Our Moon is a seismically active world with a long history of quakes stretching back to its early history. It turns out those quakes can and will affect the safety of permanent base structures for anybody planning to explore and inhabit the Moon. That's one conclusion from a study of quakes along the Lee-Lincoln fault in the Taurus-Littrow valley where the Apollo 17 astronauts landed in 1972. “The global distribution of young thrust faults like the Lee-Lincoln fault, their potential to be still active and the potential to form new thrust faults from ongoing contraction should be considered when planning the location and assessing stability of permanent outposts on the Moon,” said Smithsonian senior scientist emeritus Thomas R. Watters, lead author of the paper.
They base their work on evidence of moonquakes in the region over the past 90 million years, largely in material gathered by the Apollo astronauts. Chunks of rocks and landslides are mute proof of the power of magnitude 3.0 quakes to shift the surface materials around. Along with other active faults on the Moon, the Taurus-Littrow rocks and landslides show that our lunar companion is likely still geologically active.
Why Lunar Seismicity?
Here on Earth, we get earthquakes all the time. By some estimates, our planet shakes about 55 times a day, although many of these tremors are so weak we don't feel them. They happen largely due to plate tectonics and volcanic activity. Plates slip past each other very gradually, which releases energy that gets dissipated as an earthquake. We all know about the really famous spots on Earth for that kind of action - the San Andreas Fault line, the Ring of Fire in the Pacific, and parts of southeast Asia, for example. Volcanic activity also spurs earthquakes when underground magma causes "shudders" as it moves. Recent events such as the ongoing Kilauea eruptions in Hawai'i and those near Grindavik, Iceland, cause swarms of earthquakes as a result of that magma movement.
However, that's not how it works on the Moon. The two most likely causes for lunar quakes are tidal pulling and the continual cooling and shrinking of the Moon. The tidal quakes happen because Earth's gravity pulls on the Moon, which results in deep quakes up to hundreds of miles inside. Weaker quakes originate closer to the surface and those are generally thought to be due to lunar shrinkage. Since the Moon formed billions of years ago, it has lost about 150 feet of its diameter due to the gradual cooling after its birth. There are also very minor temblors that happen when a meteoroid slams into the surface, or when surface rocks react to heating and cooling from the Sun. All this activity describes a world that is constantly shaking and shuddering.
This artist’s concept shows the Moon’s hot interior and volcanism about 2 to 3 billion years ago. It is thought that volcanic activity on the lunar near side (the side facing Earth) helped create a landscape dominated by vast plains called mare, which are formed by molten rock that cooled and solidified. As the Moon has continued to cool, it has shrunk and its surface contracted. That causes scarps and fault lines to form. NASA/JPL-Caltech
Quakes and Risks
To understand the risk of quakes to future bases, Watters and research partner Nicholas Schmerr of the University of Maryland, studied materials from the Apollo 17 landing site. These rock samples, along with other details about rock falls and landslides on the Moon, told them that there are thousands of young thrust faults on the Moon. They point to a continual evolution of surface units, many caused by earthquake activities that create lunar thrust faults. That happens when rocks are compressed and one block is pushed up over another, generally as a result of the ongoing contraction of the Moon.
According to Watters and Schmerr, mission planners are going to have to consider those fault lines and the ongoing related lunar quakes when planning bases on the Moon. Short-term missions, like the Apollo landing, which had astronauts on the Moon for nearly 2 weeks, didn't face much danger from a quake or two. However, permanent bases face significant chances of damage during a quake, simply due to numbers. “If astronauts are there for a day, they’d just have very bad luck if there was a damaging event,” Schmerr pointed out. “But if you have a habitat or crewed mission up on the Moon for a whole decade, that’s 3,650 days times 1 in 20 million, or the risk of a hazardous moonquake becoming about 1 in 5,500. It’s similar to going from the extremely low odds of winning a lottery to much higher odds of being dealt a four of a kind poker hand.”
Taurus-Littrow valley taken by NASA’s Lunar Reconnaissance Orbiter spacecraft. The valley was explored in 1972 by the Apollo 17 mission astronauts Eugene Cernan and Harrison Schmitt. They had to zig-zag their lunar rover up and over the cliff face of the Lee-Lincoln fault scarp that cuts across this valley. Credits: NASA/GSFC/Arizona State University
Planning for Quakes
It's not just habitats and science missions that could be damaged by lunar quakes. Russia, China, and the U.S. are planning to put nuclear power plants on the Moon. Such facilities could supply all the power anyone needs for bases and exploration, but they come with a safety price and could be quite susceptible to quake damage. That's why any these and other places need to be built with tough safety margins, and not located near any active fault lines. That's going to be a tall order, considering the extent of quakes and the numbers of fault lines that thread through the Moon.
This is why the scientists' study of lunar paleoseismology is so important. Gathering evidence of past quakes (going back many millennia), as well as more recent ones, is going to help chart the safest places to build bases, habitats, and power plants. “If astronauts are there for a day, they’d just have very bad luck if there was a damaging event,” Schmerr added. “But if you have a habitat or crewed mission up on the Moon for a whole decade, that’s 3,650 days times 1 in 20 million, or the risk of a hazardous moonquake becoming about 1 in 5,500. It’s similar to going from the extremely low odds of winning a lottery to much higher odds of being dealt a four of a kind poker hand.”
For More Information
Paleoseismic Activity in the Moon's Taurus-Littrow Valley Inferred from Boulder Falls and Landslides
