SOUTHERN CALIFORNIA SEISMOGRAPHIC NETWORK A Cooperative Project of Caltech and the U.S. Geological Survey April 6, 1995 Weekly Earthquake Report for Southern California ------------------------------------------------ March 30 - April 5, 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, March 30, Pacific Standard Time, to midnight Wednesday night, April 5, 1995, Pacific Daylight Time. We recorded 305 earthquakes of M1.0 or larger during the 7-day period covered. Interestingly, none of the M3+ quakes this week was an aftershock. Perhaps the most excitement occurred Monday night, with the two feelable quakes near the San Jacinto Fault, at different locations. The first, M3.7, was in Cajon Pass at the near the intersection of the San Jacinto with the San Andreas Fault, at 10:08 pm, while the second occurred 78 minutes later near Loma Linda. Both were felt in the San Bernardino area. It turns out that neither was actually on the San Jacinto Fault itself, or the San Andreas. The first quake was produced by thrust on a northwest-striking fault, similar to the mechanism of the 1970 Lytle Creek earthquake (M5.2), that occurred in the same area in the compression zone between the San Jacinto and San Andreas Faults. The second quake was produced by a northeast-striking "normal" fault at a depth of 18 km. This could be a projection at depth of the normal Crafton Hills Fault. A normal fault is the opposite of a thrust fault, with downward motion of a block along a dipping fault plane. There were three other M3+'s this week: a M3.2 on Saturday night just south of the U.S./Mexican border, a M3.1 on Monday morning near Ridgecrest, and a M3.3 near Cholame yesterday. For more information on the Cholame quake, see the Northern California Earthquake Report. Since we have just finished the first quarter of 1995, it is time to look again at our seismicity statistics. We had the fewest number of earthquakes this quarter than we have had since before the Joshua Tree earthquake in 1992, only 3,818 inside our recording network. The following table shows the quarterly totals for the past few years, including both the total number events and the number above M3. 1st quarter 2nd quarter 3rd quarter 4th quarter all 3.0+ all 3.0+ all 3.0+ all 3.0+ 1990 3,483 56 2,627 45 2,225 27 2,403 31 1991 2,041 11 2,218 32 2,445 23 2,651 29 1992 3,618 40 10,288 744 25,000+ 781 10,700+ 166 1993 4,939 43 5,370 61 5,298 48 4,583 35 1994 10,915 417 5,978 44 5,400 50 5,040 39 1995 3,818 29 There were 399 more Northridge aftershocks this quarter, making a total of 11,426 events since the Mw6.7 (moment magnitude) mainshock on January 17, 1994. There were 1,516 more Landers/Big Bear aftershocks, for a total of over 45,000. We still have not finished processing all of them, so we do not know the exact total. By the way, if the Earth stays quiet, we expect to be finished with all the 1992 earthquakes, including most of the Landers/Big Bear aftershocks, by the end of the summer. 