Also known as moonquakes. Or earthquakes on Luna. Whatever they are called, lunaquakes are a problem we need to understand and solve for.
[This section is from the Habitats and Shelters FAQ. Which is from Book 1 of the Lunar Homesteader’s Survival Guide Series. There isn’t any point of me re-writing what I’ve already written.]
Luna isn’t geologically active like the Earth, but it does experience quakes. Luna has shrunk more than 50 meters (150 feet) over the last few hundred million years as its interior cools . This shrinkage causes the surface to wrinkle and break, forming thrust faults and releasing energy.
We’ve identified four different types of lunaquakes:
- Deep quakes originating at least 700 km below the surface . These are probably caused by tidal stress due to Earth’s gravity . Generally mild and mostly harmless .
- Impact vibrations caused by… meteorite impacts. Not much to worry about unless you happen to be in the impact zone.
- Thermal expansion quakes. The Lunar crust gets really cold during Nightspan. It expands when sunlight starts to warm it up. These quakes are low energy and not anything to worry about.
- Shallow quakes originating 20-30 km below the surface . We’re not 100% sure what causes these quakes but the guess is that it’s slip faults and perhaps slumping of material off of large crater rims. These are the ones we need to prepare for.
The Apollo seismic network (each mission placed a Passive Seismic Experiment package at their location) registered 28 shallow quakes between 1972 and 1977 . Several of these were up to 5.5 on the Richter scale . These are considered “moderate” and powerful enough to damage poorly constructed buildings . That’s not the real problem though.
The kicker is that shallow quakes can last more than 10 minutes . On Earth, even the biggest quakes stop after less than 2 minutes . More than 10 minutes of shaking, even if it’s not very powerful, can put a lot of stress on structures. It can cause fatigue that weakens the building materials and can create air leaks at seams and seals. It’s also not going to be very pleasant to experience.
Homesteads are going to have to be lunaquake resistant. The first step is figuring out where shallow lunaquakes are most prevalent. We’ve got some limited data on this but the Apollo seismic network consisted of only 4 instruments, all located in one part of Luna. We’re going to need a global network distributed over the entire surface to get the data we need. A good start would be to avoid building Homesteads near identified fault lines.
The second step is figuring out when shallow lunaquakes occur. The Apollo data shows that more shallow lunaquakes occur when Luna is at its furthest point from the Earth (apogee) . It’s not a guarantee but Homesteaders can be a little more alert during that part of Luna’s orbit.
The third, and most important, step is designing the habitats and equipment to withstand magnitude 6.5 quakes (building in a buffer because there’s a lot we don’t know) lasting 20 minutes (adding another buffer). Not just once every couple of years (or decades), but on a monthly basis.
Lunaquake resistant engineering is beyond the scope of this book but it’s something we’re going to have to research and incorporate into every design (not just habitat designs). One bit of good news is that underground structures are typically less affected by quakes. I’m not a civil engineer or a geologist but from the few articles I’ve read it seems that surface structures are more affected because of the layers of soil they sit on. Structures tied into the bedrock (like skyscrapers) are better protected. Structures inside the bedrock are even more protected. It’s unclear how Homestead habitats embedded in the mega-regolith will fare but it’s a safe guess that they will be more protected than any surface structure as they will move with the material surrounding them.