March 24, 1994 Weekly Earthquake Report for Southern California ------------------------------------------------ March 17 - 23, 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, March 17, Pacific Standard Time, to midnight Wednesday night, March 23, Pacific Standard Time. There are 531 this week. The most exciting event this week was most certainly the M5.3 Northridge aftershock on Sunday afternoon. It was felt over much of southern California, very strongly in the San Fernando Valley. It was the third largest aftershock, the first two being a M5.9 and a M5.6, both on Jan. 17. Like all but one of the major aftershocks, it had a thrust focal mechanism similar to the main shock. It occurred in the southeastern edge of the aftershock zone, in a patch that had been free of aftershocks before Sunday. Earthquakes happen on fault planes, not at points. The Northridge fault plane is not vertical but dips at an angle under the northern San Fernando Valley. Most of the aftershocks have been on or near this plane, as stress readjusts to the changes caused by the main shock, so we can use the aftershock pattern to determine where the mainshock plane was. The largest aftershocks often occur along the edges, in places that did not slip in the mainshock but were highly stressed. The "hole" observed in the aftershock pattern was probably a strong spot on the fault that resisted slip until this past Sunday. A common misconception about aftershocks is that the number and size decrease with time. Only the number decrease with time; the relative number of small to large stays the same. As the overall rate goes down, the big aftershocks become less frequent, just as the small ones do. As a rule of thumb, for every 100 magnitude 3 aftershocks, we will see one magnitude 5. Jan. 17 had 200 magnitude 3 events and 2 magnitude 5 plus events. Now, we are having only three to five magnitude 3 events each week. So one magnitude 5 every 20-30 weeks is reasonable. Having one magnitude 5 does not "use" up our probability any more than flipping heads makes tails more likely on the next flip. We still have better than a one in three chance of another magnitude 5 plus aftershock within the next year. Another quake felt this week, but not listed in this report, was a M5.0 at 5:59 pm on Tuesday, located about 25 miles ESE of Ensenada. It was felt slightly in San Diego. This location is, however, outside of our area of coverage. Further inquiries should be directed to the National Earthquake Information Service at 303-273-8500. The table lists the quakes 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 8 hrs, to the Pacific Standard Time listed. Table 1 ------- Date Time N Lat. W Long. Mag ------------------------------------------------------------- 3/17 12:52 am 34 23.6 