July 21, 1994 Weekly Earthquake Report for Southern California ------------------------------------------------ July 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, July 14, Pacific Daylight Time, to midnight Wednesday night, July 20, Pacific Daylight Time. We recorded 387 earthquakes this week. Most events, as usual, were either Northridge or Landers aftershocks. The week's largest was a M3.5 in the Canoga Park area on Sunday morning. It was felt. Numerous (58 of them, actually) very small quakes occurred in the Fontana area throughout the week. M2.3 (Monday morning) was the largest magnitude, and none of the quake apparently aroused any interest from the public. Swarm activity like this is not particularly unusual in this vicinity; over the past years, many quakes up to the lower M3's have occurred in the nearby "Ontario" location (5 mi. SE of Ontario). Another small swarm occurred in the Obsidian Butte area in northern Imperial Valley. Two of these quakes were large enough to feel easily, even though we got no inquiries: a M3.0 on Friday evening, and a M3.2 on Monday afternoon. This area has been a common source of quakes in the past, including some in the M4 range. Thirdly, there was a cluster of 10 event east of the Landers aftershock zone, 35 mi. SSW of Baker. The largest of these was M2.7. Numerous Landers aftershock have occurred east of the main rupture, delineated by the most prominent line of aftershocks, especially in the beginning of the sequence. These seem to be farther east that what have seen recently, so they may for may not be Landers aftershocks. 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 ------------------------------------------------------------- 7/14 2:28 am 34 37.8 116 36.9 2.1 22 mi. NE of Lucerne Valley 7/14 7:37 am 34 21.5 118 39.3 2.1 5 mi. SW of Magic Mtn. 7/14 7:38 am 34 21.5 118 39.4 2.5 " 7/14 10:29 am 33 59.7 117 0.0 2.1 4 mi. NNW of Beaumont 7/14 12:50 pm 34 24.6 116 28.3 2.0 20 mi. N of Yucca Valley 7/14 2:32 pm 34 37.7 116 36.7 2.7 22 mi. NE of Lucerne Valley 7/14 3:32 pm 33 57.2 116 39.1 2.4 7 mi. ENE of Cabazon 7/14 5:56 pm 34 33.8 116 34.7 2.5 22 mi. ENE of Lucerne Valley 7/14 6:30 pm 33 5.4 117 50.6 2.0 26 mi. SSW of San Clemente 7/14 10:17 pm 34 18.1 118 27.9 2.4 2 mi. NW of San Fernando 7/14 11:20 pm 34 36.8 116 34.8 2.2 23 mi. ENE of Lucerne Valley 7/14 11:31 pm 34 13.9 116 45.2 2.4 6 mi. ESE of Big Bear City 7/15 2:30 am 34 13.1 118 33.0 2.6 1 mi. SW of Northridge 7/15 7:17 am 35 30.5 118 11.7 2.1 17 mi. ESE of the town of Lake Isabella 7/15 1:09 pm 34 11.1 116 26.4 2.3 4 mi. N of Yucca Valley 7/15 7:22 pm 33 11.3 115 36.3 3.0 6 mi. SW of Niland 7/15 8:14 pm 34 17.3 118 30.7 2.0 3 mi. NNE of Northridge 7/15 11:32 pm 36 13.3 120 44.1 2.9 21 mi. WNW of Coalinga 7/16 2:40 am 32 27.2 115 28.4 2.4 14 mi. S of Calexico 7/16 2:50 am 32 27.7 115 28.6 2.1 " 7/16 3:21 am 32 27.5 115 27.1 3.0 " 7/16 3:22 am 32 27.4 115 27.3 3.1 " 7/16 3:24 am 32 26.9 115 27.4 2.8 " 7/16 7:52 am 34 16.8 118 27.7 2.2 1 mi. W of San Fernando 7/16 12:45 pm 33 55.2 118 36.9 2.2 8 mi. SSE of Malibu 7/16 12:46 pm 34 16.7 116 42.4 2.0 8 mi. E of Big Bear City 7/16 2:42 pm 34 37.5 116 33.3 2.1 25 mi. ENE of Lucerne Valley 7/16 7:16 pm 34 4.6 116 43.8 2.6 5 mi. ESE of