Mary Ann Yurjevich, age 11, of Gary, Ind., for her question;

What is a fault line?

Earthquakes tend to occur in certain regions, and the lines along which the stresses snap are called faults. Fault lines run up and down California, and they have been traced, measured and studied for many years. The earth's crust is thought to be a jacket of massive blocks, 20 to 30 miles thick. In Some places, these crustal blocks are moving in different directions, or so the experts think.

Fault lines occur where the edges of two such moving blocks meet and brush together. For years the edges are held together by friction, meantime building up stress and tension. Then suddenly the two sides of the fault spring free, causing a shuddering earthquake to shake the ground.