SOUTHERN CALIFORNIA
SEISMOGRAPHIC NETWORK
A Cooperative Project of
Caltech and the U.S. Geological Survey
January 23, 1997
Weekly Earthquake Report for Southern California
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January 16 - 22, 1997
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 818-395-6327. For daily
updates, call our Earthquake Information Hotline: 818-395-6977.
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This week's Report covers the time period from midnight Thursday
morning, January 16, Universal Time (ie. GMT), to midnight
Wednesday night, January 22, Universal Time. In local time,
the period of coverage is from 4 p.m., January 15, Pacific
Standard Time, to 4 p.m., January 22, Pacific Standard Time.
We detected and processed 288 earthquakes during the seven-day
period covered.
The total number of events was slightly smaller than it
was last week. There were four M3+ quakes this week, the largest
being M3.4. That M3.4 occurred early Tuesday evening in the
Grapevine area, about five miles north of the I5/99 interchange.
This quake was within the aftershock zone of the largest quake
in the southern half of California this century, the Mw7.5 Kern
County quake of 1952. It is difficult to use the term "after-
shock" after 45 years, but the seismicity rate in that area, the
northern front of the Tehachapi Mountaings, is still above what
it was before the 1952 mainshock.
The other three M3+'s were: a M3.0 offshore of Point Sal
last Wednesday night and a M3.1 near Yucca Valley Friday after-
noon, and a M3.0 Simi Valley quake 33 minutes after the Yucca
Valley one.
The Yucca Valley M3.1 was a Landers aftershock and was
felt in the town of Yucca Valley. Likewise, the Simi Valley
event was a Northridge aftershock and was felt in Simi Valley.
Also of interest was a small swarm of three quakes, the
largest being M2.7, in the northern end of the Brawley Seismic
Zone, which connects the northern end of the Imperial Fault with
the southern end of the San Andreas. Although it has been
relatively quiet in the last few years, the Brawley Seismic Zone
is at times one of the most active areas in the state.
For our readers who follow the weekly total numbers of
quakes, we now summarize the results from the December holidays.
The week of December 19 - 25 produced a total of 358 events,
and the following week, December 26 - January 1, produced 342.
As with the current week, the bulk of the seismicity consisted
of Landers aftershocks, activity in the Ridgecrest and Coso
areas, and a few Northridge aftershocks.
We also have a brief summary of the quarterly statistics:
Totals listed below apply to the area within a polygon bounded by
the following latitude, longitude pairs:
35.0N, 121.0W; 37.0N, 119.0W; 37.0N, 115.0W; 32.5N, 115.0W; and
32.5N, 121.0W.
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 3,690 23 4,660 50
And we also have a brief summary of the three major aftershock
sequences that were active in the fourth quarter of 1996:
Northridge
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1/17/94 - 12/31/96
Latitude 34.1 - 34.5 North, Longitude 118.3 - 118.85 West
All events 13,523 (196 of them this quarter)
3.0 - 3.9 391 (2 of them this quarter)
4.0 - 4.9 51 (none this quarter)
5.0 and up 10
Landers/Big Bear
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4/23/92 - 12/31/96
Latitude 33.8 - 35.33 North, Longitude 116.0 - 117.05 West
All events 65,380 (822 of them this quarter)
3.0 - 3.9 1,608 (8 of them this quarter)
4.0 - 4.9 169 (2 of them this quarter)
5.0 and up 22
Coso Range
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11/27/96 - 12/31/96
Latitude 36.0 - 36.25 North, Longitude 117.5 - 117.75 West
All events 1,782
3.0 - 3.9 19
4.0 - 4.9 0
5.0 and up 1
At the present rate of decay, we should expect four
Northridge and 22 Landers aftershocks of M3.0 or larger, in the
next year. The higher current rate of Landers aftershocks,
relative to Northridge, is because Landers was a bigger
earthquake. We are now out in the tails of both aftershock
sequences, and the difference in time since the mainshock is
less important than it was earlier in sequences.
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
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Date Time N Lat. W Long. Mag
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1/15 9:04 pm 35 12.3 119 1.6 2.4 11 mi. S of Bakersfield
1/15 10:58 pm 34 56.8 120 44.7 3.0 5 mi. NW of Pt. Sal
1/16 12:11 pm 36 4.0 117 38.8 2.2 16 mi. E of Coso Junction
1/16 3:49 pm 34 21.6 116 28.0 2.4 16 mi. N of Yucca Valley
1/16 7:10 pm 36 5.3 117 38.6 2.5 19 mi. SSW of Panamint
Springs
1/16 10:32 pm 34 21.2 116 27.7 2.4 16 mi. N of Yucca Valley
1/17 12:16 am 36 4.4 117 39.5 2.6 16 mi. E of Coso Junction
1/17 6:25 am 36 2.5 117 48.2 2.5 8 mi. E of Coso Junction
1/17 12:32 pm 35 37.7 118 24.6 2.0 3 mi. E of town of Lake
Isabella
1/17 2:56 pm 34 21.8 118 44.0 2.5 6 mi. NNW of Simi Valley
1/17 4:11 pm 34 9.5 116 25.5 3.1 2 mi. NNE of Yucca Valley
FELT
1/17 4:12 pm 34 30.3 116 29.6 2.5 26 mi. N of Yucca Valley
1/17 4:44 pm 34 21.3 118 44.4 3.0 5 mi. NNW of Simi Valley
FELT in Simi Valley
1/17 6:20 pm 34 24.1 119 42.5 2.0 1 mi. SSW of downtown
Santa Barbara
1/17 11:47 pm 33 60.0 116 19.2 2.1 9 mi. S of Joshua Tree
1/18 1:10 am 34 21.3 118 44.2 2.0 5 mi. NNW of Simi Valley
1/18 8:07 am 36 5.2 117 39.9 2.2 16 mi. E of Coso Junction
1/18 9:27 am 36 5.3 117 39.2 2.1 "
1/18 3:15 pm 36 5.8 117 39.7 2.5 16 mi. ENE of Coso
Junction
1/18 8:21 pm 34 37.8 116 30.6 2.0 27 mi. ENE of Lucerne
Valley
1/18 11:45 pm 36 4.7 117 39.0 2.4 16 mi. E of Coso Junction
1/19 1:02 am 34 21.7 118 44.7 2.0 6 mi. NNW of Simi Valley
1/19 5:07 pm 32 39.3 115 54.7 2.6 7 mi. SE of Ocotillo
1/19 11:54 pm 34 42.9 118 45.9 2.2 7 mi. SE of Gorman
1/20 1:06 pm 35 46.9 117 38.0 2.1 11 mi. N of Ridgecrest
1/20 2:53 pm 34 36.9 116 40.0 2.0 19 mi. NE of Lucerne
Valley
1/20 9:10 pm 36 4.7 117 39.6 2.0 16 mi. E of Coso Junction
1/21 6:01 am 36 4.8 117 39.5 2.5 "
1/21 6:59 am 33 59.5 116 59.4 2.6 3 mi. SE of Yucaipa
1/21 8:28 am 34 37.2 116 40.0 2.6 20 mi. NE of Lucerne
Valley
1/21 6:40 pm 35 4.7 118 56.8 3.4 5 mi. N of Wheeler Ridge
1/22 1:35 pm 33 15.1 115 42.0 2.3 6 mi. SSE of Bombay Beach
1/22 1:42 pm 33 15.3 115 42.0 2.7 "
1/22 1:43 pm 33 15.3 115 42.1 2.5 "
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