SOUTHERN CALIFORNIA SEISMOGRAPHIC NETWORK A Cooperative Project of Caltech and the U.S. Geological Survey July 11, 1996 Weekly Earthquake Report for Southern California ------------------------------------------------ July 3 - 10, 1996 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 Wednesday morning, July 3, Universal Time (ie. GMT), to midnight Wednesday night, July 10, Universal Time. In local time, the period of coverage is from 5 p.m., July 2, Pacific Daylight Time, to 5 p.m., July 10, Pacific Daylight Time. Because of the long holiday weekend, last week's report was issued one day earlier than usual, so this week's report covers eight days. We detected and processed 230 earthquakes during the eight-day period covered. The low seismicity continues. There was only one quake above M3 this week and only one quake felt, the same ML3.4 Landers aftershock in both cases. That quake occurred during the very early hours of July 4 and was felt in the Yucca Valley area. Since we have just passed the middle of 1996, however, there is a topic to discuss, namely our usual quarterly statistical summary. The following table lists the earthquakes in our coverage area since 1986, including both total numbers and numbers of M3 or greater. Our coverage area includes the land areas of southern California north of the U.S./Mexican border and the immediate offshore areas. The cutoff on the northwest is such that Coalinga and Mammoth Lakes are not included. Notice that only a few percent of the earthquakes we record are large enough to be felt. 1st quarter 2nd quarter 3rd quarter 4th quarter all 3.0+ all 3.0+ all 3.0+ all 3.0+ 1986 3,040 16 2,865 19 6,634 192 2,300 38 1987 1,679 24 1,887 36 2,341 38 5,748 198 1988 2,749 40 2,128 45 2,195 30 2,208 27 1989 2,528 46 2,340 21 2,323 19 2,335 26 1990 2,850 56 2,293 43 1,859 27 2,039 31 1991 1,679 10 1,843 32 2,081 21 2,337 27 1992 3,389 37 10,084 741 25,849 808 9,775 181 1993 4,799 41 5,152 58 5,039 46 4,419 34 1994 10,735 418 5,766 42 5,221 48 4,813 39 1995 3,639 28 3,788 37 8,252 88 7,733 49 1996 5,735 56 3,966 32 The variability in number per quarter is because of aftershocks to our largest earthquakes and to swarms of small and medium sized earthquakes. Aftershocks are most common immediately after a mainshock and die off rapidly with time, so the number of events peaks during a quarter containing a large earthquake, such as North Palm Springs (July 8, 1986, Mw5.9), Whittier Narrows (October 1, 1987, Mw5.9), Superstition Hills (November 23, 1987, Mw6.6), Joshua Tree (April 22, 1992, Mw6.1), Landers (June 28, 1992, Mw7.3), Northridge (January 17, 1994, Mw6.7), and Ridgecrest (September 20, 1995, M5.8). On average, the number of aftershocks is larger for larger mainshocks, with many more small events than large ones, and the total numbers die off quickly with time. Because the 1992 Landers earthquake was so large (Mw7.3), it produced the largest aftershock sequence (over 63,000 so far) since the installation of our modern seismographic network. We have just now, four years later, completed the bulk processing of those data. They still must pass through some quality assurance tests, but the total number of aftershocks is now known. We can describe the "generic California aftershock sequence" and then compare individual sequences to that. The generic sequence depends on the magnitude of the mainshock, so that a M7 mainshock will have about 10 times as many aftershocks as a M6 mainshock, for example. We can see how this works in practice by looking at the aftershocks to the Northridge