EARTHQUAKE / THE LONG ROAD BACK : Scientists Ponder Northward Concentration of Quakes : Seismology: Experts wonder if 20-year trend of temblors nearer San Gabriel and Santa Susana mountains portends a larger one to come.

As aftershocks continued to rattle the region Wednesday, scientists pondered the future effects of Monday’s magnitude 6.6 earthquake, the eighth significant, damaging temblor since 1970 to occur along or near the so-called Front Range comprising the San Gabriel and Santa Susana mountains.

Wednesday saw two of the strongest aftershocks yet, a magnitude 5.1, followed just two minutes later by another 5.1. They occurred at 1:09 p.m. and 1:11 p.m. and were the fourth and fifth aftershocks in the magnitude 5 range--not that unusual a number and intensity after such a strong quake, according to seismologists. (A magnitude 4.4 aftershock had struck at 6:09 a.m.)

The latest strong aftershocks were centered six miles northeast of Simi Valley.

Meanwhile, all the seismic activity in the last 24 years has scientists pondering if a larger quake will occur somewhere along the Front Range in the years ahead. From Lytle Creek northwest of San Bernardino, through Upland, Whittier Narrows, Sierra Madre, Pasadena, San Fernando, Northridge and coastal Ventura County, the quakes have been of at least magnitude 5, with three close to magnitude 6 and two, including Monday’s, in the strong mid-6 range.

Each of these temblors has ruptured only relatively short segments of various thrust faults. But earthquake scientists are now trying to determine whether these faults are independent of one another or are linked deep under the surface and could rupture all at once--producing a major quake.

Preliminary research, however, indicates that huge magnitude 7 or 8 quakes along the mountains have been rare. The largest in historical times, a magnitude 7, occurred at Wrightwood in the eastern San Gabriels in 1812.

Nevertheless, Caltech’s Egill Hauksson wrote two years ago in a book prepared by the Assn. of Engineering Geologists: “A future magnitude 7.7 earthquake that could rupture all seven segments or the whole length of the Sierra Madre Fault cannot be excluded.”

But Hauksson quickly added that “seismological and geological evidence for segmentation along the fault suggests that 6.4 to 6.6 earthquakes occur more frequently and that the magnitude 7.7 earthquake may be a very rare event.”

Lucille M. Jones of the U.S. Geological Survey has called the Sierra Madre Fault, extending along the San Gabriels for 75 miles, possibly the most dangerous near or in the Los Angeles Basin. But the magnitude 5.8 quake at Sierra Madre in 1991 broke only one of its seven segments.

Hauksson said this week that there is also a question whether all the quakes that have occurred along the northern edge of the Los Angeles Basin could portend a larger quake on the San Andreas Fault north and east of Los Angeles, a little more than 25 miles northeast of the epicenter of the Northridge quake.

“You can’t predict a Big One on the San Andreas based on the Front Range quakes,” Hauksson said. “But if one did occur, we’d see an oodle of papers viewing what has happened in recent years as precursors.”

Another question concerns whether the hundreds of aftershocks that have been occurring this week could suggest the possibility of a larger quake relatively soon in the vicinity of Monday’s Northridge temblor or in some particular direction away from it.

Frequent aftershocks following the magnitude 6.1 Joshua Tree quake of April 22, 1992, migrated in a northwestward direction and appeared to some scientists to have triggered the much larger 7.6 Landers quake of June 28, 1992.

Hauksson said Wednesday that a Caltech analysis indicated some migration of aftershocks from the Northridge quake about five miles in a northwestern direction toward Ventura County. He said the aftershocks have now developed “into a very energetic pattern, slightly above the normal.”

However, Hauksson told a news briefing that it is too early to reach any conclusions about the significance of this. He said a further migration would be analyzed carefully, if it develops.

Altogether, there have been more than 1,000 aftershocks since Monday morning, but the majority of them were too weak to be felt.

Most scientists at media briefings at Caltech have suggested that the Northridge quake will not lead immediately to a bigger one, and the state Office of Emergency Services has thus far abstained from issuing advisories of possible impending larger quakes, as it has occasionally done in the past.

