Ever wonder what it is that makes the sand under your feet at the beach start to glisten when you step down on it? One word: liquefaction. Basically soil or dirt that has water saturated into it can look almost like dry solid ground, but when disturbed by a large impact or ground disturbance like an earthquake, or in this case, your foot stepping down to shake things up and put pressure on the sand.
The shaking makes the lighter water molecules rise creating and instant liquid surface where moments before it was solid. This can wreck havoc on buildings that are relying on the ground staying solid to stand.