According to the passage, why might a person very close to where an earthquake begins receive no 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.