SOUTHERN CALIFORNIA SEISMOGRAPHIC NETWORK A Cooperative Project of Caltech and the U.S. Geological Survey August 31, 1995 Weekly Earthquake Report for Southern California ------------------------------------------------ August 24 - 30, 1995 Prepared by: Nick Scheckel, Seismological Laboratory Debbie Wells, 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 24, Pacific Daylight Time, to midnight Wednesday night, August 30, 1995, Pacific Daylight Time. We recorded 735 earthquakes during the 7-day period covered. The large number of earthquakes this week are due, in part, to the continuing aftershocks of the August 17, 1995 (M5.4) Ridgecrest earthquake. Almost 400 aftershocks to this earthquake occurred this week. Last week had over 1300 aftershocks so the sequence is decaying with time as aftershock sequences usually do. The largest events of the week in southern California (M4.1, M3.9 and M3.8) were also the largest aftershocks of this sequence. All three of these events occurred on Wednesday - the first two in the morning and the third in the evening - and all were felt in the epicentral area. The second figure is an updated version of last week's, showing a map view and a cross section of the sequence. Two earthquakes in the San Gabriel mountains, north of Sunland were widely felt in the Los Angeles region last Friday. Both were M3.3, one at 12:37 pm and the second at 4:50 pm. These events were located northeast of the Northridge aftershock zone and so are not considered aftershocks of that earthquake. They were actually located in the aftershock zone of the 1971 San Fernando (M6.7) earthquake. That earthquake occurred on an extension of the Sierra Madre fault, raising the San Gabriel mountains. The focal mechanisms of this week's events showed thrust faulting on approximately east-striking planes, similar to the 1971 earthquake. Two other events were felt this week. On Wednesday morning, an M2.8 woke up some folks in San Luis Obispo and later that evening an M3.5 shook some folks in Barstow. The Barstow quake was an aftershock of the M7.3 Landers earthquake. The table lists the quakes this week 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 Pacific Daylight Time or 8 hrs to the Pacific Standard Time listed. Table 1 ------- Date Time N Lat. W Long. Mag ------------------------------------------------------------- 8/24 1:54 am 35 10.9 120 35.7 2.8 3 mi. NE of Pismo Beach; FELT 8/24 2:02 am 35 45.6 117 38.5 2.4 9 mi. N of Ridgecrest 8/24 7:41 am 35 45.8 117 38.4 2.1 " 8/24 7:46 am 35 45.9 117 38.6 2.2 10 mi. N of Ridgecrest 8/24 10:24 am 35 46.3 117 39.3 2.5 " 8/24 12:16 pm 35 45.9 117 39.8 2.5 9 mi. N of Ridgecrest 8/24 2:51 pm 35 45.6 117 38.4 2.1 " 8/24 3:40 pm 35 46.5 117 39.2 2.2 10 mi. N of Ridgecrest 8/24 6:47 pm 35 19.7 118 32.9 2.0 14 mi. NNW of Tehachapi 8/24 7:59 pm 35 1.7 116 57.8 3.5 10 mi. NNE of Barstow; FELT 8/25 12:34 am 35 45.8 117 39.6 2.2 9 mi. N of Ridgecrest 8/25 3:48 am 35 47.2 117 39.8 2.0 11 mi. N of Ridgecrest 8/25 8:30 am 35 46.9 117 38.4 2.8 " 8/25 12:13 pm 34 55.1 116 46.5 2.2 14 mi. E of Barstow 8/25 