The question of whether it is safe to be underground during an earthquake does not have a simple answer. Safety levels are highly dependent on two main factors: the depth of the underground space and the specific type of structure involved. Historically, underground structures have often fared better than buildings on the surface, but this generalization hides a complex set of unique risks.
How Seismic Waves Affect Underground Structures
The ground shaking experienced on the surface is not uniformly transmitted down into the earth. Seismic waves lose energy, or attenuate, as they travel deeper into the subsurface. This dampening effect is especially noticeable for high-frequency seismic waves, which cause the sharp, violent shaking damaging to rigid surface structures. Peak ground acceleration values generally decrease with increasing depth below the surface.
Surface waves, responsible for much of the destruction above ground, are confined to the near-surface layers and diminish rapidly with depth. Structures built deep within stable rock formations often experience less intense shaking than those at the surface. Conversely, underground facilities can experience greater movement from low-frequency seismic waves, which cause a slower, rolling deformation. This movement primarily stresses the structure through compression and shear forces imposed by the surrounding earth.
Safety in Shallow Underground Environments
Shallow underground spaces, such as residential basements, utility vaults, and parking garages, present a distinct hazard profile. The primary danger is the catastrophic failure of the surface structure above. If the building overhead collapses, the shallow space can be crushed by the weight of the debris in a process sometimes called “pancaking.”
These spaces are also highly vulnerable to foundation shear and ground failure near tunnel portals or entry points. The collapse of the Daikai Subway station during the 1995 Kobe Earthquake serves as a reminder of how vulnerable even engineered shallow structures can be in soft or poorly supported soil. While the surrounding earth may protect the sides, the ceiling and entry points connected to the surface remain critical weak links.
Safety in Deep Tunnels and Mines
Highly engineered, deep subterranean environments like major subway lines or deep mines often demonstrate a higher resilience to earthquake shaking. Damage decreases significantly as the overburden depth increases due to the wave dampening effect of the rock. These facilities are designed to withstand the immense static pressure of the overlying rock mass, which contributes to their stability during seismic events.
Deep structures face a unique array of geological risks distinct from surface building collapse. Collapses can be triggered by movement along existing fault lines or fissures that cross the tunnel path, causing axial shifting and shearing of the lining. Underground mines, especially at depths exceeding 1,000 meters, are susceptible to rock bursts—sudden and violent failures of rock under high stress, sometimes triggered by seismic energy.
Secondary hazards also pose a significant threat in these deep environments. Earthquakes can cause:
- Rockfalls from unsupported ceilings or walls.
- Blocked tunnel portals due to landslides.
- Failure of ventilation and life support systems.
- Increased outgassing and risk of gas explosions in mines due to coalescing hairline cracks.
- Severe consequences for rail systems due to minor track distortion.
Immediate Survival Protocol
If you find yourself in an underground space when shaking begins, your immediate reaction should follow the standard “Drop, Cover, and Hold On” protocol. Drop to your hands and knees to prevent being thrown off balance by the sudden movement. Immediately cover your head and neck with your arms or by crawling under a sturdy object, such as a bench or table.
Hold onto your cover until the shaking stops, being prepared to move with it if the structure shifts. Avoid trying to run toward an exit or climbing stairs. If you are on a subway or transit system, remain seated or braced and wait for instructions from the train operator or emergency personnel once the shaking has completely ceased.