Between 1969 and 1972, five Apollo missions installed seismic stations at their landing sites on the nearside of the moon. Because the moon was thought to be seismically dead, the instruments were left almost as an afterthought to detect meteor strikes. But from the time the stations were switched on until they were decommissioned in 1977, they recorded hundreds of internally generated moonquakes, some as strong as magnitude 5.5 on the Richter scale.

For 40 years, scientists have scoured the Apollo seismic data for an explanation of these moonquakes. Because the moon lacks active plate tectonics, moonquakes must be driven by different forces than most quakes on Earth. Extreme temperature changes may account for the less common shallow moonquakes, but a good explanation for deep moonquakes remains elusive. Now, a new study crosses one long-standing theory about what triggers deep moonquakes off the list. Instead, the study suggests, moonquakes might have more in common with earthquakes than previously thought.

“Very early on, scientists recognized a link between moonquakes and the tidal forces exerted on the moon by the gravitational pull of the Earth,” says Bruce Bills, a geophysicist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., and co-author of the new study published in the Journal of Geophysical Research. “Moonquakes reoccur again and again in the same locations at the same time of the month, which seems to link them to the monthly tidal cycles” he says, “but so far, nobody has been able to construct a physical model or clear pattern to explain the relationship.”

The relationship between tides and moonquakes had been explored before, Bills says, but “most of the moonquake studies were done back in the 1970s soon after the data were collected, and we’ve learned a lot about geophysics and modeling since then.” By comparing data about where and when the moonquakes occurred with the well-understood cycles of tidal forcing, Bills and colleagues thought they could flesh out a moonquake pattern. But, Bills says, “it didn’t work out that way at all. Clearly there’s something else involved here besides the tides.”

One problem with the tidal forcing hypothesis, he says, is that while tides exert the same pressures from month to month on the entire moon, moonquakes only occur in limited regions. “If the tides were the only force involved in generating moonquakes, we’d expect them to be more widespread,” Bills says.

“The deep moonquake problem is similar in some ways to the deep earthquake problem,” says Cliff Frohlich, a seismologist at the University of Texas in Austin who was not involved in the new study. Deep earthquakes occur at extreme temperatures and pressures where brittle fracture, the cause of most shallow earthquakes, is not possible. So other phenomena — like dehydration embrittlement, where water and other volatiles open cracks in otherwise plastic rock, and transformational faulting, where minerals change to weaker or denser phases under extreme pressure, causing collapse — are thought to play a role, he says.

Because water has yet to be discovered on the moon, Bills and colleagues suggest mineral phase changes may make certain areas of the moon weaker, resulting in moonquakes triggered by tidal pressure. “This is one of the first broader viewpoints I’ve seen,” says Yosio Nakamura, also of the University of Texas at Austin. “Other studies have failed to find a clear relationship between tides and moonquakes, but this is the first to suggest an alternate mechanism.”

For now, Bills and colleagues plan to continue to study the Apollo data, but they say more data, collected by more instruments over a wider area of the moon, are needed to test their transformational faulting hypothesis. Frohlich and Nakamura are also hoping NASA will return to the moon and install a few dozen seismic stations. “Right now, we know virtually nothing about the moon’s interior,” Frohlich says. “So we have to make a lot of assumptions about the moon’s internal composition, which means all our theories are pretty sketchy. Sometimes so much speculation is fun, but more often, as in the case with deep moonquakes, it’s just frustrating.”