Commentary from Trinet.org:

07:26 PST, Tuesday, January 29

A series of quakes occurred last night to the northwest of San Fernando Valley. The largest ones were a M4.2 at 9:53 pm, a M3.9 at 10:00 pm, and a M3.8 at 10:08 pm. These were located about halfway between Simi Valley and Valencia. There have been 11 smaller aftershocks as well at this location.

All three of the largest quakes were felt fairly widely, the M4.2 as far away as Antelope Valley, the Grapevine area, and Orange County. The peak Modified Mercalli Intensity was IV. The M3.9 was felt as far as Ventura and Pasadena, and also had a peak Intensity of IV, over a smaller area.

The question of whether these quakes are Northridge aftershocks or not will certainly arise, and may not be so easy to answer. Technically, aftershocks continue at an ever decreasing rate until the number of quakes per week, month, or whatever is down to the level it was before the mainshock. For a quake the size of Northridge, that can be many years later. In fact, the Northridge sequence is still visible on current seismicity maps, so aftershocks are really still going on. These might be some of them. On the other hand, San Fernando Valley may be getting on with its seismic life, so to speak.

Oddly enough, in the past week, there has been a smattering of tiny quakes strewn over the San Fernando Valley. All these were less than M2.0, and so were not felt by residents. Normally, when we see such a scattered distribution, we expect to find the "quakes" are caused by wind or other extraneous noise, or telemetry problems. These quakes, however, are real and correctly located. Their appearance before the Simi Valley/Valencia activity is probably coincidence, but it's worth a note in the commentary.

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