SOUTHERN CALIFORNIA SEISMOGRAPHIC NETWORK A Cooperative Project of Caltech and the U.S. Geological Survey January 8, 1998 Weekly Earthquake Report for Southern California ------------------------------------------------ December 31, 1997 - January 7, 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. 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, December 31 Universal Time (ie. UTC), to midnight Wednesday night, January 7, Universal Time. In local time, the period of coverage is from 4 p.m., December 30, Pacific Standard Time, to 4 p.m., January 7, Pacific Standard Time. We detected 306 earthquakes during the eight-day period covered. There were four clusters of activity going on this week, two of which produced a quake above M4. The most prolific was the swarm at Obsidian Butte in the Imperial Valley. This swarm started last week with several events in the M2+ range, and the largest, M4.1, was in the early morning of December 31. As we mentioned in last week's report, this swarm belongs to the Brawley Seismic Zone, which is a band of faulting that crosses the Imperial Valley in a north/south direction, connecting the northern end of the Imperial Fault with the southern end of the San Andreas. It has been a common source of activity since the late 1980's. It produced a large swarm including a M4.7 in 1989. The right-lateral plane of the focal mechanism for this week's M4.1 strikes just a few degrees west of north, which is a typical mechanism for Obsidian Butte earthquakes. Note that the Bombay Beach swarm from last week, at the northern tip of the Brawley Seismic Zone, produced only a few more small events this week, all smaller than M2. The largest event of the week, M4.3, occurred in the Chino area and was widely felt from Santa Monica to San Bernardino. This quake occurred on Monday morning. It's strike-slip focal mechanism and hypocenter of this quake make it consistent with the Whittier Fault. However, it is almost impossible to definitively associate such a small quake with a particular fault. There were a handful of aftershocks, the largest of which, about an hour and a half later, had a magnitude of M2.5. Interestingly, two days later, a M3.1 occurred only about 11 miles away, on the other side of the Chino Hills, near Brea. It was felt sharply in the epicentral area. Because of the dis- tance between the Chino and Brea quakes, the latter cannot be considered an aftershock. It did, however, have one aftershock of its own, which was felt, even though its magnitude was only M2.5. The M3.1 had an oblique thrust focal mechanism. The fourth cluster of interest was in the western San Fernando Valley, near Canoga Park. The largest event was a M3.3 in the wee hours of Sunday morning. This quake was felt by insomniacs as far away as Simi Valley and Torrance. Although the Canoga Park events were shallow and clearly above the main rupture of the Northridge quake (Mw6.7 January 17, 1994, in case you've forgotten), they are in a location that was active during the early aftershock sequence. That makes them Northridge aftershocks. Like many events in the hanging wall of the North- ridge quake, this week's M3.3 showed an oblique normal mechanism. 