IELTS Reading 41: The Race Against the Wave

Adapted from USGS / Caltech popular science (2025-2026). IELTS Reading Section 3 difficulty, approximately 1,050 words. Source: https://www.usgs.gov/programs/earthquake-hazards/science/earthquake-early-warning-overview

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Reading Passage

A. For as long as cities have been built on fault lines, earthquakes have arrived without warning, turning a normal morning into rubble in the space of a few seconds. The old ambition was prediction — to foretell the time, place and size of a coming quake days or hours in advance — and after decades of work that dream remains elusive, because the forces that build up along a fault and then release them do not obey any simple rule a scientist can read in advance; foreshocks have proved as misleading as they were promising, and every supposed pattern has later broken down. A more modest but achievable goal has therefore emerged: not to predict the earthquake before it begins, but to detect the instant it does begin and to alert everyone who will feel the damage seconds before it arrives. Those seconds sound trivial but, against collapsing masonry, snapping pipes and toppling shelves, they are a fortune — often long enough to make the difference between a glancing injury and a fatal one. That goal is no longer theoretical. In earthquake-prone regions around the world, networks of sensors now buy those seconds automatically, and they have already begun to save lives.

B. The whole system rests on a simple fact of physics. When a fault ruptures underground, it sends out two kinds of body wave, and they travel at different speeds. The first, called the P- or primary wave, is a fast compression that races through the Earth at around six kilometres a second, pushing and pulling the rock in the direction of travel and producing a gentle rolling sensation that most people barely notice. The second, the S- or secondary wave, moves more slowly — roughly three and a half kilometres a second — but carries far more energy and shears the rock sideways, producing the violent horizontal shaking that actually topples buildings and snaps bridges. Because the P-wave is faster, it reaches a seismometer near the fault before the damaging S-wave does, however close the two may have begun together at the source. That gap between the two arrivals is precious: the farther a place is from the epicentre, the longer the interval between the harmless early wave and the destructive one, and that interval is exactly what an early-warning system puts to work.

C. The system itself is a relay race against the slower wave. A dense network of ground sensors detects the first faint P-wave and instantly relays the signal to processing centres, where algorithms estimate, within seconds, where the quake began, how large it is and which towns will feel it. That information is then sent out as an electromagnetic alert — by radio, mobile-phone network and internet — which, travelling at the speed of light, reaches distant users long before the S-wave does. A city a hundred kilometres from the epicentre may receive several seconds' notice; a city two hundred kilometres away, more than ten — enough, in the best case, to take real cover. The alert is not a forecast but a snapshot of a quake that has already started, refined and updated as more sensors come online and the true size of the rupture becomes clear. In earthquake country such systems now stand guard on ordinary days, quiet and patient, until the ground beneath a distant fault suddenly lurches and the whole chain of detection and alert springs into life within the blink of an eye, sending out its warning to everyone the shaking has not yet reached.

D. Those few seconds, brief as they are, are surprisingly useful. An alert can stop a high-speed train before it leaves a vulnerable section of track, slow a lorry on a bridge, open elevator doors so passengers are not trapped between floors, and pause delicate operations in hospitals and factories. For ordinary people, a phone alarm telling them to drop, take cover and hold on for a few seconds can be enough to move away from a falling bookshelf or a window and into a safer spot under a desk. Children in seismically active cities now practise these drills so that, when the alert arrives, the reaction is automatic rather than panicked, and valuable seconds are not lost in confusion. Yet the same physics that makes the system work also sets its sharpest limit: directly above the epicentre, the P and S waves arrive almost together, leaving almost no time to warn anyone at all. The people in the greatest danger are precisely those the system cannot reach in time — the residents of towns sitting directly over the rupture itself.

E. Honest engineers are quick to separate what these systems do from what they do not. They are early-warning systems, not prediction machines: they cannot tell a town that a quake will strike next Tuesday, only that one has just struck nearby. They can make mistakes too, underestimating a large rupture in its opening seconds or, worse, sending a false alarm that sends an entire city diving under desks for nothing, which is why the algorithms are tuned carefully and the networks made dense enough that a single faulty sensor cannot trigger a panic. Warnings are only as good as the education that accompanies them; a brilliant alert is useless if no one knows what to do when it rings, or if people simply ignore their phones and carry on as before. Still, in a century of effort to live safely alongside earthquakes, the seconds these systems now buy represent one of the few genuine gains in preparedness. They cannot stop the wave. They can, sometimes, give people just enough time to meet it with their hands braced on a desk rather than at their sides.

Questions 1-4

Choose the correct heading for paragraphs B, C, D and E from the list of headings below.

List of Headings i. The physical gap between two kinds of seismic wave ii. Why predicting earthquakes in advance remains impossible iii. How sensors and algorithms relay the alert iv. What those few seconds are good for — and their limit v. How P-waves are used to generate electricity vi. Honest limits: warning is not prediction vii. The history of building earthquake-proof houses

1. Paragraph B: ____
2. Paragraph C: ____
3. Paragraph D: ____
4. Paragraph E: ____

Questions 5-8

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

5. Why has prediction been replaced by early warning? A. Earthquakes are now too small to predict. B. The forces behind a quake cannot be read in advance with any reliability. C. Scientists prefer to wait for the shaking. D. Prediction was banned by governments.

6. Which wave causes most of the damage? A. The fast P-wave. B. The slower S-wave and surface waves. C. Electromagnetic waves. D. Radio waves.

7. Why does a city farther from the epicentre get more warning time? A. It has better internet. B. The gap between the P-wave and the S-wave grows with distance. C. It is closer to the sensors. D. The S-wave stops at the border.

8. What is a key limitation of the system? A. It never sends any alerts. B. Directly above the epicentre, the P and S waves arrive almost together. C. It works only at night. D. It requires a forecast.

Questions 9-13

Do the following statements agree with the claims of the writer?

Write:

• TRUE if the statement agrees with the information
• FALSE if the statement contradicts the information
• NOT GIVEN if there is no information on this

9. The P-wave travels faster than the S-wave.
10. Early-warning systems can accurately predict earthquakes days in advance.
11. Alerts can be used to stop trains and open elevator doors.
12. People directly above the epicentre receive the longest warning.
13. Japan has the largest earthquake early warning budget in the world.

Questions 14-15

Complete the summary below using NO MORE THAN TWO WORDS from the passage.

The fast, harmless P-wave arrives first; the slower, destructive (14) __________ wave arrives later. The alert is sent by electromagnetic waves, which travel at the speed of (15) __________.

Answers & Explanations

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