and Landers earthquakes. The number of aftershocks recorded through the second quarter of 1996 are: Northridge ---------- 1/17/94 - 6/31/96 Latitude 34.1 - 34.5 North, Longitude 118.3 - 118.85 West All events 13,138 (310 of them this quarter) 3.0 - 3.9 389 (2 of them this quarter) 4.0 - 4.9 50 (1 of them this quarter) 5.0 and up 10 Landers/Big Bear ---------------- 4/23/92 - 12/31/95 Latitude 33.8 - 35.33 North, Longitude 116.0 - 117.05 West All events 63,336 (944 of them this quarter) 3.0 - 3.9 1,592 (9 of them this quarter) 4.0 - 4.9 166 5.0 and up 22 It turns out that Landers is very close to the generic event. It has had so many aftershocks simply because it was so big. Northridge, on the other hand, has had a slightly more prolific sequence than the average M6.7. It has produced as many aftershocks as an average M6.9 mainshock. Still, however, there have been quite a bit fewer than the M7.3 Landers mainshock. The Northridge aftershocks have also been dying off slightly more rapidly than average. The net result is that, in this quarter, four years after Landers and two and a half years after Northridge, we are recording almost three times as many Landers aftershocks as Northridge aftershocks. This trend will continue. Knowing what we have observed about the rate and decay of the aftershocks, we can predict the number of aftershocks to expect in the next year. In the Landers sequnece, the next twelve months should produce 21 aftershocks between M3.0 and M3.9, inclusive. Two aftershocks of M4.0 or larger are expected, and the probability of another M5.0 aftershock in the next year is about one in four. In the Northridge sequence, the next twelve months should produce six aftershocks in the M3.0 to M3.9 range, might produce one M4.0 or larger aftershock, and will probably not produce another M5.0 aftershock. The probability of a M5.0 or greater aftershock to Northridge in the next twelve months is only one in eight. Table 1 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 ------------------------------------------------------------- 7/3 8:47 pm 35 53.7 118 6.3 2.1 13 mi. SW of Coso Junction 7/4 1:16 am 33 59.7 116 17.9 3.4 9 mi. S of Joshua Tree FELT 7/5 8:19 am 35 20.9 118 35.0 2.0 16 mi. NNW of Tehachapi 7/5 1:54 pm 33 57.2 118 20.9 2.2 1 mi. WSW of Inglewood 7/7 10:10 am 35 11.7 118 38.5 2.1 10 mi. E of Arvin 7/7 10:20 am 33 11.1 115 35.7 2.0 2 mi. ENE of Obsidian Butte, Imperial Valley 7/7 11:34 am 35 47.1 117 39.1 2.5 11 mi. N of Ridgecrest 7/7 7:23 pm 33 5.5 116 4.0 2.0 5 mi. SE of Ocotillo Wells 7/8 12:07 am 34 55.0 119 32.9 2.4 16 mi. SSW of Taft 7/8 12:50 pm 34 9.4 116 49.5 2.0 3 mi. N of Mt. San Gorgonio 7/8 2:13 pm 33 28.4 116 30.5 2.2 11 mi. ESE of Anza 7/8 3:13 pm 33 54.9 118 40.0 2.0 8 mi. S of Malibu 7/9 12:44 am 34 24.9 116 28.2 2.0 20 mi. N of Yucca Valley 7/9 12:46 am 34 24.8 116 28.4 2.4 " 7/9 6:19 am 34 22.3 116 28.3 2.0 17 mi. N of Yucca Valley 7/9 11:56 am 34 18.6 118 34.4 2.0 4 mi. NNE of Chatsworth 7/9 12:06 pm 34 29.4 118 49.9 2.1 8 mi. NE of Fillmore 7/9 12:50 pm 34 24.8 116 28.3 2.1 20 mi. N of Yucca Valley 7/9 1:47 pm 33 28.8 116 26.9 2.0 13 mi. ESE of Anza 7/9 3:59 pm 34 0.7 117 12.8 2.2 3 mi. SE of Loma Linda 7/9 4:57 pm 33 10.8 115 35.8 2.0 2 mi. ENE of Obsidian Butte, Imperial Valley 7/10 1:52 am 36 58.3 117 49.0 2.2 24 mi. ENE of Independence 7/10 1:25 pm 34 13.2 118 36.6 2.1 1 mi. NNW of Canoga Park 7/10 1:51 pm 33 57.8 118 28.2 2.0 1 mi. SW of Marina del Rey 7/10 4:49 pm 34 59.3 116 56.9 2.7 8 mi. NNE of Barstow -------------------------------------------------------------