The moon may be shrinking, scientists say.

But not to worry. It won't disappear anytime soon.

NASA scientists studying images sent back to Earth from a spacecraft orbiting the moon have found curving lines of small cliffs on the surface that they say were thrust up as the moon's sizzling hot interior cooled and shrank the surface over the past billion years.

"This is the first evidence that the moon has been shrinking, and may still be shrinking," Michael Wargo, NASA's chief lunar scientist, said during a press briefing Thursday. But it's not enough shrinkage to notice from Earth.

The cliffs, called lobate scarps, were formed by thrust faults that triggered "moonquakes" in the lunar crust, according to a report today in the journal Science. The thrust faults pushed the lunar surface upward.

The action was much the same way a major thrust fault near Los Angeles caused the deadly Northridge earthquake of 1994.

The geometry of the scarps shows that the moon's radius has decreased by at least 300 feet over the past billion years, the scientists calculate.

Quakes on the moon were first detected by seismographs carried there by astronauts on the last three Apollo missions nearly 40 years ago. Astronauts Eugene Cernan and Harrison Schmitt of Apollo 17 explored one section of what is now called the Lee-Lincoln scarp at their site near the lunar equator, where a thrust fault quake not only created the scarp, but also caused rocks to tumble down from lunar highlands above.

Now the orbiter's images have revealed 14 more lobate scarps scattered across the entire lunar surface, north and south.

"The moon is no dead body in the sky, as people have always thought, but it's tectonically active even today," said Thomas R. Watters, a planetary geologist at the Smithsonian Institution in Washington, during Thursday's briefing. "Those scarps were probably formed less than a billion years ago, they were surely forming 100 million years ago, and they may still be forming today."

Watters wrote the report in Science along with a team of scientists that included Ross A. Beyer of NASA's Ames Research Center in Mountain View and the nearby SETI Institute.

There are many theories that seek to account for the moon's origin, Beyer noted. The most widely held by scientists suggests that some giant object the size of Mars crashed into Earth after it formed about 4.5 billion years ago. A huge chunk was blasted out of the newly formed Earth, and has been orbiting our planet ever since, the theory holds.

"That early moon was hot enough," Breyer said, but was bombarded ceaselessly by asteroids and comets containing radioactive elements for billions of years more and heated still more.

The moon's surface, Breyer explained, is relatively sandy and filled with tumbled debris from impact craters, while beneath it lies the solid lunar crust.

Last year, when NASA astronomers in Mountain View slammed a space rocket into a darkly shadowed crater on the moon in a successful search for water, the Lunar Reconnaissance Orbiter orbiting above was finding fresh evidence of the moon's strange history. The orbiter has sent the high-resolution images that led to today's report about the shrinking moon.