January 20, 1994 Weekly Earthquake Report for Southern California ------------------------------------------------ January 13 - 19, 1994 Prepared by: Kate Hutton, Seismological Laboratory (kate@bombay.gps.caltech.edu) Egill Hauksson, Seismological Laboratory Lucy Jones, U.S. Geological Survey California Institute of Technology This document is a commentary on current seismic activity. The earthquakes discussed have been detected and processed automatically. All epicenters and magnitudes have been reviewed to exclude obvious blunders; however, they must still be considered preliminary. For further information, please contact the authors or the Caltech Public Relations Office at 818-395-6326. For daily updates, call our Earthquake Information Hotline: 818-395-6977. -------------------------------- This week's Report covers the time period from midnight Tuesday morning, January 13, Pacific Standard Time, to midnight Wednesday night, January 19, Pacific Standard Time. There are 857 this week, and the list is by no means complete since we are behind in the data processing. It's probably safe to say that virtually everyone in southern California was awake at 4:31 am on Monday. The MS6.6 earthquake, that originated under Northridge, in the San Fernando Valley, scared the wits out of most of us and left many standing in the middle of piles of debris. Damage was most extensive in San Fernando Valley, Simi Valley, and the northern part of Los Angeles Basin. Although we had some difficulty early in the sequence due to a phone line dropout caused by the main shock and two seismic stations destroyed by the Malibu fire in October, the Southern California Seismographic Network has recorded over 1000 aftershocks to the Northridge quake, and the staff has done preliminary analysis on about half of them. Most were small, except for 245 of them that were M3.0 or larger. Of these, 26 were M4.0 or larger, and 2 were M5.0 or larger. Their hypocenters outline a shallowly south-dipping plane, which is in agreement with the thrust focal mechanism of the main shock. The rupture started at about 9 miles depth and, during the course of the main shock, ruptured upward and northward, spreading both eastward and westward. The northern end of the fault extends under the Santa Susana Mtns. The Frew fault is one of several that could have been responsible. The rate of aftershocks is decaying as expected for a "textbook" earthquake, just slightly more energetic than the average. If the same rate of decay continues, about 7 more M5 or larger aftershocks are expected during the sequence, which will be going on for months with every decreasing frequency. Many people have questioned the relationship between the Northridge quakes and the cluster offshore of Santa Monica last week and this week. Whether they were "related" is still an open