January 27, 1994 Weekly Earthquake Report for Southern California ------------------------------------------------ January 20 - 26, 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, January 20, Pacific Standard Time, to midnight Wednesday night, January 26, Pacific Standard Time. There are 1038 this week, and the list is by no means complete since we are still behind in the data processing. There is still really only one earthquake story, namely Northridge aftershocks. The largest was the M4.6 on Friday morning, which was only slightly higher than the M4.5 on Sunday evening. Further studies of the Northridge main shock (Jan. 17, MS6.6) have confirmed the analysis of last week. The hypocenter was 1 mi. S of Northridge (34 deg. 13 min. N, 118 deg. 32 min. W), at a depth of 11 mi. (18 km). The hypocenter is the point where the mainshock started; the faulting extended up, and both east and west from that point. The rupture, as outlined by the aftershock, is roughly circular, dipping at a 40 degree angle, with its deepest point at the mainshock hypocenter and its shallowest point near Santa Clarita Valley. The rupture plane is clearly defined by the aftershocks only below about 3 mi. depth, so we infer that he rupture did not extend up to the surface. Geodetic measurements with the GPS satellite system show that the Northridge quake helped shorted San Fernando Valley, north to south, and raise the Santa Susana Mtns. by about 15 inches. The fault responsible is part of the system of faults under the northern Los Angeles Basin first recognized after the Whittier Narrows quake in 1987. We think a band of buried faults extends across the northern side of L.A. Basin and under the San Gabriel and San Fernando Valleys. The system may be underlain by a nearly horizontal fault surface, called a "detachment", which, due to its depth, may "creep" instead of store strain for earth- quakes. Various faults, called "ramps" are thought to brach upward from this detachment, both to the south and to the north. Both the Whittier Narrows quake and the Northridge quake probably occurred on these ramps. We are calling the one where the Northridge quake occurred the West San Fernando Valley thrust ramp. A summary of the cumulative aftershock activity, including last week, is as follows: 2 aftershocks of M5.0 to 5.9 35 aftershocks of M4.0 to 4.9 296 aftershocks of M3.0 to 3.9 The aftershock sequence is slightly more energetic than average, but is decaying slightly more quickly than average, which should make everyone in the Valley happy. The probability of a M5.0 or larger aftershock in the next week is about 1 in 5 (20%), and we expect to see about 1 more M5.0 or larger aftershock in the next 12 months. One or two more M4.5 or larger aftershocks are likely within the next week. The seismicity elsewhere in the region still looks lower than it has been. We believe this is not real, but rather an artifact of the preliminary data processing that we are currently doing. Since we look at only one event per trigger of the on-line system and are concentrating on aftershocks, we expect many small quakes elsewhere to stay hidden in the data until we can do a more thorough job. The table lists the quakes that were M2.0 or larger in the central part of the coverage area. The data processing is still very incomplete due to the huge number of Northridge events. Aftershocks must be at least M2.8 to be listed. This is just about the threshold for feeling quakes, if you are near the epicenter, so the table should be a good "felt list" for San Fernando Valley. 