Why the Moon Has Quakes and They Are Getting Stronger

Why the Moon Has Quakes and They Are Getting Stronger

The Moon experiences thousands of earthquakes per year — called moonquakes. Unlike Earth quakes caused by tectonic plates, moonquakes come from tidal forces, thermal expansion, and meteor impacts. And due to Earth's gravity, they're actually getting worse.

The Numbers

• 7,000 moonquakes detected by Apollo seismic network (1969-1977)
• 5 types of moonquakes identified
• Shallow quakes: Up to magnitude 5.5 on the Richter scale
• Deep quakes: Occur 700-1,000 km below the surface
• Tidal quakes: 1,300+ per year, triggered by Earth's gravitational pull

Types of Moonquakes

1. Deep moonquakes (most common):

• Depth: 700-1,000 km below surface
• Cause: Earth's tidal forces flex the Moon's interior
• Magnitude: Usually small (1-2), occasionally 4
• Pattern: Occur at predictable times related to the Moon's orbital position
• 700+ deep quakes detected per year by Apollo

2. Shallow moonquakes (most dangerous):

• Depth: 50-200 km below surface
• Cause: Internal stress and thermal expansion/contraction
• Magnitude: Up to 5.5 (would cause significant damage in a lunar base)
• Duration: Can last 10+ minutes (vs seconds for Earth quakes)
• Much longer because the Moon has no water to dampen seismic waves
• These are the biggest threat to future lunar settlements

3. Thermal moonquakes:

• Cause: Extreme temperature changes at the lunar surface
• Day side: 120°C, night side: -130°C → dramatic expansion/contraction
• Fractures and cracks in the surface rock
• Small but continuous, especially near sunrise and sunset

4. Impact moonquakes:

• Caused by meteoroids striking the surface
• Seismic waves propagate far due to the Moon's dry, rigid structure
• A basketball-sized meteoroid can create a detectable moonquake

5. Artificial moonquakes:

• Caused by Apollo missions (discarded equipment impacts)
• Used to study the Moon's internal structure

Why They're Getting Worse

Earth's gravitational influence:

• The Moon is slowly moving away from Earth (3.8 cm/year)
• Counterintuitively, this INCREASES tidal stress on certain parts of the Moon
• As the Moon's orbit changes, tidal forces shift, stressing new areas
• The Moon's interior is still cooling and contracting (losing heat since formation)
• Thermal contraction + tidal shifting = more quakes over time

Moonquakes on the surface are stronger than you'd expect:

• Lunar seismic waves travel much farther than on Earth (dry rock = no dampening)
• A magnitude 5 moonquake shakes an area 100x larger than an equivalent Earth quake
• A shallow moonquake at a proposed lunar base site could damage structures

Evidence and Discovery

Apollo missions (1969-1977):

• Astronauts placed 4 seismometers on the lunar surface
• Operated for 8 years, recording 7,000+ quakes
• Data still being analyzed today (many quakes weren't fully understood for decades)

Recent developments:

• 2024 reanalysis of Apollo data revealed previously unrecognized shallow moonquakes
• New models suggest shallow quakes are more frequent and powerful than originally thought
• NASA's Artemis program plans to deploy new seismometers
• China's Chang'e missions studying lunar geology

Implications for Lunar Settlements

Risks for Artemis/base construction:

• Shallow moonquakes are the primary seismic hazard
• Long duration (10+ minutes) vs Earth quakes (seconds)
• No building codes exist for lunar seismic resistance
• Underground habitats may be vulnerable to deep quakes
• Surface structures need to withstand sustained shaking

Design considerations:

• Flexible joints in structures to absorb movement
• Shock absorbers between habitat modules
• Seismic monitoring networks (continuous)
• Location selection based on seismic risk assessment

Fun Facts

• Moonquake waves can bounce back and forth through the Moon for over an hour (vs minutes on Earth)
• The Moon "rings like a bell" after large impacts
• Some Apollo seismometers detected the Apollo 12 LM impact from 180 km away
• The Moon has no tectonic plates but still has quakes
• Moonquakes are strongest when the Moon is at its closest point to Earth (perigee)

The Takeaway

The Moon may look dead and still, but it's seismically active — with thousands of quakes per year driven by Earth's gravity and thermal stress. As NASA and other space agencies plan permanent lunar settlements, understanding moonquakes isn't academic curiosity; it's a critical safety requirement. A magnitude 5.5 shallow moonquake lasting 10 minutes would be devastating to any surface structure. The Moon is not the stable, inert rock we imagined — it's a living, shaking world, and future astronauts will feel it.