Sandra Long, age 11, of Elberton, Georgia, for her question:

How deep do earthquakes go?

Earthquakes, as we know, are caused by sudden changes in massive blocks of the earth's crust. These enormous blocks enfold the surface of the globe like rather restless, mis¬shapen tiles. Their sizes vary from small islands to vast continental plains. Their thickness varies from five to 40 miles or more. Earth scientists still cannot explain all the stresses that cause some of these crustal blocks to shift with shuddering quakes.

The February 9 earthquake that shook Southern California was triggered by a crustal shift about ten miles below the surface. Others may be triggered by shifts nearer the surface and perhaps some are started by upheavals that occur in the very depths of the crust. In any case, the crustal upheaval of a major quake sends shivering vibrations straight down to the core of the earth and others that curve through the crust clear around the globe.