SOUTHERN CALIFORNIA SEISMOGRAPHIC NETWORK A Cooperative Project of Caltech and the U.S. Geological Survey August 15, 1996 Weekly Earthquake Report for Southern California ------------------------------------------------ August 8 - 14, 1996 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, August 8, Universal Time (ie. GMT), to midnight Wednesday night, August 14, Universal Time. In local time, the period of coverage is from 5 p.m., August 7, Pacific Daylight Time, to 5 p.m., August 14, Pacific Daylight Time. We detected and processed 285 earthquakes during the seven-day period covered. Although the total number of quakes this week was not higher than what we've seen recently, and there were only three events over M3, there were four events felt. The largest and most widely felt was a M4.3 in the Mojave Desert on Tuesday evening. It had a strike-slip focal mechanism. It was felt as far away as the Los Angeles area. We decided to call this earthquake an aftershock to the Landers event, but the decision was not clear cut. The usual criteria for an aftershock this long after the mainshock is that it occurs within the "original" aftershock zone, which is the area that saw an increase within the first few days. In the case of Landers that triggered activity as far away as Yellowstone, we further restrict the zone to be within one fault length. The Landers fault is 90 km long, so that means within 90 km of the main fault trace. Within that 90 km zone, many faults produced aftershocks and many did not. The region that produced this week's M4.5 event is east of the main fault and did NOT rupture in the Landers mainshock. It did have a M5.4 aftershock one week after the mainshock that had its own aftershock sequence. The confusing point is that aftershocks in that region stopped in 1992, and, since then, we have had only three earthquakes above M2.5 in that spot. This made us wonder if this event is a "true" aftershock, but, because a previous M5.4 aftershock was at the same place, we said yes. The reality is that all earthquakes change the probability of all other earthquakes, and the size of the change depends on distance and time. By naming one event an aftershock and another event not an aftershock we force a division in a continuous distribution. Other earthquakes felt this week include a M2.9 near Indio on Saturday, and a M2.9 and a M2.2 near Inglewood on Monday afternoon. The Indio quake was at the southern end of the Landers aftershock zone. 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 ------------------------------------------------------------- 8/7 7:43 pm 33 10.7 115 35.9 2.0 2 mi. ENE of Obsidian Butte 8/7 11:09 pm 34 56.0 116 45.5 2.1 15 mi. ENE of Barstow 8/8 6:30 am 33 10.6 115 35.9 2.1 2 mi. E of Obsidian Butte 8/8 9:27 am 35 6.0 117 29.6 2.0 26 mi. E of California City 8/8 10:02 am 35 5.9 117 29.5 2.0 " 8/8 10:03 am 35 5.9 117 29.6 2.3 " 8/8 10:40 am 32 44.5 116 2.8 2.0 3 mi. W of Ocotillo 8/8 5:43 pm 34 24.0 116 28.5 2.7 19 mi. N of Yucca Valley 8/8 6:52 pm 34 36.0 116 17.4 2.2 32 mi. N of Joshua Tree 8/8 7:39 pm 35 6.0 117 29.7 2.9 26 mi. E of California City 8/8 8:37 pm 33 15.6 116 17.2 3.3 4 mi. E of Borrego Springs 8/9 7:38 am 34 21.2 119 16.9 2.3 5 mi. NNE of Ventura 8/9 8:37 am 34 55.0 116 55.0 2.0 6 mi. ENE of Barstow 8/9 6:08 pm 34 36.0 116 17.3 2.4 31 mi. N of Joshua Tree 8/9 6:16 pm 33 59.9 118 6.5 2.1 4 mi. N of Downey 8/10 4:12 am 34 28.9 119 22.8 2.6 7 mi. WNW of Ojai 8/10 5:52 am 35 47.7 117 39.7 2.4 11 mi. N of Ridgecrest 8/10 6:20 am 33 40.3 118 4.3 2.4 2 mi. WSW of Bolsa Chica Beach 8/10 12:33 pm 33 44.4 116 9.9 2.9 3 mi. ENE of Indio; FELT 8/10 12:34 pm 33 44.4 116 9.5 2.1 " 8/10 12:35 pm 33 44.2 116 9.4 2.2 " 8/10 2:03 pm 32 52.5 115 34.4 2.1 5 mi. NNW of El Centro 8/10 5:31 pm 35 5.4 117 28.9 2.1 27 mi. E of California City 8/10 10:29 pm 35 47.8 116 5.9 2.1 15 mi. SE of Shoshone 8/10 11:29 pm 33 10.8 115 36.1 2.3 2 mi. ENE of Obsidian Butte 8/11 2:26 am 33 10.6 115 36.2 2.5 " 8/11 6:56 am 35 43.9 117 37.5 2.2 7 mi. NNE of Ridgecrest 8/11 3:57 pm 33 15.6 116 12.4 2.1 9 mi. NNW of Ocotillo Wells 8/11 6:49 pm 33 15.6 116 12.3 2.3 " 8/11 8:47 pm 33 15.8 116 17.0 2.0 5 mi. E of Borrego Springs 8/12 1:14 am 34 4.0 116 47.4 2.3 3 mi. SE of Mt. San Gorgonio 8/12 10:57 am 33 10.5 115 36.5 2.9 1 mi. ENE of Obsidian Butte 8/12 11:36 am 35 56.5 117 13.9 2.1 14 mi. NNE of Trona 8/12 1:17 pm 34 35.3 116 17.4 2.0 31 mi. N of Joshua Tree 8/12 1:25 pm 33 58.8 118 19.9 2.9 1 mi. NNW of Inglewood FELT 8/12 1:39 pm 33 59.8 118 20.9 2.2 " FELT 8/12 2:03 pm 34 1.0 116 19.0 2.2 8 mi. S of Joshua Tree 8/12 2:28 pm 33 10.8 115 36.2 2.9 2 mi. ENE of Obsidian Butte 8/13 8:40 am 34 38.2 116 30.5 2.1 28 mi. ENE of Lucerne Valley 8/13 10:29 am 34 28.2 116 30.6 2.0 24 mi. N of Yucca Valley 8/13 1:04 pm 33 10.3 115 36.4 2.2 1 mi. E of Obsidian Butte 8/13 1:12 pm 33 10.5 115 36.6 2.4 " 8/13 1:43 pm 33 31.8 118 2.0 2.3 8 mi. SW of Newport Beach 8/13 2:19 pm 33 10.5 115 36.7 2.2 1 mi. ENE of Obsidian Butte 8/13 3:30 pm 34 27.3 116 30.1 2.2 23 mi. N of Yucca Valley 8/13 5:42 pm 32 49.1 115 37.3 2.2 4 mi. WNW of El Centro 8/13 8:05 pm 34 36.0 116 16.8 4.3 32 mi. N of Joshua Tree WIDELY FELT 8/13 10:01 pm 34 36.0 116 17.0 2.2 " 8/14 12:22 am 34 35.7 116 16.6 2.2 " 8/14 12:22 am 34 6.9 116 56.6 2.0 6 mi. W of Mt. San Gorgonio 8/14 6:00 am 34 36.2 116 17.0 2.2 32 mi. N of Joshua Tree 8/14 8:04 am 32 37.7 116 6.9 2.4 10 mi. SW of Ocotillo 8/14 10:27 am 35 44.9 117 28.4 3.3 5 mi. WSW of Trona 8/14 11: 2 am 34 35.9 116 17.3 2.1 31 mi. N of Joshua Tree -------------------------------------------------------------