SOUTHERN CALIFORNIA SEISMOGRAPHIC NETWORK A Cooperative Project of Caltech and the U.S. Geological Survey April 18, 1996 Weekly Earthquake Report for Southern California ------------------------------------------------ April 11 - 17, 1996 Prepared by: Nick Scheckel, Seismological Laboratory Debbie Wells, 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 818-395-6326. For daily updates, call our Earthquake Information Hotline: 818-395-6977. -------------------------------- This week's Report covers the time period from midnight Thursday morning, April 11, Universal Time (ie. GMT), to midnight Wednesday night, April 17, Universal Time. In local time, the period of coverage is from 5 p.m., April 10, Pacific Daylight Time, to 5 p.m., April 17, Pacific Daylight Time. We detected and processed 298 earthquakes during the 7-day period covered. Seismicity this week was considerably lower than last week. There were only two quakes above M3.0; a M3.1 on Friday afternoon near the SW boundary of the Death Valley National Monument and a M3.2 Landers aftershock (M7.3, June 28th, 1992) Monday evening. Neither were felt. Now that the first quarter of the 1996 is over we have our seismicity statistics compiled. The number of earthquakes in the first quarter of 1996 is still higher than the first 2 quarters of 1995 (before the Ridgecrest sequence began), but we can see that the number of aftershocks to that sequence is dropping. The statistics include the quarterly totals for the past few years with the total number of events and the number of quakes M3 and over. 1st quarter 2nd quarter 3rd quarter 4th quarter all 3.0+ all 3.0+ all 3.0+ all 3.0+ 1990 3,483 56 2,627 45 2,225 27 2,403 31 1991 2,041 11 2,218 32 2,445 23 2,651 29 1992 3,618 40 10,288 744 25,500+ 801 10,700+ 166 1993 4,939 43 5,370 61 5,298 48 4,583 35 1994 10,915 417 5,967 43 5,400 48 5,031 40 1995 3,815 28 3,982 39 8,259 90 7,743 51 1996 5,726 49 The variations in the quarterly total is almost completely due to large numbers of aftershocks after our largest earthquakes. The number of aftershocks is largest immediately after a mainshock and then dies off with time. The mainshocks that have produced notable variations in these quarterly totals are: February 28, 1990 Upland M5.2 (with a rich aftershock sequence) June 28, 1991 Sierra Madre M5.8 April 23, 1992 Joshua Tree M6.1 June 28, 1992 Landers M7.3 January 17, 1994 Northridge M6.7 Aug/Sept 1995 Ridgecrest M5.4,5.8 The table 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 ------------------------------------------------------------- 4/10 8:49 pm 34 49.5 118 55.1 2.4 1 mi. E of Frazier Park 4/10 9:43 pm 33 41.6 116 43.7 2.4 3 mi. SSW of Idyllwild 4/11 6:43 am 36 17.3 118 20.2 2.1 20 mi. S of Mt. Whitney 4/11 2:30 pm 34 21.4 118 36.8 2.3 3 mi. SW of Valencia 4/11 6:26 pm 34 3.0 117 1.3 2.6 1 mi. NNE of Yucaipa 4/11 9:55 pm 34 17.7 116 27.2 2.0 12 mi. N of Yucca Valley 4/12 5:32 am 33 51.9 116 12.7 2.8 10 mi. N of Indio 4/12 6:51 am 33 50.8 118 16.4 2.0 1 mi. NNE of Carson 4/12 3:21 pm 35 57.7 117 1.7 3.1 14 mi. SSE of Telescope Peak 4/12 3:29 pm 35 59.9 117 47.2 2.0 9 mi. ESE of Coso Junction 4/13 6:07 am 35 10.4 118 39.3 2.1 10 mi. ESE of Arvin 4/13 9:52 am 36 37.5 118 8.6 2.0 4 mi. WNW of Lone Pine 4/13 1:24 pm 33 28.4 116 29.5 2.0 11 mi. ESE of Anza 4/13 9:41 pm 35 17.7 120 58.7 2.5 8 mi. WSW of Morro Bay 4/14 4:01 am 32 47.3 115 26.6 2.0 6 mi. E of El Centro 4/14 4:54 am 32 46.9 115 26.7 2.5 " 4/14 12:35 pm 34 36.4 116 36.5 2.2 22 mi. ENE of Lucerne Valley 4/14 3:10 pm 32 47.0 115 26.5 2.0 6 mi. E of El Centro 4/14 3:10 pm 32 46.9 115 26.3 2.2 " 4/14 3:11 pm 32 46.7 115 26.7 2.3 " 4/14 3:11 pm 32 46.8 115 26.4 2.2 " 4/14 10:00 pm 34 3.1 117 15.3 2.6 Under Loma Linda 4/15 6:13 am 34 14.7 118 29.2 2.0 1 mi. SSE of Granada Hills 4/15 7:15 am 35 45.3 117 37.8 2.2 9 mi. NNE of Ridgecrest 4/15 12:04 pm 34 37.6 116 36.5 2.0 22 mi. ENE of Lucerne Valley 4/15 9:20 pm 36 16.4 120 15.5 2.4 10 mi. NNE of Coalinga 4/15 9:38 pm 35 43.6 117 37.1 2.0 7 mi. NNE of Ridgecrest 4/15 10:07 pm 34 37.3 116 33.8 3.2 24 mi. ENE of Lucerne Valley 4/15 11:01 pm 34 58.5 116 56.8 2.6 7 mi. NE of Barstow 4/16 12:55 am 32 45.8 115 26.5 2.7 7 mi. NNE of Calexico 4/16 3:40 am 33 12.3 118 48.3 2.3 16 mi. NW of San Clemente Is. 4/16 3:55 am 35 45.2 117 37.9 2.0 9 mi. NNE of Ridgecrest 4/16 4:41 am 34 20.4 116 27.8 2.3 15 mi. N of Yucca Valley 4/16 7:06 am 35 47.9 120 19.6 2.0 5 mi. NNW of Cholame 4/16 7:18 am 34 24.2 116 28.4 2.1 19 mi. N of Yucca Valley 4/16 7:38 am 32 46.0 115 26.5 2.6 7 mi. NNE of Calexico 4/16 7:47 am 32 46.4 115 26.3 2.1 6 mi. ESE of El Centro 4/16 10:56 am 35 46.0 119 10.9 2.1 23 mi. SSW of Porterville 4/16 11:12 am 33 34.9 119 20.5 2.4 17 mi. WNW of Santa Barbara Is. 4/17 3:30 am 33 59.7 117 0.6 2.5 2 mi. SSE of Yucaipa -------------------------------------------------------------