passing overhead.

At the test site in State College, Pennsylvania, the geology is mostly limestone and dolomite, rocks that dissolve as groundwater moves through them, creating caves and sinkholes. The study revealed four weak zones where seismic waves traveled slowly, two of which matched areas where satellite radar shows ground subsidence.

Karst landscapes cover roughly 20 percent of continental land area and affect nearly a quarter of the global population. The technique may also apply to sonic booms, volcanic eruptions and even other planets such as Saturns moon Titan.

Questions 1-5

Choose the correct letter, A, B, C or D.

1. What are thunderquakes? A. Thunder caused by earthquakes B. Seismic waves from thunder energy reaching the ground C. Lightning that hits the ground D. A type of tornado
2. What is distributed acoustic sensing? A. A new microphone B. Turning fiber-optic cable into vibration sensors C. Seismology D. A type of thunder
3. How long was the cable used? A. 200 feet B. Over 2 miles C. 20 miles D. 2,000 miles
4. How deep can the technique image? A. 30 feet B. About 300 feet C. 3,000 feet D. 3 miles
5. What geology is found in State College? A. Granite B. Limestone and dolomite C. Basalt D. Sandstone

Questions 6-10

Do the following statements agree with the information? Write TRUE, FALSE or NOT GIVEN.

6. The team recorded 458 thunderquakes over two years.
7. Drilling is required for the technique.
8. Karst landscapes cover 20 percent of continental land area.
9. The weak zones all matched satellite radar subsidence data.
10. The technique could be used on other planets.

Questions 11-13

Complete the summary. Choose ONE WORD ONLY.

Fiber-optic cable is turned into thousands of vibration 11. The technique uses seismic 12 to image different depths. At State College, four weak 13 were found.

Answers

1. B 2. B 3. B 4. B 5. B
2. TRUE 7. FALSE 8. TRUE 9. FALSE 10. TRUE
3. sensors 12. dispersion 13. zones

Key Vocabulary

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