SOUTHERN CALIFORNIA SEISMOGRAPHIC NETWORK A Cooperative Project of Caltech and the U.S. Geological Survey May 1, 1997 Weekly Earthquake Report for Southern California ------------------------------------------------ April 24 - 30, 1997 Prepared by: Debbie Wells, Seismological Laboratory Nick Scheckel, 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, April 24, Universal Time (ie. GMT), to midnight Wednesday night, April 30, Universal Time. In local time, the period of coverage is from 5 p.m., April 23, Pacific Daylight Time, to 5 p.m., April 30, Pacific Daylight Time. We detected approximately 445 earthquakes during the seven-day period covered. Almost all of these events, except about 100 of magnitude less than 2.0, have been processed. It was not a good weekend for sleeping late, for many throughout the L.A. area. Two aftershocks of the Northridge earthquake (M6.7, January 17, 1994), shook many people awake and some objects off shelves on Saturday and Sunday mornings. Originally reported as M5.0 and M4.9 from our preliminary data, the final magnitudes for these earthquakes with all stations reporting, are M4.9 on Saturday and M4.8 on Sunday. These earthquakes are called aftershocks to the North- ridge earthquake because: 1) They are located in the region where aftershocks occurred in the first few days of the sequence (they are in fact at almost the exact location of a M5.1 aftershock that startled President Clinton during his visit here on January 19, 1994). 2) They are smaller than the mainshock (M6.7) and 3) They are occurring at a time when the rate of earthquakes in the aftershock zone is still significantly above the rate that was in that region before the mainshock happened. Although these earthquakes are clearly in the after- shock zone, they are probably not on the same fault as produced the mainshock. The aftershocks in this region (the second densest cluster of aftershocks in the zone) form a steeply dipping, approximately east-striking structure. They do not follow the 45 degree dipping plane (assumed to be the main- shock fault) that can be seen farther east in the aftershock zone. Both of this week's earthquakes were fairly deep (16.6 and 15.2 km., respectively) and had thrust focal mechanisms. We interpret this cluster as deformation at the edge of the mainshock rupture. These earthquakes are relatively large for this late in a sequence, but as Charles Richter said (in his book, Elementary Seismology, published in 1958) "Large, Late Aftershocks? Not uncommonly, when aftershocks have apparently subsided, what seems to be an unusually large one occurs without warning. Such a shock usually has its own aftershock sequence..." The probability of a M5 or greater aftershock in the fourth year of the Northridge sequence, assuming the rate of earlier aftershocks would continue, was 12%. This means that of 8 aftershock sequences like Northridge, 7 would not have a M5 aftershock this late but 1 of the 8 would. Once the first M5 aftershock did occur on Saturday, it triggered its own