April 21, 1994 Weekly Earthquake Report for Southern California ------------------------------------------------ April 14 - 20, 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, April 14, Pacific Daylight Time, to midnight Wednesday night, April 20, Pacific Daylight Time. There are 414 this week. It was a dull week. Even though the total number of events was not much different than it has been in the last few weeks, only one of them was above M3. That was a M3.1 Northridge aftershock on Friday afternoon, near San Fernando. It was felt. One other event was felt, also. It was a Landers aftershock on Friday evening, M2.8. A few small events occurred in other locations in the L.A. area but were all too small for anyone to feel. The expected number of M3.0 or larger Northridge aftershocks is now about three per week, so this week was particularly quiet. We will probably see more in future weeks. It is interesting to note that the expected number of M3.0 or larger Landers aftershocks is now about 1-2 per week, so both zones are expected to be active in the next few months. The reason Landers can compete with Northridge even though it has been almost two years since the mainshock is that it was a much larger mainshock The number of aftershocks at any given magnitude level scales with the magnitude of the mainshock. In the three months since the Northridge quake, we have recorded about 7,000 aftershocks (about M1.6 or so) while Landers produced 45,000 aftershocks in the first 6 months. 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 7 hrs, to the Daylight Time listed. Table 1 ------- Date Time N Lat. W Long. Mag ------------------------------------------------------------- 4/14 1:48 am 33 56.4 116 43.1 2.0 4 mi. ENE of Cabazon 4/14 5:10 am 33 51.5 116 10.4 2.0 10 mi. NNE of Indio 4/14 6:11 am 35 2.9 119 1.3 2.0 4 mi. WNW of the I5/99 interchange 4/14 2:21 pm 34 22.4 116 26.8 2.3 17 mi. N of Yucca Valley 4/14 2:37 pm 34 21.2 118 31.0 2.2 1 mi. SSE of Newhall 4/14 7:08 pm 35 58.6 117 38.7 2.0 17 mi. ESE of Coso Junction 4/14 8:40 pm 35 58.1 118 20.1 2.2 24 mi. NNE of the town of Lake Isabella 4/15 9:52 am 34 29.5 118 42.3 2.5 6 mi. WSW of Castaic Lake 4/15 4:49 pm 34 16.3 118 27.0 3.1 Under San Fernando; FELT 4/15 5:03 pm 35 16.7 117 27.7 2.4 26 mi. SSE of Ridgecrest 4/15 6:54 pm 33 58.3 117 1.3 2.1 3 mi. NW of Beaumont 4/15 7:15 pm 36 23.2 118 5.7 2.5 8 mi. NW of Olancha 4/15 7:45 pm 34 7.3 116 24.2 2.8 2 mi. E of Yucca Valley; FELT 4/15 9:02 pm 34 21.7 118 38.9 2.1 5 mi. SW of Magic Mtn. 4/15 9:08 pm 34 16.6 118 31.4 2.1 2 mi. NNE of Northridge 4/15 9:20 pm 33 21.3 116 18.9 2.0 7 mi. NNE of Borrego Springs 4/15 