Mr. Moonquake: Old Apollo data gains new relevance as scientists revisit the pioneering research of Yosio Nakamura By Lisa M. Pinsker, Jan. 11, 2006 On a cold autumn night in 1957, a young man stared up at the skies with wonder at a small satellite that would change the world, Sputnik. A member of his local astronomy club in Japan, Yosio Nakamura ended up tracking the satellite every night that fall, ultimately correctly predicting the object's orbit. The event opened up a new world of possibility to Nakamura, then in his early 20s: He now thought it would not be long before people would visit the moon and planets to study them just as people studied Earth. More than a decade later, Nakamura would be a member of the Apollo science team, and some four decades later, he would be working on the next generation of missions to the moon. But Nakamura is not an astronomer; he is a seismologist—a pioneering founder of lunar seismology, the study of "moonquakes," seismic events on the moon similar to earthquakes. Over the last 30 years, he has transformed scientists' view of the moon from a dead object in the night sky to a dynamic subject of geophysical study. "Yosio is the world’s leading authority on moonquakes," says Cliff Frohlich, associate director of the Jackson School of Geosciences' Institute for Geophysics at The University of Texas in Austin, where Nakamura was on the faculty from 1972 until earlier this year, when he retired to emeritus status. Just as studying earthquakes tells geoscientists about the structure of the planet, studying moonquakes tells them about the structure of the moon; researchers can look at how seismic waves travel to understand what the interior looks like. Only by understanding the interior of the moon can scientists fully piece together the story of how the moon and Earth formed and evolved over time. "The whole effort to understand the origin and the evolution of the moon relates directly to understanding the origin and early evolution of Earth," says Harrison Schmitt, an Apollo 17 astronaut and the only geologist to walk on the moon, who is now a professor at the University of Wisconsin in Madison.

Yosio Nakamura ___________________ Also see: Nakamura's Research Profile New computers uncover old quakes on the moon, Discover Magazine, January 2006

Detail from image of stacked seismograms of newly discovered far-side deep moonquake A285 at stations 12, 14, 15 and 16. See larger image.

Nakamura sees his work as only one step in that process to understand the solar system. "The moon is the first step beyond Earth for this endeavor, and each planet and satellite will add a new insight into this effort."

Early lunar seismology

Nakamura, who was born in 1934 in Sendai, Japan, moved to the United States in 1958 to attend graduate school at Pennsylvania State University. After completing his Ph.D. there, he took a job at General Dynamics to test aircraft components using seismology principles. At that time, the company was also supporting the U.S. space program, which was in full swing, and Nakamura learned of a group of scientists at the Lamont Geological Observatory (now Lamont-Doherty Earth Observatory) of Columbia University who were planning to establish a seismic network on the moon. Nakamura took a year’s leave of absence from General Dynamics to join the Lamont team. The team's efforts led to astronauts for the Apollo 11, 12, 14 and 16 missions installing seismometers at their landing sites, which would send the digital data back to Earth in real-time.

After Apollo 16, in 1972, the late Maurice (Doc) Ewing left Lamont to direct the newly established Galveston Geophysics Laboratory, the predecessor of the Institute for Geophysics, transferring the Apollo seismic project from Columbia University to the University of Texas. It was then, fascinated by the enormous seismic dataset, that Nakamura joined the group full time.

Although Apollo 11's instrument failed after a month, the other four stations recorded more than 12,000 seismic events through 1977. What he and colleagues would learn from that data "far exceeded our initial expectations," Nakamura says.

Unlike Earth, the moon has no plate tectonics—it is geologically quiet. Prior to the Apollo seismic project, most scientists conjectured that any seismic activity on the planetary body would be from rocks, or meteoroids, hitting the surface or from temperature changes at the surface. But the Apollo seismologists, led by Nakamura's University of Texas colleague, Gary Latham, found several types of moonquakes, at varying depths and from different sources.

Most surprising were moonquakes that seemed to be occurring nearly halfway to the center of the moon, at about 1,000 kilometers, deeper than any earthquake ever recorded. "The deep moonquakes were a complete surprise," Frohlich says. "We could not have imagined anything like them from our experience with terrestrial earthquakes."

Newly determined epicentral locations of deep moonquakes on the lunar front side. See larger images and extended caption.

The early Apollo scientists, including Nakamura, found 1,360 of these deep moonquakes through the arduous process of plotting the seismogram record on translucent papers and then using a light table to visually compare their wave patterns one by one. Even though the data were digital, computers at the time were not advanced enough to support digital analysis.