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 ------------------------------------------------------------- 12/30 6:01 pm 32 34.7 115 34.5 2.2 7 mi. SW of Calexico 12/31 4:19 am 33 11.4 115 36.6 2.0 2 mi. NE of Obsidian Butte 12/31 4:22 am 33 11.5 115 36.5 4.1 " 12/31 4:27 am 33 11.9 115 36.8 2.5 " 12/31 4:28 am 33 12.1 115 36.4 2.3 " 12/31 4:41 am 33 11.7 115 36.4 2.4 " 12/31 5:04 am 33 11.9 115 36.7 2.3 " 12/31 5:04 am 33 12.1 115 36.7 2.6 " 12/31 5:05 am 33 10.7 115 32.0 2.3 " 12/31 5:05 am 33 11.6 115 36.6 3.1 " 12/31 5:06 am 33 11.8 115 36.0 2.4 " 12/31 5:07 am 33 11.0 115 37.6 2.1 " 12/31 5:08 am 33 11.8 115 36.2 2.3 " 12/31 5:10 am 33 11.9 115 36.4 2.8 " 12/31 5:14 am 33 11.7 115 36.5 2.5 " 12/31 5:16 am 33 12.0 115 36.8 2.5 " 12/31 5:22 am 33 11.9 115 36.4 2.6 " 12/31 6:00 am 33 11.7 115 36.5 2.4 " 12/31 6:16 am 33 11.7 115 36.2 2.1 " 12/31 6:19 am 33 11.4 115 36.2 2.3 " 12/31 6:29 am 33 12.2 115 35.8 2.1 " 12/31 8:50 am 33 11.9 115 36.5 2.2 " 12/31 9:23 am 34 56.9 116 41.7 2.6 19 mi. ENE of Barstow 12/31 10:07 am 35 4.6 116 41.3 2.2 22 mi. SSE of Goldstone Lake 12/31 10:13 am 33 11.5 115 36.8 2.3 2 mi. NE of Obsidian Butte 12/31 2:32 pm 33 12.1 115 36.7 2.0 " 12/31 7:56 pm 36 14.1 120 49.5 2.9 26 mi. WNW of Coalinga 12/31 8:13 pm 32 52.3 117 40.0 2.1 23 mi. W of La Jolla 1/1 5:06 am 36 32.3 116 20.5 2.1 29 mi. ENE of Furnace Creek 1/1 12:37 pm 36 31.3 116 36.6 2.2 15 mi. ENE of Furnace Creek 1/1 8:54 pm 36 31.3 116 33.5 2.3 " 1/1 9:34 pm 34 11.8 118 38.5 2.2 2 mi. W of Canoga Park 1/1 11:06 pm 33 11.7 115 36.5 2.5 2 mi. NE of Obsidian Butte 1/2 1:07 am 36 41.3 116 12.4 2.2 39 mi. ENE of Furnace Creek 1/2 4:35 am 33 11.9 115 36.6 2.2 2 mi. NE of Obsidian Butte 1/2 12:20 pm 32 54.1 117 37.5 2.5 21 mi. W of La Jolla 1/3 4:09 pm 36 3.6 117 38.2 2.0 17 mi. E of Coso Junction 1/3 4:32 pm 34 20.1 116 28.3 2.3 15 mi. N of Yucca Valley 1/3 8:44 pm 32 38.3 117 18.3 2.1 10 mi. SSW of Mission Beach 1/4 1:11 am 34 12.0 118 38.3 3.3 1 mi. W of Canoga Park FELT 1/4 2:48 am 33 11.7 116 0.6 2.3 6 mi. SSW of Salton City 1/4 3:02 am 34 11.3 118 38.3 2.0 2 mi. WSW of Canoga Park 1/4 3:45 am 34 0.7 116 19.2 2.0 8 mi. S of Joshua Tree 1/4 2:04 pm 36 1.8 117 47.9 2.7 8 mi. E of Coso Junction 1/5 5:27 am 33 13.3 115 34.0 2.5 2 mi. NW of Obsidian Butte 1/5 6:50 am 33 11.8 115 36.7 2.1 " 1/5 10:14 am 33 57.0 117 42.5 4.3 5 mi. SSW of Chino WIDELY FELT 1/5 10:16 am 33 56.6 117 43.0 2.0 " 1/5 11:43 am 33 56.6 117 43.0 2.5 " 1/5 8:11 pm 33 56.7 117 42.8 2.3 " 1/6 5:03 am 33 35.1 117 56.8 2.0 2 mi. SSW of Newport Beach 1/6 6:31 pm 33 33.7 116 24.0 2.1 11 mi. S of Palm Desert 1/6 7:16 pm 32 7.9 116 40.1 2.5 36 mi. SE of San Ysidro 1/6 10:40 pm 33 13.6 115 33.7 2.3 2 mi. NW of Obsidian Butte 1/6 11:19 pm 32 18.1 115 14.7 3.7 29 mi. SSE of Calexico 1/7 1:16 am 33 55.0 118 31.1 2.2 5 mi. WSW of Los Angeles Airport 1/7 9:49 am 33 53.3 117 52.7 3.1 2 mi. SSE of Brea; FELT 1/7 10:16 am 33 53.3 117 52.7 2.5 " ; FELT 1/7 1:55 pm 33 11.1 115 36.3 2.0 2 mi. EN of Obsidian Butte 1/7 3:51 pm 33 11.7 115 36.7 3.0 " 1/7 3:52 pm 33 11.6 115 36.2 3.4 " 1/7 3:53 pm 33 11.7 115 36.2 2.3 " -------------------------------------------------------------