question. No physical cause and effect relationship is known, and quakes in the Santa Monica Bay are common enough that coincidence is a quite reasonable explanation. The two quakes in the table that have "West Los Angeles" locations are probably actually offshore and in the cluster, mislocated because of the high rate of Northridge aftershocks. In the same vein, the activity in the rest of the Network is probably not lower after the Northridge main shock, as the table seems to imply. The regular background activity and the week's allotment of Landers quakes are probably buried in the aftershocks and will emerge with further data processing. The table lists the quakes that were M2.0 or larger in the central part of the coverage area, during the time period before the Northridge main shock. After it, the data processing is very incomplete due to the huge number of events. Aftershocks must be at least M3.5 to be listed. Times are local times; if you want Greenwich Mean Time, add 8 hrs, to the Pacific Standard Time listed. Table 1 ------- Date Time N Lat. W Long. Mag ------------------------------------------------------------- 1/13 2:56 am 34 58.4 116 57.8 2.3 6 mi. NNE of Barstow 1/13 3:06 am 34 58.5 116 57.6 3.3 " 1/13 3:13 am 34 58.2 116 57.6 2.3 " 1/13 3:37 am 34 58.2 116 57.8 2.1 " 1/13 3:51 am 33 13.9 116 2.2 2.5 5 mi. SW of Salton City 1/13 3:52 am 34 58.2 116 57.6 2.5 6 mi. NNE of Barstow 1/13 4:20 am 34 34.2 116 34.1 2.2 23 mi. ENE of Lucerne Valley 1/13 8:51 am 34 58.4 116 57.8 2.7 6 mi. NNE of Barstow 1/13 11:55 am 33 43.4 116 50.3 2.2 7 mi. WSW of Idyllwild 1/13 11:59 am 34 8.9 116 25.9 2.2 2 mi. NNE of Yucca Valley 1/13 12:11 pm 34 58.1 116 57.5 2.2 6 mi. NNE of Barstow 1/13 2:18 pm 34 37.1 116 40.0 2.0 19 mi. NE of Lucerne Valley 1/13 5:10 pm 34 23.3 116 28.4 2.5 18 mi. N of Yucca Valley 1/13 8:09 pm 35 2.5 116 59.1 2.2 10 mi. NNE of Barstow 1/13 10:12 pm 35 29.5 118 26.6 2.9 10 mi. S of the town of Lake Isabella 1/13 10:12 pm 35 29.4 118 26.6 3.2 " 1/14 2:13 am 34 19.8 116 26.4 3.2 14 mi. N of Yucca Valley 1/14 2:35 am 34 2.5 117 5.8 3.0 4 mi. ESE of Redlands 1/14 2:52 am 34 20.5 116 28.7 2.3 15 mi. N of Yucca Valley 1/14 4:37 am 34 11.3 116 25.7 2.3 4 mi. N of Yucca Valley 1/14 4:38 am 34 11.9 116 25.6 2.3 " 1/14 4:39 am 34 11.0 116 25.9 2.1 " 1/14 4:59 am 34 37.5 116 32.6 2.4 26 mi. ENE of Lucerne Valley 1/14 6:00 am 33 14.5 116 1.9 2.0 4 mi. WSW of Salton City 1/14 2:34 pm 34 15.6 116 26.2 2.3 9 mi. N of Yucca Valley 1/14 2:45 pm 34 6.9 116 55.1 2.1 5 mi. W of Mt. San Gorgonio 1/14 4:45 pm 34 29.3 116 27.0 2.6 25 mi. N of Yucca Valley 1/14 5:24 pm 34 58.2 116 57.7 2.5 6 mi. NNE of Barstow 1/14 9:35 pm 35 17.1 118 36.7 2.3 14 mi. NW of Tehachapi 1/15 1:45 am 33 58.5 116 38.3 2.4 7 mi. W of Desert Hot Springs 1/15 2:40 am 32 45.4 115 14.9 2.1 15 mi. ENE of Calexico 1/15 11:20 am 32 52.5 117 30.9 