EARTHQUAKE: THE LONG ROAD BACK : 2 Pulses May Have Fueled Destruction, Seismologists Say : Science: Second burst of energy hit Hollywood and the Westside, sensors show. First burst was felt the most in the northern San Fernando Valley.

The Northridge earthquake pummeled large parts of Los Angeles with two distinct pulses of energy--the second and stronger of which may have accounted for some of the unexpectedly violent shaking on the Westside, scientists said Tuesday.

Researchers at the U.S. Geological Survey’s Pasadena field office said sensors showed that the second pulse occurred on the Westside and in Hollywood about five seconds after the shaking was initiated there by the arrival of secondary earthquake waves. In some other areas, it occurred after three seconds.

Seismologists Susan Hough and David Wald said the second pulse marked the most violent instant of the earthquake south of the Hollywood Hills and Santa Monica Mountains. Hough described it as a burst of energy that contributed to the damage to buildings and freeway structures caused by the quake as a whole.

Secondary waves, which cause the noticeable shaking in an earthquake, are marked by surges of energy, or pulses.

The two separate phases in the magnitude 6.6 Northridge quake were not of the same relative intensity in all parts of the city. For instance, in the northern San Fernando Valley, the first pulse caused more violent shaking than the second.

Wald said in an interview that scientists believe radio reports are exaggerating by saying that there may actually have been two 6.6 quakes occurring within seconds of one another at the same point.

“I would not venture to call them separate quakes,” he said of the two pulses, “They were part of the same rupture.” Hough agreed.

“This quake was complex, as many of them are,” Wald said. “We’ve seen this before. There are seismic events, and within those events, there are sub-events.”

Southern Californians who felt the 1987 Whittier Narrows quake may remember that there was a second, more violent pulse in certain neighborhoods in that magnitude 5.9 jolt as well. The shaking began and then suddenly intensified.

Some scientists think the 1992 Landers quake had as many as seven pulses.

The pulses of the Northridge quake are clear; they show up on the sensors put in place by the state Division of Mines and Geology at various points in the Los Angeles area.

Hough said the interval between the first pulse and the second pulse varied according to the position in the basin. Caltech seismologist Egill Hauksson added that one of the most important studies of the Northridge earthquake will be an attempt to determine the details of how energy emanated from the rupture zone to various parts of Los Angeles.

The scientists cautioned that there will be many different analyses of the Northridge earthquake, and that a full understanding of the event will not be reached for a long time.

In another theory to surface this week, Tom Henyey, executive director of the Southern California Earthquake Center at USC, said scientists are studying the possibility that although the source of the shaking was at a single point, 10 miles under Northridge, two different thrust fault planes may have been involved.

One, he said, would have been on a shallowly dipping plane, to the northeast toward Granada Hills, Sylmar and the Santa Clarita Valley, and the other would have been on a steeper fault to the northwest, toward Canoga Park and Simi Valley.

Henyey said there appear to be two distinct clumps of aftershocks, one to the northeast of the epicenter and the other to the northwest, with relatively few aftershocks in the space between them.

If two different fault planes were involved, it would help explain the confusion last week when scientists first tentatively named one fault system as responsible for the Northridge earthquake, only to name another two days later.

Wald agreed Wednesday with the other scientists that all quakes are complicated, and he said he expects to do research on the direction, or what he said may be the “double-directivity” of last week’s earthquake.

Directivity has to do with the concept that a quake often seems pointed in one direction, with most of the damage in that direction. It is often clearer, however, in cases of long faults than relatively short ones.

The Landers earthquake, for example, seemed to aim its power to the north, out into the desert, where few people lived. This seemed to be the major reason that damage to the south of the epicenter was less than might have been expected.

But the Northridge earthquake appears to have shot its energy in many different directions. Damage was extensive in such widely separated directions from the epicenter as Hollywood, the Westside, Tarzana and the Santa Clarita and Simi valleys, in addition to the San Fernando Valley communities closest to it.

“It will take us some time to look at the directivity of this earthquake,” Wald said. “But this is not shockingly different than in the past. The Sylmar quake (of 1971) was also complicated.”

A Second Pulse

Two scientists with the U.S. Geological Survey have found a distinct--and violent--pulse of the Northridge earthquake. Sensors in place before the earthquake show the second pulse hit the Westside and Hollywood about five seconds after the onset of secondary waves, which are responsible for shaking people can feel. This may help explain the destruction that occurred unexpectedly in those areas. The sensors, placed by the state Division of Mines and Geology, give three readings at each location.

Types of Waves

* Primary (P) waves travel quickly, between 3 and 4 miles a second, compressing rock ahead of them and elongating it behind. They are not immediately felt.

* Secondary (S) waves, traveling about 2 miles a second, cause undulating movement that is felt sharply at the surface.

SANTA MONICA CITY HALL SENSOR READING

Santa Monica is 13 miles south-southeast from the epicenter.

MOORPARK SENSOR READING

Moorpark, in Ventura County, is 22 miles west-northwest from the epicenter.