That was an incredibly simple and effective explanation:

>Imagine a plate of fruit-filled gelatin dessert. Tunnels are like the pieces of fruit at the base of the gelatin, while above-ground structures are like the fruit toward the top. If you shake the plate, the movement becomes more exaggerated as it flows up from the base of the gelatin. In an earthquake, this translates to tunnel movement measured in inches, while the movement above ground might be measured in feet.

Interesting. Would the safety of a tunnel depend on whether the most powerful pressure waves were traveling parallel with the tunnel or perpendicular to it?