USA 2001/2002 - January, 28th 2002

January, 28th 2002

the M4.2 Simi Valley earthquake sequence

From: http://www.trinet.org/shake/

Location Date Time Magnitude
6 miles NNE Simi Valley, California
(ID 9753485) JAN 28 2002 21:53:28 PST 4.2
6 miles W of Valencia, California
(ID 9753489) JAN 28 2002 22:00:39 PST 3.9
5 miles WSW of Valencia, California
(ID 9753497) JAN 28 2002 22:08:01 PST 3.8

A sequence of more than 40 small earthquakes has occurred 6 miles north-northeast of Simi Valley since 10 pm (PST) 28 January 2002. The mainshock of M4.2 was preceded by a M3.1 foreshock by 20 seconds. During the first 18 hours of activity, a total of one foreshock, one mainshock and 38 aftershocks have been recorded. The largest aftershocks occurred immediately following the mainshock M3.9 and M3.8, and M3.6 (12:23pm) 14 hour later. These earthquakes were widely felt in Simi Valley, San Fernando and adjacent regions.
We use seismic records to determine the orientation of the fault on which an earthquake occurs. This sequence that is occurring in the depth range of 10 to 13 km exhibits a mixture of strike-slip and thrust faulting. The preferred fault plane dips 60 degrees to the northeast. It is possible that these events are associated with the Santa Susana fault, the major thrust fault in the area with similar strike and dip. However, these are small earthquakes and could just as well be associated with smaller unrecognized oblique slip faults in the region.
Two earthquakes of M5.1 and M4.9 occurred at the same location on 26 and 27 April 1997. These earthquake sequences were also followed by numerous aftershocks and occurred at a greater depth of 12 to 17 km, about 3 to 5 km deeper than the 2002 sequence. The 1997 and 2002 sequences both reflect the ongoing compression of the Ventura basin. Furthermore, both of these sequences occurred in the aftershock zone of the Mw6.7 1994 Northridge earthquake and are considered to be part of the decaying Northridge sequence. The current rate of earthquakes in the Northridge aftershock zone is two to three times higher than the background rate, recorded before the mainshock in 1994.

From the Los Angeles Times

Quake Hits Northern Areas of the Region

A light earthquake rattled the San Fernando, Santa Clarita and Simi valleys Monday night, but there were no reports of damage.

Officials at the Caltech seismological laboratories said the magnitude 4.2 temblor occurred at 9:53 p.m. Over the next hour, at least 13 aftershocks were recorded, including a 3.9 at 10 p.m. and a 3.8 at 10:08 p.m.

They said the quake was centered about six miles west of Valencia and apparently occurred about 1.6 miles below the surface of the earth.

"It shook this facility quite mightily," said Jeff Lawrence, manager of the Ventura County sheriff's station in Simi Valley, about five miles from the epicenter. But nothing fell from walls, and Lawrence said he thought at first that a water main might have broken.

Carlos Pena, a manager of a Denny's restaurant near the center of town, said no pots or pans fell during the shaking, but patrons became uneasy.

"The customers got a little panicky," Pena said. "Most of them stood up and tried to get to the cash register to pay quickly, and one girl couldn't wait and she started to make, kind of like, screaming noises."

Simi Valley police said emergency procedures were put into effect but were not needed. Sheriff's personnel in Santa Clarita said it "wasn't that bad."

"There was just a jolt," said a dispatcher at the sheriff's station in Santa Clarita who gave her name as Officer V. Wilson.

"We only got about 15 calls, which shows it wasn't that much," she said. "It was nothing compared to the [Northridge] quake back in 1994."

The first quake was felt throughout a wide section of Southern California, including Pasadena, Santa Barbara, West Hollywood and much of the northwest side of Los Angeles.