SOUTHERN CALIFORNIA SEISMOGRAPHIC NETWORK A Cooperative Project of Caltech and the U.S. Geological Survey July 2, 1997 Weekly Earthquake Report for Southern California ------------------------------------------------ June 26 - July 1, 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, June 26, Universal Time (ie. GMT), to midnight Tuesday night, July 1, Universal Time. In local time, the period of coverage is from 5 p.m., June 25, Pacific Daylight Time, to 5 p.m., July 1, Pacific Daylight Time. We detected 223 earthquakes during the six-day period covered. The Weekly Report is coming out early this week because tomorrow is a Caltech holiday and Yours Truly does not want to come in just to write the Report. Next week's Report will cover eight days to get us back on schedule. These Weekly Reports should not be used for archival purposes. Further data processing often changes the magnitudes slightly and, especially during times of high seismicity, sometimes changes the estimates of total seismicity. For the best earthquake catalog currently available, see: http://scec.gps.caltech.edu/catalog-search.html The largest and most widely felt earthquake this week occurred on Saturday afternoon, in the San Bernardino area. It's magnitude was M4.2 and the focal mechanism was what we call "normal". Normal mechanisms indicate tension in the crust, with one block of crust dropping down with respect to the other, and they are not the most common type of event in California. However, for this particular area, they are normal, and they indicate a "graben", or dropping block of crust, between the San Andreas and San Jacinto faults. This graben is the origin of the San Bernardino Valley. This particular event was felt as far away as Pasadena and Hesperia. This week also saw activity in two places in the vicinity of the Salton Sea. The Salton City site, along the western shore, is actually near the end of a branch of the San Jacinto Fault. The largest quake in this series, M3.4 on Friday morning, showed a strike-slip mechanism, but with a strike that is close to east-west (or north-south), which is unlike the San Jacinto Fault. The largest Obsidian Butte quakes were only M2.9, but there were two of them, both on Monday. Both these events showed oblique motion, with a combination of normal and strike-slip, just what we expect from that location. Both the Salton City and Obsidian Butte sites are common sources of small events. We had a flurry of activity near the northern end of the Landers rupture, also. The largest ones were a M3.4 and a M3.2 at the familiar location northeast of Barstow, both on Sunday. They were both strike-slip events, as expected for Landers aftershocks. There was also a M2.8 on Friday, to the north of the Barstow epicenters. In addition to all this, there was a M3.4 in the Tehachapi Mtns. near Lake Isabella on Friday afternoon. It had a north-northwest right-lateral strike-slip mechanism, which is standard for that area. Lastly, there was a M3.1 Ridgecrest quake yesterday morning, at one of the usual locations in the Coso Range. Like past quakes in that area, it had a strike-slip focal mechanism. Of all these feel-able earthquakes, the San Bernardino one was the only one the produced any inquiries from the public. Notice that, in spite of the number of M3+ quakes, the total number of event is just about what it has been of late. 