The San Andreas Fault begins in the Gulf of Cali­fornia, runs north through most of the state, then turns out to sea north of San Francisco. It is the "master fault" in a zone of continually shifting, straining rock and rock fractures.

The Fault extends 20 to 30 miles down into the crust, and at various depths ranges from a few feet to a mile in width. At the surface, its presence is usually disclosed by a narrow trace-either an earth-filled depression in the terrain or a crack with raised lips where rock grinds together.

During an earthquake, rock on either side of the Fault slips horizontally in parallel but opposite direc­tions. Total slippage along the Fault since movement began about 30 million years ago is put by geologists at a minimum of 350 miles, while some individual rock formations that once were in adjoining locations on either side of the Fault have been displaced as much as 450 miles.

An earthquake relieves the stress that builds up in rocks locked together across such a fault. A major fault, geologists say, divides two plates of the crust that are in motion relative to each other. Recent studies suggest that such plate movement along the San Andreas Fault has averaged a little more than two inches per year for the last several millennia. Because this constant movement results in almost daily slippage somewhere along the Fault, nearly every Californian has experienced a small earthquake at one time or another.

Some authorities hold that numerous small earth­quakes -gradual slippages - reduce the chances for a major quake in the same region. Where rock remains locked across a section of a fault the dangers of a major quake increase with time, they say. But not all scientists concur. Some studies show that small tremors often precede big earthquakes.

All the experts agree, however, that the ceaseless movement of plates along the San Andreas Fault will lead to a large earthquake in some part of the network in the future.

Geologists believe that wherever two plates meet, or a plate plunges into a trench, earthquakes are set off. In fact, plate boundaries are best mapped through the detection of such seismic activity. A good example is the western coast of South America. There a huge oceanic plate is plunging into a deep trench beneath a continental plate. As this ocean plate dives beneath the continental plate, its action is responsible for pushing up the Andes Mountains and triggering the deep earthquakes that frequently rock Chile and Peru.

The picture on the western coast of North America is believed to be somewhat different. Here, two plates are indeed bordering one another. One, the Pacific Plate, comprises the whole northern Pacific Ocean, in­cluding a coastal sliver of California. The other in­cludes all of North America and the western half of the Atlantic Ocean. Their dividing line is the San Andreas Fault. But their driving forces (which are not fully understood) are such that these two plates are crunching slowly past one another at the rate of over two inches a year. The Pacific plate is moving in a northwesterly direction; at its remote northwestern edges it is plunging into the Aleutian trench off Alaska, pushing up the Aleutian Islands. The North American plate is heading in a southeasterly direction.

The San Andreas Fault, with its related fractures, is the weak, somewhat ragged boundary between these two massive, moving plates in the Earth's crust. Given such tremendous forces at work, how safe is it to live in the vicinity of the San Andreas Fault? Experts say that it does not really matter how close one lives to the fault-as long as one's house is built on sturdy ground. Houses built on loose or weak soil will probably not withstand severe earthquakes, no matter whether they are only 50 feet or 50 miles distant from the fault zone.

People give many reasons for deliberately building their homes in the fault zone. For one thing, the Fault often causes sags, which fill up with sediment to make nice flat depressions in otherwise hilly ground-perfect spots for building.

This region is a favorite of California's geologists because it so clearly illustrates the lateral slipping motion along the Fault. Although the fault zone has been dormant on this plain since 1857, the land is vividly scarred with a record of older rock movements.

The most graphic view is from the air. From a plane, the observer sees line upon line of streambeds - well over 150 - crossing over the Fault. Only the very youngest streams cross the Fault in a straight line. The courses of the others have been offset by as much as 1000 feet. Offsets occur during earthquakes when the rocks on either side of the Fault suddenly snap into new positions. Streams that were once straight are thus pulled apart into two channels that no longer meet, one on each side of the Fault.

Farther north, the Fault exhibits another kind of behavior. For some unknown reason - perhaps the rocks are extra tough in this region - the rocks along the Fault slip only infrequently. Stresses build up for decades before there is an earthquake, usually a severe one, relieving the strain. Geologists speak of this por­tion of the San Andreas Fault as "locked." Some believe that such locked sections of the Fault are likely to give rise to major earthquakes in the future.

Approaching the San Francisco Bay the Fault again behaves differently. In this area, rocks on either side of the Fault are creeping an inch or two a year. No one knows why the Fault locks in one place and creeps in another. But geologists have noted that earthquakes tend to be more frequent and less severe in creep zones than in locked zones.

North of San Juan Bautista, the San Andreas Fault takes a slight westward swing into the rugged Santa Cruz Mountains that lie along the Pacific coast. At this point the Fault locks again. Thereafter, it remains locked all the way to its northern end. The last violent break along this locked segment of the Fault occurred in 1906, causing the great San Francisco earthquake of that year, the worst disaster in California's history.