Special San Fernando Earthquake Edition

SAN ANDREAS BLAMED FOR SAN FERNANDO QUAKE

A 50-mile bend in the San Andreas fault north of the San Gabriel Mountains was ultimately responsible for the destructive San Fernando earthquake of February 9, even though there apparently was no movement along the big fault.

This was the opinion expressed recently by Don Anderson, director of the Seismological Laboratory of the California Institute of Technology.

"The bend tends to block and jam the general northwesterly movement (at the rate of about 2 inches a year) of that part of California that lies west of the San Andreas fault in relation to the rest of the state east of the fault," Dr. Anderson said

"The fault runs in virtually a straight line both north and south of the ben& In those areas this general northwesterly movement is punctuated by occasional horizontal slipping along the San Andreas and its associated faults, accompanied by earthquakes.

"But near the bend the horizontal slipping gets hung up. Compression builds up and instead of horizontal movement you have overthrust faulting in that region, with land thrusting over land along fault breaks, triggering earthquakes. The uplifted land eventually forms mountains."

During the Februsary 9 earthquake, the San Gabriel Mountains back of San Fernando were lifted several feet. There was considerable thrust faulting in the area, just as there was earlier during the 1952 Tehachapi and Bakersfield earthquakes.

"I think this is the way the land west of the San Andreas fault gets around the bend - by overthrusting and making mountains," Dr. Anderson said.

The San Andreas fault slices through western California for more than 600 miles. It extends virtually in a straight line southeastward from the Mendocino County coast to the southern San Joaquin Valley, where the jog occurs, then extends southeast again along the north flank of the San Gabriel Mountains. Branches of it eventually reach the Gulf of California.

"Although the bend tends to build up compression in much of southern California, the San Fernando earthquake decreased the overall compressional strain, but not very much," the geophysicist said.

Records of Caltech strain-measuring instruments at Isabella Lake and at the Nevada (atomic) Test Site showed that the earthquake did relieve compression over a wide area. Rocks at IsabelIa expanded about one millionth of an inch between the piers of a quartz strain measuring instrument. The expansion was much greater in the vicinity of the 'quake's epicenter.

Under the widely accepted geological theory of continental shift the San Andreas fault serves as part of the border of two great land masses, or plates. The theory says that the earth's land masses have been moving apart since the time when they were all in one piece. Great plates that include entire continents, and more, are moving over a plastic layer.

The massive drifting, which has been going on for millions of years, is related to the upwelling of molten rock beneath the great mid-ocean ridges. The material moves up vertically, solidifies and then spreads out to form the ocean floor, moving laterally away from the ridges as huge plates.

When two plates meet head-on, one usually is driven under the other. As a result, mountains form, deep oceanic trenches exist, the earth quakes, and volcanoes erupt. In some places, the two plates slide horizontally against each other.

The San Andreas fault and the Gulf of California are thought to separate two major plates. One extends from the mid-Atlantic westward to the San Andreas. The other extends westward from the San Andreas to the Asiatic shoreline. Part of California is on one plate, and part of it is on the other....Caltech