August 4, 1994 Weekly Earthquake Report for Southern California ------------------------------------------------ July 28 - August 3, 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, July 28, Pacific Daylight Time, to midnight Wednesday night, August 3, Pacific Daylight Time. We recorded 371 earthquakes this week. The week's largest event was a M4.9 Landers aftershock (of the June 28, 1992 Mw7.3). It occurred in an uninhabited area of the Mojave desert, but was widely felt, including at the Seismological Laboratory in Pasadena. The location was near the northern end of the mainshock rupture (although not of the aftershock zone), and it had the expected strike-slip mechanism. There has been one larger Landers aftershock this year so far, a M5.0 on June 16. 1993 contained one M5.0, on August 21. Of course, 1992 contained many (19 including the mainshock) M5.0 or larger Landers/Big Bear quakes. There were two M3.0 or larger Northridge aftershocks this week; both were felt. The first occurred on Saturday evening and had a magnitude of M3.2. The second, a M3.4, occurred early Sunday morning and woke me (KH) up in Pasadena. Like the vast majority of the Northridge aftershocks, these events showed thrust faulting on a west-northwest plane dipping south. One more, considerably smaller (M2.6), Northridge aftershock was felt in the Simi Valley area on Friday night. The Fontana swarm that has been going on for the past two weeks continued this week at a somewhat lower level. The two largest events (Friday morning and Sunday afternoon) were both M2.5. 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 ------------------------------------------------------------- 7/28 2:02 am 34 2.5 117 30.4 2.2 Fontana, 2 mi. SE of the I10/I15 interchange 7/28 3:52 am 33 24.2 116 56.6 3.2 5 mi. NW of Palomar Observatory 7/28 8:26 am 33 59.2 116 59.8 2.0 4 mi. NNW of Beaumont 7/28 1:27 pm 34 1.4 116 19.3 2.5 9 mi. SE of Yucca Valley 7/28 4:29 pm 34 21.8 116 27.5 2.3 16 mi. N of Yucca Valley 7/28 5:54 pm 34 16.8 118 34.0 2.2 3 mi. NNW of Northridge 7/28 11:32 pm 34 20.0 116 28.4 2.2 14 mi. N of Yucca Valley 7/29 5:30 am 35 59.1 118 20.8 2.6 25 mi. NNE of the town of Lake Isabella 7/29 7:03 am 34 2.9 117 30.6 2.5 Fontana, 2 mi. ESE of the I10/I15 interchange 7/29 9:55 am 34 16.4 118 27.9 2.2 1 mi. WSW of San Fernando 7/29 2:30 pm 34 8.1 116 46.7 2.2 3 mi. NE of Mt. San Gorgonio 7/29 2:50 pm 33 10.9 115 35.5 2.4 5 mi. SW of Niland 7/29 3:03 pm 34 18.2 118 25.8 2.1 1 mi. NNE of San Fernando 7/29 3:10 pm 33 11.3 115 35.5 2.1 5 mi. SW of Niland 7/29 3:29 pm 33 10.6 115 35.8 2.0 6 mi. SW of Niland 7/29 4:22 pm 34 16.9 118 24.3 2.2 1 mi. E of San Fernando 7/29 4:40 pm 34 16.9 118 24.3 2.2 " 7/29 10:34 pm 34 15.5 118 41.0 2.6 1 mi. SE of Simi Valley; FELT 7/30 3:23 am 34 9.6 116 25.9 2.2 3 mi. NNE of Yucca Valley 7/30 3:54 am 34 9.6 116 25.8 2.1 " 7/30 12:00 pm 34 23.1 116 27.4 2.3 18 mi. N of Yucca Valley 7/30 6:02 pm 33 30.9 116 33.8 2.0 6 mi. ESE of Anza 7/30 7:11 pm 32 48.4 118 4.4 2.0 15 mi. E of San Clemente Is. 7/30 8:08 pm 34 18.1 116 26.7 2.8 12 mi. N of Yucca Valley 7/30 9:35 pm 34 17.7 118 27.6 3.2 1 mi. NW of San Fernando FELT 7/30 9:36 pm 34 17.6 118 28.0 2.5 " 7/30 9:37 pm 34 17.9 118 27.4 2.9 " 7/31 4:37 am 34 16.3 118 24.7 2.2 1 mi. ESE of San Fernando 7/31 2:41 pm 34 59.2 116 56.9 2.3 8 mi. NNE of Barstow 7/31 5:47 pm 34 2.4 117 30.3 2.5 Fontana, 2 mi. SE of the I10/I15 interchange 7/31 6:55 pm 33 57.7 117 9.9 2.3 6 mi. S of Redlands 7/31 7:22 pm 34 14.6 118 33.2 2.6 1 mi. WNW of Northridge 7/31 8:07 pm 36 5.4 117 51.8 2.2 5 mi. SE of south end of Haiwee Reservoir 7/31 9:51 pm 34 59.3 116 45.4 2.6 16 mi. ENE of Barstow 7/31 10:35 pm 34 18.9 118 24.7 2.2 2 mi. NE of San Fernando 7/31 11:47 pm 32 19.4 115 13.3 2.6 28 mi. SE of Calexico 8/1 12:11 am 34 2.9 117 17.9 2.3 Fontana, 1 mi. S of the I10/I215 interchange 8/1 1:18 am 34 7.1 116 24.0 2.1 2 mi. E of Yucca Valley 8/1 1:33 am 34 10.6 116 49.7 2.1 5 mi. N of Mt. San Gorgonio 8/1 1:36 am 34 7.2 116 24.0 2.5 2 mi. E of Yucca Valley 8/1 3:00 am 34 14.5 118 35.3 2.3 3 mi. NNE of Canoga Park 8/1 8:43 am 34 18.8 116 53.2 2.3 3 mi. NNW of Big Bear City 8/1 10:29 am 33 5.4 115 46.8 2.1 9 mi. WNW of Westmorland 8/1 1:23 pm 34 20.5 117 5.0 2.3 9 mi. N of Running Springs 8/1 2:34 pm 34 38.4 116 31.0 4.9 27 mi. ENE of Lucerne Valley FELT 8/1 2:35 pm 34 37.6 116 29.6 3.0 " 8/1 3:49 pm 34 38.0 116 30.6 2.5 " 8/1 10:12 pm 34 38.2 116 30.8 2.6 " 8/2 12:13 am 34 38.2 116 30.9 2.2 " 8/2 12:48 am 34 54.3 116 55.3 2.4 5 mi. E of Barstow 8/2 9:19 am 34 2.9 117 30.6 2.0 Fontana, 2 mi. ESE of the I10/I15 interchange 8/2 11:57 am 34 38.0 116 30.7 2.0 27 mi. ENE of Lucerne Valley 8/2 10:06 pm 34 19.9 118 26.9 2.5 3 mi. N of San Fernando 8/3 12:39 am 34 5.8 116 26.6 2.2 1 mi. S of Yucca Valley 8/3 2:53 am 34 5.7 116 26.5 2.2 " 8/3 5:36 am 34 19.7 118 27.2 2.4 3 mi. NNW of San Fernando 8/3 5:39 am 34 19.7 118 27.5 2.1 " 8/3 5:40 am 34 19.7 118 27.6 3.4 " FELT 8/3 8:16 am 34 24.8 116 28.4 2.0 20 mi. N of Yucca Valley 8/3 1:51 pm 34 24.6 116 28.3 2.0 " 8/3 9:36 pm 33 17.8 116 9.6 2.0 10 mi. N of Ocotillo Wells ------------------------------------------------------------- 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.