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 ------------------------------------------------------------- 6/25 8:21 pm 35 58.7 117 40.0 2.0 16 mi. ESE of Coso Junction 6/25 9:21 pm 33 10.4 115 35.5 2.2 2 mi. E of Obsidian Butte 6/25 10:04 pm 32 25.2 115 18.1 2.6 20 mi. SSE of Calexico 6/26 11:52 am 35 54.3 117 15.2 2.0 11 mi. NE of Trona 6/26 1:04 pm 33 15.7 116 0.5 2.6 2 mi. WSW of Salton City 6/26 2:56 pm 33 10.8 115 35.1 2.0 3 mi. ENE of Obsidian Butte 6/26 6:19 pm 34 48.3 118 56.1 2.2 1 mi. SSE of Frazier Park 6/26 11:22 pm 32 43.6 115 56.2 2.3 3 mi. E of Ocotillo 6/27 12:40 am 35 54.1 117 16.0 2.0 10 mi. NNE of Trona 6/27 5:46 am 34 58.2 116 48.2 2.0 13 mi. ENE of Barstow 6/27 7:06 am 33 15.8 116 0.3 3.4 2 mi. WSW of Salton City 6/27 8:20 am 35 14.3 116 55.1 2.8 8 mi. SSW of Goldstone Lake 6/27 10:11 am 33 10.4 115 35.5 2.4 2 mi. E of Obsidian Butte 6/27 1:17 pm 35 26.5 118 18.6 3.4 16 mi. SSE of town of Lake Isabella 6/27 5:39 pm 34 37.8 116 40.1 2.5 20 mi. NE of Lucerne Valley 6/27 9:13 pm 32 38.6 118 6.4 2.3 18 mi. SE of San Clemente Is. (SE tip) 6/28 3:42 am 34 24.7 116 28.5 2.0 20 mi. N of Yucca Valley 6/28 7:34 am 35 20.2 118 31.2 2.0 14 mi. NNW of Tehachapi 6/28 8:36 am 36 0.7 117 47.4 2.0 9 mi. ESE of Coso Junction 6/28 1:53 pm 36 0.9 117 42.9 2.3 13 mi. E of Coso Junction 6/28 2:45 pm 34 10.1 117 20.2 4.2 4 mi. NNE of Rialto WIDELY FELT 6/28 2:53 pm 34 10.0 117 20.3 2.6 " 6/28 3:11 pm 34 9.9 117 20.5 2.6 " 6/28 5:21 pm 34 10.0 117 20.4 2.5 " 6/28 9:21 pm 34 17.6 118 28.9 2.2 1 mi. NE of Granada Hills 6/28 10:13 pm 34 9.5 117 20.1 2.0 4 mi. NNE of Rialto 6/29 3:47 am 35 16.4 118 35.7 2.0 12 mi. NW of Tehachapi 6/29 11:23 am 34 22.1 116 28.1 2.1 17 mi. N of Yucca Valley 6/29 2:55 pm 35 30.7 119 25.7 2.1 25 mi. N of Taft 6/29 3:49 pm 32 39.7 117 31.4 2.1 17 mi. WSW of Mission Beach 6/29 5:00 pm 33 50.8 117 39.6 2.3 5 mi. WSW of Corona 6/29 5:41 pm 32 59.8 117 43.6 2.0 24 mi. SW of Oceanside 6/29 6:05 pm 34 58.3 116 48.8 3.4 13 mi. ENE of Barstow 6/29 6:28 pm 34 58.3 116 48.9 3.2 " 6/30 10:52 am 33 10.5 115 35.5 2.3 2 mi. E of Obsidian Butte 6/30 11:10 am 33 10.5 115 35.7 2.9 " 6/30 1:58 pm 33 2.9 116 26.1 2.1 9 mi. ESE of Julian 6/30 3:01 pm 34 58.0 116 49.2 2.0 12 mi. ENE of Barstow 6/30 6:17 pm 32 37.0 117 6.5 2.4 5 mi. NW of San Ysidro 6/30 8:15 pm 33 11.1 115 35.3 2.9 3 mi. ENE of Obsidian Butte 6/30 8:16 pm 33 10.9 115 35.2 2.4 " 6/30 8:21 pm 33 10.9 115 35.5 2.3 " 7/1 1:13 am 36 4.9 117 39.3 2.1 16 mi. E of Coso Junction 7/1 6:05 am 35 47.6 117 38.2 2.9 12 mi. N of Ridgecrest 7/1 6:46 am 35 47.8 117 38.4 2.1 " 7/1 9:58 am 33 10.6 115 35.8 2.0 2 mi. E of Obsidian Butte 7/1 10:37 am 35 47.6 117 38.2 3.1 12 mi. N of Ridgecrest 7/1 10:41 am 35 47.7 117 38.3 2.0 " 7/1 11:25 am 35 47.7 117 38.3 2.1 " 7/1 1:10 pm 33 22.7 117 58.7 2.3 16 mi. S of Newport Beach -------------------------------------------------------------