SOUTHERN CALIFORNIA SEISMOGRAPHIC NETWORK A Cooperative Project of Caltech and the U.S. Geological Survey July 31, 1997 Weekly Earthquake Report for Southern California ------------------------------------------------ July 24 - 30, 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, July 24, Universal Time (ie. GMT), to midnight Wednesday night, July 30, Universal Time. In local time, the period of coverage is from 5 p.m., July 23, Pacific Daylight Time, to 5 p.m., July 30, Pacific Daylight Time. We detected 334 earthquakes during the seven-day period covered. 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 This week, the weekend in particular, was about as interesting as last week was dull. On Friday evening, eastern San Diego County experienced a M4.9 quake that was widely felt. The epicenter was along a branch of the San Jacinto Fault near Clark (dry) Lake, north of Borrego Springs. The focal mechanism was strike-slip, consistent with the San Jacinto Fault. There were only a small number of aftershocks, three above M2, to be exact; the largest was a 3.1 early the next morning. The earthquake and its aftershocks were on a deep part of the fault, 12 km (7 miles) below the Earth's surface, and probably below the part of the fault that breaks in larger (M6-7) events. Deep earthquakes on the San Jacinto fault commonly have relatively few aftershocks. The M4.9 was not an aftershock of any other event. On Saturday, three unrelated quakes occurred, two of which were felt. One occurred in the wee hours of Saturday morning, in the San Bernardino area, between the San Andreas and the San Jacinto Faults. It's focal mechanism shows that it was caused by almost vertical ("normal") motion on a very steeply dipping, north-south striking fault, inconsistent with either of the two nearby major faults. This quake was part of an ongoing sequence that began with a M4.2 on June 28. Another event (M3.1) occurred Saturday afternoon near Simi Valley. It was an aftershock of the Northridge quake, and, like almost all Northridge aftershocks, it had a thrust mechanism. The Coso Range are also experienced some activity over the weekend. There were two M3.4's, at different locations: one late last Wednesday evening, and one (you guessed it!) early Saturday morning. They were both largely strike-slip. Early Sunday morning, there was another M3.4, this time along the Garlock Fault south of Death Valley. The mechanism is difficult to determine, because the epicenter was on the fringes of the seismic network, but it could be strike-slip consistent with the Garlock Fault. Then on Tuesday afternoon, as small quake (M2.7) in the Yorba Linda area was felt. This quake was the largest member of a small cluster, which included three events above M2. To find out more about the Parkfield quake last Thursday afternoon, see the Northern California Earthquake Report. 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 ------------------------------------------------------------- 7/23 9:47 pm 36 5.9 117 52.7 3.4 5 mi. NE of Coso Junction 7/23 9:57 pm 36 5.8 117 52.5 2.2 " 7/23 11:30 pm 36 3.9 117 38.3 2.1 17 mi. E of Coso Junction 7/24 6:12 am 33 11.8 115 35.3 2.3 3 mi. NE of Obsidian Butte 7/24 1:37 pm 32 38.4 118 10.5 2.4 15 mi. SE of San Clemente Is. (SE tip) 7/24 4:43 pm 35 48.5 120 24.3 3.4 6 mi. SSE of Parkfield 7/25 12:00 am 32 24.6 115 10.9 3.0 25 mi. SE of Calexico 7/25 2:08 pm 33 11.4 115 35.5 2.5 2 mi. ENE of Obsidian Butte 7/25 8:14 pm 33 23.9 116 21.2 4.9 10 mi. N of Borrego Springs; WIDELY FELT 7/25 10:36 pm 33 11.7 115 35.5 2.2 3 mi. ENE of Obsidian Butte 7/26 1:15 am 33 11.1 115 35.8 2.1 " 7/26 3:24 am 34 9.5 117 19.8 3.7 4 mi. NNW of San Bernardino; FELT 7/26 4:06 am 34 7.1 117 26.9 2.4 1 mi. NW of Fontana 7/26 4:53 am 33 19.7 116 23.5 3.1 5 mi. NNW of Borrego Springs 7/26 5:46 am 35 45.2 118 1.1 3.4 20 mi. S of Coso Junction 7/26 6:37 am 36 5.9 117 40.0 2.0 16 mi. ENE of Coso Junction 7/26 9:24 am 34 21.1 116 28.6 2.1 16 mi. N of Yucca Valley 7/26 9:59 am 34 3.8 116 22.1 2.4 5 mi. SSW of Joshua Tree 7/26 12:44 pm 35 47.5 117 37.7 2.0 11 mi. N of Ridgecrest 7/26 1:04 pm 34 34.9 120 48.4 2.2 9 mi. W of Pt. Arguello 7/26 4:13 pm 35 47.3 117 37.8 2.0 11 mi. N of Ridgecrest 7/26 5:06 pm 34 21.4 118 41.9 3.1 5 mi. N of Simi Valley FELT 7/26 9:26 pm 34 15.4 116 27.2 2.6 9 mi. N of Yucca Valley 7/26 9:26 pm 34 15.7 116 27.3 2.2 " 7/27 1:13 am 35 36.1 116 28.6 3.4 28 mi. SSW of Shoshone 7/27 3:42 am 33 24.1 116 21.9 2.4 10 mi. N of Borrego Springs 7/27 9:05 am 33 19.6 115 41.9 2.0 2 mi. SE of Bombay Beach 7/27 1:30 pm 36 25.4 120 16.6 2.5 20 mi. NNE of Coalinga 7/27 7:23 pm 34 59.0 116 56.7 2.4 7 mi. NE of Barstow 7/28 12:23 am 32 43.4 115 57.2 2.7 2 mi. ESE of Ocotillo 7/28 3:03 pm 34 20.8 116 28.2 2.0 15 mi. N of Yucca Valley 7/28 3:06 pm 34 20.7 116 28.4 2.0 " 7/28 5:30 pm 34 33.0 116 30.0 2.1 30 mi. N of Yucca Valley 7/28 7:01 pm 34 20.8 116 28.3 2.2 15 mi. N of Yucca Valley 7/29 1:56 am 33 49.5 117 44.7 2.5 5 mi. SE of Yorba Linda 7/29 9:50 am 33 52.0 117 50.2 2.6 2 mi. SW of Yorba Linda 7/29 12:42 pm 35 43.6 117 38.5 2.3 7 mi. N of Ridgecrest 7/29 1:17 pm 33 52.5 117 49.8 2.7 1 mi. SW of Yorba Linda FELT 7/29 2:19 pm 34 23.7 116 27.3 2.3 19 mi. N of Yucca Valley 7/29 9:13 pm 34 23.2 118 38.1 2.1 4 mi. W of Valencia 7/30 12:05 am 34 19.1 117 6.7 2.0 6 mi. NE of Lake Arrowhead 7/30 6:18 am 33 16.1 116 0.4 2.4 2 mi. WSW of Salton City 7/30 7:39 am 33 21.1 116 24.6 2.5 7 mi. NNW of Borrego Springs 7/30 7:44 am 36 6.0 117 52.4 2.3 5 mi. NE of Coso Junction 7/30 8:11 am 36 12.2 118 20.3 2.2 19 mi. WSW of Olancha 7/30 3:37 pm 34 20.4 116 27.9 2.2 15 mi. N of Yucca Valley -------------------------------------------------------------