11:50 pm 34 7.5 116 23.9 2.0 2 mi. E of Yucca Valley 4/16 4:24 am 34 30.0 118 41.9 2.6 6 mi. W of Castaic Lake 4/16 5:05 am 34 16.1 118 28.1 2.1 1 mi. WSW of San Fernando 4/16 5:31 am 34 15.5 118 28.5 2.1 2 mi. SW of San Fernando 4/16 1:44 pm 36 28.1 117 54.5 2.2 12 mi. SE of Lone Pine 4/16 4:27 pm 34 1.9 117 13.5 2.0 3 mi. WSW of Redlands 4/16 8:44 pm 32 20.5 115 20.6 2.5 24 mi. SSE of Calexico 4/17 12:43 am 34 10.3 118 32.5 2.7 4 mi. S of Northridge 4/17 10:48 am 34 17.5 118 36.6 2.2 6 mi. N of Canoga Park 4/17 11:39 am 34 26.9 116 30.1 2.0 23 mi. N of Yucca Valley 4/17 11:40 am 36 1.4 117 53.1 2.1 3 mi. ESE of Coso Junction 4/17 12:10 pm 33 29.3 116 27.3 2.0 13 mi. ESE of Anza 4/17 12:16 pm 34 15.7 116 27.1 2.0 9 mi. N of Yucca Valley 4/17 2:57 pm 34 14.1 116 52.1 2.2 2 mi. SSW of Big Bear City 4/17 7:53 pm 34 16.4 118 24.6 2.3 1 mi. ESE of San Fernando 4/17 8:37 pm 34 15.6 116 27.2 2.2 9 mi. N of Yucca Valley 4/17 8:38 pm 34 15.9 116 27.1 2.3 " 4/17 8:44 pm 34 15.8 116 27.1 2.2 " 4/17 8:58 pm 34 16.0 116 27.4 2.0 " 4/18 12:43 am 34 16.3 118 27.9 2.1 1 mi. WSW of San Fernando 4/18 2:40 am 34 3.3 118 23.0 2.2 1 mi. WSW of La Brea tar pits 4/18 7:50 am 33 20.9 116 17.5 2.2 7 mi. NE of Borrego Springs 4/18 8:45 am 34 18.7 118 30.0 2.5 4 mi. WNW of San Fernando 4/18 3:55 pm 34 16.6 116 49.5 2.7 1 mi. ENE of Big Bear City 4/18 4:46 pm 33 4.0 115 57.5 2.2 11 mi. ESE of Ocotillo Wells 4/18 6:08 pm 35 1.5 116 58.2 2.1 9 mi. NNE of Barstow 4/18 6:14 pm 34 37.8 116 40.2 2.4 20 mi. NE of Lucerne Valley 4/18 7:05 pm 35 1.3 116 58.5 2.0 9 mi. NNE of Barstow 4/18 7:36 pm 36 13.4 118 8.4 2.1 8 mi. WSW of Olancha 4/18 10:03 pm 34 29.7 118 42.6 2.2 6 mi. WSW of Castaic Lake 4/18 10:30 pm 35 18.9 117 39.2 2.4 21 mi. S of Ridgecrest 4/18 10:53 pm 35 18.9 117 39.3 2.1 " 4/18 11:16 pm 35 18.8 117 39.0 2.0 " 4/19 6:22 am 34 17.8 118 27.8 2.3 1 mi. NW of San Fernando 4/19 8:39 am 34 22.8 118 41.6 2.5 7 mi. N of Simi Valley 4/19 8:51 am 34 22.9 118 41.5 2.4 " 4/19 8:54 am 34 22.2 118 40.7 2.1 " 4/19 8:11 pm 34 15.9 118 28.3 2.0 2 mi. WSW of San Fernando 4/20 1:00 am 35 59.3 120 32.9 2.6 8 mi. NW of Parkfield 4/20 2:27 am 33 59.8 116 45.3 2.1 5 mi. NNE of Cabazon 4/20 7:17 am 33 58.0 116 53.2 2.3 2 mi. N of Banning 4/20 8:55 am 34 13.2 116 45.3 2.4 6 mi. ESE of Big Bear City 4/20 1:18 pm 34 0.9 116 19.2 2.2 10 mi. SE of Yucca Valley 4/20 1:59 pm 33 59.8 116 16.9 2.7 12 mi. SE of Yucca Valley 4/20 3:06 pm 34 18.7 118 28.8 2.0 3 mi. NW of San Fernando 4/20 3:07 pm 34 19.2 118 29.2 2.0 " 4/20 7:30 pm 33 37.5 117 51.1 2.0 4 mi. E of Newport Beach 4/20 9:17 pm 34 32.9 116 30.4 2.5 29 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.