SOUTHERN CALIFORNIA SEISMOGRAPHIC NETWORK A Cooperative Project of Caltech and the U.S. Geological Survey June 29, 1995 Weekly Earthquake Report for Southern California ------------------------------------------------ June 22 - 28, 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, June 22, Pacific Daylight Time, to midnight Wednesday night, June 28, 1995, Pacific Daylight Time. We recorded 347 earthquakes of M1.0 or larger during the 7-day period covered. Many Angelenos were rudely awakened at 1:40 am on Monday by a M5.0 Northridge aftershock (initially reported as M4.9). It occurred in the Valencia area, on the northern edge of the aftershock zone of the January 17, 1994 Mw6.7 mainshock, near the northernmost and shallowest part of that quake's rupture. It was felt by a few people as far away as San Diego and Santa Barbara. Not surprisingly for a Northridge aftershock, it had a thrust focal mechanism. Of course, aftershocks are also earthquakes in their own right, and this one had two foreshocks and 40 aftershocks of its own, the largest of which was an M3.3 that occurred 18 minutes after the M5.0. At M5.0, it was one of the top nine largest Northridge aftershocks, the largest of which was the M5.6 that occured about one minute after the Mw6.7. It was the largest since March 20, 1994, when there was a M5.2. Readers who may be concerned about the size of this week's aftershock at this late date should recall that aftershocks decrease in number with time, but the relative number of small to large remains the same. Large, late aftershocks are certainly possible and expected statistically in some percentage of aftershock sequences. In the first issue of the Weekly Report in 1995, where we summarized the probabil- ities for the year: "As for the coming year, statistically speaking, the Northridge sequence should produce about 17 M3's, about two M4's, and has a about a 25% chance of producing another M5 ... By comparison, the chance of an independent M5+ earthquake somewhere is Southern California (including all the sparsely populated areas) is about 80%." There were also a few small quakes in the L.A. Basin this week, one of which was felt. That one was a M2.6 just offshore near the L.A. Airport. It occured early Sunday after- noon. Three tiny ones (largest M1.9) occurred in the Playa del Rey area. There was a San Diego quake, M2.7, early Monday morning, which was lightly felt. The largest Landers/Big Bear aftershock occurred yesterday (Wednesday) afternoon and had a magnitude of M3.1. It was a Big Bear aftershock. We got no calls on it. A small quake (M2.8) on Saturday evening, with an epicenter near Cabazon was felt in Palm Springs. 