SOUTHERN CALIFORNIA SEISMOGRAPHIC NETWORK A Cooperative Project of Caltech and the U.S. Geological Survey January 15, 1998 Weekly Earthquake Report for Southern California ------------------------------------------------ January 8 - 14, 1998 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. You will be able to get to this web site form the following home pages: http://www.gps.caltech.edu/seismo/ http://www-socal.wr.usgs.gov/ http://www.scec.org/ http://www.trinet.org/ (The last one does not exist yet, but will soon.) 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 Wednesday morning, January 8, Universal Time (ie. UTC), to midnight Wednesday night, January 15, Universal Time. In local time, the period of coverage is from 4 p.m., January 7, Pacific Standard Time, to 4 p.m., January 15, Pacific Standard Time. We detected 226 earthquakes during the seven-day period covered. The swarm at the southern end of the Salton Sea continued this week, but it is no longer confined to the area right around Obsidian Butte. In particular, it has spread to the northeast, nearer to the small town of Niland. The easternmost of these events are on trend with the San Andreas Fault; however, the San Andreas is not considered active south of its juncture with the Brawley Seismic Zone, which is at Bombay Beach twelve miles to the northwest of the swarm. The largest events were a M3.2 on Monday morning and a M3.8 late Monday afternoon. The focal mechanisms show oblique normal faulting, indicating tension in the crust. This sort of mechanism has been seen in that area before and is, in fact, expected, considering the known motion on the San Andreas to the north and on the Brawley Seismic Zone to the west. We are not getting inquiries about these events from the public, but the M3+ events are probably being felt in the epicentral area. We continued to have events in the Chino Hills and Brea area, aftershocks of last week's M4.3 Chino quake and M3.1 Brea quakes. The largest this week, however, was only M2.5 and apparently not felt. San Fernando Valley saw a little bit of action this week, also, with a M3.3 under Van Nuys on Sunday evening. Partly because of the hour that it happened, this event was fairly widely felt in the Valley and in the Hollywood area. (Any earthquake that happens during the news broadcast automatically gets more attention than its magnitude might warrant!) Focal mechanism was stike-slip. Note that this event occurred well to the southeast of the Northridge aftershock zone, so it should not be considered an aftershock. Finally, it's time for the quarterly statistics. Note that we just finished the quietest quarter since before the Landers sequence began in 1992. 