The Puente Hills fault, which scientists believe could be responsible for Friday’s 5.1 earthquake in La Habra, is considered very dangerous.
Here are some basic questions about the fault.
Q: What would be the difference in shaking between a 5.1 quake and a truly huge quake?
Friday night’s earthquake was caused by the underground fault slipping for half a second, said U.S. Geological Survey seismologist Lucy Jones, prompting about 10 seconds of shaking at the surface.
But a 7.5 quake on the Puente Hills fault could cause the fault to slip for 20 seconds — and the shaking could last far longer.
Scientists say that quake would be more destructive than the so-called Big One on the San Andreas fault.
Q: Why are scientists so worried about the fault?
The Puente Hills thrust fault is so dangerous because of its location, running from the suburbs of northern Orange County, through the San Gabriel Valley and under the skyscrapers of downtown Los Angeles before ending in Hollywood.
LOS ANGELES, Dec. 5— In a finding that may unsettle the nerves of millions of Southern Californians, geologists have discovered two major faults deep below some of the most densely developed parts of the Los Angeles metropolitan area.
The area affected includes the downtown section, the Wilshire Boulevard corridor, Beverly Hills, Santa Monica and a complex of oil refineries and power utilities near the Pacific Ocean.
The discovery, along with the gradual realization among geologists that as many as half of all the faults in the Los Angeles area may lie deep underground and have yet to be discovered, presumably doubles the risk that a devastating earthquake will eventually occur in the metropolitan area, said Don Anderson, director of the seismology laboratory at the California Insitute of Technology. But scientists do not know enough about these faults to calculate the actual risk.
State disaster officials, who have been aware of the new findings for months, said that in their planning they had long anticipated the possibility of such an event. They said Los Angeles residents should not be unduly alarmed by the discovery of the faults. Report to Be Given Today
Unlike the region’s previously known faults, which produce visible features on the earth’s surface, the new faults are ”in the basement” and have no such features at ground level, said Dr. Egill Hauksson, a research assistant professor of geophysics at the University of Southern California.
Dr. Hauksson is scheduled to give a report Tuesday describing the faults, which are 6 to 10 miles beneath the streets of Los Angeles, at a session of the American Geophysical Union’s annual meeting in San Francisco.
This is the ”first evidence that there are major buried faults beneath L.A.,” Dr. Hauksson said in a interview. ”Before we had one big fault locally, the Newport-Inglewood fault, and now we have three.”
The San Andreas fault, estimated to have a 60 percent probability of generating a catastrophic earthquake in Southern California within the next 30 years, lies 30 or more miles north and east of the Los Angeles metropolitan area. The San Andreas, which is much larger than the various faults in the immediate Los Angeles area, is believed capable of producing a much more powerful earthquake.
Scientists believe the newly discovered faults will cause serious earthquakes much less frequently than the San Andreas fault. Emergency Teams Are Ready
Dr. Richard Andrews, deputy director of the Governor’s Office of Emergency Services, who is responsible for overseeing earthquake planning in Southern California, said residents of the Los Angeles area should not worry that officials were unprepared to cope with potential dangers from the new faults. For several years now, he said, disaster preparedness officials have assumed that unknown faults capable of generating moderately severe earthquakes could exist anywhere in Los Angeles. Firefighters, police officers and medical response teams are ready for such an event, he added.
Nevertheless, in earlier written reports on seismic hazards, Dr. Andrews and others have noted that faults lying close to densely populated areas posed special risks. They have warned, for example, that a rupture of the Newport-Inglewood fault, a 40-mile surface fracture running through many populated neighborhoods, could rupture gas lines and lead to a firestorm that might kill and injure more people than would a larger earthquake on the more distant San Andreas fault.
In an interview last week, Dr. Hauksson said he began to track down the new faults in the aftermath of an earthquake that struck the town of Whittier, just east of downtown Los Angeles, on Oct. 1, 1987. The quake registered 5.9 on the Richter scale of energy released and ground motion. To everyone’s surprise, he said, the epicenter was nine miles beneath Whittier on a previously unknown fault that has turned out to be a small segment of a long underground fault extending beneath Los Angeles.
The Whittier earthquake and its aftershocks killed 8 people, injured 200 and caused an estimated $358 million in damage. Two Major Types of Faults
There are two major types of earthquake faults, Dr. Hauksson said. Strike-slip faults occur when large sections of the earth’s crust, called plates, scrape past one another in opposite directions horizontally. The San Andreas fault, which runs the length of California, is a strike-slip fault.
The two newly discovered faults are thrust faults, in which slabs of compressed rock move vertically past one another. The thrust faults tend to be smaller than strike-slip faults and often release less energy.
Because thrust faults move more slowly than strike-slip faults, Dr. Hauksson said, the interval between large earthquakes on them should be 500 to 1,000 years. The San Andreas fault, by comparison, generates large earthquakes on the average of every 160 years. Unfortunately, the historical record of Los Angeles earthquakes did not begin until the early 1800’s, he said, so it is impossible to predict when segments of the newly found thrust faults will rupture.
To look for the postulated fault, Dr. Hauksson examined seismic data from more than 200 small earthquakes, ranging from 2.5 to 4.0 on the Richter scale, that occurred in the Los Angeles area in the last 10 years.
The magnitude of earthquakes is measured on the Richter scale, on which every increase of one number represents a a tenfold increase in ground motion and a thirtyfold increase in energy released. Quakes that measure 5.0 on the Richter scale are strong enough to cause local damage, at 6.0 severe damage and at 7.0 widepread heavy damage. An earthquake with a magnitude of 8.0 is catastrophic. Different Patterns Produced
Thrust and strike-slip faults produce different patterns, Dr. Hauksson said. Using a geological technique called fault plane solution, he soon inferred the configuration of not one but two ”new” thrust faults.
One, named the Elysian fault, begins in Corona, east of Los Angeles. Its 60-mile route runs under Whittier, where the first segment was detected last year, under Dodger Stadium, beneath downtown high-rise buildings, out under Wilshire Boulevard and into Santa Monica Bay.
The second newly discovered thrust fault, named the Wilmington-Torrance fault, runs 40 miles from Newport Beach, under the oil fields at Wilmington, under Long Beach and San Pedro, beneath Palos Verdes and Torrance and out into Santa Monica Bay, Dr. Hauksson said.
The Elysian fault is fragmented into five segments, he said, and the Wilmington-Torrance fault into three segments.
The 10-year data base indicates that the two faults are active, Dr. Hauksson said. The largest segments, including one that runs from Whittier to downtown Los Angeles and out to Beverly Hills and Baldwin Hills, should be able to generate an earthquake with a magnitude of up to 6.8, he said map of faults running through Los Angeles (pg. D27)
Addie M. MIller 
