SOUTHERN CALIFORNIA SEISMOGRAPHIC NETWORK A Cooperative Project of Caltech and the U.S. Geological Survey August 24, 1995 Weekly Earthquake Report for Southern California ------------------------------------------------ August 17 - 23, 1995 Prepared by: Kate Hutton, Seismological Laboratory (kate@bombay.gps.caltech.edu) Lucy Jones, U.S. Geological Survey California Institute of Technology 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 17, Pacific Daylight Time, to midnight Wednesday night, August 23, 1995, Pacific Daylight Time. We recorded 1,568 earthquakes of during the 7-day period covered. The biggest excitement of the week was the M5.4 that occurred last Thursday afternoon, north of Ridgecrest. It was felt as far away as the L.A. area to the south, and Bishop to the north. The mechanism was mostly "normal" (dip slip with extension of the crust), with a component of strike-slip, right in line with what we know about the area. The second figure shows a map view and a cross section, which clearly shows the dipping fault. Glenn Roquemore and Kerry Sieh believe this to be part of the Airport Lake fault system. The earthquake produced a robust aftershock sequence, that is dying off in a predictable manner. The rate of decay is close to the average rate of California earthquakes. There are, however, relatively more small aftershocks compared to the number of large ones. This leads to a slope of the magnitude-frequency distribution (called the b-value) of about 1.2, higher than the California averge of 0.9 (The highest ever seen in a California aftershock sequence is 1.4, the 1986 Oceanside sequence.). The tally for the sequence so far is 1,294 events. Since Mother Nature was kind to us by allowing almost a week to process the data, these figures are more reliable than what sometimes appears for the early days of a sequence. Because the sequence is decaying at a normal rate, and because almost one week has already passed since the mainshock, coupled with the relatively few larger aftershocks, the probability of further large aftershocks is small. The chances of another M4.5 or greater is less than 10%. Eclipsed but not forgotten, a M3.2 was felt in Barstow earlier on August 17th. It was an aftershock of the June 28, 1992 Landers earthquake. The table lists the quakes this week that were M2.0 or larger in the central part of the coverage area. Only quakes M2.5 or larger are listed for the Ridgecrest sequence. Times are local times; if you want Greenwich Mean Time, add 7 hrs to the Pacific Daylight Time or 8 hrs to the Pacific Standard Time listed. Table 1 ------- Date Time N Lat. W Long. Mag ------------------------------------------------------------- 8/17 5:34 am 33 12.7 115 38.6 2.5 2 mi. N of Obsidian Butte 8/17 6:55 am 34 8.9 116 26.3 2.2 2 mi. NNE of Yucca Valley 8/17 8:06 am 34 20.6 118 34.1 2.4 3 mi. S