New Findings Move Rupture Zone North of First Location : Survey: More than 1,500 aftershocks have been recorded since Monday’s temblor.

Scientists on Friday reported finding extensive new surface cracking and likely rupture zones from the Northridge quake along the Santa Susana Mountains and in Potrero Canyon, five miles west of Castaic Junction near the Ventura County line.

A team from the Lawrence Berkeley Laboratory and UC Berkeley said they had found cracks as wide as a foot and hundreds of feet in length at the top of the Santa Susanas in an area extending about 10 miles west from the Newhall Pass, near the ruined interchange of the Antelope Valley (California 14) and Golden State (Interstate 5) freeways.

Meanwhile, the U.S. Geological Survey reported that more than a dozen scientists had made an on-site inspection of a 6- to 12-inch earth displacement in Potrero Canyon just off California 126, several miles to the north of earlier reports of evidence of quake activity. Two USGS scientists at a Caltech news briefing called this a likely rupture zone from the quake.

The sites of cracking, land displacement and a new cluster of aftershocks in the magnitude 4 range are all helping researchers delineate the exact extent of the Northridge quake. Generally speaking, this is a northwest trending oblong extending perhaps 20 miles from the northeast San Fernando Valley toward Ventura County.

The epicenter of the quake has been officially put at 34 degrees, 13 minutes north and 118 degrees, 32 minutes west, or just northwest of the intersection of Roscoe and Reseda boulevards, about one mile south southwest of the center of Northridge.

Although UC Berkeley seismologists said they believe the quake reached a magnitude 6.8, Caltech and the USGS stuck with the original 6.6 figure. Magnitude estimates may change as more stations from around the world report their instrumental readings.

Jim Mori, director of the USGS Pasadena field office, said that the aftershocks, although annoying and even scary to many residents, are not deviating from a normal pattern for quakes of such size as the magnitude 6.6 last Monday.

More than 1,500 aftershocks had been recorded through Friday afternoon. He said that a close analysis of the aftershocks since Monday showed there have actually been only two quakes in the magnitude 5 range (a smaller figure than had been given earlier), about 30 in the 4 range and about 270 others in the 3 range.

Caltech and the USGS said Friday that the aftershock zone extends 20 miles east-west, from San Fernando through the Santa Susana Mountains, and for about 18 miles from Northridge in the south to the Santa Clarita Valley in the north.

“From the seismological point of view, this was a very small earthquake,” Mori said of the main shock. Worldwide in an average year, he said, there are about 150 such quakes.

What made this one different was that it occurred in an urban area and the damage and casualties were dramatic.

Dan Ponti and Ken Hudnut, two USGS scientists, said the discovery of a likely rupture zone in Potrero Canyon, north of the Santa Susanas, indicates that a previously undiscovered fault lies in the canyon and that there was ground movement along this fault. Similar signs of movement were reported Tuesday along the Mission Hills Fault, south of the Santa Susanas.

It is common in such earthquakes that movement on one fault will trigger movement on others, and the aftershocks reflect this, scientists said.

Cracks in Potrero Canyon run in rows of about 10 to 20 yards in open areas and at the bottom of the hills in the canyon.

Geologists who first went into the canyon Wednesday said they expect to examine the surface disturbance for five more days.

David Schwartz of the USGS, who was there Friday, said, “It’s possible this is the tip of the fault that produced the Northridge earthquake.”

About 3,000 head of cattle graze in the area.

Ponti said that the surface rupture in Potrero Canyon extends about four miles. But the team of Berkeley scientists said in a statement that the cracking they had noted in the Santa Susanas extended even further.

They also noted signs of some cracking in the hills to the northeast of Antelope Valley-Golden State freeway interchange and postulated that the same forces that caused the cracks had caused the interchange to collapse.

Pat Williams, a geologist with the Lawrence Berkeley Laboratory, said the cracks made it appear that the Santa Susana range was being pulled apart, which would be consistent with the notion that the quake squeezed the San Fernando Valley, with mountains from both the north and south of it moving closer together.

Refining previous figures, scientists Friday estimated the Santa Susanas were raised by about 15 inches in the quake and that the Castaic Junction dropped by 3 1/2 inches. The cracking most likely resulted from rumpling of the earth’s crust as it was pushed together, much like a fold in a carpet, with the fractures occurring across the rumple’s surface, Williams said Friday.

A retired mining geologist, Frank Cox, who was hiking in the Santa Susanas on Friday, reported that he had come upon an extensive network of fractures and rifts on a steep ridge above Omelveny Park near the mouth of Bee Canyon in Granada Hills. He said that in some spots, the hard-packed earth had been uplifted a foot and in other areas cracks had created a jigsaw puzzle effect.