February 10, 1994 Weekly Earthquake Report for Southern California ------------------------------------------------ February 3 - 9, 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 Tuesday morning, February 3, Pacific Standard Time, to midnight Wednesday night, February 9, Pacific Standard Time. There are 866 this week. The data processing, although not totally complete, is more so than it has been since the Northridge quake. This allows a larger number of small events to be included on the map and in the weekly total. The total does not mean that the activity has increased since last week; in fact, it has decrease substantially. Our near return to our regular data procedures also accounts for the reappearance of substantial numbers of quakes in other (non- Northridge) parts of the network. Again, the relative quiet since Jan. 17 was due to our not having processed all the events yet. This week, in addition to the M3.4 in the Anza segment of the San Jacinto fault near Borrego Springs, the Landers/Big Bear aftershock zone produced 2 events that exceeded M3.0. That should serve as a reminder that aftershocks normally continue for years following a major quake. This week shows a slight increase, over what we have seen recently, in other parts of the greater L.A. area, including Malibu, the L.A. Basin, and Monrovia/Baldwin Park area. All the quakes were small and none were reported to us as felt. It is normal for areas surrounding a big earthquake to increase, or in some cases decrease, their activity slightly as the crust readjusts to the new stress regime. This often happens in old aftershock zones; however, that appears not to be the case this time. Experience has shown that aftershocks follow statistically predictable patterns. The Northridge sequence is following these patterns well, so we can predict how many aftershocks can be expected over the next, say, year. Aftershocks come in all magnitudes, but smaller magnitudes are much more common. The largest Northridge aftershock, so far, was the M5.6 event that occurred 11 hr after the main shock. The totals, so far, are 5 aftershocks above M5.0 44 aftershocks of M4.0 to M4.9 343 aftershocks of M3.0 to M3.9 Three of the M5's occurred on the day of the main shock, Jan. 17, one occurred on Jan. 19, and one occurred on Jan. 29. One of the Jan. 17 M5's was previously listed as a M4.9. Another, that occurred 1 min. after the main shock and was felt by nearly everyone (but is difficult to see in the records due to the main shock) has been newly added the list. The relative numbers of small to large aftershocks is almost exactly average and implies 1 M5 for every 90 M3's. The rate of aftershocks died off quickly with time, as expected. Even the second day had many fewer (56 above M3) than the first (188 above M3). The Northridge sequence is dying off slightly more quickly than average but is also more active than average for California. By comparison, the '71 San Fernando sequence was smaller than average, but the '33 Long Beach sequence was even more energetic than Northridge. By fitting an equation to the 3 weeks