SOUTHERN CALIFORNIA SEISMOGRAPHIC NETWORK A Cooperative Project of Caltech and the U.S. Geological Survey April 13, 1995 Weekly Earthquake Report for Southern California ------------------------------------------------ April 6 - 12, 1995 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 818-395-6326. For daily updates, call our Earthquake Information Hotline: 818-395-6977. -------------------------------- This week's Report covers the time period from midnight Thursday morning, April 6, Pacific Daylight Time, to midnight Wednesday night, April 12, 1995, Pacific Daylight Time. We recorded 271 earthquakes of M1.0 or larger during the 7-day period covered. There were four M3+ quakes this week, all of them aftershocks of the June 28, 1992 Landers quake. The largest one, M3.7, occurred early Monday morning. It might have been felt, even though it was in a fairly unpopulated area; however, we got no calls. Its focal mechanism showed strike-slip faulting, like most of the Landers aftershocks. Last Thursday evening, there was a M3.1 in the same general vicinity, also apparently unnoticed by the public. On Sunday afternoon, a M3.3 nearer to Yucca Valley was felt, as was a M3.4 on Tuesday morning. The latter was near the southern end of the Landers aftershock zone. Its focal mechanism showed northeast-striking normal faulting. This type of mechanism was common in all of the Little San Bernardino Mountains before the Joshua Tree earthquake and is similar to the mechanism of the M3.2 earthquake that occurred west of the Landers/Joshua Tree aftershock zone on March 27. This week, the largest Northridge aftershock was only M2.3. Since the Northridge mainshock (Mw6.7) was quite a bit smaller than the Landers aftershock (Mw7.3), the former's after- shocks have overall been fewer and have been dying off more quickly. We have already passed the point in time when the weekly rate of Landers aftershocks is higher than that of Northridge. Recent Weekly Reports have been reflecting this trend. Given the present rates of aftershocks and their rates of decay, we expect the Landers sequence to be noticeable on our maps for another five years, while Northridge may have returned to the background rate in another two years. 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 ------------------------------------------------------------- 4/6 1:32 am 33 48.8 116 11.9 2.5 6 mi. N of Indio 4/6 11:38 am 35 4.9 119 4.0 2.8 Grapevine area, 7 mi. NW of the I5/99 interchange 4/6 11:39 am 35 4.8 119 5.0 2.1 " 4/6 5:22 pm 35 49.3 117 45.7 2.4 14 mi. NNW of Ridgecrest 4/6 7:01 pm 34 12.0 116 26.0 2.2 5 mi. N of Yucca Valley 4/6 7:04 pm 32 28.7 115 49.5 2.0 20 mi. SSE of Ocotillo 4/6 8:17 pm 34 21.9 116 28.2 3.1 17 mi. N of Yucca Valley 4/6 11:38 pm 34 12.2 118 38.2 2.0 1 mi. W of Canoga Park 4/7 5:50 am 35 58.6 117 18.8 2.4 14 mi. NNE of Trona 4/7 9:51 am 34 37.4 116 36.0 2.0 23 mi. ENE of Lucerne Valley 4/7 11:00 am 34 23.2 118 37.2 2.0 3 mi. SW of Magic Mtn. 4/7 2:30 pm 34 37.9 116 30.4 2.4 28 mi. ENE of Lucerne Valley 4/7 4:20 pm 35 43.5 118 29.5 2.3 6 mi. NNW of the town of Lake Isabella 4/7 4:52 pm 33 0.6 116 5.4 2.1 9 mi. SSE of Ocotillo Wells 4/7 8:06 pm 34 16.7 119 57.5 2.3 10 mi. SSW of Isla Vista 4/7 8:41 pm 33 11.6 115 33.9 2.2 Obsidian Butte, 4 mi. SW of Niland 4/7 9:07 pm 34 17.1 119 56.9 2.0 10 mi. SSW of Isla Vista 4/8 2:27 am 34 16.2 119 56.6 2.4 " 4/8 9:29 am 34 24.8 116 28.4 2.3 20 mi. N of Yucca Valley 4/8 1:19 pm 34 22.3 116 28.1 2.1 17 mi. N of Yucca Valley 4/8 5:17 pm 33 10.3 115 37.0 2.0 6 mi. WNW of Calipatria 4/8 10:45 pm 36 15.2 120 16.2 2.5 9 mi. NNE of Coalinga 4/9 5:26 am 34 7.2 116 55.1 2.4 5 mi. WNW of Mt. San Gorgonio 4/9 4:01 pm 34 2.4 116 19.3 3.3 8 mi. SE of Yucca Valley FELT 4/9 7:46 pm 34 17.1 118 27.3 2.3 1 mi. WNW of San Fernando 4/9 7:55 pm 35 55.9 117 40.0 2.5 Coso Range, 17 mi. ESE of Coso Junction 4/9 7:56 pm 35 55.9 117 40.0 2.7 " 4/9 9:57 pm 33 11.5 115 36.4 2.3 Obsidian Butte, 6 mi. WSW of Niland 4/9 10:02 pm 33 11.4 115 36.8 2.9 " 4/9 11:58 pm 34 59.2 116 57.2 2.2 7 mi. NNE of Barstow 4/10 1:17 am 33 11.6 115 35.4 2.0 Obsidian Butte, 5 mi. SW of Niland 4/10 2:26 am 33 11.4 115 35.5 2.0 " 4/10 2:41 am 34 10.1 116 26.2 2.0 3 mi. N of Yucca Valley 4/10 7:01 am 34 24.7 116 28.2 3.7 20 mi. N of Yucca Valley 4/10 7:10 am 36 1.7 118 13.4 2.0 Coso Range, 15 mi. W of Coso Junction 4/10 9:27 am 34 28.5 116 29.7 2.1 24 mi. N of Yucca Valley 4/10 11:42 am 34 59.0 116 56.8 2.5 7 mi. NNE of Barstow 4/10 1:26 pm 34 14.8 118 29.0 2.1 2 mi. ENE of Northridge 4/10 5:00 pm 33 20.3 116 14.6 2.3 9 mi. NE of Borrego Springs 4/11 2:27 am 34 23.1 116 27.4 2.0 18 mi. N of Yucca Valley 4/11 3:06 am 34 22.3 116 27.2 2.2 " 4/11 8:56 am 33 54.4 116 17.5 3.4 13 mi. NNW of Indio; FELT 4/11 10:30 am 34 22.1 116 27.5 2.1 17 mi. N of Yucca Valley 4/11 11:45 am 34 21.7 116 27.5 2.1 " 4/11 6:55 pm 36 5.5 117 40.2 2.6 Coso Range, 15 mi. ESE of Haiwee Reservoir 4/12 3:23 am 33 54.4 116 17.5 2.5 13 mi. NNW of Indio 4/12 8:47 pm 34 18.3 118 25.6 2.1 1 mi. NNE of San Fernando 4/12 8:52 pm 33 32.9 115 54.3 2.4 2 mi. NE of North Shore 4/12 10:07 pm 34 56.5 116 46.7 2.0 14 mi. ENE of Barstow 4/12 11:53 pm 35 18.3 118 30.6 2.6 12 mi. NNW of Tehachapi -------------------------------------------------------------