Number of known moonquakes tripled with discovery in Apollo archive‍

The discovery in the Apollo archive has tripled the number of known moonquakes. This significant finding sheds new light on lunar seismic activity, enhancing our understanding of the Moon's geological processes. The data unearthed from historical missions like Apollo provides valuable insights into celestial bodies beyond Earth.

Within 22,000 new strikes are dozens of mysterious quakes concentrated in the north

THE WOODLANDS, TEXAS—The Moon suddenly seems more alive. From 1969 to 1977, seismometers left on the lunar surface by the Apollo astronauts detected thousands of distinctive “moonquakes.” Now, half a century later, a new analysis has cut through the noise in the old data and nearly tripled the number of moonquakes, adding more than 22,000 new quakes to 13,000 previously identified ones.

The finding, presented last week here at the Lunar and Planetary Science Conference, shows “that the Moon may be more seismically and tectonically active today than we had thought,” says Jeffrey Andrews-Hanna, a geophysicist at the University of Arizona unaffiliated with the work, which is in review at the Journal of Geophysical Research. “It is incredible that after 50 years we are still finding new surprises in the data.”

The Apollo missions placed two kinds of seismometers on the surface: ones capable of capturing the 3D motion of long-period seismic waves, and others that recorded more rapid shaking. All of the previously known moonquakes came from the long-period instrument; the short-period data sets contained so much noise, generated by temperature swings between the lunar days and nights, and errors from the radio transmission of the data, that they were indecipherable to 1980s technology. For decades they sat largely untouched. “Literally no one checked all of the short-period data before,” says Keisuke Onodera, a seismologist at the University of Tokyo who conducted the new archival analysis.

Yosio Nakamura, a geophysicist now retired from the University of Texas (UT) at Austin who worked on the Apollo program, long suspected there were more quakes hidden within the data. “We thought there must be many, many more,” he says. “But we couldn’t find them.”

The fact that the data were still around to analyze is a testament to Nakamura. After NASA turned off the functioning seismometers to invest instead in Skylab and the Space Shuttle, the agency lost interest in the records—and eventually even lost some of them. But UT maintained its own archive on 12,000 reel-to-reel tapes. Nakamura set out to transfer the data to cassette tapes in the 1990s, but after a reviewer stated that the agency already “knew everything” about the data, NASA declined to fund the effort. Eventually, the Japan Aerospace Exploration Agency supported the work. More recently, Nakamura helped NASA scientists redigitize the records and make them available online in standard seismic formatting.

To pluck the moonquakes from the short-period data, Onodera didn’t need to apply any fancy artificial intelligence algorithms. Instead, he used well-established “denoising” techniques that, much like noise-canceling headphones, subtract a moving average of the record to catch the remaining signal. It took 3 months to hunt down some 30,000 candidates. Onodera inspected each one to confirm it had the expected seismic shape of a moonquake. “That was the tough part,” he says.

Most moonquakes are thought to be little tremors from meteorites or small asteroids bombarding the surface, or very deep shudders caused regularly by Earth’s tidal pull. But among the 22,000 new quakes were 46 mysterious ones that were relatively shallow, seeming to come from 10 or more kilometers below the surface. The 28 similar moonquakes of this kind previously identified by the long-period seismometers seemed exceptionally strong, up to magnitude 5.5—enough to present a hazard to astronauts. The new quakes are smaller in magnitude, but Alice Turner, a UT seismologist, is confident that they belong to the same class. She developed her own algorithm to pick out shallow quakes from Onodera’s data and found the same ones. “We now have two independent ways of identifying that these signals are shallow moonquakes,” she says.

The combined 74 quakes have the potential to revise understanding of the Moon’s interior, Onodera says. Most of the new shallow quakes were seen by the Apollo 15 seismometer, the farthest north of the Apollo stations. This suggests the quakes could be regionally concentrated in the north, a pattern not seen before. It’s a cool observation, Turner says, although more work needs to be done to ensure that the Apollo 15 site isn’t biasing the results. “But the Moon is weird,” she says. “I don’t see any problem with it being a real thing.”

The number of smaller shallow moonquakes also changes the “b-value” of the Moon—the ratio of small-to-large quakes on an exponential scale. On Earth, this ratio varies by region: In the fractured landscapes near tectonic plate boundaries, the number of small quakes is high relative to large ones, whereas in the middle of plates, quakes are fewer but more are large. If these shallow moonquakes occur along near-surface faults created as the Moon cools and shrinks, as some researchers have suggested, Onodera’s data should show far more small quakes than large. Instead, the moonquakes’ b-value seems to resemble that of intraplate quakes, suggesting a deeper origin, Turner says. Onodera suspects the quakes could have something to do with ancient magmatic intrusions into the lunar crust in the northern hemisphere spotted a decade ago by NASA’s GRAIL mission—although what could be triggering the quakes now, long after the magma cooled, is a mystery.

Confirmation of Onodera’s work may come when NASA lands a seismometer on the Moon’s far side next year, as part of its commercial lunar delivery program. But until then, Nakamura is heartened to see the surge of interest in the Apollo data. “Who knows?” he says. “There may still be more data hidden in there.”

Sourced from Science