SOUTHERN CALIFORNIA SEISMOGRAPHIC NETWORK A Cooperative Project of Caltech and the U.S. Geological Survey September 28, 1995 Weekly Earthquake Report for Southern California ------------------------------------------------ September 21 - 27, 1995 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 818-395-6326. For daily updates, call our Earthquake Information Hotline: 818-395-6977. -------------------------------- This week's Report covers the time period from midnight Thursday morning, September 21, Pacific Daylight Time, to midnight Wednesday night, September 27, 1995, Pacific Daylight Time. We recorded 1,035 earthquakes during the 7-day period covered. There are about 700 more yet to be processed. However, all of these are less than M2.0, and most are less than M1.5. The bulk of the activity this week consists of after- shocks to last Wednesday afternoon's ML5.8 Ridgecrest quake. The second figure shows the distribution of aftershocks on a map view and cross section. From this, it is apparent that the ML5.8 occurred on a nearly vertical fault. The focal mechanism (southernmost of the two "beach ball" diagrams) agrees with this, showing a large component of right-lateral strike slip. This is in contrast to the August 17 M5.4 quake, which was on a west southwesterly dipping plane and showed a large component of normal faulting. We conclude that the two M5+ quakes (August 17 and September 20) occurred on different, but adjacent, faults. The northernmost of the two "beach balls" corresponds to the ML4.9 aftershock that occurred on Sunday evening. The geographic location is indicated by the large square just to the east of the focal diagram on the map. This event appears to have been on yet another fault. A geologic map shows a plethora of small faults in this vicinity. Glenn Roquemore reports a centimeter or two of surface rupture on one of them, but unfortunately not one that is easy to relate geographically to the hypocenters. Ridgecrest was, however, not the only active area this week. A small "swarm" occurred within the Landers aftershock zone, 9 miles north of Yucca Valley, beginning on in the wee hours of Saturday morning. It peaked at 9:03 am the same day with a M3.4 that showed mostly strike-slip faulting. As of this Report the swarm is still coninuing at a low level. We got no "felt" reports on the M3.4, but a M2.8 on Saturday evening and a M2.9 Wednesday morning were both felt. Another small sequence of note occurred on Sunday night, beginning with a M3.0 near North Shore on the Salton Sea. A large number (for a M3.0) of small