118 37.8 2.4 3 mi. SW of Magic Mtn. 3/17 5:02 am 34 18.0 118 27.4 2.2 1 mi. NW of San Fernando 3/17 6:25 am 34 13.5 118 36.6 2.0 1 mi. N of Canoga Park 3/17 10:01 am 34 23.7 118 37.7 3.0 2 mi. SW of Magic Mtn. 3/17 4:58 pm 34 0.6 116 19.1 2.1 10 mi. SE of Yucca Valley 3/17 6:25 pm 33 42.7 116 49.6 2.2 6 mi. WSW of Idyllwild 3/17 7:01 pm 34 16.2 118 33.0 2.4 2 mi. NNW of Northridge 3/17 7:51 pm 35 4.4 117 0.4 2.0 12 mi. N of Barstow 3/17 8:10 pm 34 27.6 118 36.3 2.0 2 mi. NNW of Magic Mtn. 3/18 12:27 am 34 13.6 118 36.5 2.2 1 mi. N of Canoga Park 3/18 5:06 am 34 18.0 118 25.3 2.4 1 mi. NE of San Fernando 3/18 9:44 am 34 21.6 118 36.4 2.2 4 mi. SSW of Magic Mtn. 3/18 9:58 am 34 14.6 118 27.5 2.8 2 mi. SSW of San Fernando 3/18 11:35 am 34 17.4 118 28.6 2.0 2 mi. WNW of San Fernando 3/18 1:31 pm 34 13.5 118 37.8 2.0 2 mi. NW of Canoga Park 3/18 2:19 pm 34 16.5 118 27.9 2.1 1 mi. WSW of San Fernando 3/18 3:08 pm 35 2.2 117 40.4 2.0 28 mi. E of Mojave 3/18 7:53 pm 34 18.7 118 28.1 2.4 2 mi. NW of San Fernando 3/18 10:09 pm 34 15.7 118 39.0 2.1 3 mi. ESE of Simi Valley 3/19 6:25 am 34 15.4 118 35.5 2.3 4 mi. N of Canoga Park 3/19 7:37 am 36 34.6 120 9.6 2.3 23 mi. WSW of downtown Fresno 3/19 7:52 am 34 15.6 118 35.5 2.5 4 mi. N of Canoga Park 3/19 9:49 am 33 24.7 116 26.2 2.4 11 mi. NNW of Borrego Springs 3/19 1:05 pm 34 17.7 118 30.6 2.6 4 mi. NNE of Northridge 3/19 4:11 pm 34 14.1 118 36.5 2.5 2 mi. N of Canoga Park 3/19 5:04 pm 34 15.5 118 42.0 2.0 1 mi. SSE of Simi Valley 3/19 5:16 pm 33 15.4 116 22.2 2.2 Under Borrego Springs 3/19 5:40 pm 32 39.0 115 53.3 2.2 8 mi. SE of Ocotillo 3/19 8:49 pm 34 23.0 118 25.4 2.4 7 mi. N of San Fernando 3/19 10:48 pm 34 20.9 118 42.0 3.0 4 mi. N of Simi Valley 3/19 10:51 pm 33 58.5 117 12.3 2.0 5 mi. SSW of Redlands 3/20 5:29 am 34 8.7 118 30.2 2.2 6 mi. SSE of Northridge 3/20 11:26 am 34 19.5 118 32.8 2.2 3 mi. SSW of Newhall 3/20 12:23 pm 34 21.7 118 38.3 2.6 5 mi. SW of Magic Mtn. 3/20 12:31 pm 33 58.4 117 12.3 2.2 5 mi. SSW of Redlands 3/20 1:17 pm 34 15.0 118 28.2 2.1 2 mi. SW of San Fernando 3/20 1:20 pm 34 13.9 118 28.5 5.3 3 mi. E of Northridge; FELT 3/20 1:27 pm 34 13.6 118 28.9 2.3 " 3/20 1:39 pm 34 13.4 118 30.0 2.1 " 3/20 1:41 pm 34 13.6 118 29.3 2.1 " 3/20 1:48 pm 34 13.2 118 29.0 2.0 " 3/20 2:08 pm 34 12.4 118 30.7 2.0 2 mi. SSE of Northridge 3/20 2:12 pm 34 13.9 118 29.0 2.1 2 mi. E of Northridge 3/20 2:28 pm 34 23.8 116 27.6 2.0 19 mi. N of Yucca Valley 3/20 2:39 pm 34 14.0 118 28.9 2.2 2 mi. E of Northridge 3/20 3:55 pm 34 13.1 118 29.7 2.2 2 mi. ESE of Northridge 3/20 4:11 pm 35 57.9 117 37.9 2.5 18 mi. ESE of Coso Junction 3/20 5:29 pm 34 13.9 118 29.4 2.6 2 mi. E of Northridge 3/20 8:00 pm 34 13.5 118 30.4 2.7 " FELT 3/20 8:14 pm 34 54.8 116 53.2 2.4 7 mi. ENE of Barstow 3/20 10:19 pm 34 23.9 