SOUTHERN CALIFORNIA SEISMOGRAPHIC NETWORK A Cooperative Project of Caltech and the U.S. Geological Survey February 27, 1997 Weekly Earthquake Report for Southern California ------------------------------------------------ February 20 - 26, 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, February 20, Universal Time (ie. GMT), to midnight Wednesday night, February 26, Universal Time. In local time, the period of coverage is from 4 p.m., February 19, Pacific Standard Time, to 4 p.m., February 26, Pacific Standard Time. We detected and processed 277 earthquakes during the seven-day period covered. The total number of earthquakes this week is just about average for the past few months. There were six quakes above M3. The first occurred early last Friday morning, just off- shore south of Santa Barbara. The M3.6 mainshock was fairly widely felt in Santa Barbara. It was relatively shallow (4 km depth) and, like most events in the Santa Barbara Channel, resulted from pure thrust faulting on an east-striking plane. The second M3+ occurred Saturday evening in the Little San Bernardino Mountains northeast of Coachella Valley, and with it two aftershocks that were above M3 (M3.5 and M3.1). These were felt in the Coachella Valley area. This location is east of the San Andreas Fault by 5 miles. The Little San Bernardino Mountains are characterized by many, young, small, north-striking faults slaying off from the San Andreas fault, and a few larger east-striking faults. These faults are a common source of swarms of small earthquakes. This weeks focal mechanisms show north- striking, right-lateral or east-striking, left-lateral strike- slip motion, compatible with these faults. On Monday evening, there was an unremarkable M3.1 in the southern Sierra Nevada, east Lake Isabella near the eastern edge of the mountain range. And finally, very early Wednesday morning there was a M3.2 offshore of Pt. Arguello. In the case of the Lake Isabella quake and the Pt. Arguello quake, the epicenters were in sparsely populated areas and we got no inquiries. Also felt were two smaller quakes: a M2.7 north of Twentynine Palms on Thursday evening, a M2.4 near Palm Springs on Friday night. Remarkably, none of these M3+ or felt earthquakes was a Landers or Northridge aftershock. 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 ------------------------------------------------------------- 2/19 4:51 pm 33 51.2 119 47.3 2.1 17 mi. SSE of Santa Cruz Is. 2/20 1:07 am 36 4.5 117 38.5 2.1 17 mi. E of Coso Junction 2/20 1:56 pm 36 5.0 117 39.5 2.1 16 mi. E of Coso Junction 2/20 2:32 pm 36 34.3 118 8.9 2.3 5 mi. WSW of Lone Pine 2/20 3:38 pm 34 47.0 116 44.0 2.3 17 mi. ESE of Barstow 2/20 8:14 pm 34 27.8 115 53.2 2.7 24 mi. NNE of Twentynine Palms; FELT 2/20 10:23 pm 36 11.2 120 12.1 2.6 9 mi. ENE of Coalinga 2/21 5:23 am 34 23.8 119 41.8 3.6 1 mi. S of downtown Santa Barbara; FELT 2/21 10:46 am 34 23.4 119 41.5 2.4 2 mi. SSE of downtown Santa Barbara 2/21 9:39 pm 36 4.0 117 53.8 2.2 3 mi. ENE of Coso Junction 2/21 10:27 pm 33 51.3 116 31.7 2.4 2 mi. NNE of Palm Springs FELT 2/21 11:36 pm 36 5.4 117 40.0 2.2 16 mi. ENE of Coso Junction 2/22 12:22 am 34 26.7 116 31.2 2.4 22 mi. N of Yucca Valley 2/22 8:42 am 33 53.9 117 3.4 2.1 5 mi. WSW of Beaumont 2/22 7:43 pm 33 44.3 116 2.6 3.8 11 mi. N of Mecca; FELT 2/22 7:44 pm 33 44.3 116 2.6 3.5 " 2/22 7:48 pm 33 44.3 116 2.5 3.1 " 2/23 6:49 pm 36 2.3 117 42.8 2.4 13 mi. E of Coso Junction 2/24 12:11 am 33 15.7 116 0.4 2.3 2 mi. WSW of Salton City 2/24 12:23 am 34 0.6 116 19.2 2.5 8 mi. S of Joshua Tree 2/24 3:47 am 36 5.4 117 39.7 2.0 16 mi. E of Coso Junction 2/24 4:14 am 32 27.9 116 12.5 2.1 22 mi. SSW of Ocotillo 2/24 4:53 am 35 1.6 119 4.1 2.1 7 mi. WNW of Wheeler Ridge 2/24 5:04 am 35 1.5 119 4.8 2.3 7 mi. W of Wheeler Ridge 2/24 5:05 am 36 5.4 117 39.8 2.6 16 mi. E of Coso Junction 2/24 12:40 pm 36 5.5 117 39.8 2.3 " 2/24 2:08 pm 36 5.4 117 40.0 2.4 " 2/24 3:22 pm 34 19.3 116 51.2 2.0 3 mi. N of Big Bear City 2/24 3:42 pm 35 31.4 118 26.3 2.1 8 mi. SSE of town of Lake Isabella 2/24 5:23 pm 35 18.8 118 33.5 2.1 14 mi. NNW of Tehachapi 2/24 7:07 pm 36 5.4 117 39.9 2.3 16 mi. E of Coso Junction 2/24 9:38 pm 35 40.7 118 6.4 3.1 20 mi. E of town of Lake Isabella 2/24 9:53 pm 35 40.9 118 6.3 2.4 " 2/24 10:48 pm 36 5.2 117 38.2 2.3 16 mi. E of Coso Junction 2/24 11:05 pm 36 5.3 117 39.8 2.1 " 2/25 4:25 pm 34 35.3 116 36.7 2.0 21 mi. ENE of Lucerne Valley 2/25 11:20 pm 34 29.4 116 28.4 2.5 25 mi. N of Yucca Valley 2/26 12:55 am 34 24.3 120 45.8 2.9 13 mi. SSW of Pt. Arguello 2/26 1:03 am 34 26.7 120 42.8 3.2 9 mi. SSW of Pt. Arguello 2/26 1:52 am 36 40.5 118 55.6 2.1 31 mi. NE of Visalia 2/26 2:19 am 36 5.9 117 39.8 2.5 16 mi. E of Coso Junction 2/26 2:19 am 36 5.7 117 39.7 2.0 " 2/26 3:10 am 32 53.7 115 40.5 2.2 10 mi. SSW of Westmorland 2/26 5:56 am 33 59.8 117 34.9 2.0 6 mi. SE of Ontario 2/26 6:39 am 33 19.0 116 11.4 2.0 12 mi. NNW of Ocotillo Wells 2/26 9:43 am 34 1.0 117 14.1 2.0 2 mi. SSE of Loma Linda -------------------------------------------------------------