What is the central idea of the passage? (A Few Seconds of Warning)
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.
What is the central idea of the passage?
- P waves travel faster than S waves and rarely cause damage.
- Scientists will soon be able to predict earthquakes months in advance.
- Earthquakes cannot be predicted, but a warning system can detect one in progress and give people a few useful seconds.Correct answer
- People who live close to a fault should not rely on earthquake warnings.
Why “Earthquakes cannot be predicted, but a warning system can detect one in progress and give people a few useful seconds.” is correct
A central idea has to cover the whole passage, not one paragraph of it. This passage opens by separating prediction from detection, explains how detection works, describes who gets warned, and ends with what the seconds of warning allow. Only the answer that includes both the limit (no prediction) and the benefit (a few useful seconds) covers all of it.
Why the other answers are wrong
- P waves travel faster than S waves and rarely cause damage. — This is a supporting detail from the second paragraph that explains how the system works.
- Scientists will soon be able to predict earthquakes months in advance. — The passage says the opposite in its first two sentences: no one can predict an earthquake.
- People who live close to a fault should not rely on earthquake warnings. — The passage says people very close may get no warning, but it never tells anyone not to rely on the system.
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.
This is an independent study resource. It is not affiliated with, endorsed by, or connected to the California Department of Education or the official ELPAC program. All questions here are original practice material written to match the published task formats.