aftershocks followed. This location is of interest because it is very close to, if not on, the locked portion of the San Andreas Fault. Earthquakes in this size range have occurred before, however, without disastrous consequences. In particular, there was a very similar cluster on December 14, 1989 at almost the same location. The table lists the quakes this week that were M2.0 or larger in the central part of the coverage area, except for the Ridgecrest aftershocks since the ML5.8, which must be M2.5 or larger to be listed. 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. The second figure shows a map view and cross section of the Ridgrest activity so far. Table 1 ------- Date Time N Lat. W Long. Mag ------------------------------------------------------------- 9/21 12:01 am 35 44.1 117 38.0 2.7 8 mi. NNE of Ridgecrest 9/21 12:09 am 35 46.2 117 38.5 2.9 10 mi. N of Ridgecrest 9/21 12:26 am 35 47.0 117 39.4 3.1 11 mi. N of Ridgecrest 9/21 12:29 am 35 43.6 117 37.4 3.1 7 mi. NNE of Ridgecrest 9/21 12:46 am 35 45.5 117 38.2 3.9 9 mi. N of Ridgecrest 9/21 12:57 am 35 45.4 117 38.0 4.1 9 mi. NNE of Ridgecrest 9/21 12:58 am 35 45.5 117 38.2 3.1 9 mi. N of Ridgecrest 9/21 12:59 am 35 45.5 117 38.3 3.0 " 9/21 12:59 am 35 45.6 117 38.0 2.7 9 mi. NNE of Ridgecrest 9/21 2:13 am 35 45.0 117 38.4 2.9 8 mi. N of Ridgecrest 9/21 2:14 am 35 44.5 117 38.2 2.6 " 9/21 2:27 am 35 44.4 117 38.0 2.7 8 mi. NNE of Ridgecrest 9/21 3:07 am 35 45.6 117 39.0 2.5 9 mi. N of Ridgecrest 9/21 3:23 am 35 43.7 117 37.4 2.5 7 mi. NNE of Ridgecrest 9/21 3:25 am 33 46.5 116 8.3 2.6 6 mi. NE of Indio 9/21 7:11 am 35 44.6 117 38.0 3.5 8 mi. NNE of Ridgecrest 9/21 7:11 am 35 44.4 117 38.0 3.6 " 9/21 7:18 am 35 44.4 117 37.9 2.6 " 9/21 7:21 am 33 25.1 118 3.3 3.2 15 mi. SSW of Newport Beach 9/21 7:31 am 35 44.3 117 37.6 2.5 8 mi. NNE of Ridgecrest 9/21 7:32 am 35 43.9 117 37.8 2.7 7 mi. NNE of Ridgecrest 9/21 10:03 am 34 13.4 118 35.3 2.0 1 mi. NNE of Canoga Park 9/21 11:49 am 35 44.0 117 37.8 2.6 7 mi. NNE of Ridgecrest 9/21 2:10 pm 35 46.1 117 39.1 3.6 10 mi. N of Ridgecrest 9/21 3:00 pm 32 54.5 115 40.4 2.4 9 mi. SSW of Westmorland 9/21 4:48 pm 35 45.7 117 38.5 4.0 9 mi. N of Ridgecrest 9/21 4:53 pm 35 45.8 117 38.2 2.7 " 9/21 5:06 pm 35 45.9 117 37.6 2.6 10 mi. NNE of Ridgecrest 9/21 6:15 pm 35 46.5 117 37.4 2.5 " 9/22 4:33 am 35 45.6 117 38.1 2.6 9 mi. N of Ridgecrest 9/22 5:02 am 32 37.6 115 55.2 2.0 8 mi. SSE of Ocotillo 9/22 5:27 am 36 16.7 120 13.0 2.5 12 mi. NE of Coalinga 9/22 10:43 am 35 43.8 117 37.8 2.6 7 mi. NNE of Ridgecrest 9/22 11:26 am 35 46.5 117 39.3 2.5 10 mi. N