Experience shows that underground structures, especially deep ones, are far less vulnerable to earthquakes than superficial ones. The latter are endangered by earthquakes due to the fact that the motion of the ground can be amplified by the response of the structure to such an extent that the induced strains damage the structure. The earthquake waves can also be amplified within soft superficial strata. In addition, loose water-saturated soil may loose its strength (so-called liquefaction), and this can lead to landslides or failure of foundations and retaining walls. In contrast, deep buried structures, especially flexible ones, are not expected to oscillate independently of the surrounding ground, i.e. amplification of the ground motion can be excluded. This is manifested by the relatively low earthquake damage of tunnels.
Of course, the portals may be damaged by earthquake-induced landslides. Very revealing on earthquake effects is the report of what happened to the driving of a 7 m diameter tunnel in the underground of Los Angeles during the San Fernando M 6.7 earthquake in 1971:
‘The earthquake caused an outage of electrical power that caused the tunnel pumps to stop. Amid the attendant confusion and anxiety, the miners made their way to the locomotive and drove 5 miles out of the tunnel in pitch darkness. This means that the rails were not significantly distorted to cause a derailment. However, Southern Pacific Railroad tracks on the surface were distorted and broken.’