aftershock sequence, including the M4.8 on Sunday and over 120 other events above M1.5. Aftershocks to one earthquake can also be foreshocks to another, but now, five days after the M4.9, the probability of an even larger earthquake occurring in the next week is below 1%. The ground motions produced by this earthquake were recorded at over 60 stations of the new digital seismographic network in southern California called TriNet (a cooperative project of Caltech, U.S. Geological Survey, and the State Department of Conservation Division of Mines and Geology). The ground motions are defined as a percentage of the force of gravity. An acceleration of 1%g means the ground is pushing on you with 1% of the force that the gravity of the earth pulls on you. Ground accelerations recorded in Saturday's earth- quake were 14%g at Sylmar, and 7.6%g at Granada Hills. On Sunday, the shaking at Sylmar was 11%g and 5.9%g at Granada Hills. A detailed color map and cross sections of the sequence are available on the World Wide Web at: http://www.scecdc.scec.org/newafter.html One other, small quake was felt this week. It was a M2.6 on Thursday morning, felt near the epicenter, in the Malibu area. 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 ------------------------------------------------------------- 4/23 7:06 pm 36 3.0 117 39.8 2.7 15 mi. E of Coso Junction 4/23 8:44 pm 36 12.6 118 3.8 2.6 5 mi. SSW of Olancha 4/24 5:32 am 34 1.9 118 53.7 2.6 2 mi. ESE of Leo Carrillo State Beach; FELT 4/24 7:21 am 33 58.4 116 39.8 2.0 6 mi. SW of Morongo Valley 4/24 6:15 pm 36 2.8 117 39.7 2.0 16 mi. E of Coso Junction 4/25 12:00 am 35 47.8 117 38.2 2.4 12 mi. N of Ridgecrest 4/25 12:54 am 35 47.9 117 38.3 2.5 " 4/25 11:56 am 34 58.2 116 48.5 2.4 13 mi. ENE of Barstow 4/25 12:26 pm 34 58.2 116 48.5 3.0 " 4/25 1:40 pm 32 58.1 116 0.5 2.5 14 mi. SSE of Ocotillo Wells 4/25 11:48 pm 35 44.6 117 37.9 2.4 8 mi. NNE of Ridgecrest 4/26 3:37 am 34 22.1 118 40.3 4.9 6 mi. NNE of Simi Valley WIDELY FELT 4/26 3:40 am 34 22.5 118 40.2 3.9 " 4/26 3:47 am 34 22.3 118 39.5 2.4 5 mi. WSW of Valencia 4/26 3:53 am 34 22.2 118 38.9 2.2 " 4/26 3:54 am 34 22.5 118 39.1 3.0 " 4/26 3:57 am 34 22.6 118 38.6 2.6 4 mi. WSW of Valencia 4/26 3:58 am 34 22.2 118 37.7 2.4 " 4/26 4:08 am 34 22.9 118 38.8 2.4 " 4/26 4:10 am 34 22.5 118 39.0 3.1 5 mi. WSW of Valencia 4/26 4:10 am 34 22.0 118 39.3 2.6 " 4/26 4:16 am 34 22.6 118 38.6 2.5 4 mi. WSW of Valencia 4/26 4:16 am 34 22.7 118 39.6 2.0 5 mi. WSW of Valencia 4/26 4:33 am 34 22.5 118 38.6 3.0 4 mi. WSW of Valencia 4/26 4:55 am 34 22.4 118 39.9 3.9 6 mi. NNE of Simi Valley 4/26 5:01 am 34 22.1 118 39.0 2.3 5 mi. WSW of Valencia 4/26 5:21 am 34 22.5 118 39.4 2.4 " 4/26 5:29 am 34 22.3 118 39.6 2.0 " 4/26 5:31 am 34 22.6 118 38.9 2.3 " 4/26 6:02 am 34 21.6 118 39.9 2.2 6 mi. NNE of Simi Valley 4/26 6:08 am 34 21.9 118 39.1 2.0 5 mi. WSW of Valencia 4/26 7:31 am 34 22.6 118 38.8 2.8 4 mi. WSW of Valencia 4/26 8:51 am 34 22.3 118 39.0 2.2 5 mi. WSW of Valencia 4/26 9:13 am 34 22.0 118 41.0 3.3 6 mi. NNE of