August 18, 1994 Weekly Earthquake Report for Southern California ------------------------------------------------ August 11 - 17, 1994 Prepared by: Kate Hutton, Seismological Laboratory (kate@bombay.gps.caltech.edu) Lucy Jones, U.S. Geological Survey California Institute of Technology This document is a commentary on current seismic activity. The earthquakes discussed have been detected and processed automatically. All epicenters and magnitudes have been reviewed to exclude obvious blunders; however, they must still be considered preliminary. 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, August 11, Pacific Daylight Time, to midnight Wednesday night, August 17, Pacific Daylight Time. We recorded 325 earthquakes this week. Probably the most interesting activity this week occurred at the very southern end of the Landers aftershock zone, about 2 mi. from the San Andreas fault. The largest one, a M3.8 during the wee hours Monday morning, was felt in Palm Desert. This quake had a "normal" focal mechanism, which indicates extension of the crust. It was probably on one of the many north-south striking normal faults that run through the Little San Bernardino Mtns., where the Landers fault trend runs into the San Andreas fault. There were a total of 27 events above M1.0. Because of the proximity to the San Andreas, this little sequence caused a few eyebrows to be raised at the Seismo Lab. Immediate foreshocks are generally located within 10 km of their mainshock, so any activity very close to a large fault is of concern. In this case, however, the Little San Bernardino Mountains have produced several similar swarms in the past. A comparison with past seismicity suggests that the probability of this swarm being foreshocks is less than 1%. There was also another M3.8 this week, just south of San Clemente Is. just after midnight on Saturday. It was a single event and, if it was felt, no-one bothered to call. Later, on Saturday evening, there was a M3.4 near Santa Barbara Is., which was not felt either, as far as we know. A few more quakes occurred in the Fontana sequence, the largest one being a M2.7 early Friday morning. Most of the rest of the week's activity consisted of, as usual, Landers and Northridge aftershocks. The table lists the quakes 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 Daylight Time listed. Table 1 ------- Date Time N Lat. W Long. Mag ------------------------------------------------------------- 8/11 2:49 am 34 3.2 117 30.6 2.1 Fontana, 2 mi. ESE of the I10/I15 interchange 8/11 9:49 am 34 9.0 116 26.8 3.0 2 mi. N of Yucca Valley 8/11 8:04 pm 34 22.8 116 27.5 2.3 18 mi. N of Yucca Valley 8/11 9:36 pm 34 3.0 117 30.5 2.0 Fontana, 2 mi. ESE of the I10/I15 interchange 8/11 11:27 pm 33 50.4 116 51.0 2.0 6 mi. SSE of Banning 8/12 1:50 am 34 9.4 116 25.4 2.2 2 mi. NNE of Yucca Valley 8/12 1:52 am 34 10.6 116 25.5 2.2 4 mi. NNE of Yucca Valley 8/12 5:27 am 33 59.4 117 32.7 2.7 Fontana, 5 mi. S of the I10/I15 interchange 8/12 2:31 pm 34 15.2 116 49.9 2.4 1 mi. ESE of Big Bear City 8/12 7:10 pm 35 2.3 116 59.8 2.7 10 mi. N of Barstow 8/12 7:34 pm 34 16.6 118 27.1 2.1 Under San Fernando 8/13 4:43 am 33 59.5 116 16.1 2.9 13 mi. SE of Yucca Valley 8/13 4:51 am 34 13.6 