December 8, 1994 Weekly Earthquake Report for Southern California ------------------------------------------------ December 1 - 7, 1994 Prepared by: Kate Hutton, Seismological Laboratory (kate@bombay.gps.caltech.edu) 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 Thursday morning, December 1, Pacific Standard Time, to midnight Wednesday night, December 7, Pacific Standard Time. We recorded 353 earth- quakes of M1.0 or larger during the 7-day period covered. The biggest excitement of the week occurred at the eastern edge of the Northridge aftershock zone near San Fernando, with a cluster of events on Monday evening. The two largest, a M3.6 and a M4.5, were widely felt in the greater L.A. area. Like the Northridge mainshock and most of the other aftershocks, both of these had thrust mechanisms, with the southern block sliding up and over the northern block. The M4.5 was the largest aftershock since the March 20 M5.2; however, there was one of comparable size (M4.4) on May 25. The last M4+ aftershock was a M4.1 on June 14. Many people were surprised to find aftershocks of this size so long after the mainshock, because the expect aftershocks to get progressively smaller with time. Actually, they become progressively less frequent with time, but still maintain the same proportions of size. So, a M4 is quite possible one year later, and in fact is perfectly normal for the aftershock sequence to a quake as large as Northridge (M6.7). In fact, we had computed aftershock probabilities for the coming year (1995): about 18 M3's, about two M4's, and about a 25% chance of a M5 aftershock. Of course, there will be many smaller than M3. The table lists the quakes this week that were M2.0 or larger in the central part of the coverage area. Times are local times; if you want Greenwich Mean Time, add 7 hrs to the Pacific Daylight Time or 8 hrs to the Pacific Standard Time listed. Table 1 ------- Date Time N Lat. W Long. Mag ------------------------------------------------------------- 12/1 12:12 am 34 17.4 116 23.7 2.1 12 mi. NNE of Yucca Valley 12/1 2:33 am 34 15.1 116 26.6 2.1 9 mi. N of Yucca Valley 12/1 4:50 am 34 19.7 116 28.1 2.1 14 mi. N of Yucca Valley 12/1 5:47 am 34 18.7 118 25.3 2.4 2 mi. NNE of San Fernando 12/1 8:19 am 34 1.8 116 19.7 2.1 9 mi. SE of Yucca Valley 12/1 10:07 am 34 33.5 116 31.6 2.2 25 mi. ENE of Lucerne Valley 12/1 11:46 am 34 1.7 116 19.8 2.5 9 mi. SE of Yucca Valley 12/1 12:51 pm 35 4.0 116 58.3 2.3 12 mi. NNE of Barstow 12/1 4:36 pm 34 16.0 118 33.4 2.8 2 mi. NNW of Northridge 12/1 4:38 pm 34 33.7 116 32.5 2.5 24 mi. ENE of Lucerne Valley 12/1 6:10 pm 34 37.0 116 29.5 2.0 28 mi. ENE of Lucerne Valley 12/1 7:46 pm 34 17.6 118 27.4 2.1 1 mi. NW of San Fernando 12/1 9:18 pm 34 2.6 116 48.7 2.0 3 mi. SSE of Mt. San Gorgonio 12/1 9:48 pm 34 18.8 118 25.2 2.0 2 mi. NNE of San