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 ------------------------------------------------------------- 6/22 4:35 am 34 56.5 116 55.7 2.2 6 mi. NE of Barstow 6/22 9:46 am 34 56.5 116 55.6 2.3 " 6/22 10:06 am 34 56.3 116 55.8 2.0 " 6/22 5:18 pm 34 22.6 116 28.0 2.4 17 mi. N of Yucca Valley 6/22 7:55 pm 34 18.2 118 27.8 2.5 2 mi. NW of San Fernando 6/22 8:54 pm 34 18.4 118 27.3 2.3 " 6/23 12:17 am 35 2.3 116 60.0 3.1 10 mi. N of Barstow 6/23 3:18 am 36 21.1 120 15.2 3.0 15 mi. NNE of Coalinga 6/23 9:24 pm 35 18.2 118 29.8 2.2 12 mi. NNW of Tehachapi 6/24 12:16 am 34 22.9 116 28.1 2.1 18 mi. N of Yucca Valley 6/24 1:38 am 34 29.1 120 41.4 2.4 6 mi. SSW of Pt. Arguello 6/24 4:49 am 34 24.0 116 27.9 2.3 19 mi. N of Yucca Valley 6/24 8:08 am 36 8.3 120 7.7 3.3 12 mi. E of Coalinga 6/24 11:42 am 34 29.3 116 30.5 2.6 25 mi. N of Yucca Valley 6/24 12:50 pm 34 24.9 118 59.1 2.1 3 mi. WNW of Fillmore 6/24 6:44 pm 35 59.1 117 38.2 2.4 Coso Range, 17 mi. ESE of Coso Junction 6/24 7:16 pm 34 12.9 118 35.2 2.0 1 mi. NE of Canoga Park 6/24 7:23 pm 33 59.6 116 44.0 2.8 5 mi. NNE of Cabazon FELT 6/24 11:16 pm 36 0.6 117 48.9 2.3 Coso Range, 7 mi. ESE of Coso Junction 6/25 1:06 am 33 59.5 116 55.9 2.1 5 mi. NNE of Beaumont 6/25 11:22 am 34 18.2 118 28.0 2.0 2 mi. NW of San Fernando 6/25 1:44 pm 33 54.4 118 27.9 2.6 3 mi. SSW of Los Angeles Airport 6/25 3:31 pm 35 44.2 118 28.7 2.1 6 mi. N of the town of Lake Isabella 6/25 3:36 pm 34 20.2 116 28.6 2.0 15 mi. N of Yucca Valley 6/26 12:19 am 34 23.6 118 39.8 3.0 4 mi. WSW of Magic Mtn. 6/26 1:40 am 34 23.6 118 40.1 5.0 5 mi. WSW of Magic Mtn. WIDELY FELT 6/26 1:46 am 34 25.0 118 38.2 2.0 2 mi. W of Magic Mtn. 6/26 1:47 am 34 23.9 118 38.3 2.0 4 mi. WSW of Magic Mtn. 6/26 1:50 am 34 23.8 118 39.3 2.0 " 6/26 1:58 am 34 24.6 118 38.4 3.3 " 6/26 3:36 am 34 24.2 118 38.4 2.1 " 6/26 3:43 am 34 23.6 118 39.6 2.0 " 6/26 5:51 am 32 43.6 117 10.5 2.7 2 mi. SE of the I5/I8 interch., San Diego FELT 6/26 7:25 am 33 33.5 118 15.5 2.4 10 mi. SSE of Pt. Fermin, San Pedro 6/26 11:26 am 32 57.5 117 47.0 2.1 28 mi. SW of Oceanside 6/26 6:22 pm 34 55.0 116 54.4 2.0 6 mi. ENE of Barstow 6/26 6:26 pm 32 53.0 115 57.5 2.4 10 mi. N of Ocotillo 6/26 8:17 pm 32 53.0 115 57.7 2.2 " 6/27 1:06 am 34 22.8 116 27.3 2.7 18 mi. N of Yucca Valley 6/27 3:40 am 34 9.0 116 52.2 2.0 4 mi. NW of Mt. San Gorgonio 6/27 4:54 am 32 18.8 115 13.2 2.7 29 mi. SSE of Calexico 6/27 10:45 am 34 19.3 116 27.8 2.2 14 mi. N of Yucca Valley 6/27 11:02 am 35 2.3 117 0.1 2.3 10 mi. N of Barstow 6/27 11:03 am 34 19.6 116 27.9 2.0 14 mi. N of Yucca Valley 6/27 1:34 pm 33 22.1 116 23.6 2.3 7 mi. N of Borrego Springs 6/27 1:52 pm 34 23.5 116 27.9 2.4 18 mi. N of Yucca Valley 6/28 2:51 am 34 23.5 116 28.0 2.8 18 mi. N of Yucca Valley 6/28 6:48 am 36 8.1 120 8.4 2.6 12 mi. E of Coalinga 6/28 7:12 am 36 8.1 120 7.0 2.8 13 mi. E of Coalinga 6/28 9:42 am 33 2.0 115 34.2 2.5 3 mi. E of Westmorland 6/28 10:14 am 33 2.0 115 35.1 2.1 2 mi. E of Westmorland 6/28 10:17 am 34 23.8 118 39.3 2.6 4 mi. WSW of Magic Mtn. FELT 6/28 10:43 am 33 41.3 118 13.7 2.1 3 mi. ESE of Pt. Fermin, San Pedro 6/28 2:29 pm 34 21.4 116 54.2 3.1 6 mi. SSE of Lucerne Valley 6/28 9:57 pm 34 11.6 118 33.9 2.1 2 mi. E of Canoga Park -------------------------------------------------------------