SOUTHERN CALIFORNIA SEISMOGRAPHIC NETWORK A Cooperative Project of Caltech and the U.S. Geological Survey March 20, 1997 Weekly Earthquake Report for Southern California ------------------------------------------------ March 13 - 19, 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, March 13, Universal Time (ie. GMT), to midnight Wednesday night, March 19, Universal Time. In local time, the period of coverage is from 4 p.m., March 12, Pacific Standard Time, to 4 p.m., March 19, Pacific Standard Time. We detected and processed 309 earthquakes during the seven-day period covered. The main excitement this week was a M5.0 Landers aftershock on Tuesday morning. This quake was widely felt throughout the epicentral region and as far away as Orange County. We initially assigned it a magnitude of M5.4, but closer inspection of the data indicates that M5.0 is a better estimate. This week's quake was the 23rd M5+ quake in the continuing Joshua Tree/Landers/Big Bear sequence. The last one previous was on June 16, 1994. This week's quake was located at the southern end of a cluster of aftershocks called the Barstow cluster, which is part of the aftershock sequence of the June 28, 1992 Mw7.3 Landers earthquake. This epicenter is at the northern end of the Calico Fault. The focal mechanism indicates right-lateral slip on a strike consistent with the Calico Fault. Strong ground motions for this quake were recorded at over 60 stations in the new digital seismographic network called TriNet (a cooperative project of Caltech, the U.S. Geological Survey, and the California Division of Mines and Geology). The maximum ground acceleration recorded was 8.2%g, meaning that the peak force of the earthquake motion was 8.2% of the force of gravity, at Flash Peak, 19 kilometers west of the epicenter. Peak ground motions were used in an automated algorithm to produce contour maps of the ground shaking within a few minutes of the earthquake, and the maps were posted on the World Wide Web for access by the general public, as well as emergency services personnel. Aftershocks are earthquakes and will produce their own aftershocks. This quake has produced five so far that were M3 and above. The largest of these was a M3.7, at the same epi- center, slightly less than one hour later. It was felt in Barstow. If the present rate of aftershocks continues, we would expect to see one or two more aftershocks of M3 or larger in the next week. The Barstow aftershock also had three foreshocks during the night before: a M3.1 at 1:47 am, a M1.9 at 2:04 am, and a M2.0 at 7:07 am, 17 minutes before the M5.0. 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 ------------------------------------------------------------- 3/12 4:59 pm 34 22.3 116 27.3 2.2 17 mi. N of Yucca Valley 3/12 8:35 pm 34 13.7 118 57.1 2.0 5 mi. E of Camarillo 3/12 10:39 pm 36 6.0 117 39.6 2.1 16 mi. E of Coso Junction 3/13 2:59 am 32 54.2 115 31.8 2.3 4 mi. S of Brawley 3/13 4:47 am 33 44.3 116 2.6 2.1 11 mi. N of Mecca 3/13 10:53 am 34 37.5 116 39.4 2.1 20 mi. NE of Lucerne Valley 3/13 2:02 pm 33 57.3 116 21.3 2.0 8 mi. E of Desert Hot Springs 3/13 4:45 pm 36 2.7 117 39.8 2.6 15 mi. E of Coso Junction 3/14 1:32 am 35 6.3 119 6.7 2.3 11 mi. NW of Wheeler Ridge 3/14 9:37 am 32 56.7 116 20.6 2.3 17 mi. ESE of Julian 3/14 6:06 pm 34 1.7 116 21.3 2.5 7 mi. SSW of Joshua Tree 3/14 10:25 pm 34 18.1 118 35.7 2.0 3 mi. N of Chatsworth 3/15 7:01 pm 36 5.6 117 40.2 2.4 15 mi. E of Coso Junction 3/15 9:50 pm 33 42.4 116 45.5 2.1 3 mi. SW of Idyllwild 3/16 10:58 am 34 1.2 117 4.7 2.0 2 mi. WSW of Yucaipa 3/16 5:26 pm 33 19.5 116 24.8 2.1 5 mi. NNW of Borrego Springs 3/16 5:30 pm 34 15.1 118 27.3 2.1 2 mi. N of Panorama City 3/16 6:56 pm 35 5.5 119 6.9 2.4 11 mi. WNW of Wheeler Ridge 3/16 11:50 pm 34 12.3 117 24.5 2.0 1 mi. SSW of Devore 3/17 8:36 am 35 0.1 118 54.7 2.0 1 mi. E of Wheeler Ridge 3/17 12:11 pm 36 34.9 117 59.5 2.0 4 mi. ESE of Lone Pine 3/17 10:56 pm 34 22.8 116 27.7 2.0 18 mi. N of Yucca Valley 3/18 12:33 am 36 7.4 118 3.1 2.1 7 mi. NW of Coso Junction 3/18 12:59 am 36 1.9 119 41.6 2.5 30 mi. SW of Visalia 3/18 1:47 am 34 58.1 116 49.4 3.1 12 mi. ENE of Barstow 3/18 5:23 am 33 1.1 116 1.2 2.0 10 mi. SE of Ocotillo Wells 3/18 7:07 am 34 58.1 116 49.6 2.0 12 mi. ENE of Barstow 3/18 7:24 am 34 58.1 116 49.3 5.0 " WIDELY FELT 3/18 7:27 am 34 58.2 116 49.7 2.7 " 3/18 7:37 am 34 57.8 116 50.0 2.2 " 3/18 7:39 am 34 58.4 116 49.4 2.8 " 3/18 7:40 am 34 57.7 116 50.6 2.3 " 3/18 8:19 am 34 58.3 116 49.5 3.7 " FELT 3/18 8:40 am 34 58.4 116 49.2 3.4 " FELT 3/18 9:03 am 34 58.2 116 49.2 2.1 " 3/18 9:20 am 34 58.2 116 49.3 2.1 " 3/18 9:35 am 34 58.2 116 50.0 2.1 " 3/18 9:50 am 34 58.1 116 49.1 2.5 " 3/18 9:50 am 34 58.9 116 49.7 2.0 " 3/18 10:18 am 34 58.0 116 49.5 2.1 " 3/18 10:22 am 34 58.5 116 49.5 2.3 " 3/18 10:26 am 34 58.2 116 49.3 2.0 " 3/18 1:09 pm 34 58.6 116 50.0 2.3 " 3/18 1:09 pm 33 31.1 118 18.9 2.5 12 mi. N of Avalon, Catalina Island 3/18 1:28 pm 34 58.1 116 49.6 3.1 12 mi. ENE of Barstow 3/18 3:25 pm 34 58.1 116 49.6 2.2 " 3/18 4:44 pm 34 58.3 116 49.8 2.2 " 3/18 5:17 pm 34 57.6 116 50.8 2.1 " 3/18 5:55 pm 34 58.3 116 49.1 2.5 " 3/18 7:53 pm 34 58.4 116 49.6 2.1 " 3/18 8:45 pm 34 58.1 116 49.3 3.4 " 3/18 9:44 pm 33 31.1 118 18.7 2.1 12 mi. N of Avalon, Catalina Island 3/19 5:08 am 34 18.8 116 28.0 2.4 13 mi. N of Yucca Valley 3/19 2:34 pm 34 57.7 116 49.7 2.0 12 mi. ENE of Barstow -------------------------------------------------------------