2.1 15 mi. W of La Jolla 1/15 11:34 am 34 19.2 116 38.6 2.9 12 mi. ENE of Big Bear City 1/15 9:28 pm 33 42.6 116 42.5 3.0 2 mi. S of Idyllwild 1/15 11:51 pm 34 38.0 116 40.1 2.1 20 mi. NE of Lucerne Valley 1/16 4:08 am 34 34.3 116 34.4 2.0 22 mi. ENE of Lucerne Valley 1/16 7:35 am 34 10.7 116 25.7 2.0 4 mi. NNE of Yucca Valley 1/17 4:30 am 34 12.9 118 32.3 6.6 1 mi. S of Northridge 1/17 4:39 am 4.5 Northridge aftershock zone 1/17 4:40 am 4.8 " 1/17 4:41 am 4.0 " 1/17 4:49 am 34 18.8 118 25.8 3.5 2 mi. NNE of San Fernando 1/17 4:51 am 34 23.6 118 26.3 3.8 5 mi. ENE of Newhall 1/17 4:53 am 34 22.7 116 26.4 2.5 18 mi. N of Yucca Valley 1/17 4:54 am 3.5 Northridge aftershock zone 1/17 4:57 am 34 21.2 118 25.5 3.7 5 mi. N of San Fernando 1/17 4:59 am 34 19.4 118 28.9 3.7 3 mi. NW of San Fernando 1/17 4:59 am 35 9.6 118 25.7 3.1 2 mi. NNE of Tehachapi 1/17 5:01 am 34 21.3 118 37.3 3.8 5 mi. SSW of Magic Mtn. 1/17 5:06 am 34 14.7 118 33.0 3.9 1 mi. WNW of Northridge 1/17 5:08 am 34 18.0 118 27.5 3.6 1 mi. NW of San Fernando 1/17 5:17 am 34 18.3 118 28.9 3.6 2 mi. WNW of San Fernando 1/17 5:22 am 34 21.9 118 37.2 3.5 4 mi. SSW of Magic Mtn. 1/17 5:25 am 34 20.0 118 30.3 3.5 3 mi. SSE of Newhall 1/17 5:26 am 34 19.3 118 27.2 4.5 2 mi. NNW of San Fernando 1/17 5:28 am 34 15.1 118 34.8 3.5 2 mi. WNW of Northridge 1/17 5:29 am 34 20.5 118 28.5 3.8 3 mi. SE of Newhall 1/17 5:32 am 34 19.0 118 25.4 3.7 2 mi. NNE of San Fernando 1/17 5:37 am 34 21.0 118 35.8 3.9 5 mi. S of Magic Mtn. 1/17 5:44 am 34 21.0 118 33.0 3.8 2 mi. SW of Newhall 1/17 5:45 am 34 21.6 118 37.5 3.9 4 mi. SSW of Magic Mtn. 1/17 5:56 am 34 16.9 118 37.4 4.3 5 mi. N of Canoga Park 1/17 6:03 am 34 21.8 118 37.7 3.6 4 mi. SSW of Magic Mtn. 1/17 6:06 am 34 18.0 118 32.4 3.7 4 mi. N of Northridge 1/17 6:08 am 34 18.7 118 26.0 3.7 2 mi. N of San Fernando 1/17 6:14 am 34 20.1 118 26.1 4.3 3 mi. N of San Fernando 1/17 6:20 am 33 57.7 118 29.7 2.7 2 mi. WSW of Marina del Rey 1/17 6:26 am 34 22.7 118 28.1 3.6 3 mi. E of Newhall 1/17 6:33 am 34 18.1 118 29.4 3.5 3 mi. WNW of San Fernando 1/17 6:50 am 34 19.1 118 27.8 3.8 2 mi. NNW of San Fernando 1/17 7:07 am 34 18.7 118 29.5 4.2 3 mi. NW of San Fernando 1/17 7:07 am 34 21.5 118 31.8 4.1 1 mi. S of Newhall 1/17 7:10 am 34 19.0 118 26.8 3.9 2 mi. NNW of San Fernando 1/17 7:14 am 34 21.2 118 27.2 3.7 5 mi. N of San Fernando 1/17 7:24 am 34 22.1 118 36.7 3.5 4 mi. SSW of Magic Mtn. 1/17 7:42 am 34 18.9 118 24.7 3.8 2 mi. NNE of San Fernando 1/17 7:45 am 34 22.1 118 37.0 3.8 4 mi. SSW of Magic Mtn. 1/17 7:45 am 34 25.3 118 36.9 3.7 1 mi. W of Magic Mtn. 1/17 7:46 am 3.7 northern San Fernando Valley 1/17 7:54 am 34 22.4 118 37.3 4.5 3 mi. SSW of Magic Mtn. 1/17 8:16 am 34 17.4 118 28.7 3.7 2 mi. WNW of San Fernando 1/17 8:19 am 34 20.9 118 27.0 3.5 4 mi. N of San Fernando 1/17 9:21 am 34 19.1 118 25.9 3.5 2 mi. N of