Fernando 12/2 2:50 am 36 10.2 120 43.8 3.3 20 mi. W of Coalinga 12/2 2:51 am 36 9.3 120 44.1 3.1 " 12/2 3:24 am 33 17.6 115 42.8 2.1 Brawley Seismic Zone, 3 mi. SSE of Bombay Beach 12/2 3:44 am 33 18.0 115 43.2 2.4 " 12/2 3:55 am 33 17.9 115 43.1 2.5 " 12/2 9:00 am 36 1.8 118 10.0 2.0 12 mi. W of Coso Junction 12/2 11:36 am 33 17.9 115 43.2 2.1 3 mi. SSE of Bombay Beach 12/2 11:59 am 34 28.6 116 30.8 2.1 25 mi. N of Yucca Valley 12/2 3:49 pm 35 31.1 118 26.7 2.5 8 mi. S of the town of Lake Isabella 12/2 6:43 pm 34 0.8 116 19.0 2.1 10 mi. SE of Yucca Valley 12/2 10:27 pm 34 21.6 118 42.1 2.7 5 mi. N of Simi Valley 12/3 12:45 am 34 23.6 118 38.4 2.0 3 mi. SW of Magic Mtn. 12/3 12:51 am 34 23.5 118 38.6 2.3 " 12/3 1:23 am 34 23.6 118 38.5 2.1 " 12/3 3:12 am 35 17.3 119 12.3 2.0 13 mi. SW of downtown Bakersfield 12/3 7:40 am 32 22.9 115 24.1 2.6 20 mi. SSE of Calexico 12/3 11:41 am 34 20.7 116 28.3 2.2 15 mi. N of Yucca Valley 12/3 3:30 pm 34 18.5 116 26.6 2.2 13 mi. N of Yucca Valley 12/3 6:16 pm 36 7.2 117 50.3 2.1 6 mi. ESE of Haiwee Reservoir 12/4 8:19 am 34 21.8 118 45.0 2.0 6 mi. NNW of Simi Valley 12/4 9:14 am 34 0.1 117 33.3 2.2 2 mi. S of the I60/I15 interchange 12/4 12:36 pm 32 23.9 118 42.9 2.6 36 mi. SW of San Clemente Is. 12/4 1:24 pm 34 20.8 116 27.5 2.1 15 mi. N of Yucca Valley 12/5 1:02 am 34 33.9 116 32.7 2.7 24 mi. ENE of Lucerne Valley 12/5 1:30 am 36 1.3 120 30.0 2.4 9 mi. NNW of Parkfield 12/5 6:50 am 34 18.0 116 25.1 2.1 12 mi. N of Yucca Valley 12/5 9:18 am 34 18.4 118 33.4 2.1 5 mi. NNW of Northridge 12/5 4:51 pm 33 32.7 116 46.6 2.0 6 mi. W of Anza 12/5 5:09 pm 35 1.9 117 0.1 2.5 10 mi. N of Barstow 12/5 7:19 pm 35 21.0 118 37.1 2.2 15 mi. NE of Arvin 12/5 7:36 pm 34 17.6 118 23.4 3.6 3 mi. ENE of San Fernando 12/5 7:47 pm 34 17.5 118 23.0 2.3 " 12/5 7:48 pm 34 17.9 118 23.3 2.9 " 12/5 7:48 pm 34 17.6 118 23.4 4.5 " 12/5 8:08 pm 34 17.3 118 23.1 2.0 " 12/5 8:09 pm 34 17.5 118 23.2 2.3 " 12/5 8:09 pm 34 17.4 118 23.1 2.6 " 12/5 10:15 pm 33 28.3 116 34.1 2.0 8 mi. SE of Anza 12/6 12:05 am 34 17.7 118 23.1 2.5 3 mi. ENE of San Fernando 12/6 12:05 am 34 17.9 118 22.9 2.3 " 12/7 1:08 am 34 18.5 118 27.6 2.9 2 mi. NW of San Fernando 12/7 4:16 am 34 4.5 116 23.0 2.0 4 mi. SE of Yucca Valley 12/7 7:40 am 35 45.6 118 3.7 2.8 20 mi. SSW of Coso Junction 12/7 7:49 am 33 57.7 116 49.8 2.1 3 mi. NE of Banning 12/7 8:41 am 36 4.2 117 51.2 2.4 5 mi. ENE of Coso Junction 12/7 2:33 pm 36 9.7 117 55.9 2.3 1 mi. NNE of south end of Haiwee Reservoir 12/7 3:14 pm 36 4.2 117 51.3 2.0 5 mi. ENE of Coso Junction 12/7 3:24 pm 34 18.9 118 26.5 2.1 2 mi. N of San Fernando 12/7 5:27 pm 34 12.8 117 34.3 2.1 6 mi. SE of Mt. Baldy 12/7 8:33 pm 34 14.7 116 24.7 2.1 8 mi. NNE of Yucca Valley ------------------------------------------------------------- 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.