December 15, 1994 Weekly Earthquake Report for Southern California ------------------------------------------------ December 8 - 14, 1994 Prepared by: Kate Hutton, Seismological Laboratory (kate@bombay.gps.caltech.edu) Lucy Jones, U.S. Geological Survey California Institute of Technology This document is a commentary on current seismic activity. The earthquakes discussed have been detected and processed automatically. All epicenters and magnitudes have been reviewed to exclude obvious blunders; however, they must still be considered preliminary. 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, December 8, Pacific Standard Time, to midnight Wednesday night, December 14, Pacific Standard Time. We recorded 343 earth- quakes of M1.0 or larger during the 7-day period covered. Although the total number of earthquakes this week was about the same as last week, the proportion of M3+ quakes was slightly higher than usual. We had six of them, at various locations. Only two of them were aftershocks: the M3.3 quake on Saturday morning, a Landers aftershock, and the M3.2 quake on Tuesday morning, a Big Bear aftershock. The first was felt in Barstow and the second one in the Big Bear Lake area. The other four M3+ events were scattered: a M3.6 on Friday evening near Ojai, which was felt; a M3.4 early Sunday morning near Marina del Rey, which was felt; a M3.6 on Monday morning in the Imperial Valley; and a M3.7 near Ridgecrest, also on Monday morning. None of these quakes were in places that surprised us, and none were part of extensive swarms. Both the Ojai and Marina del Rey quakes occurred on approximately east-striking faults. Several systems of such faults are evident in both areas. The San Cayatano and Oak Ridge systems both go through the Ojai area. Several researchers have inferred blind thrust faults in the Marina area, based on the sedimentary geology and on the presence of small events (like this week's) with thrust focal mechanisms. The Imperial Valley quake was near Obsidian Butte, the site of hundreds on M3+ earthquakes in the last decade. The Ridgecrest quake was also in a perennial hot spot. Both these quakes had approximately strike-slip mechanisms, as expected for those locations. The fact that these particular quakes all happened in one week is undoubtably explained by statistical fluctuation. Random events, such as coin flips and earthquakes, sometimes show "runs", which the human mind almost always perceives as significant. Actually, the total number of non-aftershocks has been a slightly lower than average this year. We normally expect to see about 100 independent M3+ quakes each year in our coverage area. This year we've only had 65. So the overall seismicity is only higher than normal because there are two major aftershock sequences going on. 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 ------------------------------------------------------------- 12/8 5:50 am 35 49.7 116 49.4 2.0 27 mi. SSE of Telescope Peak, Panamint Range 12/8 8:38 pm 34 10.2 116 26.2 2.3 3 mi. N of Yucca Valley 12/8 8:56 pm 34 57.0 116 55.8 2.7 6 mi. NE of Barstow 12/8 10:01 pm 34 21.6 118 38.9 2.2 5 mi. SW of Magic Mtn. 12/9 3:50 pm 34 58.6 116 57.3 2.1 7 mi. NNE of Barstow 12/9 4:09 pm 35 5.4 118 17.6 2.7 7 mi. WNW of Mojave 12/9 6:34 pm 34 26.8 119 20.1 2.0 5 mi. W of Ojai 12/9 6:34 pm 34 26.6 119 20.6 3.6 " FELT 12/10 12:05 am 33 15.5 115 39.1 