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 ------------------------------------------------------------- 1/20 12:24 am 33 44.5 120 9.8 2.1 14 mi. SSW of Santa Rosa Is. 1/20 12:58 am 34 18.3 118 30.5 3.3 4 mi. NNE of Northridge 1/20 2:07 am 34 17.1 118 30.9 2.8 3 mi. NNE of Northridge 1/20 3:06 am 34 22.5 118 31.1 3.1 Under Newhall 1/20 3:36 am 34 18.3 118 26.3 2.8 1 mi. N of San Fernando 1/20 3:37 am 34 18.8 118 25.5 3.3 2 mi. NNE of San Fernando 1/20 4:26 am 34 18.3 118 26.5 3.3 1 mi. N of San Fernando 1/20 5:56 am 34 22.0 118 42.4 3.2 6 mi. N of Simi Valley 1/20 6:19 am 34 22.1 118 31.5 3.4 Under Newhall 1/20 7:05 am 34 18.8 118 26.2 2.9 2 mi. N of San Fernando 1/20 7:11 am 34 20.4 118 26.5 2.8 4 mi. N of San Fernando 1/20 8:37 am 34 15.1 118 27.8 3.0 2 mi. SW of San Fernando 1/20 8:41 am 34 22.0 118 27.9 2.8 3 mi. ESE of Newhall 1/20 9:36 am 34 12.8 118 36.6 2.8 1 mi. NNW of Canoga Park 1/20 10:48 am 34 22.5 118 30.9 3.3 Under Newhall 1/20 10:56 am 34 19.3 118 25.5 3.0 2 mi. NNE of San Fernando 1/20 12:42 pm 34 17.1 118 33.8 3.1 3 mi. NNW of Northridge 1/20 12:45 pm 34 17.2 118 33.3 2.7 " 1/20 12:48 pm 34 14.7 118 35.8 3.0 3 mi. N of Canoga Park 1/20 1:29 pm 33 54.0 116 17.5 2.4 13 mi. NNW of Indio 1/20 2:04 pm 34 15.4 118 27.7 3.5 2 mi. SW of San Fernando 1/20 4:07 pm 34 21.9 118 32.3 3.3 1 mi. SSW of Newhall 1/20 4:09 pm 34 18.0 118 28.0 3.1 1 mi. NW of San Fernando 1/20 5:35 pm 34 17.8 118 25.8 3.1 1 mi. NNE of San Fernando 1/20 6:04 pm 34 22.9 118 30.3 3.1 1 mi. E of Newhall 1/20 6:06 pm 34 22.7 118 30.7 2.9 Under Newhall 1/20 6:24 pm 34 26.3 117 26.3 2.4 10 mi. SW of Victorville 1/20 7:35 pm 34 19.4 118 30.8 3.0 3 mi. SSE of Newhall 1/20 9:29 pm 34 21.0 118 33.6 3.0 2 mi. SW of Newhall 1/20 9:49 pm 34 17.0 118 34.7 2.8 4 mi. NW of Northridge 1/20 9:56 pm 34 16.3 118 39.0 3.4 3 mi. E of Simi Valley 1/20 11:53 pm 34 21.5 118 31.5 2.8 1 mi. S of Newhall 1/21 12:10 am 35 8.4 117 36.5 2.0 33 mi. S of Ridgecrest 1/21 12:40 am 34 21.5 118 40.8 3.0 5 mi. NNE of Simi Valley 1/21 12:42 am 34 16.7 118 39.3 2.9 3 mi. E of Simi Valley 1/21 2:43 am 34 22.6 118 38.2 2.8 4 mi. SW of Magic Mtn. 1/21 4:43 am 34 22.3 118 29.5 3.0 2 mi. ESE of Newhall 1/21 5:22 am 34 16.4 118 35.6 2.8 4 mi. NW of Northridge 1/21 10:39 am 34 18.3 118 27.3 4.6 1 mi. NNW of San Fernando 1/21 10:42 am 34 18.6 118 28.4 4.1 " 1/21 10:52 am 34 18.1 118 27.1 4.1 " 1/21 10:53 am 34 17.8 118 27.5 4.3 " 1/21 10:54 am 34 17.8 118 27.7 3.2 " 1/21 10:57 am 34 17.5 118 28.4 3.6 2 mi. WNW of San Fernando 1/21 11:14 am 34 20.0 118 38.2 3.3 5 mi. NE of Simi Valley 1/21 11:27 am 34 17.8 118 27.7 2.8 1 mi. NW of San Fernando 1/21 11:46 am 34 17.8 118 28.2 2.8 2 mi. WNW of San Fernando 1/21 11:54 am 34 18.6 118 28.8 2.6 3 mi. NW of San Fernando 1/21 1:02 pm 34 19.9 118 38.2 3.1 5 mi. NE of Simi Valley 1/21 1:26 pm 34 17.1 118 25.1 3.3 1 mi. ENE of San Fernando 1/21 3:54 pm 34 34.3 116 27.6 2.5 31 mi. N of Yucca Valley 1/21 6:18 pm 34 12.7 118 31.3 2.9 1 mi. SSE of Northridge 1/21 7:34 pm 34 21.0 118 29.4 3.2 