Large tracts along California's San Andreas Fault have begun to expand toward the Pacific Ocean, suggesting to scientists at the U.S. Geological Survey that a large earthquake is more imminent than they thought.

So great has the land expansion been in the region of California known as the Palmdale bulge that it has addeda quarter of an acre to a 15-mile stretch of the Palmdale region in the last six months. The land mass increase covers both sides of the San Andreas Fault, which cuts through the Palmdale bulge.

"It is premature to speculate on the exact implications for future earthquakes," Dr. C. Barry Raleigh of the USGS said yesterday, "but if this recent pattern of expansion persists it will mean a large earthquake in the not-distant future."

Speaking at the annual meeting in San Francisco of the American Geophysical Union, Raleigh said that USGS geologists had not determined if the westerly expansion of the land is due to rises in elevation of the hills bordering the fault or to a decrease in density of rock along the fault. Either way, it would show up as a land expansion.

"It's not as if new land were rising up out of the surface," Raleigh said. "We just don't know how it's expanding."

Geologists discovered the phenomenon early this year when scientists at the Jet Propulsion Laboratory in Pasadena noticed changes in distances between radio telescopes at various locations in California, especially those near Palmdale, a region northeast of Los Angeles that has undergone mysterious episodes of uplift and collapse in the last four years.

Measuring the angles on which radio noise comes to earth from quasars outside our galaxy, JPL scientists are able to pinpoint distances to less than an inch between telescopes located as far as 200 miles apart. The distant quasars are used as references because scientists figure they have stayed in the same places in space for millions of years.

JPL's Dr. A. E. Neil told the AGU meeting yesterday that the distance between radio telescopes at Pasadena and Goldstone in the Mojave Desert grew by eight inches in little over six months, which is a remarkable growth over a stretch of 120 miles. Continental drift often is measured in terms of inches per million years.

JPL's method of measurement involved recording radio noise from 10 quasars at the two telescope locations. By comparing the times of arriving signals, changes in distance between the telescopes from one month to the next can be measured.

When USGS scientists heard of the JPL work, they set out laser rangfinders on each side of the San Andreas Fault near Palmdale to find out if the land was moving westward. Lasers shining beams on reflectors placed at high points are used to check displacements in the earth's crust along a 500 mile stretch of the San Andreas Fault.

"Sure enough, the recent measurements we did in Palmdale agreed with the JPL measurements," Raleigh said yesterday by telephone. "Not only did our data show westward expansion, it also showed the same amount of strain, about eight inches in the last six months."

Significantly, the westward expansion began six years after laser and telescope measurements showed that California had been shrinking in a north-south direction along the San Andreas Fault for six straight years. The shrinking stopped in 1978 and no change was noticed until last March, when the westerly expansion was first picked up by the telescopes.

What this means, Raleigh said, is that the strains along the fault are increasing and shifting their weight. If this trend continues, it could mean an imminent earthquake of some size for California.

"Imagine two blocks of wood with a diagonal cut across the surface where, if I press down, the blocks tend to lock up," Raleigh said. "On the other hand, if I push from the sides I tend to unlock the two blocks and they'll slide apart. That's what might be happening here."