of after- shocks so far, we can estimate the number to be expected in the future. We expect about 10 aftershocks of M3.0 or larger in the next week, of which 1 will probably be above M4.0, within the Northridge aftershock zone. We will probably have 1 more aftershock above M5.0 in the next year. The table lists the quakes that were M2.0 or larger in the central part of the coverage area. Aftershocks must be at least M2.5 to be listed. Times are local times; if you want Greenwich Mean Time, add 8 hrs, to the Pacific Standard Time listed. Table 1 ------- Date Time N Lat. W Long. Mag ------------------------------------------------------------- 2/3 12:21 am 34 18.3 118 26.1 3.0 1 mi. N of San Fernando 2/3 3:19 am 34 26.1 116 28.6 2.3 22 mi. N of Yucca Valley 2/3 4:32 am 34 18.0 118 29.6 2.8 3 mi. WNW of San Fernando 2/3 4:33 am 34 17.5 118 30.4 2.5 4 mi. NNE of Northridge 2/3 6:18 am 34 1.4 118 55.7 2.6 1 mi. SSE of Leo Carrillo State Beach 2/3 6:24 am 34 15.8 116 27.0 2.2 9 mi. N of Yucca Valley 2/3 7:40 am 34 57.5 116 55.4 2.0 7 mi. NE of Barstow 2/3 8:09 am 36 2.6 117 53.6 2.4 3 mi. E of Coso Junction 2/3 8:23 am 34 18.0 118 26.3 4.2 1 mi. N of San Fernando WIDELY FELT 2/3 10:00 am 34 18.1 118 27.3 2.5 1 mi. NNW of San Fernando 2/3 3:30 pm 34 11.0 116 25.0 2.2 4 mi. NNE of Yucca Valley 2/3 3:38 pm 33 14.8 116 19.6 2.9 2 mi. ESE of Borrego Springs 2/3 4:10 pm 36 29.3 116 54.5 3.5 4 mi. NW of Furnace Creek, Death Valley 2/3 4:21 pm 36 26.0 116 59.8 2.4 " 2/3 4:50 pm 34 45.0 116 25.4 2.3 36 mi. NE of Lucerne Valley 2/3 5:34 pm 34 10.2 116 25.0 2.3 3 mi. NNE of Yucca Valley 2/3 6:23 pm 36 26.2 116 59.2 2.1 4 mi. NW of Furnace Creek, Death Valley 2/3 8:49 pm 34 17.9 118 24.7 3.0 1 mi. NE of San Fernando 2/3 10:33 pm 34 16.5 118 37.5 3.5 5 mi. NNW of Canoga Park; FELT 2/4 12:00 am 34 22.6 116 27.4 2.2 17 mi. N of Yucca Valley 2/4 12:38 am 32 55.3 116 13.5 2.8 16 mi. SSW of Ocotillo Wells 2/4 12:39 am 33 56.5 118 22.7 2.0 2 mi. WSW of Hollywood Park Racetrack 2/4 3:08 am 34 10.0 116 25.2 2.5 3 mi. NNE of Yucca Valley 2/4 5:03 am 34 9.4 116 25.8 2.4 " 2/4 6:26 am 34 16.0 118 24.2 3.3 2 mi. ESE of San Fernando; FELT 2/4 8:25 am 34 18.4 118 36.9 2.7 5 mi. ENE of Simi Valley 2/4 8:28 am 34 19.9 116 49.8 2.0 4 mi. NNE of Big Bear City 2/4 12:15 pm 34 18.5 118 26.5 3.0 1 mi. N of San Fernando 2/4 10:45 pm 33 0.8 117 50.7 2.3 31 mi. SSW of San Clemente 2/4 11:11 pm 34 13.7 118 36.1 2.5 1 mi. N of Canoga Park 2/5 12:17 am 34 22.5 118 38.2 2.8 4 mi. SW of Magic Mtn. 2/5 12:29 am 34 14.5 117 6.3 2.3 2 mi. N of Running Springs 2/5 12:51 am 34 22.1 118 39.0 3.0 4 mi. SW of Magic Mtn.; FELT 2/5 12:51 am 34 24.7 118 40.9 3.7 " FELT 2/5 12:58 am 34 22.5 118 38.1 2.9 " 2/5 12:58 am 34 22.2 118 38.6 2.8 " 2/5 12:59 am 34 22.3 118 39.1 2.7 " 2/5 1:53 am 34 22.3 118 38.0 2.7 " 2/5 2:48 am 32 36.8 115 54.8 2.1 9 mi. SSE of Ocotillo 2/5 3:19 am 34 18.5 118 31.6 2.8 4 mi. S of Newhall 2/5 5:03 am 33 25.4 116 22.6 2.9 11 mi. N of Borrego Springs 2/5 7:20 am 34 37.3 116 37.6 2.5 21 mi. NE of Lucerne Valley 2/5 11:48 am 34 19.2 116 27.3 2.4 13 mi. N of Yucca Valley 2/5 11:54 am 34 18.2 