2 mi. SE of Newhall 1/21 9:24 pm 35 17.9 117 40.0 2.1 22 mi. S of Ridgecrest 1/21 9:27 pm 34 28.6 116 29.8 2.8 24 mi. N of Yucca Valley 1/21 9:41 pm 34 15.4 118 30.6 3.0 1 mi. NE of Northridge 1/21 10:12 pm 34 34.0 116 31.3 2.1 25 mi. ENE of Lucerne Valley 1/22 1:03 am 34 21.6 118 33.3 3.1 1 mi. SW of Newhall 1/22 5:31 am 34 9.2 116 25.6 2.0 2 mi. NNE of Yucca Valley 1/22 5:49 am 34 18.9 118 31.1 2.9 4 mi. S of Newhall 1/22 8:58 am 34 37.4 116 33.4 2.0 25 mi. ENE of Lucerne Valley 1/22 3:14 pm 34 18.8 118 24.7 3.2 2 mi. NE of San Fernando 1/22 3:36 pm 34 15.3 118 27.5 3.0 2 mi. SSW of San Fernando 1/22 6:42 pm 33 49.9 118 34.3 2.1 9 mi. WNW of Palos Verdes Point 1/22 6:48 pm 34 20.2 118 31.0 3.2 2 mi. SSE of Newhall 1/22 6:56 pm 33 41.6 116 49.0 3.0 6 mi. WSW of Idyllwild 1/22 8:00 pm 34 18.1 118 27.6 3.0 1 mi. NW of San Fernando 1/22 9:14 pm 33 58.9 118 21.5 2.0 1 mi. SSE of Baldwin Hills 1/22 10:57 pm 34 22.7 118 41.7 3.1 6 mi. N of Simi Valley 1/23 12:39 am 34 22.0 118 32.1 3.0 Under Newhall 1/23 12:41 am 34 17.5 118 27.6 3.8 1 mi. WNW of San Fernando 1/23 12:43 am 34 18.5 118 23.7 2.7 3 mi. NE of San Fernando 1/23 12:55 am 34 18.0 118 25.7 4.0 1 mi. NNE of San Fernando 1/23 6:52 am 34 16.8 118 31.5 3.3 3 mi. N of Northridge 1/23 6:52 am 34 23.8 118 42.1 3.2 6 mi. WSW of Magic Mtn. 1/23 7:59 am 34 15.6 118 36.5 3.2 4 mi. N of Canoga Park 1/23 2:05 pm 34 8.1 116 52.0 2.6 3 mi. NW of Mt. San Gorgonio 1/23 2:58 pm 34 35.8 116 37.3 2.4 21 mi. ENE of Lucerne Valley 1/23 3:37 pm 34 17.8 118 25.4 2.8 1 mi. NE of San Fernando 1/23 5:24 pm 34 34.1 116 27.4 2.6 31 mi. N of Yucca Valley 1/23 6:41 pm 34 15.1 118 28.5 3.6 2 mi. SW of San Fernando 1/23 7:42 pm 34 18.1 118 35.1 3.0 5 mi. NNW of Northridge 1/23 8:15 pm 34 20.7 118 33.1 4.5 2 mi. SSW of Newhall 1/23 9:50 pm 34 21.5 118 37.7 4.2 4 mi. SSW of Magic Mtn. 1/23 9:54 pm 34 21.8 118 37.6 4.1 " 1/23 10:33 pm 34 20.0 118 28.1 2.8 3 mi. NNW of San Fernando 1/24 2:48 am 34 21.0 118 33.7 3.1 2 mi. SW of Newhall 1/24 4:39 am 34 16.7 118 27.8 3.0 1 mi. W of San Fernando 1/24 5:47 am 34 22.8 116 27.5 3.0 18 mi. N of Yucca Valley 1/24 6:44 am 34 9.7 116 46.7 2.3 5 mi. NNE of Mt. San Gorgonio 1/24 9:52 am 34 22.2 118 39.1 3.2 5 mi. SW of Magic Mtn. 1/24 10:05 am 34 22.1 118 39.2 3.4 " 1/24 10:16 am 34 21.5 118 33.7 3.3 2 mi. SW of Newhall 1/25 3:39 am 35 17.9 117 39.9 2.1 22 mi. S of Ridgecrest 1/25 4:09 am 3.1 San Fernando Valley 1/25 9:17 am 34 19.0 118 30.3 3.4 4 mi. SSE of Newhall 1/25 10:42 am 3.3 San Fernando Valley 1/25 11:45 am 34 57.7 117 44.9 2.1 24 mi. ESE of Mojave 1/25 6:17 pm 34 16.1 118 38.3 3.0 4 mi. E of Simi Valley 1/25 7:53 pm 34 22.5 118 39.9 2.9 5 mi. SW of Magic Mtn. 1/25 8:04 pm 34 15.3 118 28.1 2.9 2 mi. SW of San Fernando 1/26 4:28 am 34 18.1 118 28.2 3.8 2 mi. NW of San Fernando 1/26 5:53 am 34 10.8 118 37.5 2.9 1 mi. SW of Canoga Park 1/26 8:07 am 34 22.3 118 37.5 2.8 4 mi. SSW of Magic Mtn. 1/26 9:09 am 34 22.5 118 31.2 3.5 Under Newhall 1/26 9:59 am 34 12.8 118 36.8 3.1 1 mi. NNW of Canoga Park 1/26 3:43 pm 35 21.1 117 39.4 2.0 18 mi. S of Ridgecrest 1/26 3:53 pm 34 5.5 116 51.7 2.3 2 mi. WSW of Mt. San Gorgonio 1/26 8:43 pm 34 21.8 118 28.9 3.5 2 mi. ESE of Newhall ------------------------------------------------------------- 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.