August 25, 1994 Weekly Earthquake Report for Southern California ------------------------------------------------ August 18 - 24, 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, August 18, Pacific Daylight Time, to midnight Wednesday night, August 24, Pacific Daylight Time. We recorded 406 earthquakes this week. Most of the action this week was with small Northridge aftershocks. The week's largest event was a M3.5 last Thursday night, on the Simi Valley end of the aftershock zone, with the usual thrust focal mechanism on a northwest-southeast striking plane. It was felt, as was a M3.1 aftershock on Friday morning at the other end of the zone, near San Fernando. The public also noticed a small swarm of Northridge aftershocks, at the southwest corner of the aftershock zone, near Canoga Park. There were 28 events in the swarm, 9 of which were felt. The largest one was a M3.1 on Thursday evening. Five events in the upper M2's happened on Friday afternoon between 3 and 5 pm. The swarm events were all shallow, 3 to 4 km depth (2 to 3 miles), so they occurred above the main Northridge rupture plane. The week's second largest event was a Landers aftershock (M3.4) that occurred early Sunday morning in a remote area of the Mojave desert. There were considerably fewer Indio events and considerably fewer Fontana events than in previous weeks. 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 ------------------------------------------------------------- 8/18 2:26 am 36 8.1 120 4.8 2.8 15 mi. E of Coalinga 8/18 5:48 am 35 26.1 118 37.6 2.2 16 mi. SSW of the town of Lake Isabella 8/18 10:44 am 34 3.6 118 51.6 2.0 4 mi. ENE of Leo Carrillo State Beach 8/18 1:45 pm 36 40.8 118 3.6 2.7 5 mi. N of Lone Pine 8/18 9:30 pm 32 40.8 115 55.7 2.0 5 mi. SE of Ocotillo 8/18 9:41 pm 34 13.0 118 39.0 3.1 2 mi. WNW of Canoga Park FELT 8/18 11:56 pm 34 14.6 118 44.9 3.5 3 mi. SW of Simi Valley; FELT 8/19 12:19 am 33 54.5 116 16.7 2.0 13 mi. NNW of Indio 8/19 12:59 am 32 41.2 115 55.5 2.1 5 mi. SE of Ocotillo 8/19 9:39 am 34 18.5 118 25.6 3.1 1 mi. NNE of San Fernando FELT 8/19 12:26 pm 34 12.6 118 39.0 2.5 2 mi. WNW of Canoga Park FELT 8/19 12:47 pm 34 12.7 118 39.2 2.5 " FELT 8/19 3:46 pm 34 13.1 118 38.5 2.9 " FELT 8/19 3:50 pm 34 12.8 118 38.9 2.9 " FELT 8/19 3:53 pm 34 12.4 118 38.8 2.0 " 8/19 3:58 pm 34 12.4 118 39.1 2.6 " FELT 8/19 4:15 pm 34 13.0 118 39.1 2.5 " FELT 8/19 4:31 pm 34 13.0 118 38.9 2.7 " FELT 8/19 4:37 pm 33 48.8 116 12.2 2.0 6 mi. N of Indio 8/19 5:46 pm 35 25.2 117 11.7 2.9 17 mi. WNW of Goldstone (dry) Lake 8/20 1:31 am 34 22.2 118 39.9 2.5 5 mi. SW of Magic Mtn. 8/20 2:47 am 35 39.2 118 24.5 2.5 3 mi. ENE of the town of Lake Isabella 8/20 9:20 am 32 1.1 114 53.0 3.1 49 mi. SSW of Yuma, Arizona 8/20 1:22 pm 33 22.6 116 23.1 3.0 8 mi. N of Borrego Springs 8/20 5:43 pm 33 48.8 116 11.9 2.0 6 mi. N of Indio 8/20 6:05 pm 34 27.2 115 49.2 2.0 25 mi. NNE of Twentynine Palms 8/21 12:55 am 33 32.3 116 48.8 2.1 8 mi. W of Anza 8/21 3:07 am 33 33.3 116 38.9 2.0 1 mi. E of Anza 8/21 3:09 am 34 3.4 116 47.9 2.0 3 mi. SSE of Mt. San Gorgonio 8/21 5:52 am 34 35.5 116 37.0 3.4 21 mi. ENE of Lucerne Valley 8/21 10:51 am 34 38.0 116 30.8 2.4 27 mi. ENE of Lucerne Valley 8/21 3:54 pm 32 53.1 116 12.5 2.4 16 mi. NW of Ocotillo 8/21 5:07 pm 33 24.6 116 52.9 2.0 3 mi. NNW of Palomar Observatory 8/21 11:15 pm 34 21.1 118 27.1 2.1 4 mi. N of San Fernando 8/22 8:13 am 34 14.6 118 29.8 2.1 2 mi. ENE of Northridge 8/22 11:22 am 34 1.5 117 9.0 2.1 2 mi. SE of Redlands 8/22 1:27 pm 34 13.0 118 39.0 2.8 2 mi. WNW of Canoga Park FELT 8/22 2:27 pm 34 21.3 117 37.8 2.3 Under Wrightwood 8/22 3:03 pm 34 12.9 118 39.3 2.1 3 mi. WNW of Canoga Park 8/22 3:47 pm 34 21.3 116 27.4 2.5 16 mi. N of Yucca Valley 8/22 11:18 pm 34 12.6 118 38.9 2.0 2 mi. WNW of Canoga Park 8/23 4:39 am 34 18.0 118 32.5 2.3 4 mi. N of Northridge 8/23 10:24 am 35 1.7 120 58.8 2.4 24 mi. SSW of Morro Bay 8/23 10:55 am 34 9.0 116 50.5 2.3 3 mi. NNW of Mt. San Gorgonio 8/23 2:36 pm 34 23.0 118 34.8 2.1 2 mi. S of Magic Mtn. 8/23 5:09 pm 34 18.8 116 26.5 2.4 13 mi. N of Yucca Valley 8/23 5:15 pm 32 9.4 115 1.7 2.5 44 mi. SSW of Yuma, Arizona 8/23 7:23 pm 34 3.5 117 29.8 2.2 Fontana, 2 mi. ESE of the I10/I15 interchange 8/23 7:24 pm 35 19.4 118 34.2 2.1 15 mi. NNW of Tehachapi 8/23 7:52 pm 34 12.4 118 38.8 2.0 2 mi. W of Canoga Park 8/23 8:15 pm 35 30.6 117 29.6 2.2 12 mi. SE of Ridgecrest 8/24 4:52 am 34 3.5 117 30.1 2.1 Fontana, 2 mi. ESE of the I10/I15 interchange 8/24 8:01 am 34 3.3 117 30.0 2.2 " 8/24 8:09 am 35 1.5 116 58.7 2.0 9 mi. NNE of Barstow 8/24 10:09 am 34 21.0 116 27.3 2.0 16 mi. N of Yucca Valley 8/24 11:49 am 34 2.3 116 18.2 2.6 9 mi. SE of Yucca Valley 8/24 3:37 pm 36 7.7 120 25.5 2.5 3 mi. WSW of Coalinga 8/24 8:59 pm 33 20.8 116 13.0 2.4 11 mi. NE of Borrego Springs 8/24 9:15 pm 34 3.5 117 29.8 2.3 Fontana, 2 mi. ESE of the I10/I15 interchange ------------------------------------------------------------- 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.