单项选择题

What if an earthquake were coming, and you had just a little bit of warning It might be only 15 seconds, maybe 30--but it might be enough time to take cover under a table, or find the safety offered by a door frame.
Italian scientists say they have figured out a way to measure the shock waves from a quake just two seconds after they begin, and, from that, calculate the earthquake’s strength, location, and potential for damage in populated regions.
American researchers have been at work on this as well, hoping to set up an early warning system in California and other earthquake-prone areas.
If researchers succeed, they might be able to place seismometers up and down fault lines, and measure the P-waves--the primary shock waves from an earthquake-- several critical seconds before they reach the places where people live. P-waves may be useful for warnings; they are not nearly as strong as secondary vibrations--known as S-waves--that do most of the damage. It typically takes about 10 seconds for P-waves to spread 40 miles. S-waves travel about half as fast.
But a warning signal, transmitted electronically from a seismometer near the epicenter (震中), would spread at the speed of light. That could mean extra time for gas companies to close automatic valves on their supply pipes, electric companies to isolate the vulnerable sections of their grid and people to run for cover.
Many geologists have been skeptical that the first P-waves can tell very much about the strong vibrations to follow. But the new research suggests they’re reliable for setting off warnings.
"We can determine the magnitude within a couple of seconds of initiation of rupture and predict the ground motion from seconds to tens of seconds before it’s felt," said Richard Allen, an assistant professor of earth and planetary science at the University of California, Berkeley, when he and colleagues published a paper on early warnings last year in the journal Nature.
They calculate that in the worst earthquakes, a well-designed system could give San Francisco, for instance, an extra 20 seconds to put systems in safe mode. That is, if the warnings are accurate, nobody wants a very expensive system that constantly gives false alarms.
That’s where the Italian researchers come in. They report that in just the first two seconds after a major earthquake, they can get enough data for an accurate picture of where it will spread, and how violent it will be.
Could all this make a difference when a major earthquake strikes Science has been frustrated in its attempts to predict when one will happen, so, as many geologists argue, every nanosecond’s warning will help.
Which of the following statements is true about P-waves