of Valencia 8/17 8:26 am 34 55.9 116 54.9 3.2 6 mi. ENE of Barstow FELT 8/17 11:12 am 34 19.7 116 50.8 2.4 4 mi. N of Big Bear City 8/17 12:47 pm 34 12.1 117 36.6 2.1 6 mi. SSE of Mt. Baldy 8/17 3:39 pm 35 46.6 117 39.7 5.4 10 mi. N of Ridgecrest FELT 8/17 3:41 pm 35 47.4 117 40.7 4.2 11 mi. N of Ridgecrest 8/17 3:43 pm 35 47.0 117 39.8 3.9 " 8/17 3:47 pm 35 47.5 117 40.2 2.5 " 8/17 3:49 pm 35 47.2 117 40.5 2.8 " 8/17 3:49 pm 35 46.9 117 40.7 3.2 " 8/17 3:55 pm 35 46.6 117 40.1 2.7 10 mi. N of Ridgecrest 8/17 3:59 pm 35 45.9 117 40.1 2.6 9 mi. N of Ridgecrest 8/17 3:59 pm 35 45.8 117 40.2 2.5 " 8/17 3:59 pm 35 46.2 117 40.3 2.6 10 mi. N of Ridgecrest 8/17 4:03 pm 35 45.8 117 39.0 2.7 9 mi. N of Ridgecrest 8/17 4:06 pm 35 47.0 117 39.3 2.5 11 mi. N of Ridgecrest 8/17 4:17 pm 35 45.8 117 39.2 2.7 9 mi. N of Ridgecrest 8/17 4:18 pm 35 47.2 117 39.8 2.5 11 mi. N of Ridgecrest 8/17 4:19 pm 35 47.2 117 39.8 2.7 " 8/17 4:28 pm 35 47.2 117 40.0 2.7 " 8/17 4:33 pm 35 45.8 117 39.6 3.6 9 mi. N of Ridgecrest 8/17 4:34 pm 35 46.7 117 40.2 2.7 10 mi. N of Ridgecrest 8/17 4:36 pm 35 46.6 117 40.4 3.2 " 8/17 4:37 pm 35 46.0 117 39.8 2.5 " 8/17 4:43 pm 35 46.9 117 40.4 3.0 11 mi. N of Ridgecrest 8/17 4:45 pm 35 46.4 117 39.3 2.5 10 mi. N of Ridgecrest 8/17 4:46 pm 35 47.0 117 40.1 2.7 11 mi. N of Ridgecrest 8/17 4:46 pm 35 48.2 117 40.7 2.9 12 mi. N of Ridgecrest 8/17 4:54 pm 35 45.8 117 39.1 2.7 9 mi. N of Ridgecrest 8/17 4:59 pm 35 46.8 117 39.5 3.0 11 mi. N of Ridgecrest 8/17 5:05 pm 35 46.3 117 39.5 2.7 10 mi. N of Ridgecrest 8/17 5:10 pm 35 46.3 117 39.3 2.6 " 8/17 5:29 pm 35 46.3 117 39.3 3.0 " 8/17 5:40 pm 35 46.1 117 39.3 2.5 " 8/17 5:42 pm 35 45.8 117 39.7 2.5 9 mi. N of Ridgecrest 8/17 6:00 pm 35 46.1 117 39.3 2.5 10 mi. N of Ridgecrest 8/17 6:10 pm 34 1.6 118 58.9 2.0 2 mi. WSW of Leo Carrillo State Beach 8/17 6:51 pm 35 46.3 117 39.2 2.5 10 mi. N of Ridgecrest 8/17 6:56 pm 35 47.4 117 39.8 3.2 11 mi. N of Ridgecrest 8/17 7:01 pm 35 46.3 117 39.0 2.7 10 mi. N of Ridgecrest 8/17 7:05 pm 35 47.4 117 39.9 2.6 11 mi. N of Ridgecrest 8/17 7:11 pm 35 46.3 117 40.1 2.8 10 mi. N of Ridgecrest 8/17 7:12 pm 35 47.3 117 39.7 2.5 11 mi. N of Ridgecrest 8/17 7:33 pm 35 47.0 117 39.9 3.3 " 8/17 8:08 pm 35 47.4 117 39.7 2.5 " 8/17 8:13 pm 35 47.0 117 40.0 2.7 " 8/17 8:22 pm 35 47.0 117 39.8 3.1 " 8/17 8:27 pm 35 47.2 117 40.0 2.6 " 8/17 8:28 pm 35 47.0 117 39.7 2.5 " 8/17 8:33 pm 35 46.0 117 39.4 2.7 10 mi. N of Ridgecrest 8/17 8:34 pm 35 46.0 117 39.3 2.5 " 8/17 8:37 pm 35 46.5 117 39.1 2.6 " 8/17 8:51 pm 35 46.2 117 39.2 3.5 " 8/17 9:54 pm 35 46.4 117 40.0 3.0 " 8/17 10:03 pm 35 47.2 117 39.4 3.5 11 mi. N of Ridgecrest 8/17 11:26 pm 32 42.2 117 33.7 2.2 18 mi. WSW of Mission Beach 8/17 11:50 pm 35 47.0 117 40.3 3.1 11 mi. N of Ridgecrest 8/18 1:37 am 35 45.8 117 39.7 2.5 9 mi. N of Ridgecrest 8/18 2:03 am 35 45.7 117 39.2 2.5 " 8/18 