SOUTHERN CALIFORNIA SEISMOGRAPHIC NETWORK A Cooperative Project of Caltech and the U.S. Geological Survey July 17, 1997 Weekly Earthquake Report for Southern California ------------------------------------------------ July 10 - 16, 1997 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 818-395-6327. For daily updates, call our Earthquake Information Hotline: 818-395-6977. -------------------------------- This week's Report covers the time period from midnight Thursday morning, July 10, Universal Time (ie. GMT), to midnight Wednesday night, July 16, Universal Time. In local time, the period of coverage is from 5 p.m., July 9, Pacific Daylight Time, to 5 p.m., July 16, Pacific Daylight Time. We detected 291 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 The largest earthquake this week was a felt M3.5 near San Bernardino on Saturday morning. It was an aftershock of the M4.2 earthquake that we reported on two weeks ago on June 28. Like its mainshock, this earthquake was a normal faulting earthquake on one of the many north-striking normal faults that lie between the San Jacinto and San Andreas faults. In the wee hours on Monday there was a M3.2 near Ocotillo and the border with Baja California. We received no reports of this earthquake being felt. It was located very near the Elsinore fault aand had a strike-slip focal mechanism consistent with right-lateral strike=slip on the Elsinore fault. This was the week for M3.0 earthquakes. There were 4 of them: One on Thursday evening near Palmdale, early morning on Saturday near Twentynine Palms, an aftershock of the M5.8 Ridgecrest earthquake (September 20, 1995) on Monday morning and one south of the border outside our network. We again did not receive reports that any of these were felt. The Palmdale event was located just 3 miles south of the San Andreas fault, but had a focal mechanism showing thrust motion on a plane subparallel to the San Andreas. This is the typical focal mechanism for this region. These earthquakes are probably occuring on secondary faults that form around the San Andreas fault. 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 ------------------------------------------------------------- 7/ 9 10:40 pm 35 47.9 117 38.2 2.0 12 mi. N of Ridgecrest 7/10 2:26 am 32 41.2 118 6.2 2.2 16 mi. ESE of San Clemente Is. (SE tip) 7/10 9:02 am 35 46.2 117 39.2 2.0 10 mi. N of Ridgecrest 7/10 1:00 pm 35 46.1 117 39.1 2.0 " 7/10 6:45 pm 34 8.9 117 20.1 2.0 3 mi. NE of Rialto 7/10 11:40 pm 34 28.5 118 4.9 3.0 7 mi. SSE of Palmdale 7/11 8:42 am 33 20.3 116 23.3 2.2 5 mi. N of Borrego Springs 7/12 4:13 am 34 1.5 116 5.5 3.0 8 mi. SSW of Twentynine Palms 7/12 4:37 am 33 58.4 118 55.6 2.0 4 mi. S of Leo Carrillo State Beach 7/12 9:46 am 34 9.2 117 20.1 2.7 3 mi. NNE of Rialto 7/12 10:41 am 32 11.3 115 46.3 2.1 36 mi. SSW of Calexico 7/12 11:05 am 34 9.3 117 19.8 3.5 3 mi. NNW of San Bernardino FELT 7/12 11:07 am 34 8.9 117 20.1 2.0 3 mi. NE of Rialto 7/12 1:22 pm 35 47.8 117 38.2 2.0 12 mi. N of Ridgecrest 7/12 4:31 pm 35 52.3 118 5.5 2.1 14 mi. SW of Coso Junction 7/12 6:29 pm 34 9.1 117 19.9 2.8 3 mi. NW of San Bernardino 7/13 5:38 am 33 38.3 117 50.3 2.1 2 mi. SSW of Irvine 7/13 11:42 am 32 12.3 115 48.2 2.5 36 mi. SSW of Calexico 7/13 12:41 pm 32 5.4 114 59.8 3.0 47 mi. SSW of Yuma, Arizona 7/14 3:20 am 32 37.9 115 54.2 3.2 8 mi. SE of Ocotillo 7/14 4:34 am 32 37.4 115 54.0 2.1 9 mi. SE of Ocotillo 7/14 7:54 am 35 46.3 117 39.4 3.0 10 mi. N of Ridgecrest 7/14 10:03 am 36 4.8 117 37.5 2.1 18 mi. E of Coso Junction 7/14 1:28 pm 33 10.5 115 35.5 2.0 2 mi. E of Obsidian Butte 7/14 5:04 pm 34 25.9 118 36.6 2.5 3 mi. NW of Valencia 7/14 7:57 pm 33 55.1 116 48.1 2.7 Under Cabazon 7/15 1:22 am 35 47.9 117 38.1 2.2 12 mi. N of Ridgecrest 7/15 2:37 am 33 41.1 116 45.1 2.2 4 mi. SSW of Idyllwild 7/15 5:49 am 34 9.1 117 20.1 2.1 3 mi. NNE of Rialto 7/15 7:34 am 34 2.3 117 14.3 2.9 1 mi. SE of Loma Linda 7/16 9:05 am 35 47.4 117 39.0 2.0 11 mi. N of Ridgecrest 7/16 12:48 pm 33 56.4 118 37.5 2.7 7 mi. SSE of Malibu 7/16 12:51 pm 32 8.9 115 50.3 2.2 41 mi. SSE of Ocotillo 7/16 1:00 pm 35 26.8 118 18.8 2.2 15 mi. SSE of town of Lake Isabella 7/16 1:03 pm 33 15.8 116 8.4 2.2 8 mi. N of Ocotillo Wells -------------------------------------------------------------