San Fernando 1/17 9:56 am 34 13.5 118 34.6 4.5 2 mi. NE of Canoga Park 1/17 10:20 am 34 17.0 118 27.9 3.5 1 mi. W of San Fernando 1/17 10:32 am 34 17.4 118 30.4 3.7 3 mi. NNE of Northridge 1/17 10:51 am 34 17.9 118 26.1 3.7 1 mi. NNE of San Fernando 1/17 11:23 am 34 16.7 118 34.6 3.5 3 mi. NW of Northridge 1/17 11:35 am 34 19.0 118 26.8 3.9 2 mi. N of San Fernando 1/17 11:43 am 34 22.0 118 38.0 4.1 4 mi. SSW of Magic Mtn. 1/17 11:46 am 34 22.7 118 38.1 3.5 " 1/17 11:53 am 34 13.3 116 46.4 2.1 5 mi. ESE of Big Bear City 1/17 11:58 am 34 22.3 118 37.8 3.6 4 mi. SSW of Magic Mtn. 1/17 12:02 pm 34 24.0 118 34.0 3.8 2 mi. SE of Magic Mtn. 1/17 12:05 pm 34 20.4 118 32.0 3.7 2 mi. S of Newhall 1/17 12:11 pm 34 19.4 118 31.3 3.6 " 1/17 12:17 pm 34 19.9 118 31.4 3.5 " 1/17 12:32 pm 3.7 Northridge aftershock zone 1/17 12:35 pm 3.5 " 1/17 12:38 pm 34 18.5 118 28.3 3.5 2 mi. NW of San Fernando 1/17 12:39 pm 34 17.5 118 29.8 3.6 3 mi. WNW of San Fernando 1/17 12:46 pm 34 18.0 118 34.9 4.7 5 mi. NNW of Northridge 1/17 12:50 pm 34 20.8 118 29.6 3.8 2 mi. SE of Newhall 1/17 2:07 pm 34 20.6 118 29.4 3.5 " 1/17 2:11 pm 34 15.2 118 35.9 3.5 3 mi. N of Canoga Park 1/17 2:19 pm 34 22.0 118 37.2 3.9 4 mi. SSW of Magic Mtn. 1/17 2:21 pm 34 20.2 118 28.3 3.5 4 mi. NNW of San Fernando 1/17 2:24 pm 34 20.3 118 31.8 3.5 2 mi. S of Newhall 1/17 2:31 pm 34 20.2 118 26.3 3.9 3 mi. N of San Fernando 1/17 2:50 pm 34 18.7 118 29.3 3.5 3 mi. NW of San Fernando 1/17 2:57 pm 34 21.0 118 36.1 3.5 5 mi. S of Magic Mtn. 1/17 3:33 pm 34 19.3 118 42.2 5.3 3 mi. N of Simi Valley 1/17 3:36 pm 34 20.9 118 40.0 3.6 5 mi. NNE of Simi Valley 1/17 3:38 pm 34 22.4 118 31.3 3.4 Under Newhall 1/17 3:49 pm 34 20.7 118 40.0 3.9 5 mi. NNE of Simi Valley 1/17 4:01 pm 34 20.4 118 41.5 3.5 " 1/17 4:36 pm 34 16.0 118 28.8 3.9 2 mi. WSW of San Fernando 1/17 4:39 pm 34 22.8 118 33.8 4.0 2 mi. W of Newhall 1/17 4:40 pm 3.8 Northridge aftershock zone 1/17 4:43 pm 34 22.7 118 41.5 4.9 7 mi. N of Simi Valley 1/17 4:45 pm 34 22.4 118 43.8 3.5 " 1/17 5:17 pm 34 22.9 118 42.1 3.6 " 1/17 6:05 pm 34 1.5 116 52.6 2.3 5 mi. SSW of Mt. San Gorgonio 1/17 7:34 pm 34 22.1 118 38.8 3.6 5 mi. SW of Magic Mtn. 1/17 8:01 pm 34 21.1 118 37.6 4.2 5 mi. SSW of Magic Mtn. 1/17 8:31 pm 34 21.8 118 27.0 3.7 4 mi. ESE of Newhall 1/17 8:43 pm 34 18.0 118 29.3 3.6 3 mi. WNW of San Fernando 1/17 9:19 pm 34 20.2 118 40.6 3.7 4 mi. NNE of Simi Valley 1/17 9:54 pm 33 41.5 116 43.8 2.7 3 mi. SSW of Idyllwild 1/17 10:29 pm 34 18.0 118 26.2 3.7 1 mi. N of San Fernando 1/17 11:11 pm 34 19.0 118 26.7 3.5 " 1/17 11:23 pm 34 20.0 118 37.1 4.0 6 mi. SSW of Magic Mtn. 1/17 11:53 pm 34 20.6 118 36.2 3.5 5 mi. S of Magic Mtn. 1/18 1:05 am 34 12.1 116 47.0 2.4 5 mi. SE of Big Bear City 1/18 1:09 am 33 58.1 116 23.0 2.2 6 mi. E of Desert Hot Springs 1/18 1:41 am 34 13.3 118 31.1 3.6 1 mi. SE of Northridge 1/18 