SOUTHERN CALIFORNIA SEISMOGRAPHIC NETWORK A Cooperative Project of Caltech and the U.S. Geological Survey October 23, 1997 Weekly Earthquake Report for Southern California ------------------------------------------------ October 16 - 22, 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. -------------------------------- ***************************************************************** If you receive this Report by FAX and if you have recently had a change of AREA CODE, please notify us ASAP. We have no other way to know which fax numbers need to be changed!! Actually, if your area code is 818 and it HASN'T changed, it would be helpful to know that, too. ***************************************************************** This week's Report covers the time period from midnight Thursday morning, October 16, Universal Time (ie. UTC), to midnight Wednesday night, October 22, Universal Time. In local time, the period of coverage is from 5 p.m., October 15, Pacific Daylight Time, to 5 p.m., October 22, Pacific Daylight Time. We detected 216 earthquakes during the seven-day period covered. 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 four M3+ events this week. The largest was offshore, between Santa Barbara Is. and Catalina. It occurred on Saturday afternoon, and had a magnitude of 3.4. We received no inquiries. Due to the sparsity of seismic stations in the area (confined to the islands), the focal mechanism is difficult to determine. There were two quake felt. The first was a M3.0 North- ridge aftershock last Thursday morning. It was felt in the San Fernando Valley. Unlike most Northridge aftershocks, this one had a strike-slip mechanism. It was relatively shallow (4 kilometers depth) and probably represents deformation of the hanging wall above the mainshock's fault. A M3.0 Landers aftershock occurred early Wednesday morning; it was felt in the Yucca Valley and Landers area. Unlike most Landers aftershocks, but similar to some of the background activity at this location, this event had a normal mechanism. There was one more M3+ event, in the early hours of last Thursday. It was a M3.1 deep (12 kilometers) below the San Gorgonio Pass, near Cabazon. We received no inquiries about that one either, so we presume it was not felt. This quake also had a normal focal mechanism. Following is our routine quarterly summary of the seismic activity. Note that, due to the fact that it had a significantly smaller mainshock (Mw6.7 compared to Mw7.3), the Northridge seismicity rate is now much lower than the Landers seismicity rate, even though the Northridge quake happened more recently. 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 1977 3,699 39 3,276 54 3,193 31 Northridge ---------- 1/17/94 - 9/30/97 All events 14,402 (198 of them this quarter) 3.0 - 3.9 409 (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 - 9/30/97 All events 67,535 (689 of them this quarter) 3.0 - 3.9 1,632 (1 of them this quarter) 4.0 - 4.9 171 (2 of them this quarter) 4.1 on Sept. 19 4.4 on Sept. 28 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 ------------------------------------------------------------- 10/15 8:49 pm 34 15.1 117 40.4 2.2 2 mi. SSW of Mt. Baldy 10/16 2:05 am 32 52.7 115 41.7 2.5 10 mi. NW of El Centro 10/16 2:37 am 34 1.2 116 44.9 3.1 7 mi. NNE of Cabazon 10/16 5:59 am 33 53.5 119 36.9 2.3 11 mi. SSW of Santa Cruz Is. (E end) 10/16 7:17 am 35 47.5 117 38.7 2.0 11 mi. N of Ridgecrest 10/16 9:02 am 34 13.4 118 37.2 3.0 1 mi. NNW of Canoga Park FELT 10/17 7:43 pm 32 19.4 115 11.9 2.8 29 mi. SE of Calexico 10/17 10:09 pm 33 11.3 115 34.0 2.3 4 mi. SW of Niland 10/18 6:19 am 33 21.0 116 26.2 2.1 7 mi. NNW of Borrego Springs 10/18 1:57 pm 33 25.1 118 44.6 3.4 18 mi. ESE of Santa Barbara Is. 10/18 9:18 pm 33 48.5 119 17.4 2.3 20 mi. SE of Santa Cruz Is. (E end) 10/18 9:20 pm 33 56.1 116 1.4 2.0 14 mi. S of Twentynine Palms 10/18 9:38 pm 34 59.2 116 57.2 2.7 7 mi. NNE of Barstow 10/19 12:37 am 33 37.9 116 42.5 2.1 5 mi. NNW of Anza 10/21 4:15 am 33 15.8 116 18.3 2.1 3 mi. E of Borrego Springs 10/21 9:48 am 33 11.5 115 36.2 2.0 2 mi. NE of Obsidian Butte 10/22 7:09 am 34 18.1 116 27.1 3.0 12 mi. N of Yucca Valley FELT 10/22 8:42 am 33 58.3 116 35.1 2.0 5 mi. S of Morongo Valley 10/22 11:14 am 32 43.0 118 4.9 2.5 16 mi. ESE of San Clemente Is. (SE tip) -------------------------------------------------------------