SOUTHERN CALIFORNIA SEISMOGRAPHIC NETWORK A Cooperative Project of Caltech and the U.S. Geological Survey February 6, 1997 Weekly Earthquake Report for Southern California ------------------------------------------------ January 30 - February 5, 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. -------------------------------- This week's Report covers the time period from midnight Thursday morning, January 30, Universal Time (ie. GMT), to midnight Wednesday night, February 5, Universal Time. In local time, the period of coverage is from 4 p.m., January 20, Pacific Standard Time, to 4 p.m., February 5, Pacific Standard Time. We detected and processed 260 earthquakes during the seven-day period covered. Although the total number of events this week was pretty average, there were quite a few M3+'s. Aside from the Coalinga quake on Monday afternoon, which is not really in our coverage area (see the Northern California Weekly Report for more information), there were six. The first was a M3.0 Ridgecrest quake, a late aftershock of the sequences that were prominent in 1995 and 1996, early last Thursday morning. Later the same morning, there was a M3.2 in the Coso sequence, an aftershock of last year's November 27 M5.3. Both earthquakes had focal mechanisms similar to their mainshocks, right-lateral strike-slip motion on northwest-striking planes. The next quake of the week to be felt was the M3.5 near Yorba Linda, very early Friday morning. It was fairly widely felt in Orange County and the Inland Empire. The focal mechanism was right-lateral strike-slip on a west- northwest striking fault plane. It was located very near the Whittier Fault. This quake was not a member of any our on-going aftershock sequences. On Saturday afternoon, there was another M3.5, located just south of the U.S./Mexico border, near the town of Ocotillo. This quake was apparently felt in the El Centro area. It also was not a member of any on-going aftershock sequence. The focal mechanism was strike-slip, with one of the possible fault planes consistent with the prevailing northwest/southeast of the nearby faults. On Sunday afternoon, a M3.0 Landers aftershock was felt in the Yucca Valley area. Like most Landers aftershocks, it had a strike-slip mechanism. The last felt earthquake of the week was a M3.2 North -ridge aftershock, near Chatsworth, very early Monday morning. It was widely felt in the San Fernando Valley. It occurred on a very shallowly-dipping thrust fault. Its depth was shallow, above the main thrust fault of the Northridge earthquake. 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 ------------------------------------------------------------- 1/29 6:27 pm 36 5.0 117 38.0 2.4 Coso Range, 19 mi. SSW of Panamint Springs 1/30 1:56 am 33 54.7 117 47.0 2.0 2 mi. NE of Yorba Linda 1/30 2:12 am 36 3.5 117 39.2 2.6 16 mi. E of Coso Junction 1/30 2:52 am 33 54.7 117 46.9 2.0 2 mi. NE of Yorba Linda 1/30 5:06 am 35 44.9 117 38.3 3.0 8 mi. N of Ridgecrest 1/30 7:44 am 34 1.3 117 2.4 2.0 Under Yucaipa 1/30 10:48 am 36 4.9 117 38.0 3.2 20 mi. SSW of Panamint Springs 1/31 12:34 am 36 4.8 117 38.1 2.1 " 1/31 4:25 am 33 54.7 117 47.1 3.5 2 mi. NE of Yorba Linda FELT 1/31 6:03 am 32 23.5 115 7.0 2.6 29 mi. SE of Calexico 1/31 6:06 am 32 23.5 115 7.3 2.6 " 2/1 1:04 pm 34 33.9 116 30.0 2.2 31 mi. N of Yucca Valley 2/1 1:49 pm 32 28.6 115 28.2 2.4 13 mi. S of Calexico 2/1 4:19 pm 32 37.3 115 55.1 3.5 8 mi. SSE of Ocotillo FELT 2/1 5:36 pm 32 28.0 115 28.0 2.2 14 mi. S of Calexico 2/1 6:20 pm 36 11.1 120 11.2 3.9 10 mi. ENE of Coalinga 2/1 8:10 pm 33 55.6 116 2.9 2.0 14 mi. S of Twentynine Palms 2/2 1:39 am 35 5.7 117 29.5 2.4 11 mi. NE of Boron 2/2 2:00 am 35 5.5 117 29.4 2.4 " 2/2 3:08 pm 34 37.5 116 33.0 2.0 25 mi. ENE of Lucerne Valley 2/2 3:41 pm 36 5.9 118 13.7 2.1 16 mi. WNW of Coso Junction 2/2 3:52 pm 33 29.2 116 46.7 2.8 7 mi. SW of Anza 2/2 9:16 pm 34 2.8 116 23.8 3.0 5 mi. SSE of Yucca Valley; FELT 2/3 3:04 am 34 18.0 118 34.0 3.2 3 mi. NE of Chatsworth FELT 2/3 11:09 am 34 6.8 117 27.2 2.1 1 mi. WNW of Fontana 2/3 12:14 pm 34 31.3 115 49.6 2.0 29 mi. NNE of Twentynine Palms 2/3 5:23 pm 36 7.0 118 14.1 2.3 17 mi. SW of Olancha 2/3 7:52 pm 33 15.7 116 0.3 2.2 2 mi. WSW of Salton City 2/4 2:32 pm 33 20.0 116 25.1 2.1 6 mi. NNW of Borrego Springs 2/4 10:30 pm 36 4.1 117 38.6 2.0 17 mi. E of Coso Junction -------------------------------------------------------------