2:15 am 35 47.3 117 38.6 2.9 11 mi. N of Ridgecrest 8/18 2:45 am 33 26.3 117 3.7 2.0 11 mi. ENE of Fallbrook 8/18 3:00 am 34 16.2 116 26.5 2.4 10 mi. N of Yucca Valley 8/18 4:31 am 35 46.9 117 39.6 2.7 11 mi. N of Ridgecrest 8/18 5:13 am 35 46.4 117 40.0 3.0 10 mi. N of Ridgecrest 8/18 5:29 am 35 46.5 117 40.0 2.5 " 8/18 7:16 am 35 46.5 117 38.3 2.7 " 8/18 7:18 am 35 46.5 117 38.3 2.6 " 8/18 7:24 am 35 47.0 117 39.5 2.6 11 mi. N of Ridgecrest 8/18 7:58 am 35 46.1 117 39.4 2.6 10 mi. N of Ridgecrest 8/18 8:57 am 35 46.5 117 38.2 2.5 " 8/18 10:07 am 35 46.0 117 39.5 2.5 9 mi. N of Ridgecrest 8/18 12:11 pm 35 45.3 117 39.4 3.0 " 8/18 1:53 pm 35 46.4 117 39.5 2.5 10 mi. N of Ridgecrest 8/18 3:13 pm 35 46.1 117 39.2 2.5 " 8/18 4:07 pm 35 46.1 117 39.0 2.5 " 8/18 8:54 pm 35 45.0 117 38.8 2.8 8 mi. N of Ridgecrest 8/18 9:01 pm 33 24.2 116 26.0 3.2 10 mi. NNW of Borrego Springs 8/19 5:11 am 35 46.7 117 39.0 3.2 10 mi. N of Ridgecrest 8/19 6:29 am 32 57.2 117 45.0 2.2 27 mi. SW of Oceanside 8/19 6:59 am 35 46.6 117 38.7 2.6 10 mi. N of Ridgecrest 8/19 7:29 am 35 46.3 117 39.1 2.5 " 8/19 8:52 am 35 46.5 117 38.6 2.5 " 8/19 5:46 pm 35 46.8 117 39.2 2.5 " 8/19 7:30 pm 35 46.5 117 38.8 2.7 " 8/19 8:26 pm 35 46.6 117 39.1 2.5 " 8/19 11:28 pm 35 45.6 117 40.5 2.7 9 mi. N of Ridgecrest 8/20 12:16 am 34 35.5 116 36.6 2.3 21 mi. ENE of Lucerne Valley 8/20 2:17 am 35 28.5 120 45.3 2.6 4 mi. W of Atascadero 8/20 2:31 am 34 50.1 117 13.2 2.4 11 mi. WSW of Barstow 8/20 9:23 am 33 24.2 116 26.3 2.8 10 mi. NNW of Borrego Springs 8/20 11:42 am 35 27.6 120 46.0 2.4 5 mi. WSW of Atascadero 8/20 12:39 pm 35 47.4 117 38.1 2.6 11 mi. N of Ridgecrest 8/20 2:07 pm 34 54.7 117 23.6 2.1 21 mi. W of Barstow 8/20 7:43 pm 35 46.2 117 39.7 2.5 10 mi. N of Ridgecrest 8/20 8:07 pm 35 47.1 117 39.9 2.8 11 mi. N of Ridgecrest 8/20 9:44 pm 35 46.7 117 39.0 2.5 10 mi. N of Ridgecrest 8/21 3:15 am 34 25.7 116 31.0 2.0 21 mi. N of Yucca Valley 8/21 6:09 am 34 22.4 116 27.9 2.3 17 mi. N of Yucca Valley 8/21 8:02 am 35 47.5 117 40.0 2.6 11 mi. N of Ridgecrest 8/21 9:42 am 35 47.6 117 40.0 2.6 " 8/21 12:52 pm 35 45.7 117 39.4 2.6 9 mi. N of Ridgecrest 8/22 8:01 am 33 10.4 115 36.6 2.5 1 mi. E of Obsidian Butte 8/22 8:08 am 33 10.5 115 36.1 2.4 2 mi. E of Obsidian Butte 8/22 9:20 am 34 37.9 116 39.1 2.2 21 mi. NE of Lucerne Valley 8/22 9:26 am 33 10.6 115 36.5 2.8 1 mi. ENE of Obsidian Butte 8/22 9:57 am 33 40.8 116 43.5 2.2 4 mi. S of Idyllwild 8/22 10:14 am 33 10.6 115 36.3 2.6 1 mi. ENE of Obsidian Butte 8/23 12:57 am 35 46.5 117 39.3 3.4 10 mi. N of Ridgecrest 8/23 4:13 am 34 53.1 117 44.7 2.0 20 mi. SE of California City 8/23 8:49 am 33 35.6 117 16.4 2.3 6 mi. SSE of Lake Elsinore 8/23 12:11 pm 33 30.7 116 30.3 2.2 10 mi. ESE of Anza 8/23 8: 8 pm 34 36.4 116 37.9 2.2 21 mi. ENE of Lucerne Valley -------------------------------------------------------------