of Ridgecrest 9/22 11:47 am 35 47.1 117 39.4 2.5 11 mi. N of Ridgecrest 9/22 1:44 pm 35 47.0 117 40.5 3.0 " 9/22 1:48 pm 35 44.1 117 39.3 2.5 7 mi. N of Ridgecrest 9/22 3:48 pm 35 46.0 117 39.1 3.1 10 mi. N of Ridgecrest 9/22 6:02 pm 35 45.4 117 39.0 2.6 9 mi. N of Ridgecrest 9/22 6:55 pm 35 44.1 117 39.6 2.9 7 mi. N of Ridgecrest 9/22 9:38 pm 35 47.0 117 40.4 2.5 11 mi. N of Ridgecrest 9/23 1:20 am 34 15.1 116 26.0 2.4 9 mi. N of Yucca Valley 9/23 1:21 am 34 15.4 116 26.4 2.2 " 9/23 6:09 am 34 25.2 116 30.0 2.2 21 mi. N of Yucca Valley 9/23 8:08 am 34 15.6 116 26.3 2.4 9 mi. N of Yucca Valley 9/23 8:09 am 34 15.6 116 26.0 2.8 " 9/23 8:23 am 34 32.4 116 31.8 2.1 2 mi. SW of Galway Lake 9/23 8:32 am 34 15.5 116 26.4 2.2 9 mi. N of Yucca Valley 9/23 8:42 am 34 15.2 116 26.1 2.1 " 9/23 8:43 am 34 15.2 116 26.1 2.1 " 9/23 8:46 am 34 15.3 116 26.2 2.1 " 9/23 9:00 am 34 15.3 116 26.3 2.3 " 9/23 9:03 am 34 15.1 116 26.2 3.4 " 9/23 9:23 am 34 8.5 116 27.7 2.6 1 mi. NNW of Yucca Valley 9/23 9:30 am 34 8.5 116 27.8 2.8 " FELT 9/23 9:57 am 34 15.6 116 26.4 2.1 9 mi. N of Yucca Valley 9/23 10:07 am 34 15.4 116 26.3 2.3 " 9/23 10:26 am 34 22.8 116 27.3 2.0 18 mi. N of Yucca Valley 9/23 11:54 am 34 15.4 116 26.5 2.1 9 mi. N of Yucca Valley 9/23 11:57 am 34 15.4 116 26.3 2.0 " 9/23 1:20 pm 34 15.9 116 26.4 2.3 10 mi. N of Yucca Valley 9/23 1:29 pm 35 46.4 117 39.4 2.5 10 mi. N of Ridgecrest 9/23 1:35 pm 34 15.5 116 26.5 2.3 9 mi. N of Yucca Valley 9/23 1:46 pm 35 47.0 117 38.8 2.6 11 mi. N of Ridgecrest 9/23 7:40 pm 35 44.1 117 39.5 2.8 7 mi. N of Ridgecrest 9/23 11:35 pm 34 15.1 116 26.2 2.8 9 mi. N of Yucca Valley FELT 9/23 11:46 pm 33 47.1 116 8.6 2.2 6 mi. NE of Indio 9/24 6:15 am 35 47.5 117 39.6 3.7 11 mi. N of Ridgecrest FELT 9/24 6:43 am 34 15.3 116 26.4 2.0 9 mi. N of Yucca Valley 9/24 7:34 am 35 43.6 117 37.4 3.2 7 mi. NNE of Ridgecrest 9/24 9:28 am 34 22.7 119 23.7 2.2 7 mi. E of Carpinteria 9/24 10:11 am 34 30.1 116 30.8 2.0 26 mi. N of Yucca Valley 9/24 10:11 am 34 15.4 116 26.5 2.1 9 mi. N of Yucca Valley 9/24 10:17 am 34 15.1 116 26.4 2.1 " 9/24 10:17 am 34 15.1 116 26.4 2.0 " 9/24 10:58 am 35 44.7 117 38.2 2.5 8 mi. N of Ridgecrest 9/24 10:59 am 35 44.6 117 38.3 2.5 " 9/24 11:21 am 34 27.8 116 30.6 2.0 24 mi. N of Yucca Valley 9/24 12:33 pm 34 15.4 116 26.4 2.0 9 mi. N of Yucca Valley 9/24 1:14 pm 32 19.4 115 15.4 3.4 27 mi. SSE of Calexico 9/24 1:21 pm 35 46.5 117 37.8 2.6 10 mi. N of Ridgecrest 9/24 1:39 pm 35 45.5 117 38.8 2.9 9 mi. N of Ridgecrest 9/24 6:16 pm 34 15.3 116 26.3 2.2 9 mi. N of Yucca Valley 9/24 6:53 pm 34 8.9 116 25.9 2.2 2 