SOUTHERN CALIFORNIA SEISMOGRAPHIC NETWORK A Cooperative Project of Caltech and the U.S. Geological Survey December 31, 1997 Weekly Earthquake Report for Southern California ------------------------------------------------ December 18 - 30, 1997 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 626-395-6327. For daily updates, call our Earthquake Information Hotline: 626-395-6977. -------------------------------- ****************************************************************** As many of you know by now, the U.S. Geological Survey in Menlo Park, the office which originates the Northern California Earthquake Report, is planning to discontinue the paper (and faxed) Report at the end of January 1998. We plan to follow their initiative. As of the beginning of February 1998, we will be replacing our weekly 'treeware' product with a Web site, which will include the same map, table, and commentary as the current Weekly Report, plus other features. The Web page will be updated more often than once per week. There will be no paper copy or usenet distribution. We hope we are giving our Users sufficient notice to obtain World Wide Web access for themselves. At minimum, most public libraries now provide Web access, along with the capability to print copies of Web pages. ****************************************************************** This week's Report covers the time period from midnight Thursday morning, December 18 Universal Time (ie. UTC), to midnight Wednesday night, December 30, Universal Time. In local time, the period of coverage is from 4 p.m., December 17, Pacific Standard Time, to 4 p.m., December 30, Pacific Standard Time. Because of the Christmas holiday last week, we skipped a Report and will be covering almost two weeks in this one. We detected 379 earthquakes during the thirteen-day period covered. Next week we will revert to the regular schedule until the end of January. These Weekly Reports should not be used for archival purposes. Further data processing often changes the magnitudes slightly and, especially during times of high seismicity, sometimes changes the estimates of total seismicity. For the best earthquake catalog currently available, see: http://scec.gps.caltech.edu/catalog-search.html There were three M3+ quakes this week, two of which were in the northern Imperial Valley. The other one was a M3.7 in the Inyo Mountains east of the Owens Valley, late at night on Thursday the 18th. Since this is an extremely sparsely populated area, no-one reported feeling it. The Imperial Valley activity occurred at two sites under the Salton Sea. Two M3+ events occurred on Friday (M3.6) and Saturday (M3.2) near Bombay Beach, 4 miles south of the southern end of the San Andreas fault. Several M2+ events (and a M4.1 this morning that will be described in detail next week) were located at the southern end of the Salton Sea at Obsidian Butte, otherwise known as the Salton Sea Wildlife Preserve. Both of these locations are in a grouping of earthquakes called the Brawley Seismic Zone (BSZ). This BSZ links the northen end of the Imperial Fault with the southern end of the San Andreas. Individual faults in this zone can not be mapped because of the thick sediments of the Imperial Valley. However, the BSZ was one of the most active sites in California through the 