Simi Valley 4/27 12:00 am 32 37.6 115 54.3 2.0 9 mi. SE of Ocotillo 4/27 1:04 am 34 23.0 118 39.5 2.1 5 mi. W of Valencia 4/27 1:56 am 36 7.4 118 16.7 2.1 18 mi. SW of Olancha 4/27 2:25 am 35 59.9 118 16.6 2.1 18 mi. W of Coso Junction 4/27 4:09 am 34 22.6 118 38.9 4.8 5 mi. WSW of Valencia WIDELY FELT 4/27 4:12 am 34 22.6 118 39.2 2.7 " 4/27 4:13 am 34 22.5 118 38.4 2.5 4 mi. WSW of Valencia 4/27 4:13 am 34 22.3 118 38.5 2.6 " 4/27 4:16 am 34 22.2 118 39.2 2.0 5 mi. WSW of Valencia 4/27 4:17 am 34 22.5 118 39.0 2.8 " 4/27 4:20 am 34 22.4 118 39.0 2.2 " 4/27 4:22 am 34 22.9 118 38.5 2.5 4 mi. WSW of Valencia 4/27 4:27 am 34 22.2 118 39.0 2.0 5 mi. WSW of Valencia 4/27 4:30 am 34 22.8 118 38.2 2.2 4 mi. WSW of Valencia 4/27 4:31 am 34 22.8 118 38.5 3.5 " 4/27 4:31 am 34 23.8 118 39.3 3.4 5 mi. W of Valencia 4/27 4:31 am 34 22.3 118 38.8 3.4 5 mi. WSW of Valencia 4/27 4:33 am 34 22.6 118 38.8 2.3 4 mi. WSW of Valencia 4/27 4:34 am 34 22.7 118 38.2 2.3 " 4/27 4:37 am 34 22.3 118 38.1 2.3 " 4/27 4:39 am 34 22.6 118 39.0 2.3 5 mi. WSW of Valencia 4/27 4:51 am 34 22.3 118 39.1 2.7 " 4/27 5:14 am 34 22.5 118 38.3 2.0 4 mi. WSW of Valencia 4/27 5:18 am 34 22.7 118 38.0 2.4 " 4/27 5:22 am 34 22.5 118 38.9 2.5 5 mi. WSW of Valencia 4/27 5:25 am 34 22.4 118 37.9 2.0 4 mi. WSW of Valencia 4/27 5:37 am 34 22.5 118 39.0 2.0 5 mi. WSW of Valencia 4/27 5:44 am 34 22.7 118 39.0 2.1 " 4/27 8:16 am 34 22.8 118 37.0 2.0 3 mi. WSW of Valencia 4/27 8:18 am 34 22.7 118 38.0 3.3 4 mi. WSW of Valencia 4/27 8:20 am 34 22.4 118 38.3 2.0 " 4/27 10:55 am 34 22.0 118 40.7 2.0 6 mi. NNE of Simi Valley 4/27 11:23 am 36 0.9 118 26.8 2.9 26 mi. N of town of Lake Isabella 4/27 11:53 am 34 22.8 118 38.6 2.3 4 mi. WSW of Valencia 4/27 12:36 pm 34 22.6 118 38.3 2.1 " 4/27 12:38 pm 34 22.9 118 38.7 2.0 4 mi. W of Valencia 4/27 2:34 pm 34 22.9 118 38.8 2.5 " 4/27 2:44 pm 34 23.1 118 38.7 2.4 " 4/27 3:28 pm 34 4.8 116 22.3 2.0 5 mi. SW of Joshua Tree 4/27 6:20 pm 34 23.0 118 38.5 3.1 4 mi. W of Valencia 4/27 6:32 pm 34 22.8 118 38.3 2.0 4 mi. WSW of Valencia 4/27 7:36 pm 33 50.3 117 2.7 2.0 7 mi. SSW of Beaumont 4/27 8:28 pm 33 19.9 116 17.8 2.1 6 mi. NE of Borrego Springs 4/27 10:14 pm 33 18.0 116 27.0 2.2 5 mi. NW of Borrego Springs 4/27 10:16 pm 34 22.5 118 37.5 2.0 3 mi. WSW of Valencia 4/28 8:25 am 34 22.5 118 38.2 2.2 4 mi. WSW of Valencia 4/28 2:58 pm 34 37.5 116 31.8 2.4 26 mi. ENE of Lucerne Valley 4/28 4:04 pm 34 22.1 118 40.7 2.1 6 mi. NNE of Simi Valley 4/28 10:00 pm 34 22.0 118 41.3 2.1 6 mi. N of Simi Valley 4/29 12:02 pm 34 37.0 116 39.6 2.2 20 mi. NE of Lucerne Valley 4/29 12:31 pm 34 57.6 116 50.1 2.2 11 mi. ENE of Barstow 4/29 4:26 pm 32 28.4 115 15.6 2.6 19 mi. SE of Calexico 4/30 3:13 am 33 30.8 118 1.2 2.8 8 mi. SW of Newport Beach 4/30 4:37 am 34 22.8 118 40.5 2.1 7 mi. NNE of Simi Valley 4/30 7:56 am 35 20.7 119 19.6 2.4 15 mi. NNE of Taft 4/30 12:12 pm 34 22.7 118 38.1 2.6 4 mi. WSW of Valencia 4/30 12:12 pm 36 2.2 117 40.5 2.3 15 mi. E of Coso Junction -------------------------------------------------------------