SOUTHERN CALIFORNIA SEISMOGRAPHIC NETWORK A Cooperative Project of Caltech and the U.S. Geological Survey September 21, 1995 Weekly Earthquake Report for Southern California ------------------------------------------------ September 14 - 20, 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, September 14, Pacific Daylight Time, to midnight Wednesday night, September 20, 1995, Pacific Daylight Time. We recorded 614 earthquakes during the 7-day period covered. The most excitement for the week was clearly the ML5.8 Ridgecrest quake which happened yesterday afternoon. It was felt widely as far away as the L.A. area, with a few scattered inquiries from Las Vegas and even San Diego. Like the similar M5.4 event on August 17, '95, it had a focal mechanism that showed a combination of strike-slip and normal (extentional) faulting. A preliminary look at the data suggests that it may have been on the same fault as the August quake, rupturing an adjacent patch of fault to the southeast and slightly shallower. This activity is simliar to the earthquakes that occurred in the same region in 1981-1982, which was the last time we saw significant earthquakes near China Lake. In that sequence, a "cluster" of earthquakes continued on and off for more than a year. The earthquakes in that cluster migrated over several different faults around Indian Wells Valley. This month's earthquakes are larger than any seen in 1982 (the largest then was M4.9) and are located to the east of the 1982 events. We received many questions about whether it was an "aftershock" of the August event or not. Actually, we could call the August quake a foreshock of the ML5.8, since it was smaller, but the term "foreshock" is usually reserved for earthquakes closer in time (less than 3-5 days). We could also call the two M5+ events a pair of mainshocks. The point is that these two quakes are close enough in time and space that they must be related. The terms foreshock, mainshock and aftershock are semantic terms created by humans, used to describe a wide range of possible earthquakes. Given the patterns observed in this region (i.e., the long cluster of earthquakes in several locations in 1981-1982), we have decided to call both of these two events mainshocks, and when needed explain that the first was a preshock of the second. A "preshock" occurs further back in time than a foreshock, like the 1992 M6.1 Joshua Tree "preshock", two months before the M7.3 Landers earthquake. The M5.8 mainshock of September 20 is generating its own aftershock sequence. The rate of aftershocks has already begun to decrease as we expect. Eighteen hours after the mainshock, we see an aftershock sequence close to the average for California. Given this, an aftershock larger than than the largest we have seen so far (a M4.1 on just after midnight on Thursday morning) would not be surprising. It is always problematic when a large