Ruth Ann Harrison, age 9, of Nellis, W. Virginia, for her question:

What causes an earthquake?

To the people of Charleston, earthquakes seem to happen in far away places. We hear about many such disasters way down in South America. These happen where the high Andes Mountains swoop down to dip their toes in the Pacific Ocean. Most earthquakes happen in certain places called earthquake zones. And many of the world's earthquake zones are where tall mountains meet the sea.

Experts tell us that earthquakes happen deep down in the earth's crust. This crust is a rocky layer that wraps around the globe somewhat like the peel of an orange. In some places it is cracked and broken into great slabs. Sometimes the sides of two slabs rub together and this forms a weak, shaky region in the crust. Earthquakes happen when two slabs shake loose and move in different directions. If this happens with a sudden jerk, a shuddering earthquake shakes the ground for miles and miles and miles