SOUTHERN CALIFORNIA SEISMOGRAPHIC NETWORK A Cooperative Project of Caltech and the U.S. Geological Survey April 10, 1997 Weekly Earthquake Report for Southern California ------------------------------------------------ April 3 - 9, 1997 Prepared by: Nick Scheckel, Seismological Laboratory Debbie Wells, Seismological Laboratory (kate@bombay.gps.caltech.edu) Jim Mori, 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, April 3, Universal Time (ie. GMT), to midnight Wednesday night, April 9, Universal Time. In local time, the period of coverage is from 4 p.m., April 2, Pacific Standard Time, to 5 p.m., April 9, Pacific Daylight Time. We detected and processed 286 earthquakes during the seven-day period covered. This week we had two earthquakes greater than M3.0 and both were felt. The first, which was felt moderately throughout the Los Angeles Basin, was a M3.3 near Inglewood early Friday morning. It occurred near the Newport-Inglewood fault and with the focal mechanism, we inferred right-lateral strike-slip faulting with a strike of 40 degrees west of north. It had two felt aftershocks, a M2.4 about 9 minutes later and a M2.5 the following morning. The second, a M3.2, was near Mt. Palomar, in northern San Diego County. It occurred early Sunday morning near the Agua Caliente Fault, an offshoot of the Elsinore Fault. Below we 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 1997 3,699 39 Brief summaries of the two largest aftershock sequences. Northridge (M6.7, January 17, 1994) ---------- 1/17/94 - 3/31/97 Latitude 34.1 - 34.5 North, Longitude 118.3 - 118.85 West All events 13,685 (162 this quarter) 3.0 - 3.9 393 ( 2 this quarter) 4.0 - 4.9 51 ( 0 this quarter) 5.0 and up 10 ( 0 this quarter) Landers/Big Bear (M7.3/M6.4, June 28, 1992) ---------------- 4/23/92 - 3/31/97 Latitude 33.8 - 35.33 North, Longitude 116.0 - 117.05 West All events 66,131 (751 this quarter) 3.0 - 3.9 1,617 ( 9 this quarter) 4.0 - 4.9 169 ( 0 this quarter) 5.0 and up 23 ( 1 this quarter) The current rate of Landers aftershocks is higher than Northridge aftershocks, even though it has been a longer time since the Landers mainshock. We are in the "tails" of both aftershock sequences, so that the higher rate for the Landers sequence is because of the greater size of its mainshock. 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 ------------------------------------------------------------- 4/ 2 11:30 pm 34 26.3 120 43.3 2.7 10 mi. SSW of Pt. Arguello 4/ 3 12:06 am 32 21.5 115 7.2 2.5 30 mi. SE of Calexico 4/ 3 9:32 am 34 50.7 118 58.3 2.2 2 mi. NW of Frazier Park 4/ 3 9:48 am 36 3.7 118 49.0 2.0 11 mi. E of Porterville 4/ 3 7:03 pm 34 23.0 116 28.0 2.1 18 mi. N of Yucca Valley 4/ 3 7:27 pm 35 44.5 117 40.9 2.0 8 mi. N of Ridgecrest 4/ 3 7:59 pm 33 5.9 117 56.6 2.1 28 mi. SSW of Dana Point 4/ 4 1:26 am 33 59.0 118 21.3 3.3 1 mi. N of Inglewood FELT 4/ 4 1:33 am 34 22.9 116 28.3 2.0 18 mi. N of Yucca Valley 4/ 4 1:35 am 33 59.2 118 21.4 2.4 2 mi. N of Inglewood FELT 4/ 4 2:24 am 36 6.3 117 39.5 2.1 16 mi. ENE of Coso Junction 4/ 4 8:20 am 35 32.2 118 18.7 2.1 11 mi. SE of town of Lake Isabella 4/ 4 9:33 am 36 4.1 117 38.7 2.1 17 mi. E of Coso Junction 4/ 4 1:58 pm 32 59.1 117 47.0 2.1 27 mi. WSW of Oceanside 4/ 4 6:55 pm 35 26.6 118 45.6 2.1 16 mi. NNE of Arvin 4/ 4 7:12 pm 33 5.9 117 56.1 2.5 28 mi. SSW of Dana Point 4/ 4 9:58 pm 34 19.5 119 21.9 2.8 4 mi. NW of Ventura 4/ 5 5:51 am 32 57.0 115 44.7 2.6 9 mi. SW of Westmorland 4/ 5 6:21 am 33 1.6 117 48.9 2.1 29 mi. SSW of San Clemente 4/ 5 6:33 am 33 59.2 118 21.5 2.5 2 mi. N of Inglewood FELT 4/ 5 12:21 pm 33 41.5 118 21.5 2.0 3 mi. WSW of Pt. Fermin, San Pedro 4/ 6 4:41 am 33 25.3 116 57.0 3.2 6 mi. NW of Palomar Observatory; FELT 4/ 6 5:53 am 32 21.7 118 14.8 2.1 32 mi. S of San Clemente Is. (SE tip) 4/ 6 3:09 pm 34 28.0 120 37.1 2.0 7 mi. SSE of Pt. Arguello 4/ 6 9:46 pm 36 29.9 120 36.7 2.6 28 mi. NNW of Coalinga 4/ 6 10:39 pm 36 5.7 117 39.5 2.1 16 mi. ENE of Coso Junction 4/ 6 11:56 pm 36 5.7 117 39.5 2.0 " 4/ 7 3:30 pm 36 5.2 117 39.4 2.1 " 4/ 8 12:11 am 35 21.6 118 36.9 2.0 16 mi. NE of Arvin 4/ 8 3:01 am 35 2.0 118 57.8 2.2 2 mi. NNW of Wheeler Ridge 4/ 8 4:17 am 34 10.8 116 26.4 2.2 4 mi. N of Yucca Valley 4/ 8 1:33 pm 34 21.5 118 38.9 2.0 5 mi. WSW of Valencia 4/ 8 3:33 pm 36 4.1 117 51.1 2.2 5 mi. ENE of Coso Junction 4/ 8 7:51 pm 36 12.4 120 5.8 2.5 15 mi. ENE of Coalinga 4/ 8 9:16 pm 33 28.4 116 27.2 2.6 15 mi. NNW of Borrego Springs -------------------------------------------------------------