116 27.7 2.9 19 mi. N of Yucca Valley 3/20 11:51 pm 34 13.3 118 37.1 2.0 1 mi. NNW of Canoga Park 3/21 1:20 am 34 9.1 116 25.8 2.5 2 mi. NNE of Yucca Valley 3/21 1:47 am 34 15.1 118 36.4 2.2 3 mi. N of Canoga Park 3/21 2:16 am 34 21.2 118 52.3 2.6 4 mi. SE of Fillmore 3/21 2:31 am 34 9.3 116 25.8 2.7 2 mi. NNE of Yucca Valley 3/21 2:57 am 34 19.9 118 26.8 2.0 3 mi. N of San Fernando 3/21 3:44 am 34 19.6 118 28.4 2.0 " 3/21 6:32 am 34 17.7 118 26.0 2.5 Under San Fernando 3/21 7:38 am 36 11.9 119 33.9 2.2 17 mi. WSW of Visalia 3/21 10:13 am 32 5.5 115 19.7 3.0 41 mi. SSE of Calexico 3/21 10:31 am 32 49.0 115 37.4 2.0 4 mi. WNW of El Centro 3/21 12:54 pm 34 20.1 118 36.1 2.2 5 mi. SW of Newhall 3/21 2:26 pm 34 9.2 116 25.9 2.1 2 mi. NNE of Yucca Valley 3/21 3:58 pm 34 11.9 118 36.5 2.2 Under Canoga Park 3/21 4:43 pm 34 56.5 116 39.7 2.3 20 mi. E of Barstow 3/21 5:08 pm 34 18.2 118 26.6 2.6 1 mi. N of San Fernando 3/21 11:50 pm 35 4.2 119 10.1 2.2 12 mi. WNW of the I5/99 interchange 3/22 1:16 am 34 17.5 118 25.4 2.0 1 mi. NE of San Fernando 3/22 3:04 am 34 14.2 118 27.9 2.1 3 mi. SSW of San Fernando 3/22 3:38 am 34 35.4 116 16.9 2.2 33 mi. NNE of Yucca Valley 3/22 5:06 am 33 30.5 116 27.9 2.5 12 mi. ESE of Anza 3/22 5:07 am 34 13.4 118 36.2 2.4 1 mi. N of Canoga Park 3/22 7:32 am 34 18.7 118 26.0 2.8 2 mi. N of San Fernando 3/22 7:55 am 34 13.6 118 29.7 2.6 2 mi. ESE of Northridge 3/22 8:52 am 34 9.7 116 25.1 2.3 3 mi. NNE of Yucca Valley 3/22 11:21 am 34 19.2 118 35.1 2.0 5 mi. SW of Newhall 3/22 1:03 pm 34 13.3 118 36.5 2.5 1 mi. N of Canoga Park 3/22 2:55 pm 34 17.2 118 34.9 2.1 4 mi. NW of Northridge 3/22 3:54 pm 34 24.8 116 30.5 2.1 20 mi. N of Yucca Valley 3/22 6:26 pm 34 12.1 118 30.9 2.2 2 mi. SSE of Northridge 3/22 6:27 pm 34 13.9 118 29.5 2.2 2 mi. E of Northridge 3/22 7:19 pm 34 18.6 118 32.5 2.0 4 mi. S of Newhall 3/22 7:19 pm 34 19.0 118 32.4 2.7 " 3/22 11:39 pm 34 17.4 118 30.2 2.0 4 mi. NNE of Northridge 3/23 5:05 am 34 23.3 116 27.6 2.8 18 mi. N of Yucca Valley 3/23 6:14 am 34 16.2 118 28.1 2.4 1 mi. WSW of San Fernando 3/23 9:00 am 34 17.4 118 27.6 3.3 1 mi. WNW of San Fernando FELT 3/23 9:31 am 34 17.0 118 27.1 2.5 " 3/23 10:44 am 34 15.1 118 28.9 2.1 3 mi. SW of San Fernando 3/23 11:03 am 34 23.1 116 27.3 2.3 18 mi. N of Yucca Valley 3/23 1:05 pm 34 12.8 118 29.7 2.0 2 mi. SE of Northridge 3/23 2:24 pm 34 19.5 116 28.3 2.1 14 mi. N of Yucca Valley 3/23 4:28 pm 34 17.2 118 31.4 2.5 3 mi. N of Northridge 3/23 4:47 pm 32 25.6 115 26.9 2.4 16 mi. S of Calexico 3/23 5:19 pm 34 23.8 116 27.4 2.2 19 mi. N of Yucca Valley 3/23 5:31 pm 34 16.5 118 28.1 2.0 1 mi. WSW of San Fernando 3/23 7:21 pm 34 21.8 116 28.2 2.7 16 mi. N of Yucca Valley 3/23 10:27 pm 34 23.0 116 28.0 2.0 18 mi. N 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.