EARTHQUAKE / The Long Road Back : 4.5 Aftershock Breaks 4-Day Lull : Temblor: Officials will reinspect buildings near the northern San Fernando Valley epicenter. Post-quake patterns remain normal, seismologists say.

A magnitude 4.5 aftershock centered two miles west of Granada Hills at 9:19 a.m. Thursday broke a four-day lull of only scattered mild aftershocks in the magnitude 3 range, but scientists reiterated that such moderately strong quakes are to be expected.

Seismologist Lucile M. Jones of the U.S. Geological Survey said that the number of aftershocks in any given day has been slowly diminishing since the 6.6 temblor Jan. 17, but that quite a few more magnitude 4s and perhaps one more magnitude 5 are likely in coming months.

“The aftershocks are following a very normal pattern for Californian earthquakes,” read a statement issued Thursday afternoon by four institutions that are actively studying the geology of the Northridge quake--Caltech, the U.S. Geological Survey, the Southern California Earthquake Center at USC and the Jet Propulsion Laboratory.

“If the aftershocks continue at this rate, the probability of a magnitude 5 aftershock in the next week is about one in five,” the statement read. “We expect one or two more aftershocks of magnitude 4.5 or greater in the next week.”

Jones noted that a number of historic earthquakes in California have been followed by a “large late aftershock” that has done some damage and injury. For instance, the magnitude 7.7 Tehachapi earthquake of July 21, 1952, that killed 12 people was followed a month later by a magnitude 5.8 temblor centered close to Bakersfield that killed two more.

In the 1933 Long Beach 6.3 earthquake, a damaging 5.4 aftershock came seven months after the main shock, and in the 1983 Coalinga 6.5 earthquake, a 6.0 aftershock came three months later near the town of Avenal.

The Avenal earthquake was larger in relation to the Coalinga main shock than is usually the case with aftershocks. Jones reported this week that historically, “the difference in magnitude between the main shock and largest aftershock can range from 0.1 to 3 or more, but averages 1.2.”

This means that the largest average aftershock to have been expected from the 6.6 Northridge temblor would have been 5.4. But that magnitude has already been exceeded, by the 5.6 shock that occurred on the afternoon of the quake.

Thursday morning’s aftershock, which authorities said would prompt some reinspection of buildings for damage in the area near its north Valley epicenter, was the 36th between magnitude 4.0 and 4.9 since the Northridge earthquake.

During the same period, there have been two aftershocks between 5.0 and 5.6 and more than 300 between 3.0 and 3.9. Altogether there have been more than 3,150 aftershocks, still a long way from the 50,000 that have accumulated in the 19 months since the major Landers earthquake of June 28, 1992.

A quake must reach approximately magnitude 2.5 before most people feel it in their ordinary activities, and the vast majority of shocks are less than that.

But aftershocks, regardless of size, are studied closely by earthquake experts because they often delineate as clearly as anything the extent of the area directly involved in the earthquake, and by determining their relative depth, scientists can calculate the fault plane of the quake. Aftershocks also can demonstrate movement that has been triggered on other faults.

Scientists have put the depth of the origin of the Northridge main shock at about 11 miles. By locating the aftershocks, they believe that the unnamed thrust fault involved extends up, east and west from this point.

“It is a circular plane, tilted at an angle of about 40 degrees from the earth’s surface,” the researchers said in a statement. “Its deepest point is at the main shock focus and its shallowest point is near the Santa Clarita Valley. . . . (But) we think the main shock fault does not extend up to the surface.”

Signs of surface disturbance that have been found in Granada Hills, along the Santa Susana range and in Potrero Canyon west of Castaic Junction probably represent movements on other faults, some of the scientists say.

In a new observation Thursday, the four institutions added that they believe that the Northridge earthquake “occurred within the system of faults under the northern Los Angeles Basin that was recognized after the Whittier Narrows earthquake of 1987. We think this band of buried faults extends across the northern side of the basin and under the San Gabriel and San Fernando valleys,” they said.

Times staff writer Bettina Boxall contributed to this story.

Adding Aftershocks

The magnitude 4.5 aftershock Thursday morning merely added to the list of seismic activity since the Jan. 17 Northridge earthquake. Some aftershock totals:

* 2 aftershocks magnitude 5.0-5.6

* 36 aftershocks magnitude 4.0-4.9

* 302 aftershocks magnitude 3.0-3.9

* 3,150 total aftershocks