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 ------------------------------------------------------------- 3/30 5:44 am 32 57.4 117 42.1 2.1 24 mi. SW of Oceanside 3/30 6:03 am 34 22.0 116 27.7 2.4 17 mi. N of Yucca Valley 3/30 6:08 am 34 22.7 116 27.9 2.2 " 3/30 6:08 am 34 22.3 116 27.4 2.2 " 3/30 10:10 am 33 32.8 115 53.9 2.3 2 mi. ENE of North Shore 3/30 1:45 pm 34 13.5 118 22.9 2.0 2 mi. NW of Burbank Airport 3/30 3:20 pm 34 8.1 116 35.7 2.1 8 mi. W of Yucca Valley 3/30 6:48 pm 33 1.0 116 14.9 2.2 11 mi. SW of Ocotillo Wells 3/31 3:22 am 34 41.0 120 15.8 2.1 9 mi. NW of Solvang 3/31 6:43 am 33 50.2 118 48.6 2.0 11 mi. S of Pt. Dume 3/31 10:02 am 33 5.7 116 4.2 2.5 4 mi. SE of Ocotillo Wells 4/1 5:30 am 34 9.3 116 25.3 2.1 2 mi. NNE of Yucca Valley 4/1 7:57 am 33 58.1 118 41.1 2.3 4 mi. S of Malibu 4/1 9:01 am 32 57.8 115 52.9 2.0 17 mi. NNE of Ocotillo 4/1 11:41 am 35 59.2 117 60.0 2.5 4 mi. SW of Coso Junction 4/1 1:51 pm 33 43.6 118 0.8 2.0 5 mi. E of Seal Beach 4/1 4:16 pm 36 9.6 118 7.4 2.8 10 mi. SW of Olancha 4/1 5:04 pm 34 11.5 116 49.6 2.0 5 mi. SSE of Big Bear City 4/1 7:55 pm 32 39.4 115 54.5 2.0 7 mi. SE of Ocotillo 4/1 10:02 pm 34 47.7 116 17.8 2.1 34 mi. SSW of Baker 4/1 11:49 pm 32 28.1 115 28.3 2.3 13 mi. S of Calexico 4/1 11:49 pm 32 27.9 115 28.2 3.2 " 4/2 9:13 am 36 10.5 118 7.2 2.0 9 mi. SW of Olancha 4/2 9:17 am 36 10.2 118 7.6 2.0 " 4/2 11:27 am 34 24.4 116 28.2 2.0 19 mi. N of Yucca Valley 4/2 3:37 pm 35 21.3 118 37.2 2.0 15 mi. NE of Arvin 4/3 12:54 am 35 38.7 118 58.0 2.1 17 mi. N of downtown Bakersfield 4/3 5:27 am 33 59.5 116 48.0 2.6 4 mi. N of Cabazon 4/3 9:13 am 35 50.9 117 37.9 3.1 15 mi. N of Ridgecrest 4/3 10:21 am 34 23.3 116 28.1 2.2 18 mi. N of Yucca Valley 4/3 10:27 am 34 58.9 116 57.1 2.2 7 mi. NNE of Barstow 4/3 10:47 am 33 1.5 115 34.5 2.1 2 mi. ESE of Westmorland 4/3 10:47 am 33 43.8 118 39.3 2.7 13 mi. WSW of Palos Verdes Point 4/3 7:09 pm 34 23.3 116 27.9 2.0 18 mi. N of Yucca Valley 4/3 10:08 pm 34 16.0 117 28.1 3.7 Cajon Pass, 4 mi. NW of the I15/I215 interchange 4/3 11:26 pm 34 3.2 117 15.5 3.5 Loma Linda, 2 mi. ESE of the I10/I215 interchange 4/4 12:21 am 32 44.3 116 5.1 2.3 5 mi. W of Ocotillo 4/4 12:36 am 36 2.4 117 52.3 2.3 4 mi. E of Coso Junction 4/4 2:25 pm 35 50.3 117 44.2 2.2 15 mi. NNW of Ridgecrest 4/4 2:44 pm 34 8.7 117 1.5 2.0 6 mi. SE of Running Springs 4/4 5:12 pm 36 1.1 117 40.4 2.3 15 mi. E of Coso Junction 4/4 6:14 pm 34 33.0 116 31.3 2.0 25 mi. ENE of Lucerne Valley 4/5 12:15 am 34 8.6 116 26.0 2.0 1 mi. NNE of Yucca Valley 4/5 5:40 am 34 2.9 117 2.3 2.0 8 mi. NNW of Beaumont 4/5 1:20 pm 35 46.2 120 19.8 3.3 3 mi. NNW of Cholame 4/5 2:49 pm 32 30.0 115 26.9 2.2 11 mi. SSE of Calexico 4/5 3:20 pm 33 11.3 115 35.9 2.1 5 mi. SW of Niland 4/5 10:24 pm 34 23.0 116 27.1 2.4 18 mi. N of Yucca Valley 4/5 11:12 pm 32 28.0 115 28.0 2.0 14 mi. S of Calexico -------------------------------------------------------------