quake occurs right before the Weekly Report is issued. We have not finished the data processing of the aftershocks, so the final weekly total will be somewhat larger than the 614 included here. We believe, however, that we have a good handle on everything about M2.5. Some revisions are likely, however. The table lists the quakes this week that were M2.0 or larger in the central part of the coverage area, except for the Ridgecrest aftershocks since the ML5.8, which must be M2.5 or larger to be listed. 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. The second figure shows a map view and cross section of the Ridgrest activity so far. The two stars are the M5+ quakes; the more southeasterly and shallower one is this week's earthquake. Table 1 ------- Date Time N Lat. W Long. Mag ------------------------------------------------------------- 9/14 12:29 am 32 48.4 115 42.2 2.9 8 mi. W of El Centro 9/14 3:59 am 35 48.0 117 38.2 3.4 12 mi. N of Ridgecrest 9/14 11:59 am 33 18.7 118 18.0 2.3 2 mi. SSE of Avalon, Catalina Island 9/14 12:09 pm 34 22.3 116 28.4 2.0 17 mi. N of Yucca Valley 9/14 2:45 pm 35 45.7 117 38.7 2.1 9 mi. N of Ridgecrest 9/14 3:33 pm 33 34.3 117 57.0 2.3 3 mi. SSW of Newport Beach 9/14 5:06 pm 35 44.8 117 38.2 2.5 8 mi. N of Ridgecrest 9/14 5:57 pm 34 44.3 120 2.5 2.1 11 mi. NNE of Solvang 9/14 9:14 pm 34 44.6 116 42.8 2.0 20 mi. ESE of Barstow 9/15 12:22 am 35 47.3 117 38.5 2.1 11 mi. N of Ridgecrest 9/15 4:29 am 35 46.4 117 39.2 2.0 10 mi. N of Ridgecrest 9/15 9:43 pm 34 2.5 116 19.2 2.1 6 mi. S of Joshua Tree 9/15 9:56 pm 34 9.6 116 25.8 2.3 3 mi. NNE of Yucca Valley 9/16 12:53 am 33 10.6 115 39.2 2.9 1 mi. WNW of Obsidian Butte 9/16 5:53 am 32 2.2 116 18.7 2.1 51 mi. SSW of Ocotillo 9/16 9:54 am 33 58.8 116 40.2 2.0 6 mi. SW of Morongo Valley 9/16 10:48 am 34 1.9 116 19.2 2.4 7 mi. S of Joshua Tree 9/16 12:26 pm 32 1.0 116 15.1 2.7 51 mi. SSW of Ocotillo 9/16 2:08 pm 35 45.7 117 38.7 2.6 9 mi. N of Ridgecrest 9/16 3:16 pm 35 47.2 117 39.3 2.1 11 mi. N of Ridgecrest 9/16 3:22 pm 35 45.5 117 38.6 2.1 9 mi. N of Ridgecrest 9/16 4:13 pm 35 47.0 117 39.4 2.3 11 mi. N of Ridgecrest 9/16 5:54 pm 33 57.3 116 58.3 2.1 1 mi. N of Beaumont 9/16 6:34 pm 32 24.9 115 24.9 2.8 18 mi. SSE of Calexico 9/16 7:48 pm 32 27.2 115 25.6 3.2 15 mi. SSE of Calexico 9/16 8:07 pm 34 48.4 118 58.0 2.0 1 mi. SW of Frazier Park 9/16 10:35 pm 33 51.7 117 3.1 2.2 6 mi. SW of Beaumont 9/17 12:21 am 33 18.5 116 18.5 2.5 5 mi. NE of Borrego Springs 9/17 3:55 am 33 54.6 116 17.5 2.2 14 mi. NNW of Indio 9/17 9:20 am 33 56.4 116 20.1 2.2 9 mi. E of Desert Hot Springs 9/17 10:59 am 35 7.2 118 33.3 2.0 6 mi. W of Tehachapi 9/17 10:59 am 32 7.4 116 39.8 2.5 36 mi. SE of San Ysidro 9/17 11:35 am 34 22.1 118 40.2 2.0 6 mi. NNE of Simi Valley 9/17 12:30 pm 32 18.5 115 13.6 2.9 29 mi. SSE of Calexico 9/17 5:28 pm 35 47.7 117 37.9 2.0 12 mi. N of Ridgecrest 9/18 3:58 am 33 3.5 116 30.0 2.2 