The magnitude 6.1 earthquake Thursday followed 11 months of abnormally low geological activity in the earthquake-prone Southern California area extending from the Mexican border to Lake Isabella north of Bakersfield, scientists said.
"Normally, there is about one magnitude 4 earthquake every month in this area," said Caltech seismologist Kate Hutton. But there has not been such an earthquake since last November.
That makes it the longest earthquake-free period in Southern California that seismologists have observed in 55 years, or as long as Caltech has been monitoring earthquakes in the region.
"We were just sitting around talking about it over coffee yesterday morning," said seismologist Lucille Jones of the U.S. Geological Survey office at Caltech. "We have no knowledge of the meaning of such a phenomenon."
The epicenter of Thursday's earthquake was in Rosemead, near the intersection of Rosemead Boulevard and Garvey Avenue, according to Caltech.
That site is in a fault system that has not had an earthquake larger than magnitude 5 since 1929.
The quake also was the largest to have ever struck so close to downtown Los Angeles, Hutton said.
The 7:42 a.m. earthquake resulted when the land mass on the northeast side of the Whittier Fault forced its way slightly over the top of the land on the southwest side of the fault, Hutton said.
This same type of vertical motion caused the magnitude 6.4 Sylmar earthquake in 1971.
Thursday's quake, like the Sylmar temblor, was not directly linked to the San Andreas Fault. But it was powered by the same geological forces that created the San Andreas Fault--the scraping together of two large continental "plates" that float on the Earth's molten core and are trying to move past each other in opposite directions, Jones said.
The Whittier Fault is a part of the large Southern California fault system that also includes a portion of the long San Andreas Fault.
One of the plates that makes up the San Andreas Fault, the North American plate, stretches from the fault to the middle of the Atlantic Ocean and is trying to move in a southeasterly direction. The Pacific plate, which extends from the fault westward past Japan, is trying to move northwest. At the same time, the plates also are pushing against one another.
Normally, these plates stay locked together by friction until the force becomes so great that they "slip" horizontally to release the stress. That slippage causes an earthquake.
But Thursday's quake involved more of a vertical motion--about six miles underground--as one plate tried to slip under the other, Hutton said.
A Caltech team dispatched to Rosemead on Thursday found no evidence of ground rupturing.
By mid-afternoon, scientists had recorded three aftershocks above magnitude 4, and at least 16 that registered between magnitude 3 and 4.
The chances of additional aftershocks of magnitude 5 or more were estimated by Hutton to be about 1%.
But perhaps more ominously, Thursday's quake was nowhere near large enough to relieve significant amounts of the stress along the San Andreas Fault, Jones said.
The large earthquake--possibly measuring 7 or 8 on the Richter scale--"is still coming," she said. "And quakes like this will have no effect on it. . . . "
Thursday's earthquake was in a fault zone where scientists expect one or more temblors of magnitude 7 "anytime from tomorrow to 100 years in the future," said Jones.
The National Earthquake Information Center in Golden, Colo., rated the shock at magnitude 6.1, and sensors at UC Berkeley recorded it as being 6.0.
That difference is not surprising, said Clarence Allen, director of the Caltech Seismology Laboratory, because both locations were simply doing "a rough and dirty" calculation.
The different readings also reflect the different types of ground that the earthquake waves travel through before they reach the seismometers.
Ironically, Caltech scientists did not have a good estimate of the magnitude Thursday because they were only about seven miles northwest of the epicenter. That was too close because the initial shock sent the recorders on many of their supersensitive automated detectors off-scale so that they could not get any readings.
Thus the seismologists had to fall back on older, less-sensitive instruments that record data on photographic film. All told, Caltech and the U.S. Geological Survey have 280 monitoring stations in Southern California.
But even the older instruments in the immediate area were unable to yield an accurate reading. So researchers had to drive to Riverside, and beyond, to collect the photographic film on which the data was recorded. That information was not yet available late Thursday afternoon.
The epicenter is roughly at the northwest end of the Whittier Fault, which runs northwest from the Santa Ana Narrows near Yorba Linda to the Whittier Narrows at the base of the Whittier Hills.
The Whittier Fault is a branch of the much larger Elsinore Fault, which runs from the Chino area southeast into Mexico. The Elsinore Fault is one of a series of faults that run parallel to the San Andreas Fault and is part of the same general system, although it is not directly connected.
There has not been an earthquake greater than magnitude 4 along the Elsinore Fault in this century, Jones said, but some data indicate there may have been one in the 18th Century.
An earthquake with a magnitude of about 5 struck Whittier in 1929, Hutton said. The precise magnitude and the epicenter are not known because seismologists had no recording instruments then. A 4.6 temblor struck in Walnut about seven miles southeast of Thursday's earthquake on Jan. 1, 1976.