A swarm of 10 earthquakes that shook a mountainous area west of the Santa Clarita Valley over a 48-hour period ending Wednesday were aftershocks of the 1994 Northridge quake.
And scientists said they probably won't be the last.
"They were in the Northridge aftershock zone," said Lucy Jones, scientist in charge of the Pasadena office of the U.S. Geological Survey. "We are still above the level of quake frequency preceding Northridge in that area, a definition of aftershocks, but we are basically toward the end of the aftershock sequence."
The latest jolts, all of which were centered five miles west of Valencia, began at 12:31 a.m. Monday, with a magnitude 3.2 temblor, and reached their strongest at 11:02 p.m. Tuesday, with a magnitude 3.9 quake. The last quake in the series took place at 12:43 a.m. Wednesday.
The 3.9 was felt as far away as Santa Monica, Pasadena and central Los Angeles, but there were no reports of damage or injuries.
The epicentral area has been the site of many of the approximately 16,000 aftershocks that have followed the Jan. 17, 1994, Northridge main shock.
Caltech seismologist Egill Hauksson noted Wednesday that the epicenters of the latest quakes were very close to the epicenters of the aftershocks of magnitude 5.1 and 4.9 that took place April 26 and 27, 1997, and to a 4.2 that occurred Jan. 29, 2002.
While the magnitude 6.7 Northridge quake caused 57 deaths and an estimated $40 billion in damage, about 70% of its energy was dissipated not in metropolitan Los Angeles but in the mountains north of the San Fernando and west of the Santa Clarita valleys. Had the thrust quake been directed south, rather than toward the mountains to the north, it would have probably caused even more damage and casualties.
Jones noted that the strongest aftershock Tuesday night occurred when it was most noticeable, before people had gone to bed and while they were quietly watching the news or relaxing in their homes. As a result, more than 1,500 people described feeling the quake on the Geological Survey's Web site, http://quake.wr.usgs.gov.
People from the San Fernando and Santa Clarita valleys who reported their observations on the site said the quake felt like an "intensity IV" on the modified Mercali scale. At that level, hanging objects in homes swing, windows, dishes and doors rattle, glasses clink and crockery clashes.