Debra Vyse, age 13, of Peterborough, Ontario, Canada, for her question:

What is the San Andreas Fault?

This fault, as no doubt you suspect, is a geological feature associated with earthquakes. Similar weak crustal zones around the Mediterranean were noted by early geologists. Later, others were traced in the New World, especially around the rims o€ the Pacific Ocean. One of the most famous of these is the San Andreas Fault. It extends from Mexico, through central California to north of San Francisco. There it leaves the land and travels on through the ocean floor.

Geologists see it as a fractured zone where enormous crustal plates are pushing against each other. Stupendous pressures build up deep underground. In time they become critical and the stresses are relieved by sudden earthquakes. The San Andreas causes numerous small quakes in California and occasionally a big one. Farther south, similar fault zones caused the recent disaster in Nicaragua and major earthquakes in Peru.