3:35 am 34 13.0 118 36.3 4.1 1 mi. N of Canoga Park 1/18 5:24 am 34 18.5 118 34.0 4.3 5 mi. NNW of Northridge 1/18 6:17 am 34 17.2 118 29.5 3.5 3 mi. W of San Fernando 1/18 6:56 am 34 40.3 116 42.5 2.6 20 mi. NE of Lucerne Valley 1/18 7:01 am 34 22.0 118 27.5 3.5 4 mi. ESE of Newhall 1/18 7:19 am 34 13.0 118 35.4 3.8 1 mi. NE of Canoga Park 1/18 7:23 am 34 22.6 118 33.6 4.7 1 mi. W of Newhall 1/18 7:42 am 34 18.1 118 35.5 3.5 5 mi. NW of Northridge 1/18 7:51 am 34 14.6 118 27.9 3.9 3 mi. SSW of San Fernando 1/18 8:23 am 34 22.1 118 34.0 3.8 2 mi. WSW of Newhall 1/18 8:34 am 34 10.7 117 24.2 3.0 1 mi. S of Glen Helen Regional Park 1/18 10:03 am 34 19.9 118 27.0 3.6 3 mi. N of San Fernando 1/18 10:46 am 33 57.4 118 26.5 3.1 West Los Angeles 1/18 3:00 pm 34 21.1 116 27.8 2.0 16 mi. N of Yucca Valley 1/18 8:40 pm 34 21.6 118 34.3 4.3 2 mi. WSW of Newhall 1/18 8:43 pm 34 21.9 118 42.5 4.1 5 mi. N of Simi Valley 1/18 9:14 pm 34 17.6 118 28.0 3.6 1 mi. WNW of San Fernando 1/18 11:14 pm 34 17.2 118 28.5 3.9 2 mi. W of San Fernando 1/18 11:58 pm 34 14.3 118 32.2 3.2 Under Northridge 1/19 1:13 am 34 18.5 118 43.8 3.7 2 mi. NNW of Simi Valley 1/19 4:38 am 34 19.7 118 25.6 3.5 3 mi. NNE of San Fernando 1/19 5:12 am 33 13.1 115 37.1 2.4 5 mi. WSW of Niland 1/19 6:09 am 34 12.9 118 30.6 4.4 2 mi. SE of Northridge 1/19 6:46 am 3.9 Northridge aftershock zone 1/19 7:03 am 34 17.8 118 28.4 3.6 2 mi. WNW of San Fernando 1/19 10:53 am 33 34.0 116 42.8 2.7 2 mi. WNW of Anza 1/19 11:01 am 34 6.0 118 29.5 2.0 West Los Angeles 1/19 11:50 am 34 17.2 118 26.7 3.7 Under San Fernando 1/19 1:09 pm 34 21.7 118 41.9 5.0 5 mi. N of Simi Valley 1/19 1:11 pm 34 22.6 118 37.1 4.6 3 mi. SSW of Magic Mtn. 1/19 1:34 pm 34 22.2 118 40.5 3.5 6 mi. NNE of Simi Valley 1/19 5:18 pm 35 35.8 118 13.7 2.6 13 mi. ESE of the town of Lake Isabella 1/19 7:52 pm 33 59.3 116 40.1 2.1 8 mi. NE of Cabazon 1/19 9:09 pm 33 13.1 115 37.4 2.4 6 mi. WSW of Niland 1/19 9:58 pm 3.6 Northridge aftershock zone 1/19 10:58 pm 3.8 Northridge aftershock zone ------------------------------------------------------------- Figure 1. A map of southern California showing the earthquakes recorded during the past week by the Caltech/USGS Seismic Network. Major faults are marked, as well as the metropolitan areas of Los Angeles (L.A.), Palm Springs (P.S.), San Diego (S.D.), and Santa Barbara (S.B.). The circles denote the earthquakes, the size of the circle indicating the magnitude. Figure 2. A more detailed map showing the locations of some of the best located Northridge aftershocks. Northridge itself is at the large symbol at the southern end of the zone, and the many faults shown are in the mountainous terrain north of San Fernando Valley. Figure 3. A cross section showing the same aftershocks plotted in Figure 2. The viewer is looking from the west, through the Earth's crust.