118 31.7 2.5 4 mi. N of Northridge 2/5 1:18 pm 34 6.9 118 30.0 2.9 Mandeville Canyon, 4 mi. NW of Westwood Village 2/5 2:38 pm 34 56.5 116 47.4 2.2 13 mi. ENE of Barstow 2/5 4:43 pm 33 2.7 117 50.6 2.3 29 mi. SSW of San Clemente 2/5 7:15 pm 34 34.0 116 24.8 3.2 31 mi. N of Yucca Valley 2/5 8:30 pm 34 56.4 116 47.1 2.5 13 mi. ENE of Barstow 2/5 8:34 pm 32 38.8 115 53.6 2.0 8 mi. SE of Ocotillo 2/6 1:22 am 33 46.4 120 8.5 2.1 12 mi. S of Santa Rosa Is. 2/6 2:00 am 34 22.4 118 39.7 3.2 5 mi. SW of Magic Mtn. 2/6 3:22 am 33 25.0 117 0.9 2.6 9 mi. WNW of Palomar Observatory 2/6 4:19 am 36 4.5 117 52.4 2.0 4 mi. ENE of Coso Junction 2/6 4:23 am 34 2.2 116 56.9 2.0 7 mi. NNE of Beaumont 2/6 5:19 am 34 14.4 118 26.3 3.2 2 mi. S of San Fernando; FELT 2/6 5:19 am 34 17.5 118 28.6 4.0 2 mi. WNW of San Fernando WIDELY FELT 2/6 5:21 am 34 17.4 118 28.6 3.7 2 mi. WNW of San Fernando; FELT 2/6 6:04 am 34 56.4 116 47.4 2.5 13 mi. ENE of Barstow 2/6 6:19 am 34 17.5 118 28.5 2.6 2 mi. WNW of San Fernando 2/6 8:33 am 36 26.3 116 59.3 2.1 7 mi. W of Furnace Creek, Death Valley 2/6 8:51 am 34 10.9 116 24.5 2.3 4 mi. NNE of Yucca Valley 2/6 10:10 am 34 17.6 116 55.1 2.4 3 mi. N of town of Big Bear Lake 2/6 10:11 am 33 47.7 118 7.7 2.0 2 mi. NE of downtown Long Beach 2/6 11:09 am 34 12.4 116 48.1 2.6 5 mi. SE of Big Bear City 2/6 6:02 pm 34 57.5 116 46.8 2.1 14 mi. ENE of Barstow 2/6 7:51 pm 33 48.7 116 59.8 2.4 4 mi. NNW of Hemet 2/6 8:13 pm 34 22.1 118 26.3 2.5 5 mi. N of San Fernando 2/7 6:33 am 32 12.4 117 44.2 2.7 55 mi. SSE of San Clemente Is. 2/7 2:52 pm 34 19.7 118 37.3 2.9 5 mi. NE of Simi Valley 2/7 5:18 pm 34 37.3 116 36.3 2.2 22 mi. ENE of Lucerne Valley 2/7 6:07 pm 34 13.8 118 35.9 2.6 2 mi. N of Canoga Park 2/7 6:46 pm 33 13.7 116 11.7 2.1 6 mi. NNW of Ocotillo Wells 2/7 7:17 pm 34 53.3 116 54.8 2.0 6 mi. E of Barstow 2/7 9:50 pm 32 8.3 117 49.3 2.9 56 mi. SSE of San Clemente Is. 2/7 9:52 pm 34 1.8 118 50.2 2.2 2 mi. NW of Pt. Dume 2/7 9:59 pm 34 57.3 116 55.9 2.2 6 mi. NE of Barstow 2/7 10:30 pm 34 17.6 118 30.8 2.7 4 mi. NNE of Northridge 2/7 10:42 pm 34 13.0 118 37.4 2.5 1 mi. NW of Canoga Park 2/7 11:06 pm 34 20.4 118 36.2 2.5 5 mi. S of Magic Mtn. 2/8 3:16 am 34 20.4 118 32.3 2.9 2 mi. SSW of Newhall 2/8 6:23 am 34 21.3 118 43.1 2.8 5 mi. N of Simi Valley 2/8 11:31 am 35 9.3 118 33.7 2.4 6 mi. WNW of Tehachapi 2/8 3:13 pm 35 18.4 118 32.7 2.5 13 mi. NNW of Tehachapi 2/8 4:09 pm 34 19.8 118 38.0 2.6 5 mi. NE of Simi Valley 2/8 9:05 pm 33 14.4 116 6.6 2.1 6 mi. N of Ocotillo Wells 2/8 9:51 pm 32 59.6 116 20.9 2.1 18 mi. S of Borrego Springs 2/9 12:48 am 36 26.6 116 58.8 2.3 7 mi. W of Furnace Creek, Death Valley 2/9 2:18 am 34 51.4 116 58.8 2.3 3 mi. SE of Barstow 2/9 7:31 am 34 5.8 116 24.7 2.0 2 mi. SE of Yucca Valley 2/9 7:47 am 33 28.0 116 27.1 3.4 15 mi. NNW of Borrego Springs 2/9 7:49 am 34 15.3 116 45.5 3.7 5 mi. E of Big Bear City; FELT 2/9 1:52 pm 34 22.8 118 37.1 2.5 3 mi. SSW of Magic Mtn. 2/9 8:19 pm 34 14.3 118 37.5 2.5 2 mi. NNW of Canoga Park 2/9 11:43 pm 34 14.4 118 34.7 3.1 2 mi. W of Northridge; 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.