Which detail from the passage best explains why an alert can reach people before the strongest shaking does?
Read an Informational Passage · ELPAC Grades 9–10 reading. The correct answer, and the reasoning behind it.
A longer factual text, with questions about the main idea, supporting details and vocabulary.
Read the passage. Then answer the questions.
A Few Seconds of Warning
No one can predict an earthquake. Scientists cannot say that a large one will strike next month, or even next year. What they can do, it turns out, is notice an earthquake that has already begun and warn people before the strongest shaking reaches them. The warning lasts seconds, not minutes, but a surprising amount can happen in a few seconds.
The idea depends on a difference between two kinds of waves. When a fault slips, it sends out a fast, mild wave called a P wave and a slower, stronger wave called an S wave. The P wave moves through rock much faster, so it arrives first and rarely does damage. The S wave, arriving after it, produces most of the shaking that cracks walls and throws objects off shelves.
California has placed a network of sensors across the state to listen for P waves. When several sensors near the start of an earthquake detect one, computers estimate where the quake began and how strong it is, and then send out an alert. Electronic signals travel far faster than any wave through rock, so the alert can outrun the shaking.
How much warning a person gets depends on distance. Someone far from where the earthquake began may have twenty seconds or more. Someone very close may get no warning at all, because the strong shaking reaches them before the system has finished its calculation. This is one reason the system matters most for large earthquakes: a large quake shakes a wide area, so many people are far enough away to benefit.
A few seconds are enough for more than most people expect. A person can drop to the ground, take cover under a sturdy table and hold on, which protects against the most common earthquake injuries, those caused by falling objects. Trains can begin to slow down. Elevators can be programmed to stop at the nearest floor and open their doors. A surgeon can lift an instrument away from a patient.
None of this makes an earthquake harmless. But it turns the first moments of one from a complete surprise into something people have a few seconds to meet.
The question
Choose the best answer.
Which detail from the passage best explains why an alert can reach people before the strongest shaking does?
- “Electronic signals travel far faster than any wave through rock.”Correct answer
- “The S wave, arriving after it, produces most of the shaking.”
- “A large quake shakes a wide area.”
- “Trains can begin to slow down.”
Why ““Electronic signals travel far faster than any wave through rock.”” is correct
This tests choosing the evidence that answers a 'why' question. The system detects the earthquake through the first, weak wave, but the alert still has to beat the strong wave to people's phones, and it can only do that because electronic signals move much faster than waves through rock. The passage makes the link itself with 'so the alert can outrun the shaking'.
Why the other answers are wrong
- “The S wave, arriving after it, produces most of the shaking.” — This explains why the S wave is dangerous, but not how an alert gets ahead of it.
- “A large quake shakes a wide area.” — This explains why large earthquakes benefit most from warnings, not why a warning can arrive in time.
- “Trains can begin to slow down.” — This is one use of the warning after it arrives, not a reason it arrives early.
Practice more of this
More reading practice for Grades 9–10, or see the ELPAC practice test for grades 9–10. For other grades, see free ELPAC practice tests by grade.
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