Science: Pent-Up Fault

A famous possession in which California takes no particular pride is the San Andreas Fault, a great crack in the bedrock that leads from the Mexican border to Point Arena, no miles northwest of San Francisco, and out to sea. When the fault slips, it causes a major earthquake. Last week Seismologist Charles F. Richter of Caltech outraged chambers of commerce by warning that the San Andreas Fault has been gathering pressure for a major bust-loose.

Twice in recorded history the fault has relieved itself. In 1857, a serious earthquake centered around Fort Tejon, north of Los Angeles. The second movement, in 1906, leveled much of San Francisco. Since then, according to Richter, the fault has been relatively peaceful. Minor California earthquakes have been caused by lesser faults.

There is evidence that pressure in the rock has been building up. Survey monuments on opposite sides of the great fault have been pulling apart at the mean rate of 2 in. per year. Such slow shifting does not relieve forces; it is a sign that they are increasing relentlessly. Within the last fortnight, California has had two moderate earthquakes, with many lesser shocks following them. Richter does not know whether they are warning forerunners of a really big jolt. But he and his colleagues feel that the San Andreas Fault has been quiet so long that it cannot be trusted much longer.