The view by scientists of what may happen several years ahead is often less optimistic.

Hauksson has pointed out that the Front Range earthquakes since 1970 represent a distinct change from the pattern of quakes in and around Los Angeles in the early years of the century.

Records show that there have been three major phases of seismic activity in the Los Angeles-Orange County area since 1920.

From 1920 to 1941, all of the most significant earthquakes occurred in southern or southwestern parts of the basin.

In a second phase, from 1941 to 1971, there were no quakes over magnitude 4.8 anywhere in the basin.

Since then, in the third phase, all the earthquakes in the 5 and 6 magnitude range have been at or near the northern edge of the basin, near the San Gabriel or Santa Susana mountains.

Tracking the Quakes

Between 1920 and 1941, virtually all significant earthquakes in the Los Angeles area were in the southerly or southwest parts of the basin. After the magnitude 6.4 Sylmar-San Fernando quake in 1971, there have been seven sizable, damaging quakes at the northern edge of the basin. Scientists now ponder whether the shift may not portend a larger quake along the San Gabriel or Santa Susana Mountains, or possibly a big one on the San Andreas fault north and east of Los Angeles.

Researched by KENNETH REICH and VICKY McCARGAR / Los Angeles Times

How Earthquake Magnitude Is Measured

There are 270 seismometers positioned throughout Southern California. By examining the interval between waves of quakes, seismologists can identify the epicenter and the magnitude. A look at the three main types of waves, the sequence in which they occur and how they are measured for magnitude:

Primary wave: Travels through earth. When it hits the surface, it causes buildings and other structures to contract and expand.

Secondary wave: Travels through earth, moving rock back and forth, up and down. At earth’s surface, it shakes structures violently.

Long waves

Love wave: Originates and travels along earth’s surface, shaking structures from side to side.

Rayleigh wave: Originates and travels along earth’s surface, rolling like swells on the ocean.

How Magnitude Scale Works

Seismologists determine the magnitude of an earthquake by measuring two factors: time and amplitude. How Wednesday’s 5.1 aftershocks appeared on a seismograph:

Base line (no quake activity)

Primary wave begins

Secondary wave begins

Time lapse: 24 seconds

Amplitude: Highest peak 33 millimeters

This chart measures time lapse between primary and secondary waves as well as distance in kilometers of that particular seismometer from the epicenter.

A straight line is drawn between the distance and amplitude charts to determine magnitude. Magnitude: 5.1

This chart measures amplitude, or the highest peak from the base line.

How Seismometer Works

Paper-covered drum makes a complete rotation every 15 minutes, records 24 hours of data.

Horizontal Earth Motion

Vertical Earth Motion

A Quake’s Energy

Earthquakes release energy that scientists measure in metric units called ergs. One erg is the amount of energy it takes to move one gram of mass one centimeter in one second. Some examples of an earthquake’s energy force: Energy released (in millions of ergs): 20,000,000,000,000,000,000 Magnitude (not to scale): 9 Energy released (in millions of ergs): 600,000,000,000,000,000 Magnitude (not to scale): 8 Example or equivalent: 1933, largest quake recorded, Japan (8.9) Example or equivalent: 1906 San Francisco earthquake (8.3) Energy released (in millions of ergs): 20,000,000,000,000,000 Magnitude (not to scale): 7 Example or equivalent: Energy generated by Niagara Falls in four months Energy released (in millions of ergs): 600,000,000,000,000 Magnitude (not to scale): 6 Example or equivalent: Energy to launch 2 million NASA space shuttles Energy released (in millions of ergs): 20,000,000,000,000 Magnitude (not to scale): 5 Example or equivalent: Energy created by atomic bomb test in 1945 Energy released (in millions of ergs): 600,000,000,000 Magnitude (not to scale): 4 Example or equivalent: Kiloton of TNT exploding Energy released (in millions of ergs): 20,000,000,000 Magnitude (not to scale): 3 Example or equivalent: Smallest quakes commonly felt Source: The World Book Encyclopedia, Caltech, U.S. Geological Survey, Associated Press

Researched by APRIL JACKSON and CAROLINE LEMKE / Los Angeles Times