1st quarter 2nd quarter 3rd quarter 4th quarter all 3.0+ all 3.0+ all 3.0+ all 3.0+ 1986 3,040 16 2,865 19 6,634 192 2,300 38 1987 1,679 24 1,887 36 2,341 38 5,748 198 1988 2,749 40 2,128 45 2,195 30 2,208 27 1989 2,528 46 2,340 21 2,323 19 2,335 26 1990 2,850 56 2,293 43 1,859 27 2,039 31 1991 1,679 10 1,843 32 2,081 21 2,337 27 1992 3,389 37 10,084 741 25,849 808 9,775 181 1993 4,799 41 5,152 58 5,039 46 4,419 34 1994 10,735 418 5,766 42 5,221 48 4,813 39 1995 3,639 28 3,788 37 8,252 88 7,733 49 1996 5,735 56 3,966 32 3,690 23 4,660 50 1997 3,699 39 3,276 54 3,193 31 2,443 27 Northridge ---------- 1/17/94 - 12/31/97 All events 14,532 (130 of them this quarter) 3.0 - 3.9 413 (4 of them this quarter) 4.0 - 4.9 52 (none this quarter) 5.0 and up 11 (none this quarter) Landers/Big Bear ---------------- 4/23/92 - 12/31/97 All events 68,282 (646 of them this quarter) 3.0 - 3.9 1,637 (5 of them this quarter) 4.0 - 4.9 172 (1 of them this quarter) 4.1 on Dec. 5 5.0 and up 23 (none this quarter) 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 ------------------------------------------------------------- 1/7 4:12 pm 33 12.1 115 36.5 2.3 2 mi. NE of Obsidian Butte 1/8 2:42 am 36 26.4 117 53.3 2.3 12 mi. NNE of Olancha 1/8 4:54 am 32 46.8 115 34.4 2.1 1 mi. SW of El Centro 1/8 6:57 am 33 11.5 115 36.3 2.2 2 mi. NE of Obsidian Butte 1/8 8:37 am 33 11.0 115 36.4 2.5 " 1/8 3:54 pm 33 52.9 117 52.9 2.4 2 mi. SE of Brea 1/8 10:35 pm 33 53.5 117 52.5 2.5 " 1/8 10:35 pm 33 53.8 117 51.5 2.2 " 1/10 9:44 am 33 56.9 117 42.8 2.0 5 mi. SSW of Chino 1/11 5:51 am 32 6.2 115 46.7 2.7 42 mi. SSW of Calexico 1/11 8:48 pm 34 23.0 116 28.0 2.1 18 mi. N of Yucca Valley 1/11 10:36 pm 34 11.2 118 27.9 3.3 Under Van Nuys FAIRLY WIDELY FELT 1/11 10:50 pm 34 35.0 118 55.6 2.0 12 mi. N of Fillmore 1/11 11:08 pm 33 56.3 116 1.4 2.0 13 mi. S of Twentynine Palms 1/12 4:36 am 33 14.1 115 33.6 2.3 2 mi. W of Niland 1/12 4:54 am 34 34.2 116 32.4 2.3 24 mi. ENE of Lucerne Valley 1/12 8:00 am 33 14.0 115 34.1 3.2 2 mi. W of Niland 1/12 8:01 am 33 14.1 115 33.9 2.7 " 1/12 8:02 am 33 14.9 115 33.4 2.2 " 1/12 8:03 am 33 14.6 115 33.7 2.0 " 1/12 8:11 am 33 13.9 115 33.4 2.0 " 1/12 8:55 am 33 14.6 115 33.9 2.0 " 1/12 10:06 am 33 14.2 115 33.9 2.7 " 1/12 10:10 am 33 14.7 115 33.4 2.1 " 1/12 12:48 pm 33 13.9 115 33.0 2.5 " 1/12 1:12 pm 33 13.8 115 33.6 2.5 " 1/12 2:45 pm 35 55.7 117 13.1 2.0 13 mi. NE of Trona 1/12 3:11 pm 33 13.6 115 33.5 2.6 2 mi. W of Niland 1/12 5:09 pm 33 14.5 115 34.1 3.8 " 1/12 5:12 pm 33 14.2 115 33.2 2.7 " 1/12 5:16 pm 33 14.6 115 33.7 2.2 " 1/12 5:21 pm 35 40.9 117 36.2 2.6 5 mi. NE of Ridgecrest 1/12 5:21 pm 33 14.3 115 33.8 2.7 2 mi. W of Niland 1/12 5:28 pm 33 14.7 115 33.8 2.8 " 1/12 5:49 pm 33 13.3 115 33.5 2.0 " 1/12 6:02 pm 33 14.7 115 33.7 2.2 " 1/12 6:02 pm 33 14.0 115 33.5 2.5 " 1/12 7:37 pm 35 53.0 117 21.8 2.9 8 mi. N of Trona 1/12 8:37 pm 33 15.0 115 33.5 2.0 2 mi. WNW of Niland 1/13 10:37 am 32 33.3 115 34.0 2.3 8 mi. SSW of Calexico 1/13 1:16 pm 35 1.3 118 57.0 2.4 1 mi. NNW of Wheeler Ridge 1/13 7:54 pm 34 49.4 117 7.9 2.5 7 mi. SW of Barstow 1/13 10:33 pm 32 2.5 116 13.4 2.6 49 mi. SSW of Ocotillo 1/14 12:24 am 33 12.1 115 34.4 2.2 4 mi. SW of Niland 1/14 8:41 am 35 4.2 118 24.4 2.1 4 mi. SSE of Tehachapi 1/14 11:14 am 33 10.7 115 36.5 2.1 1 mi. ENE of Obsidian Butte 1/14 1:46 pm 35 48.3 118 1.5 2.5 17 mi. SSW of Coso Junction -------------------------------------------------------------