Complicating this analysis, Frohlich says, was the odd nature of the seismic waves. "When seismologists first saw the lunar data, they were expecting that it would be like freshman seismology Earth data," with distinct textbook wave patterns. What they found with the lunar data instead were very dispersed, "messy" signals, with waves that "would just ring," lasting from tens of minutes to hours and making it hard to analyze the signals.

Despite these challenges, however, the early lunar seismologists discovered that the moonquakes repeatedly came from about 100 locations and rates that mirrored the 27-day orbit of the moon around Earth. They determined that deep quakes were thus the result of tidal forces. "Although the moon does not have oceans, the solid part of the moon also experiences the same forces of the tides we’re experiencing here," Nakamura says. "That means that the shape of the moon changes at monthly intervals, and when the moon’s shape changes, stress inside the moon causes moonquakes."

Unanswered questions

Nakamura says that he and colleagues were fortunate to have the minimum data necessary to locate these deep moonquakes, in addition to a handful of shallow quakes. The early analyses, however, were not enough: Nakamura had many unanswered questions about the nature of the events and their implications for the moon's interior structure.

Because Apollo scientists needed the data streaming in real-time, all the seismic stations were on the side of the moon facing Earth, so there was no data from the "far side." Although Nakamura initially found one location, or "nest," of repeating moonquakes on the far side of the moon, just over the boundary between the Earth-facing and far sides of the moon, he was not sure whether there were any more nests on the far side, or whether the moon's core was preventing seismic waves from the far side from registering on the Apollo seismometers. He needed more data.

By the 1980s, most of the money had dried up from Apollo research, Frohlich says, and only Nakamura and a few of his close associates at UT were working on moonquakes. It was largely through the persistence of those few individuals, he says, that the original data were preserved.

In 2000, the advent of high-powered desktop computers allowed Nakamura to revisit the data and develop new digital techniques to sift through the thousands of lunar seismic signals and compare the wave patterns. "It’s a story about how the computers caught up with the data," Frohlich says.

Jim Dorman (top image, second from right) , a member of the Apollo team and associate director of UTIG's Galveston Lab, and the 1970s-era computing power of the Institute for Geophysics during its Galveston phase. See larger images.

In recent years, Nakamura has used his computer techniques to find many more deep moonquakes, bringing the total to 7,245, and many more nests, now at 240. While some of the new nests are on the far side of the moon, there are still far fewer on the far side than the near side, leaving the lingering question about the nature of the moon's core. "This may be a problem left for future lunar missions," Nakamura says.

Indeed, Nakamura is now part of the scientific development team on a proposal to put a global seismic network on the moon. Clive Neal, a visiting scientist at the Lunar and Planetary Institute in Houston, Texas, is heading up the project, which he hopes will be part of NASA's larger initiative to return to the moon, mars and "beyond," as directed in 2002 by President Bush. "Because the Apollo seismic array was in a very narrow region and only on one side of the moon, we can’t see into the lunar interior very far," he says. A global network, he says, would enable scientists to see deeper.

When Neal first started looking into the project, he made many calls to Nakamura to get a handle of the Apollo seismic data. "I’ve been very inspired by his enthusiasm for the subject, even though we haven’t had any new seismic data since 1977," Neal says.

Important to the current proposal, Neal says, is that the Apollo data has shown the possibility for very large moonquakes, as large as magnitude-5, from the 28 shallow quakes Nakamura found. With the president's space initiative calling for a permanent habitat on the moon, having more data to pinpoint possible large events is vital. "If you’re trying to maintain an airtight habitat, having it rocked around by a magnitude-5 may not be a good thing, so we need to find out where these moonquakes actually occur."

Lunar origins

But more exciting to Neal than the hazards application is the prospect of better understanding the lunar interior, and thus the formation of the moon. The leading hypothesis of how the moon formed is that a Mars-sized asteroid hit Earth more than 4.5 billion years ago, throwing out debris that coalesced into Earth’s satellite.

Astronaut and geologist Schmitt, however, says that recent research about the moon's mantle, some of which draws on Nakamura's seismic work, calls that hypothesis into question. A new seismic network is "clearly one of the best ways to test that hypothesis, and it should be tested in the return to the moon science."

Harrison says that the moon tells us most about the time from 3.8 billion years back to the beginning of the solar system. "This is the period of time on Earth that we know least about," he says, "and it is also the period of time in which the organic precursors to life were forming on Earth in an extraordinarily violent impact environment."

Nakamura is excited about the potential to learn about the moon's past through a global seismic network, but he says that much work still remains in analyzing the old data. Right now, he is trying to determine what causes the 28 shallow moonquakes, by better locating more such events. "The more you learn about the moon based on existing data, the more questions arise that need to be answered by future missions," he says.

Lisa Pinsker is managing editor of Geotimes magazine, published by the American Geological Institute.