SOUTHERN CALIFORNIA SEISMOGRAPHIC NETWORK A Cooperative Project of Caltech and the U.S. Geological Survey August 28, 1997 Weekly Earthquake Report for Southern California ------------------------------------------------ August 21 - 27, 1997 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 818-395-6327. For daily updates, call our Earthquake Information Hotline: 818-395-6977. -------------------------------- ***************************************************************** If you receive this Report by FAX and if you have recently had a change of AREA CODE, please notify us ASAP. We have no other way to know which fax numbers need to be changed!! ***************************************************************** This week's Report covers the time period from midnight Thursday morning, August 21, Universal Time (ie. UTC), to midnight Wednesday night, August 27, Universal Time. In local time, the period of coverage is from 5 p.m., August 20, Pacific Daylight Time, to 5 p.m., August 27, Pacific Daylight Time. We detected 253 earthquakes during the seven-day period covered. These Weekly Reports should not be used for archival purposes. Further data processing often changes the magnitudes slightly and, especially during times of high seismicity, sometimes changes the estimates of total seismicity. For the best earthquake catalog currently available, see: http://scec.gps.caltech.edu/catalog-search.html There were only two quakes about M3 this week. The first was a Northridge 'aftershock' last Wednesday evening, M3.3. It was fairly widely felt in the San Fernando Valley. The focal mechanism was, surprisingly, strike slip with nearly north-south and east-west planes. At 7 km depth, this quake was shallower than the actual Northridge rupture at that location, so some may care to challenge its aftershock status. Technically, however, its epicenter was in the Northridge aftershock zone, and it occurred while the earthquake rate is still higher than is usual for that region. There was one very tiny (M2.1) aftershock to the aftershock that was also felt. The other M3+ was a M3.4 in the southern Sierra Nevada, near Lake Isabella on Sunday morning. This focal mechanism is ambiguous, due to the fact that it is on the northern edge of our Network; it could be either strike-slip or 'normal', indicating tension in the crust. Quakes in this area seldom attract much attention. Table 1 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 ------------------------------------------------------------- 8/20 6:29 pm 34 10.9 118 31.5 3.3 1 mi. SSE of Reseda FELT 8/20 7:51 pm 34 10.4 118 31.3 2.1 1 mi. NW of Encino FELT 8/21 12:08 am 34 19.9 116 28.0 2.0 14 mi. N of Yucca Valley 8/21 5:18 am 34 10.4 118 31.4 2.0 1 mi. NW of Encino 8/21 9:03 am 33 38.9 116 42.5 2.1 6 mi. S of Idyllwild 8/21 9:31 pm 32 20.0 115 7.8 2.5 31 mi. SE of Calexico 8/22 1:43 am 33 11.9 115 34.5 2.4 4 mi. ENE of Obsidian Butte 8/22 3:02 am 32 11.6 115 41.4 2.7 34 mi. SSW of Calexico 8/22 12:19 pm 35 35.8 118 21.9 2.1 6 mi. ESE of town of Lake Isabella 8/22 12:46 pm 32 11.3 115 41.0 2.8 34 mi. SSW of Calexico 8/22 1:06 pm 34 16.0 118 28.7 2.0 1 mi. ESE of Granada Hills 8/22 1:57 pm 33 9.3 115 57.8 2.0 8 mi. S of Salton City 8/22 4:54 pm 33 21.2 116 26.3 2.0 7 mi. NNW of Borrego Springs 8/23 12:41 am 34 19.4 116 27.3 2.3 14 mi. N of Yucca Valley 8/23 6:05 am 35 46.8 117 40.1 2.5 10 mi. N of Ridgecrest 8/23 10:49 am 32 9.8 115 47.4 2.7 38 mi. SSW of Calexico 8/23 12:38 pm 33 15.5 116 0.4 2.2 2 mi. WSW of Salton City 8/23 12:46 pm 32 38.1 115 54.0 2.1 8 mi. SE of Ocotillo 8/23 2:32 pm 34 1.3 116 48.5 2.1 5 mi. S of Mt. San Gorgonio 8/23 7:52 pm 32 16.0 115 17.4 2.6 30 mi. SSE of Calexico 8/23 11:42 pm 32 10.2 115 40.6 2.8 36 mi. SSW of Calexico 8/24 2:04 am 32 4.7 115 5.2 2.5 47 mi. SSE of Calexico 8/24 3:11 am 35 47.0 118 3.0 2.2 18 mi. SSW of Coso Junction 8/24 6:43 am 35 47.1 117 39.7 2.1 11 mi. N of Ridgecrest 8/24 10:43 am 35 35.6 118 21.7 3.4 6 mi. ESE of town of Lake Isabella 8/24 2:43 pm 32 25.4 116 31.0 2.1 31 mi. ESE of Chula Vista 8/24 2:56 pm 34 51.0 117 37.0 2.1 10 mi. S of Boron 8/24 7:38 pm 32 44.0 115 56.2 2.1 3 mi. E of Ocotillo 8/24 7:51 pm 35 52.0 119 50.1 2.4 27 mi. ENE of Cholame 8/24 9:39 pm 32 15.7 115 9.1 2.5 34 mi. SE of Calexico 8/25 12:28 am 35 51.6 118 24.7 2.0 15 mi. NNE of town of Lake Isabella 8/25 7:31 am 32 40.1 115 45.2 2.0 14 mi. SW of El Centro 8/25 10:09 am 32 49.4 115 36.8 2.2 4 mi. WNW of El Centro 8/25 1:26 pm 36 11.2 118 1.5 2.0 6 mi. S of Olancha 8/25 6:45 pm 32 17.4 115 18.0 2.9 28 mi. SSE of Calexico 8/26 5:05 am 34 0.9 118 9.7 2.3 1 mi. SSE of East Los Angeles 8/26 9:29 am 33 42.3 116 49.0 2.0 6 mi. WSW of Idyllwild 8/26 4:41 pm 34 10.2 117 20.4 2.0 4 mi. NNE of Rialto 8/26 5:11 pm 34 27.4 119 13.9 2.1 1 mi. NE of Ojai 8/26 11:00 pm 34 9.9 117 20.2 2.1 4 mi. NNE of Rialto 8/27 4:37 am 35 47.1 117 36.7 2.2 11 mi. NNE of Ridgecrest 8/27 7:51 am 36 43.2 116 18.3 2.2 35 mi. ENE of Furnace Creek 8/27 8:29 am 34 8.8 117 20.1 2.0 3 mi. NE of Rialto 8/27 9:41 am 34 9.1 116 26.0 2.2 2 mi. NNE of Yucca Valley 8/27 11:32 am 33 57.4 117 49.4 2.2 3 mi. SSW of Diamond Bar 8/27 12:26 pm 32 12.6 115 47.4 2.2 36 mi. SSW of Calexico 8/27 1:02 pm 33 57.5 117 49.0 2.0 2 mi. S of Diamond Bar 8/27 1:26 pm 33 9.5 115 40.0 2.1 1 mi. WSW of Obsidian Butte 8/27 4:29 pm 34 9.5 117 19.9 2.1 4 mi. NNW of San Bernardino -------------------------------------------------------------