SOUTHERN CALIFORNIA SEISMOGRAPHIC NETWORK A Cooperative Project of Caltech and the U.S. Geological Survey May 11, 1995 Weekly Earthquake Report for Southern California ------------------------------------------------ May 4 - 10, 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, May 4, Pacific Daylight Time, to midnight Wednesday night, May 10, 1995, Pacific Daylight Time. We recorded 272 earthquakes of M1.0 or larger during the 7-day period covered. The biggest excitement, and the biggest earthquake, this week was a M4.8 Landers aftershock at 4:03 am on Sunday. It was widely felt in the Palm Springs, Indio, and Yucca Valley areas, with sporadic reports from as far away as Los Angeles and San Diego. Initially, we reported it at M5.0, but further work has yielded the slightly lower magnitude. This quake is considered an aftershock in the Joshua Tree (M6.1 on April 23, 1992)/Landers (M7.3 sequence. The aftershock zone of the Joshua Tree earthquake was reactivated by the Landers earth- quake, so we group them together into one sequence. We have included an extra map this week, showing the earthquakes of this region in 1994 and 1995. This week's earthquake is the circle located due west of the end of the Blue Cut fault. The Joshua Tree aftershocks were primarily located between the Pinto Mountain and the Blue Cut faults. The Landers earthquake ruptured faults north of the Pinto Mountain fault with a small amount of surface offsets just south of the Pinto Mountain. Even though most of the Landers faulting was to the north, the earthquake rate in the Joshua Tree zone and south of the Joshua Tree zone to the San Andreas fault increased dramatically after Landers. This week's earthquake occurred near the Blue Cut fault at the southern end of the Joshua Tree zone. Its focal mechanism was strike-slip, consistent with right-lateral motion on the north-northwest striking fault plane responsible for the Joshua Tree earthquake. On Sunday afternoon, there was a M3.1 at the same location, which was felt in the Palm Springs area. Recall that we said in the January 5, 1995 Weekly Report that we expected, statistically, about three M4's from the Landers/Big Bear sequence during the course of this year, and about a 25% chance of M5. Aside from this excitement in the desert, there is nothing remarkable about the week's seismicity. We had one more small aftershock of the tiny Inglewood sequence of last week (largest M3.1): it was a M2.1 on Thursday evening. The table 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 ------------------------------------------------------------- 5/4 5:06 am 34 56.6 116 53.3 2.0 8 mi. ENE of Barstow 5/4 5:24 am 34 28.6 116 30.8 2.0 25 mi. N of Yucca Valley 5/4 8:30 am 34 24.8 116 33.8 2.0 21 mi. NNW of Yucca Valley 5/4 10:18 am 34 0.1 116 18.3 2.1 11 mi. SE of Yucca Valley 5/4 2:33 pm 36 3.2 117 30.3 2.2 19 mi. S of Panamint Springs 5/4 4:28 pm 34 28.7 116 29.9 2.0 25 mi. N of Yucca Valley 5/4 9:22 pm 33 57.0 118 21.7 2.1 1 mi. W of Hollywood Park Racetrack, Inglewood 5/5 6:10 am 34 21.2 116 27.3 2.1 16 mi. N of Yucca Valley 5/5 6:20 am 36 2.2 117 31.1 2.0 21 mi. S of Panamint Springs 5/5 4:20 pm 32 30.6 115 17.3 2.0 16 mi. SE of Calexico 5/5 5:14 pm 33 21.6 116 23.2 2.4 7 mi. N of Borrego Springs 5/6 7:15 pm 34 46.8 116 17.3 2.0 35 mi. SSW of Baker 5/7 2:15 am 34 27.0 116 30.2 2.2 23 mi. N of Yucca Valley 5/7 4:03 am 33 54.3 116 17.3 4.8 13 mi. NNW of Indio; WIDELY FELT 5/7 4:20 am 33 54.7 116 18.3 2.0 " 5/7 4:55 am 33 54.2 116 17.9 2.4 " 5/7 5:21 am 33 54.5 116 18.0 2.3 " 5/7 8:52 am 33 54.5 116 17.6 2.0 " 5/7 10:52 am 34 23.7 118 35.0 2.0 1 mi. S of Magic Mtn. 5/7 12:07 pm 33 56.4 116 18.5 2.4 14 mi. NNW of Indio 5/7 1:20 pm 34 35.5 116 36.6 2.0 21 mi. ENE of Lucerne Valley 5/7 3:05 pm 33 54.3 116 17.8 3.1 14 mi. NNW of Indio; FELT 5/8 11:10 am 33 28.0 116 25.9 2.7 14 mi. NNW of Borrego Springs 5/8 12:10 pm 33 53.3 116 17.1 2.6 12 mi. NNW of Indio 5/8 11:38 pm 34 0.1 119 2.1 2.0 6 mi. WSW of Leo Carrillo State Beach 5/9 3:38 am 34 22.1 116 28.2 2.7 17 mi. N of Yucca Valley 5/9 7:27 am 34 10.8 116 25.7 2.0 4 mi. NNE of Yucca Valley 5/9 9:40 am 34 17.0 118 25.0 2.0 1 mi. E of San Fernando 5/9 10:05 am 35 55.9 118 23.8 2.0 20 mi. N of the town of Lake Isabella 5/9 11:53 am 34 6.9 117 19.2 2.2 1 mi. WNW of downtown San Bernardino 5/9 7:00 pm 35 46.4 117 44.1 2.3 11 mi. NNW of Ridgecrest 5/9 10:25 pm 35 2.2 117 0.2 2.1 10 mi. N of Barstow 5/10 6:02 am 34 1.9 116 39.5 2.1 10 mi. NE of Cabazon 5/10 1:13 pm 35 50.9 117 37.9 2.7 15 mi. N of Ridgecrest 5/10 1:20 pm 33 48.1 116 57.5 2.2 3 mi. N of Hemet 5/10 1:20 pm 35 50.7 117 37.8 2.4 15 mi. N of Ridgecrest 5/10 3:47 pm 33 54.4 116 17.6 2.1 13 mi. NNW of Indio 5/10 6:05 pm 34 56.4 116 40.2 2.4 20 mi. E of Barstow -------------------------------------------------------------