May 19, 1994 Weekly Earthquake Report for Southern California ------------------------------------------------ May 12 - 18, 1994 Prepared by: Kate Hutton, Seismological Laboratory (kate@bombay.gps.caltech.edu) Lucy Jones, U.S. Geological Survey California Institute of Technology This document is a commentary on current seismic activity. The earthquakes discussed have been detected and processed automatically. All epicenters and magnitudes have been reviewed to exclude obvious blunders; however, they must still be considered preliminary. 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 12, Pacific Daylight Time, to midnight Wednesday night, May 18, Pacific Daylight Time. We recorded 375 earthquakes this week. Aftershocks of the Northridge and Landers/Big Bear sequences continued as expected this week. Four Northridge events, all at different locations within the aftershock zone, were felt: a M2.9 on Sunday morning, a M3.0 on Wednesday morning, and a M2.8 and a M3.6 on Wednesday evening. At this time in the decay of the sequence, 4 months after the mainshock, we still expect 2 to 3 aftershocks of M3.0 or larger. This week's reality is just what was expected. A small cluster of 3 events occurred on Friday evening, just offshore about halfway between Dockweiler and Manhattan Beaches. The largest 2, a M2.5 and a M3.2, were felt by the public. We do not believe there is any particular significance to these events. Quakes of this size happen several times per year in the Santa Monica Bay, expecially in the fold-and-thrust belt associated with the Palos Verdes faut, which is where these quakes occurred. These were relatively deep, at about 13 kilometers. The focal mechanism for the largest shows oblique reverse slip, meaning a mixture of strike-slip and thrusting, which is common for the small earthquakes in the Santa Monica Bay. It is interesting that, over the last two decades, we have seen many small earthquakes with oblique motion, but all of the larger events (above about M4.5) are either strike-slip or thrust. In summary, the week's seismicity was very normal. The table lists the quakes 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 Daylight Time listed. Table 1 ------- Date Time N Lat. W Long. Mag ------------------------------------------------------------- 5/12 1:35 am 32 47.0 115 27.4 2.0 5 mi. E of El Centro 5/12 4:12 am 34 36.5 116 38.8 2.2 20 mi. NE of Lucerne Valley 5/12 3:11 pm 34 36.2 116 38.4 2.2 " 5/12 3:26 pm 34 35.3 116 37.1 2.0 " 5/13 1:55 am 34 25.1 120 45.4 2.3 12 mi. SSW of Pt. Arguello 5/13 2:40 am 34 16.7 118 27.4 2.2 1 mi. WSW of San Fernando 5/13 2:40 am 34 16.6 118 27.8 2.2 " 5/13 2:58 am 34 6.8 116 59.5 2.1 9 mi. SE of Running Springs 5/13 10:15 am 32 56.8 116 17.9 2.2 16 mi. SW of Ocotillo Wells 5/13 2:58 pm 36 34.2 118 22.1 2.1 4 mi. W of Mt. Whitney 5/13 3:58 pm 33 28.5 116 29.7 2.1 11 mi. ESE of Anza 5/13 7:26 pm 33 54.9 118 26.4 2.5 2 mi. S of Los Angeles Airport; FELT 5/13 7:35 pm 33 54.8 118 26.3 3.2 " FELT 5/13 8:52 pm 33 55.2 118 26.8 2.0 " 5/14 12:22 am 34 7.4 116 23.9 2.0 2 mi. E of Yucca Valley 5/14 1:29 am 34 16.9 116 41.0 2.8 9 mi. E of Big Bear City 5/14 6:11 am 34 18.2 118 28.3 2.1 2 mi. NW of San Fernando 5/14 1:10 pm 34 6.8 116 59.3 2.7 9 mi. SE of Running Springs 5/14 3:52 pm 34 6.9 116 59.3 2.3 " 5/14 5:47 pm 34 14.3 118 29.1 2.1 2 mi. E of Northridge 5/14 7:20 pm 34 22.6 118 34.7 2.4 3 mi. S of Magic Mtn. 5/14 9:35 pm 35 56.6 118 20.5 2.0 22 mi. NNE of the town of Lake Isabella 5/15 4:04 am 32 37.0 117 6.6 2.2 5 mi. NW of San Ysidro 5/15 10:45 am 34 13.7 118 28.8 2.9 3 mi. E of Northridge; FELT 5/15 5:05 pm 34 14.5 118 34.8 2.2 2 mi. W of Northridge 5/15 5:05 pm 34 23.4 118 38.2 2.4 3 mi. SW of Magic Mtn. 5/16 12:22 am 34 14.9 118 35.0 2.0 2 mi. WNW of Northridge 5/16 12:44 am 34 15.7 118 28.6 2.1 2 mi. WSW of San Fernando 5/16 1:40 am 34 19.8 118 37.1 3.7 6 mi. SSW of Magic Mtn. 5/16 1:42 am 34 19.5 118 37.0 2.4 " 5/16 2:10 am 34 19.8 118 37.0 2.6 " 5/16 10:01 am 34 15.2 118 28.4 2.1 2 mi. SW of San Fernando 5/16 1:53 pm 34 13.1 118 34.5 2.1 2 mi. NE of Canoga Park 5/17 7:00 am 34 12.5 116 26.9 2.1 6 mi. N of Yucca Valley 5/17 7:08 am 36 1.5 117 24.7 2.6 17 mi. N of Trona 5/17 4:13 pm 33 54.4 117 7.6 2.2 8 mi. W of Beaumont 5/18 1:47 am 34 18.5 116 26.4 2.3 13 mi. N of Yucca Valley 5/18 1:47 am 34 18.3 116 26.5 2.4 " 5/18 5:58 am 34 17.8 118 33.4 2.0 4 mi. NNW of Northridge 5/18 7:01 am 34 15.4 118 34.9 2.5 3 mi. WNW of Northridge 5/18 9:51 am 33 58.4 116 58.2 2.6 3 mi. N of Beaumont 5/18 11:54 am 34 17.7 118 33.3 3.0 4 mi. NNW of Northridge FELT 5/18 8:02 pm 34 13.9 118 36.5 2.8 2 mi. N of Canoga Park FELT 5/18 11:12 pm 34 19.4 118 26.2 3.6 2 mi. N of San Fernando FELT ------------------------------------------------------------- Figure 1. A map of southern California showing the earthquakes recorded during the past week by the Caltech/USGS Seismic Network. Major faults are marked, as well as the metropolitan areas of Los Angeles (L.A.), Palm Springs (P.S.), San Diego (S.D.), and Santa Barbara (S.B.). The circles denote the earthquakes, the size of the circle indicating the magnitude.