Did your brick “building” topple over? Did your ping-pong ball rise to the surface? Did the sands flow like a liquid?
Sands feel solid because grains touch and support each other. Between the sand grains are pores—empty spaces that make up to 50 percent of the volume of the sand. Often, these spaces are filled with water, called groundwater.
When loose or unconsolidated sediments are shaken, they try to settle into new positions. However, when seismic waves from an earthquake hit an area, the sand and water are rapidly compressed. This can cause the water pressure in the ground to go up significantly. Ground failure happens when this high-pressure water causes a reduction of friction between sand grains. When grain-to-grain contact is lost, sediments can flow like liquid. This phenomenon is called liquefaction.
In the case of the brick building, liquefaction causes uneven support of the brick’s base, so it topples over. As for the ping-pong ball “storage tank,” its density is less than that of the surrounding sediments. It’s held underground by the weight of the solid sediments above—until an earthquake takes place. If the sediment undergoes liquefaction, the buoyancy of the ping-pong ball causes it to float up and through the temporarily liquid sediment.