SOUTHERN CALIFORNIA SEISMOGRAPHIC NETWORK A Cooperative Project of Caltech and the U.S. Geological Survey January 23, 1997 Weekly Earthquake Report for Southern California ------------------------------------------------ January 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 818-395-6327. For daily updates, call our Earthquake Information Hotline: 818-395-6977. -------------------------------- This week's Report covers the time period from midnight Thursday morning, January 16, Universal Time (ie. GMT), to midnight Wednesday night, January 22, Universal Time. In local time, the period of coverage is from 4 p.m., January 15, Pacific Standard Time, to 4 p.m., January 22, Pacific Standard Time. We detected and processed 288 earthquakes during the seven-day period covered. The total number of events was slightly smaller than it was last week. There were four M3+ quakes this week, the largest being M3.4. That M3.4 occurred early Tuesday evening in the Grapevine area, about five miles north of the I5/99 interchange. This quake was within the aftershock zone of the largest quake in the southern half of California this century, the Mw7.5 Kern County quake of 1952. It is difficult to use the term "after- shock" after 45 years, but the seismicity rate in that area, the northern front of the Tehachapi Mountaings, is still above what it was before the 1952 mainshock. The other three M3+'s were: a M3.0 offshore of Point Sal last Wednesday night and a M3.1 near Yucca Valley Friday after- noon, and a M3.0 Simi Valley quake 33 minutes after the Yucca Valley one. The Yucca Valley M3.1 was a Landers aftershock and was felt in the town of Yucca Valley. Likewise, the Simi Valley event was a Northridge aftershock and was felt in Simi Valley. Also of interest was a small swarm of three quakes, the largest being M2.7, in the northern end of the Brawley Seismic Zone, which connects the northern end of the Imperial Fault with the southern end of the San Andreas. Although it has been relatively quiet in the last few years, the Brawley Seismic Zone is at times one of the most active areas in the state. For our readers who follow the weekly total numbers of quakes, we now summarize the results from the December holidays. The week of December 19 - 25 produced a total of 358 events, and the following week, December 26 - January 1, produced 342. As with the current week, the bulk of the seismicity consisted of Landers aftershocks, activity in the Ridgecrest and Coso areas, and a few Northridge aftershocks. We also have a brief summary of the quarterly statistics: Totals listed below apply to the area within a polygon bounded by the following latitude, longitude pairs: 35.0N, 121.0W; 37.0N, 119.0W; 37.0N, 115.0W; 32.5N, 115.0W; and 32.5N, 121.0W. 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 And we also have a brief summary of the three major aftershock sequences that were active in the fourth quarter of 1996: Northridge ---------- 1/17/94 - 12/31/96 Latitude 34.1 - 34.5 North, Longitude 118.3 - 118.85 West All events 13,523 (196 of them this quarter) 3.0 - 3.9 391 (2 of them this quarter) 4.0 - 4.9 51 (none this quarter) 5.0 and up 10 Landers/Big Bear ---------------- 4/23/92 - 12/31/96 Latitude 33.8 - 35.33 North, Longitude 116.0 - 117.05 West All events 65,380 (822 of them this quarter) 3.0 - 3.9 1,608 (8 of them this quarter) 4.0 - 4.9 169 (2 of them this quarter) 5.0 and up 22 Coso Range ---------- 11/27/96 - 12/31/96 Latitude 36.0 - 36.25 North, Longitude 117.5 - 117.75 West All events 1,782 3.0 - 3.9 19 4.0 - 4.9 0 5.0 and up 1 At the present rate of decay, we should expect four Northridge and 22 Landers aftershocks of M3.0 or larger, in the next year. The higher current rate of Landers aftershocks, relative to Northridge, is because Landers was a bigger earthquake. We are now out in the tails of both aftershock sequences, and the difference in time since the mainshock is less important than it was earlier in sequences. 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 ------------------------------------------------------------- 1/15 9:04 pm 35 12.3 119 1.6 2.4 11 mi. S of Bakersfield 1/15 10:58 pm 34 56.8 120 44.7 3.0 5 mi. NW of Pt. Sal 1/16 12:11 pm 36 4.0 117 38.8 2.2 16 mi. E of Coso Junction 1/16 3:49 pm 34 21.6 116 28.0 2.4 16 mi. N of Yucca Valley 1/16 7:10 pm 36 5.3 117 38.6 2.5 19 mi. SSW of Panamint Springs 1/16 10:32 pm 34 21.2 116 27.7 2.4 16 mi. N of Yucca Valley 1/17 12:16 am 36 4.4 117 39.5 2.6 16 mi. E of Coso Junction 1/17 6:25 am 36 2.5 117 48.2 2.5 8 mi. E of Coso Junction 1/17 12:32 pm 35 37.7 118 24.6 2.0 3 mi. E of town of Lake Isabella 1/17 2:56 pm 34 21.8 118 44.0 2.5 6 mi. NNW of Simi Valley 1/17 4:11 pm 34 9.5 116 25.5 3.1 2 mi. NNE of Yucca Valley FELT 1/17 4:12 pm 34 30.3 116 29.6 2.5 26 mi. N of Yucca Valley 1/17 4:44 pm 34 21.3 118 44.4 3.0 5 mi. NNW of Simi Valley FELT in Simi Valley 1/17 6:20 pm 34 24.1 119 42.5 2.0 1 mi. SSW of downtown Santa Barbara 1/17 11:47 pm 33 60.0 116 19.2 2.1 9 mi. S of Joshua Tree 1/18 1:10 am 34 21.3 118 44.2 2.0 5 mi. NNW of Simi Valley 1/18 8:07 am 36 5.2 117 39.9 2.2 16 mi. E of Coso Junction 1/18 9:27 am 36 5.3 117 39.2 2.1 " 1/18 3:15 pm 36 5.8 117 39.7 2.5 16 mi. ENE of Coso Junction 1/18 8:21 pm 34 37.8 116 30.6 2.0 27 mi. ENE of Lucerne Valley 1/18 11:45 pm 36 4.7 117 39.0 2.4 16 mi. E of Coso Junction 1/19 1:02 am 34 21.7 118 44.7 2.0 6 mi. NNW of Simi Valley 1/19 5:07 pm 32 39.3 115 54.7 2.6 7 mi. SE of Ocotillo 1/19 11:54 pm 34 42.9 118 45.9 2.2 7 mi. SE of Gorman 1/20 1:06 pm 35 46.9 117 38.0 2.1 11 mi. N of Ridgecrest 1/20 2:53 pm 34 36.9 116 40.0 2.0 19 mi. NE of Lucerne Valley 1/20 9:10 pm 36 4.7 117 39.6 2.0 16 mi. E of Coso Junction 1/21 6:01 am 36 4.8 117 39.5 2.5 " 1/21 6:59 am 33 59.5 116 59.4 2.6 3 mi. SE of Yucaipa 1/21 8:28 am 34 37.2 116 40.0 2.6 20 mi. NE of Lucerne Valley 1/21 6:40 pm 35 4.7 118 56.8 3.4 5 mi. N of Wheeler Ridge 1/22 1:35 pm 33 15.1 115 42.0 2.3 6 mi. SSE of Bombay Beach 1/22 1:42 pm 33 15.3 115 42.0 2.7 " 1/22 1:43 pm 33 15.3 115 42.1 2.5 " -------------------------------------------------------------