SOUTHERN CALIFORNIA
SEISMOGRAPHIC NETWORK
A Cooperative Project of
Caltech and the U.S. Geological Survey
July 11, 1996
Weekly Earthquake Report for Southern California
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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.
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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
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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
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