12:37 pm 34 21.9 118 17.4 3.3 7 mi. N of Sunland; FELT 8/25 4:50 pm 34 21.9 118 17.7 3.3 7 mi. N of Sunland; FELT 8/25 5:16 pm 34 21.8 118 17.8 2.0 7 mi. N of Sunland 8/25 5:44 pm 35 45.6 117 39.7 2.1 9 mi. N of Ridgecrest 8/25 11:41 pm 34 18.1 118 26.9 2.9 1 mi. NNW of San Fernando 8/26 12:44 am 34 21.7 118 17.6 2.2 7 mi. N of Sunland 8/26 4:09 am 34 29.9 116 31.3 2.0 26 mi. N of Yucca Valley 8/26 6:20 am 33 58.8 116 34.4 2.1 4 mi. S of Morongo Valley 8/26 9:13 am 35 45.8 117 39.7 2.2 9 mi. N of Ridgecrest 8/26 2:37 pm 36 13.8 118 18.3 2.2 17 mi. WSW of Olancha 8/26 4:43 pm 34 11.3 116 48.0 2.1 6 mi. NNE of Mt. San Gorgonio 8/27 12:02 am 35 59.8 118 21.2 2.5 25 mi. NNE of town of Lake Isabella 8/27 3:13 am 35 47.4 117 40.0 2.1 11 mi. N of Ridgecrest 8/27 3:41 am 35 3.2 116 47.3 2.1 17 mi. NE of Barstow 8/27 7:36 am 35 45.5 117 38.5 2.0 9 mi. N of Ridgecrest 8/27 11:32 am 35 46.6 117 39.3 2.9 10 mi. N of Ridgecrest 8/27 2:53 pm 33 55.3 118 40.5 2.0 7 mi. S of Malibu 8/27 4:16 pm 35 45.9 117 39.0 2.6 9 mi. N of Ridgecrest 8/27 4:31 pm 35 45.9 117 38.7 2.0 " 8/27 9:30 pm 35 47.0 117 38.0 2.1 11 mi. N of Ridgecrest 8/28 1:36 am 36 53.4 118 14.2 2.4 6 mi. NNW of Independence 8/28 4:49 am 34 38.0 116 40.0 2.2 20 mi. NE of Lucerne Valley 8/28 6:52 am 35 45.6 117 39.1 2.1 9 mi. N of Ridgecrest 8/28 7:47 am 35 59.8 118 21.1 2.2 25 mi. NNE of town of Lake Isabella 8/28 7:22 pm 35 46.5 117 39.2 2.1 10 mi. N of Ridgecrest 8/28 7:54 pm 35 47.5 117 44.3 2.4 12 mi. NNW of Ridgecrest 8/29 6:14 am 35 46.2 117 39.3 2.1 10 mi. N of Ridgecrest 8/29 7:25 am 35 45.7 117 39.3 2.1 9 mi. N of Ridgecrest 8/29 8:41 am 35 47.3 117 39.6 2.4 11 mi. N of Ridgecrest 8/29 12:02 pm 35 45.2 117 39.2 2.2 9 mi. N of Ridgecrest 8/29 12:11 pm 33 30.9 116 34.2 2.3 6 mi. ESE of Anza 8/29 12:23 pm 35 47.3 117 39.9 2.1 11 mi. N of Ridgecrest 8/29 12:35 pm 35 46.7 117 39.2 2.2 10 mi. N of Ridgecrest 8/29 4:44 pm 35 46.4 117 38.9 2.2 " 8/30 4:51 am 36 41.3 121 17.9 2.5 12 mi. SSE of Hollister 8/30 5:35 am 35 46.4 117 39.1 2.1 10 mi. N of Ridgecrest 8/30 8:26 am 34 22.6 116 27.4 2.3 17 mi. N of Yucca Valley 8/30 8:29 am 35 47.5 117 38.5 4.1 11 mi. N of Ridgecrest; FELT 8/30 8:51 am 35 47.2 117 38.3 2.6 11 mi. N of Ridgecrest 8/30 8:54 am 35 47.7 117 38.4 3.9 12 mi. N of Ridgecrest; FELT 8/30 8:57 am 34 1.2 117 6.7 2.4 4 mi. ESE of Redlands 8/30 8:58 am 35 47.1 117 38.0 2.7 11 mi. N of Ridgecrest 8/30 9:00 am 35 47.5 117 37.9 2.1 " 8/30 10:56 am 35 48.0 117 38.3 2.8 12 mi. N of Ridgecrest 8/30 10:57 am 35 48.0 117 38.6 2.6 " 8/30 11:03 am 35 47.0 117 38.6 2.0 11 mi. N of Ridgecrest 8/30 11:15 am 35 46.1 117 38.4 2.4 10 mi. N of Ridgecrest 8/30 11:20 am 35 47.7 117 38.4 2.3 12 mi. N of Ridgecrest 8/30 2:07 pm 35 47.5 117 37.1 2.4 12 mi. NNE of Ridgecrest 8/30 3:18 pm 35 48.1 117 38.2 2.4 12 mi. N of Ridgecrest 8/30 6:58 pm 35 47.6 117 38.7 3.8 11 mi. N of Ridgecrest; FELT 8/30 7:17 pm 35 47.7 117 38.0 3.0 12 mi. N of Ridgecrest 8/30 7:55 pm 35 47.9 117 38.2 2.6 " 8/30 11:08 pm 35 1.5 118 59.6 3.2 3 mi. WNW of Wheeler Ridge -------------------------------------------------------------