Mt. San Gorgonio 7/16 8:05 pm 34 47.1 116 17.7 2.7 35 mi. SSW of Baker 7/16 8:06 pm 34 46.8 116 17.3 2.6 " 7/16 9:12 pm 34 14.9 118 44.8 2.0 3 mi. SW of Simi Valley 7/16 9:50 pm 34 16.3 116 46.5 2.0 4 mi. E of Big Bear City 7/17 1:12 am 35 58.3 118 19.0 2.1 24 mi. NNE of the town of Lake Isabella 7/17 1:28 am 33 54.3 117 2.3 2.0 3 mi. WSW of Beaumont 7/17 2:15 am 35 43.3 117 33.2 2.5 9 mi. NE of Ridgecrest 7/17 2:41 am 34 47.2 116 17.3 2.4 35 mi. SSW of Baker 7/17 8:22 am 34 18.6 118 25.8 2.0 2 mi. NNE of San Fernando 7/17 8:29 am 34 14.0 118 35.4 3.5 2 mi. NNE of Canoga Park FELT 7/17 9:04 am 34 15.6 118 36.8 2.3 4 mi. N of Canoga Park 7/17 9:50 am 34 59.3 117 2.5 2.1 6 mi. N of Barstow 7/17 12:25 pm 34 19.7 118 28.6 2.2 3 mi. NW of San Fernando 7/17 2:01 pm 34 2.5 116 18.4 2.0 9 mi. SE of Yucca Valley 7/17 3:57 pm 34 32.6 116 32.3 2.1 24 mi. ENE of Lucerne Valley 7/17 8:19 pm 33 48.7 116 11.7 2.4 6 mi. N of Indio 7/18 12:27 am 34 23.4 116 28.0 2.1 18 mi. N of Yucca Valley 7/18 2:07 am 33 48.9 116 11.6 2.0 7 mi. N of Indio 7/18 2:45 am 34 47.4 116 17.5 2.1 35 mi. SSW of Baker 7/18 3:59 am 34 3.4 117 29.9 2.0 2 mi. ESE of the I10/I15 interchange 7/18 5:52 am 34 1.9 116 26.4 2.1 6 mi. S of Yucca Valley 7/18 7:12 am 34 3.0 117 30.5 2.0 2 mi. ESE of the I10/I15 interchange 7/18 7:33 am 35 25.3 118 46.1 2.5 15 mi. NNE of Arvin 7/18 10:59 am 34 18.8 116 26.5 2.2 13 mi. N of Yucca Valley 7/18 11:02 am 34 33.5 116 30.3 2.6 30 mi. N of Yucca Valley 7/18 11:04 am 34 33.8 116 30.6 2.1 " 7/18 1:34 pm 33 10.5 115 38.0 2.1 Obsidian Butte area, 7 mi. WNW of Calipatria 7/18 2:59 pm 34 47.4 116 17.5 2.1 35 mi. SSW of Baker 7/18 5:31 pm 33 11.4 115 31.2 2.2 Obsidain Butte area, 3 mi. S of Niland 7/18 5:34 pm 33 11.2 115 36.0 3.2 Obsidian Butte area, 5 mi. SW of Niland 7/18 9:13 pm 34 2.8 117 30.4 2.3 2 mi. ESE of the I10/I15 interchange 7/19 12:37 am 34 33.7 116 17.5 2.4 31 mi. NNE of Yucca Valley 7/19 4:09 am 36 9.4 118 4.0 2.2 6 mi. WNW of south end of Haiwee Reservoir 7/19 4:51 am 34 22.0 116 28.0 2.2 17 mi. N of Yucca Valley 7/19 9:30 am 34 18.1 116 27.3 2.2 12 mi. N of Yucca Valley 7/19 10:51 am 34 17.7 118 27.5 2.0 1 mi. NW of San Fernando 7/19 11:49 am 32 40.1 115 54.3 2.1 6 mi. SE of Ocotillo 7/19 2:08 pm 34 18.9 116 24.0 2.1 13 mi. N of Yucca Valley 7/19 4:40 pm 34 5.8 118 31.9 2.0 5 mi. NNW of Santa Monica 7/19 6:22 pm 34 13.2 118 30.6 2.1 1 mi. SE of Northridge 7/20 12:07 am 33 56.2 116 49.2 2.2 2 mi. WNW of Cabazon 7/20 4:34 am 34 20.9 118 33.2 2.8 2 mi. SW of Newhall 7/20 4:48 am 34 16.0 116 24.0 2.4 10 mi. NNE of Yucca Valley 7/20 4:51 am 34 0.3 116 19.0 2.0 10 mi. SE of Yucca Valley 7/20 7:25 am 34 15.3 118 27.4 2.2 2 mi. SSW of San Fernando 7/20 7:31 am 34 47.8 116 17.9 2.0 34 mi. SSW of Baker 7/20 12:53 pm 34 37.4 116 38.6 2.4 21 mi. NE of Lucerne Valley 7/20 2:58 pm 34 20.7 116 28.2 2.2 15 mi. N of Yucca Valley 7/20 3:16 pm 33 53.9 117 52.1 2.6 2 mi. ESE of Brea 7/20 3:24 pm 34 2.8 117 30.7 2.2 2 mi. SE of the I10/I15 interchange 7/20 3:29 pm 36 9.3 117 51.3 2.1 Coso Range, 5 mi. ENE of south end of Haiwee Reservoir 7/20 3:38 pm 33 18.5 116 17.6 2.2 5 mi. NE of Borrego Springs ------------------------------------------------------------- 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.