118 37.2 2.2 2 mi. NNW of Canoga Park 8/13 10:36 am 35 56.3 116 27.1 2.0 10 mi. WSW of Shoshone 8/13 12:51 pm 32 47.5 118 24.5 3.8 4 mi. WSW of San Clemente Is. 8/13 1:57 pm 34 17.5 118 26.4 2.1 Under San Fernando 8/13 9:16 pm 33 31.5 119 15.6 3.4 11 mi. WNW of Santa Barbara Is. 8/13 11:19 pm 36 25.2 117 28.4 2.9 5 mi. N of Panamint Springs 8/14 2:17 am 33 51.9 120 5.0 2.4 5 mi. SSE of Santa Rosa Is. 8/14 5:27 am 34 21.6 118 39.3 3.1 5 mi. SW of Magic Mtn. 8/14 5:36 am 34 21.7 118 39.3 2.3 " 8/14 5:43 am 33 0.6 116 21.6 2.0 17 mi. S of Borrego Springs 8/14 5:47 am 34 15.1 118 30.0 2.0 2 mi. ENE of Northridge 8/14 5:53 am 34 4.0 116 41.8 2.2 7 mi. ESE of Mt. San Gorgonio 8/14 6:24 am 34 21.2 118 39.8 2.1 5 mi. NNE of Simi Valley 8/14 6:27 am 34 21.4 118 40.2 2.0 " 8/14 6:58 am 34 21.8 118 39.3 2.2 5 mi. SW of Magic Mtn. 8/14 6:58 am 34 21.0 118 40.5 2.1 5 mi. NNE of Simi Valley 8/14 8:50 am 34 33.6 116 33.0 2.5 23 mi. ENE of Lucerne Valley 8/14 8:50 am 34 32.7 116 33.0 2.0 " 8/14 12:56 pm 34 23.4 116 27.4 2.8 18 mi. N of Yucca Valley 8/14 2:46 pm 34 25.2 116 33.0 2.0 21 mi. NNW of Yucca Valley 8/14 2:48 pm 34 23.4 116 28.3 2.1 18 mi. N of Yucca Valley 8/14 3:49 pm 34 21.8 118 30.0 2.0 1 mi. SE of Newhall 8/14 5:03 pm 33 44.0 118 19.8 2.0 2 mi. NW of Pt. Fermin, San Pedro 8/15 12:34 am 32 59.3 117 48.2 2.0 28 mi. WSW of Oceanside 8/15 1:07 am 33 48.7 116 12.1 3.8 6 mi. N of Indio 8/15 1:08 am 33 48.5 116 12.3 3.1 " 8/15 1:58 am 34 15.0 116 25.9 2.0 9 mi. N of Yucca Valley 8/15 2:51 am 33 48.6 116 11.9 2.1 6 mi. N of Indio 8/15 6:40 am 34 18.4 116 26.3 2.0 13 mi. N of Yucca Valley 8/15 7:01 am 34 18.5 117 7.8 2.0 5 mi. NE of Lake Arrowhead 8/15 9:29 am 34 11.6 117 26.7 2.1 2 mi. WSW of Glen Helen Regional Park, Cajon Pass 8/15 9:51 am 33 48.6 116 12.2 2.5 6 mi. N of Indio 8/15 5:46 pm 34 20.5 118 28.3 2.0 4 mi. SE of Newhall 8/15 10:14 pm 34 14.1 116 45.1 2.1 6 mi. ESE of Big Bear City 8/15 11:46 pm 34 0.8 116 19.2 2.0 10 mi. SE of Yucca Valley 8/16 4:34 am 32 44.7 118 29.0 2.0 9 mi. WSW of San Clemente Is. 8/16 12:38 pm 34 27.7 115 50.8 2.4 25 mi. NNE of Twentynine Palms 8/16 1:21 pm 34 30.0 117 24.4 2.1 6 mi. WSW of Victorville 8/16 2:49 pm 34 10.4 116 26.4 2.0 3 mi. N of Yucca Valley 8/16 4:39 pm 36 2.9 117 56.4 2.0 Under Coso Junction 8/16 9:33 pm 33 48.5 116 11.8 2.1 6 mi. N of Indio 8/17 1:56 am 33 48.5 116 11.8 2.3 6 mi. N of Indio 8/17 5:57 am 34 37.0 116 37.3 2.5 21 mi. ENE of Lucerne Valley 8/17 7:30 am 34 16.0 118 42.7 2.2 Under Simi Valley 8/17 9:28 am 34 11.5 116 50.9 2.3 5 mi. S of Big Bear City 8/17 9:28 am 34 11.5 116 50.7 2.0 " 8/17 3:27 pm 34 23.6 116 28.1 2.1 19 mi. N of Yucca Valley 8/17 4:59 pm 36 39.5 118 24.0 2.6 8 mi. NW of Mt. Whitney 8/17 9:41 pm 34 0.2 116 19.0 2.0 10 mi. SE of Yucca Valley 8/17 10:04 pm 34 21.9 118 39.0 2.4 5 mi. SW of Magic Mtn. ------------------------------------------------------------- Figure 1. A map of southern California showing the earthquakes recorded during the past week by the Caltech/USGS Seismic Network. Major faults are marked, as well as the metropolitan areas of Los Angeles (L.A.), Palm Springs (P.S.), San Diego (S.D.), and Santa Barbara (S.B.). The circles denote the earthquakes, the size of the circle indicating the magnitude.