1970's and 1980's. For the last five years, since the 1992 Landers earth- quake (Mw7.3), most of the zone has been much quieter than in the past. The one exception has been the Obsidian Butte site. A large swarm that included a M4.7 earthquake occurred at this site in 1989. Ever since then, Obsidian Butte has been a common source of M2 and M3 events. This week's M3 events near Bombay Beach are more uncommon. The Brawley Seismic Zone ends at the San Andreas fault and this week's events are only 4 miles from the San Andreas. The last time a M3 event occurred this far north in the Brawley Seismic Zone was a M3.8 earthquake in April 1986. 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 ------------------------------------------------------------- 12/17 8:22 pm 33 29.4 116 30.1 2.6 10 mi. ESE of Anza 12/18 12:35 am 36 12.8 120 46.3 2.8 23 mi. WNW of Coalinga 12/18 11:07 pm 36 27.4 117 39.5 3.7 22 mi. ENE of Olancha 12/19 4:52 am 36 7.6 117 41.2 2.0 15 mi. ENE of Coso Junction 12/19 10:23 pm 34 1.6 117 29.9 2.0 6 mi. SSW of Fontana 12/21 4:34 am 36 7.7 117 41.0 2.3 15 mi. ENE of Coso Junction 12/21 7:20 am 34 3.0 118 58.9 2.1 2 mi. W of Leo Carrillo State Beach 12/21 8:41 am 34 5.9 117 0.2 2.3 4 mi. NNE of Yucaipa 12/22 6:45 am 32 0.3 116 21.9 3.2 54 mi. SSW of Ocotillo 12/22 10:47 am 36 5.3 117 50.7 2.0 6 mi. ENE of Coso Junction 12/22 8:49 pm 36 12.9 117 51.2 2.1 9 mi. ESE of Olancha 12/23 12:56 pm 33 29.0 116 30.3 2.1 10 mi. ESE of Anza 12/24 7:47 am 36 4.8 117 38.2 2.2 17 mi. E of Coso Junction 12/24 8:46 am 33 43.8 119 10.9 2.7 19 mi. NNW of Santa Barbara Is. 12/24 6:29 pm 36 26.5 117 52.5 2.6 13 mi. NNE of Olancha 12/25 7:16 am 33 21.0 116 26.3 2.0 7 mi. NNW of Borrego Springs 12/26 3:53 am 34 5.8 116 60.0 2.2 4 mi. NNE of Yucaipa 12/26 3:54 am 34 5.8 116 59.8 2.1 " 12/26 7:05 am 33 19.2 115 41.3 3.6 3 mi. SE of Bombay Beach 12/26 8:33 pm 34 0.0 117 34.6 2.7 6 mi. SE of Ontario 12/27 2:55 am 35 55.9 117 13.4 2.2 14 mi. NE of Trona 12/27 6:02 am 33 59.8 117 34.8 2.3 6 mi. SE of Ontario 12/27 8:42 am 33 19.1 115 41.0 2.0 3 mi. SE of Bombay Beach 12/27 8:56 am 33 19.1 115 41.6 3.2 " 12/27 8:58 am 33 19.3 115 41.4 2.0 " 12/27 8:58 am 33 18.6 115 41.1 2.4 " 12/27 9:13 am 33 18.6 115 41.5 2.3 " 12/27 9:14 am 33 19.0 115 41.6 2.1 " 12/27 9:34 am 34 25.9 120 53.9 2.3 17 mi. SW of Pt. Arguello 12/27 9:50 am 33 9.9 115 39.0 2.9 Under Obsidian Butte 12/27 10:01 am 33 10.1 115 38.8 2.0 " 12/27 11:40 am 33 9.9 115 39.3 2.3 " 12/27 12:38 pm 33 18.8 115 41.6 2.3 3 mi. SE of Bombay Beach 12/27 12:46 pm 33 19.1 115 41.5 2.1 " 12/28 12:12 am 33 11.0 116 2.6 2.8 5 mi. ENE of Ocotillo Wells 12/28 9:37 am 32 19.8 115 13.6 2.5 28 mi. SE of Calexico 12/28 11:45 am 34 5.7 116 60.0 2.0 4 mi. NNE of Yucaipa 12/28 2:41 pm 36 29.0 117 37.8 2.1 25 mi. NE of Olancha 12/29 12:28 am 33 59.4 116 42.5 2.1 6 mi. NE of Cabazon 12/29 2:29 am 33 8.2 116 28.1 2.2 8 mi. ENE of Julian 12/29 5:23 am 35 55.2 118 1.3 2.5 9 mi. SSW of Coso Junction 12/29 5:53 am 32 36.2 115 19.3 2.9 11 mi. ESE of Calexico 12/29 11:21 am 36 4.7 117 38.7 2.4 17 mi. E of Coso Junction 12/29 12:23 pm 35 7.2 116 39.7 2.0 20 mi. SE of Goldstone Lake 12/29 12:48 pm 36 4.7 117 38.7 2.4 17 mi. E of Coso Junction 12/29 3:33 pm 32 58.8 115 55.6 2.2 17 mi. NNE of Ocotillo -------------------------------------------------------------