5 mi. ESE of Julian 9/18 5:58 am 35 45.7 117 38.8 2.4 9 mi. N of Ridgecrest 9/18 2:37 pm 35 46.5 117 38.8 2.0 10 mi. N of Ridgecrest 9/18 3:06 pm 35 47.4 117 39.5 2.1 11 mi. N of Ridgecrest 9/18 3:38 pm 35 47.4 117 39.2 2.2 " 9/18 11:26 pm 35 46.8 117 38.2 2.3 " 9/19 12:41 am 33 1.3 117 45.4 2.1 24 mi. WSW of Oceanside 9/19 1:49 am 34 14.8 118 41.2 2.0 2 mi. SSE of Simi Valley 9/19 7:49 am 35 22.2 118 30.9 2.6 17 mi. NNW of Tehachapi 9/19 10:22 am 34 9.8 116 25.5 2.0 3 mi. NNE of Yucca Valley 9/19 12:34 pm 34 31.7 118 46.7 2.5 9 mi. W of Castaic Lake dam 9/19 1:10 pm 35 47.8 117 37.9 2.4 12 mi. N of Ridgecrest 9/19 1:20 pm 35 47.2 117 39.8 2.5 11 mi. N of Ridgecrest 9/19 1:23 pm 32 58.2 117 48.0 2.1 28 mi. WSW of Oceanside 9/19 3:24 pm 34 59.0 116 58.7 2.0 6 mi. NNE of Barstow 9/19 6:39 pm 32 37.4 115 54.9 2.6 8 mi. SSE of Ocotillo 9/20 12:13 am 34 15.1 116 26.2 2.3 9 mi. N of Yucca Valley 9/20 1:21 am 34 15.2 116 26.0 2.0 " 9/20 1:48 am 34 20.7 116 28.1 2.0 15 mi. N of Yucca Valley 9/20 3:03 am 34 0.9 117 0.8 2.2 1 mi. SE of Yucaipa 9/20 3:44 am 35 46.0 117 38.9 2.3 10 mi. N of Ridgecrest 9/20 9:38 am 35 47.0 117 38.0 2.0 11 mi. N of Ridgecrest 9/20 10:54 am 35 47.5 117 39.1 2.0 " 9/20 4:27 pm 35 45.6 117 38.3 5.8 9 mi. N of Ridgecrest VERY WIDELY FELT 9/20 4:30 pm 35 43.8 117 36.7 3.7 8 mi. NNE of Ridgecrest 9/20 4:32 pm 35 45.3 117 37.7 3.9 9 mi. NNE of Ridgecrest 9/20 4:37 pm 35 46.4 117 39.8 3.6 10 mi. N of Ridgecrest 9/20 4:53 pm 35 45.3 117 39.2 3.1 " 9/20 4:55 pm 35 46.2 117 39.4 2.8 10 mi. N of Ridgecrest 9/20 4:55 pm 35 46.4 117 39.1 2.9 " 9/20 4:56 pm 35 46.8 117 39.9 3.9 " 9/20 5:16 pm 35 46.5 117 38.4 3.4 " 9/20 5:31 pm 35 45.1 117 39.7 3.2 8 mi. N of Ridgecrest 9/20 5:33 pm 35 46.5 117 39.7 2.5 10 mi. N of Ridgecrest 9/20 5:45 pm 35 45.1 117 39.0 2.7 8 mi. N of Ridgecrest 9/20 5:45 pm 35 45.1 117 38.9 2.6 9 mi. N of Ridgecrest 9/20 5:46 pm 35 46.7 117 38.9 2.7 10 mi. N of Ridgecrest 9/20 5:46 pm 35 46.6 117 39.0 2.8 " 9/20 6:06 pm 35 44.8 117 38.0 2.9 8 mi. NNE of Ridgecrest 9/20 6:13 pm 35 45.0 117 38.3 3.0 8 mi. N of Ridgecrest 9/20 7:07 pm 35 45.8 117 39.0 2.5 9 mi. N of Ridgecrest 9/20 7:13 pm 35 44.7 117 38.6 2.9 8 mi. N of Ridgecrest 9/20 7:16 pm 35 46.1 117 39.0 3.1 10 mi. N of Ridgecrest 9/20 7:17 pm 35 46.8 117 39.3 2.5 " 9/20 7:48 pm 35 44.5 117 37.7 2.7 8 mi. NNE of Ridgecrest 9/20 8:09 pm 35 46.0 117 39.0 3.4 10 mi. N of Ridgecrest 9/20 8:30 pm 35 46.2 117 39.2 2.7 " 9/20 8:51 pm 35 45.6 117 38.2 2.6 9 mi. N of Ridgecrest 9/20 8:56 pm 35 44.2 117 37.6 2.5 8 mi. NNE of Ridgecrest 9/20 9:23 pm 35 45.8 117 39.3 3.1 9 mi. N of Ridgecrest 9/20 9:28 pm 35 43.7 117 37.6 2.5 7 mi. NNE of Ridgecrest 9/20 9:50 pm 35 43.8 117 37.5 2.6 " 9/20 10:10 pm 35 46.0 117 38.7 3.1 10 mi. N of Ridgecrest 9/20 10:27 pm 35 46.2 117 39.3 2.7 " 9/20 10:57 pm 35 45.3 117 38.9 2.7 9 mi. N of Ridgecrest -------------------------------------------------------------