2.0 7 mi. SE of Bombay Beach 12/10 12:20 am 34 5.7 116 24.0 2.0 3 mi. ESE of Yucca Valley 12/10 3:04 am 33 53.5 118 42.8 2.1 9 mi. SE of Pt. Dume 12/10 9:54 am 34 58.8 116 57.1 3.3 7 mi. NNE of Barstow; FELT 12/10 4:06 pm 34 16.6 118 40.3 2.3 2 mi. E of Simi Valley 12/10 4:08 pm 34 0.7 116 19.2 2.8 10 mi. SE of Yucca Valley 12/10 7:42 pm 34 26.5 119 20.7 2.3 5 mi. W of Ojai 12/10 9:12 pm 34 30.0 116 30.9 2.3 26 mi. N of Yucca Valley 12/10 10:44 pm 34 28.3 117 50.9 2.1 15 mi. WNW of Wrightwood 12/11 12:38 am 34 46.6 116 17.9 2.0 36 mi. SSW of Baker 12/11 1:40 am 35 52.2 117 48.7 2.3 14 mi. SSE of Coso Junction 12/11 2:48 am 33 59.3 118 26.1 3.4 1 mi. NE of Marina del Rey FELT 12/11 8:15 am 34 37.5 116 33.5 2.0 25 mi. ENE of Lucerne Valley 12/11 8:59 am 34 2.0 117 13.2 2.0 2 mi. WSW of Redlands 12/11 11:07 am 34 22.0 116 27.8 2.1 17 mi. N of Yucca Valley 12/11 1:31 pm 34 49.8 116 36.0 2.3 24 mi. E of Barstow 12/11 2:23 pm 32 50.0 117 39.5 2.2 23 mi. W of La Jolla 12/11 6:18 pm 34 23.8 116 28.0 2.3 19 mi. N of Yucca Valley 12/11 8:37 pm 34 46.9 116 19.6 2.1 36 mi. SSW of Baker 12/11 8:58 pm 34 23.5 118 38.6 2.1 3 mi. SW of Magic Mtn. 12/11 9:04 pm 32 2.3 116 21.6 2.9 52 mi. SSW of Ocotillo 12/11 9:50 pm 35 56.5 120 29.2 2.9 4 mi. NW of Parkfield 12/11 10:06 pm 34 47.6 116 18.3 2.5 35 mi. SSW of Baker 12/11 10:07 pm 34 2.0 118 4.1 2.0 Whittier Narrows Recreation Area 12/11 11:24 pm 34 15.6 116 24.8 2.0 9 mi. N of Yucca Valley 12/12 1:37 am 34 26.1 119 20.0 2.2 4 mi. W of Ojai 12/12 1:37 am 32 37.0 115 54.8 2.0 9 mi. SSE of Ocotillo 12/12 3:41 am 33 58.1 117 14.7 2.3 3 mi. NE of Moreno Valley 12/12 5:05 am 33 11.6 115 34.6 3.6 Obsidian Butte area, 4 mi. SW of Niland 12/12 5:09 am 33 10.7 115 33.2 2.1 " 12/12 5:15 am 33 11.6 115 34.5 2.8 " 12/12 6:04 am 33 11.4 115 34.3 2.0 " 12/12 7:50 am 34 47.8 116 18.3 2.5 34 mi. SSW of Baker 12/12 9:49 am 34 8.4 116 25.8 2.8 1 mi. NNE of Yucca Valley FELT 12/12 11:48 am 35 41.9 117 39.5 3.7 5 mi. N of Ridgecrest FELT 12/12 11:49 am 35 41.8 117 39.3 2.5 " 12/12 1:18 pm 35 41.9 117 39.6 2.5 " 12/12 5:24 pm 34 12.7 118 35.8 2.0 Under Canoga Park 12/12 8:25 pm 33 45.9 117 54.4 2.4 3 mi. S of Disneyland 12/12 8:44 pm 34 20.8 116 28.0 2.0 15 mi. N of Yucca Valley 12/12 11:18 pm 33 53.0 119 26.5 2.0 11 mi. SSE of Santa Cruz Is. 12/13 12:40 am 34 47.0 116 18.2 2.0 35 mi. SSW of Baker 12/13 3:07 am 33 54.4 116 46.0 2.3 1 mi. SE of Cabazon 12/13 5:05 am 34 22.2 116 28.0 2.1 17 mi. N of Yucca Valley 12/13 8:43 am 34 37.3 116 32.2 2.3 26 mi. ENE of Lucerne Valley 12/13 9:51 am 34 7.6 116 53.0 3.2 3 mi. WNW of Mt. San Gorgonio FELT 12/13 3:53 pm 34 23.8 118 37.7 2.0 2 mi. SW of Magic Mtn. 12/13 6:52 pm 34 12.3 116 52.4 2.0 3 mi. SE of town of Big Bear Lake 12/13 7:31 pm 35 5.3 119 5.6 2.2 9 mi. WNW of the I5/99 interchange 12/13 7:51 pm 33 54.8 119 38.0 2.3 10 mi. SW of Santa Cruz Is. 12/14 4:09 pm 34 20.0 116 28.1 2.5 14 mi. N of Yucca Valley 12/14 4:49 pm 34 25.6 116 30.9 2.3 21 mi. N of Yucca Valley ------------------------------------------------------------- Figure 1. A map of southern California showing the earthquakes recorded during the past week by the Caltech/USGS Seismic Network. Major faults are marked, as well as the metropolitan areas of Los Angeles (L.A.), Palm Springs (P.S.), San Diego (S.D.), and Santa Barbara (S.B.). The circles denote the earthquakes, the size of the circle indicating the magnitude.