mi. NNE of Yucca Valley 9/24 7:00 pm 33 30.3 115 52.1 3.0 4 mi. ESE of North Shore 9/24 7:06 pm 33 30.3 115 52.1 2.0 " 9/24 7:13 pm 33 30.3 115 51.8 2.0 " 9/24 7:25 pm 33 30.2 115 51.9 2.0 " 9/24 7:28 pm 33 29.9 115 51.7 2.2 " 9/24 9:21 pm 35 48.5 117 37.1 3.6 13 mi. NNE of Ridgecrest FELT 9/24 9:28 pm 35 46.3 117 38.8 2.6 10 mi. N of Ridgecrest 9/24 9:47 pm 35 48.5 117 37.0 4.9 13 mi. NNE of Ridgecrest FELT 9/24 9:53 pm 35 48.4 117 36.8 2.7 " 9/24 10:16 pm 34 18.0 118 27.5 2.1 1 mi. NW of Pacoima 9/24 10:47 pm 35 48.2 117 37.1 3.3 12 mi. NNE of Ridgecrest FELT 9/24 11:30 pm 35 48.7 117 37.0 3.3 13 mi. NNE of Ridgecrest FELT 9/25 2:02 am 33 30.4 115 51.9 2.3 4 mi. ESE of North Shore 9/25 2:56 am 34 15.4 116 26.2 2.0 9 mi. N of Yucca Valley 9/25 3:33 am 35 44.3 117 38.2 3.6 8 mi. NNE of Ridgecrest FELT 9/25 4:17 am 34 15.3 116 26.2 2.5 9 mi. N of Yucca Valley 9/25 6:43 am 35 31.3 118 24.6 2.1 8 mi. SSE of town of Lake Isabella 9/25 6:54 am 35 46.3 117 39.2 2.2 10 mi. N of Ridgecrest 9/25 7:03 am 34 8.9 116 26.1 2.3 2 mi. NNE of Yucca Valley 9/25 8:33 am 33 2.8 116 21.9 2.7 14 mi. S of Borrego Springs 9/25 10:28 am 34 9.2 116 25.9 2.0 2 mi. NNE of Yucca Valley 9/25 10:28 am 34 8.6 116 25.9 2.5 1 mi. NNE of Yucca Valley 9/25 11:02 am 34 15.6 116 26.3 2.2 9 mi. N of Yucca Valley 9/25 2:06 pm 34 15.3 116 26.3 2.3 " 9/25 5:46 pm 35 45.6 117 37.3 2.6 9 mi. NNE of Ridgecrest 9/25 6:07 pm 34 8.7 116 25.8 2.3 2 mi. NNE of Yucca Valley 9/25 6:07 pm 34 8.6 116 26.0 2.2 1 mi. NNE of Yucca Valley 9/25 10:12 pm 33 29.9 115 51.6 2.1 4 mi. ESE of North Shore 9/26 4:32 am 34 16.0 118 26.2 2.0 1 mi. S of Pacoima 9/26 7:15 am 34 13.7 118 34.8 2.2 2 mi. SE of Chatsworth 9/26 7:34 am 34 13.9 117 28.7 2.1 Lytle Creek, 4 mi. W of Devore 9/26 9:43 am 35 47.6 117 39.7 2.7 11 mi. N of Ridgecrest 9/26 1:44 pm 35 46.4 117 38.2 2.6 10 mi. N of Ridgecrest 9/26 5:55 pm 35 45.8 117 39.0 2.5 9 mi. N of Ridgecrest 9/26 6:30 pm 35 43.8 117 37.6 2.5 7 mi. NNE of Ridgecrest 9/26 11:15 pm 34 8.8 116 25.9 2.3 2 mi. NNE of Yucca Valley 9/27 4:34 am 35 47.0 117 38.2 2.6 11 mi. N of Ridgecrest 9/27 5:30 am 34 10.3 116 26.1 2.4 3 mi. N of Yucca Valley 9/27 7:27 am 33 47.8 118 49.6 2.2 14 mi. S of Pt. Dume 9/27 7:50 am 34 15.4 116 26.3 2.9 9 mi. N of Yucca Valley FELT 9/27 7:51 am 34 15.4 116 26.2 2.1 " 9/27 12:55 pm 34 22.0 118 26.5 2.0 5 mi. N of San Fernando 9/27 5:25 pm 35 46.0 117 38.6 3.1 10 mi. N of Ridgecrest FELT 9/27 8:45 pm 33 2.5 116 26.0 2.0 9 mi. ESE of Julian 9/27 9:22 pm 34 16.0 116 27.3 2.3 10 mi. N of Yucca Valley -------------------------------------------------------------