SOUTHERN CALIFORNIA
SEISMOGRAPHIC NETWORK
A Cooperative Project of
Caltech and the U.S. Geological Survey
December 5, 1996
Weekly Earthquake Report for Southern California
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November 27 - December 4, 1996
Prepared by: Kate Hutton, Seismological Laboratory
(kate@bombay.gps.caltech.edu)
Lucy Jones, Jim Mori, 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, November 27, Universal Time (ie. GMT), to midnight
Wednesday night, December 4, Universal Time. In local time,
the period of coverage is from 4 p.m., November 26, Pacific
Daylight Time, to 4 p.m., December 4, Pacific Daylight Time.
Due to the Thanksgiving holiday, last week's Report was one day
early, making this week's longer than usual. We detected and
processed 1,052 earthquakes during the six-day period covered.
Due to a combination of high seismicity and a four-day weekend,
about 100 small events are not processed yet; so this total is
a lower bound on the week's seismicity.
The major events of the week were: a M4.1 Landers after-
shock late last Wednesday afternoon, along with a few aftershocks
up to M3.1, and a Mw5.3 in the Coso Range east of Coso Junction,
with a large number of aftershocks, up to M3.7.
The M4.1 was a typical Landers aftershock, with a mostly
north northwest trending strike-slip mechanism. It was located
very close to the epicenter of the Mw6.1 Joshua Tree quake that
preceded the Landers earthquake by two months. This week's event
was felt fairly widely, in the Yucca Valley and Desert Hot
Springs area.
The Coso earthquake occurred in the area of the Coso
geothermal fields, where there are frequent small quakes.
This region has had vigorous swarms in the past, consisting of
many small earthquakes with several M4's, in 1981, 1982-83, 1985
and 1992. The 1995-96 Ridgecrest sequence that included 3 M5+
events occurred in an area 15 to 20 miles to the south. Focal
mechanisms from both the first motions and the long-period
surface waves showed a strike-slip solution with northeast and
northwest nodal planes. The distribution of relocated after-
shocks seems to be elongated in a northwest-southeast direction,
suggesting right-lateral slip on a northwest trending fault
plane. The Coso earthquake is producing a robust aftershock
sequence, as is typical of the Owens Valley region. Given this
rate of activity, we would expect to see a few more aftershocks
above M3 in the next couple of weeks.
In hindsight, we identified a small "foreshock" sequence
that occurred 27 hours before the mainshock and had 3 M2 earth-
quakes with several smaller events (see last week's Report). The
closest telemetered strong-motion instrument recorded 2% g at
Ridgecrest, about 25 miles to the south.
Table 1 lists the quakes this week that were M2.0 or
larger in the central part of the coverage area. To make the
list managably short, Coso quakes are listed only if they are
M2.5 or larger. 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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11/26 5:42 pm 33 57.2 116 18.8 4.1 12 mi. S of Joshua Tree
FELT
11/26 5:44 pm 33 57.5 116 18.5 3.0 "
11/26 5:51 pm 33 57.9 116 18.9 2.0 "
11/26 6:16 pm 33 57.2 116 18.8 3.1 "
11/26 6:24 pm 33 57.4 116 18.8 2.3 "
11/26 9:06 pm 33 57.5 116 18.7 2.2 "
11/27 12:11 am 33 45.9 118 27.1 2.1 1 mi. WSW of Palos Verdes
Point
11/27 12:18 am 33 57.4 116 18.8 2.4 12 mi. S of Joshua Tree
11/27 12:22 am 33 57.3 116 18.7 2.0 "
11/27 12:50 am 33 57.3 116 18.9 2.2 "
11/27 1:19 am 33 12.6 116 2.0 2.2 6 mi. SW of Salton City
11/27 9:45 am 34 12.1 117 24.6 2.2 1 mi. SSW of Devore
11/27 10:01 am 34 14.8 119 40.4 2.5 12 mi. S of downtown Santa
Barbara
11/27 12:07 pm 36 1.4 117 51.5 2.0 5 mi. ESE of Coso Junction
11/27 12:17 pm 36 4.5 117 39.0 5.3 Coso Range, 16 mi. E of
Coso Junction
WIDELY FELT
11/27 12:20 pm 36 3.2 117 39.3 2.5 "
11/27 12:24 pm 36 4.7 117 38.9 2.5 "
11/27 12:25 pm 36 5.5 117 39.0 2.8 "
11/27 12:39 pm 36 5.1 117 39.2 2.5 "
11/27 12:46 pm 36 4.2 117 39.9 2.6 "
11/27 12:46 pm 36 5.2 117 38.8 2.9 "
11/27 2:29 pm 36 4.5 117 38.1 3.7 "
11/27 2:30 pm 36 3.8 117 38.0 2.9 "
11/27 2:31 pm 36 5.0 117 38.8 2.5 "
11/27 2:48 pm 36 4.3 117 39.3 2.6 "
11/27 3:41 pm 32 29.9 115 17.0 3.1 17 mi. SE of Calexico
11/27 3:45 pm 32 29.6 115 16.8 2.2 "
11/27 3:48 pm 32 29.0 115 16.8 2.4 "
11/27 3:55 pm 36 5.0 117 39.3 2.5 16 mi. E of Coso Junction
11/27 4:27 pm 36 3.8 117 40.3 2.5 "
11/27 4:40 pm 36 5.3 117 39.2 2.9 "
11/27 4:45 pm 36 4.6 117 39.3 2.7 "
11/27 4:45 pm 36 5.2 117 38.5 2.5 "
11/27 4:53 pm 36 3.9 117 38.4 2.5 "
11/27 4:56 pm 36 5.3 117 39.2 2.7 "
11/27 5:00 pm 33 59.8 116 17.9 2.2 9 mi. S of Joshua Tree
11/27 5:01 pm 36 4.6 117 39.1 2.7 16 mi. E of Coso Junction
11/27 5:18 pm 36 4.1 117 38.5 2.5 "
11/27 5:47 pm 34 56.3 116 39.9 2.0 20 mi. E of Barstow
11/27 8:52 pm 36 5.1 117 39.0 2.6 16 mi. E of Coso Junction
11/27 8:59 pm 36 5.0 117 39.0 2.6 "
11/27 11:41 pm 36 3.9 117 38.3 2.6 "
11/28 12:49 am 36 4.8 117 38.4 3.5 "
11/28 12:50 am 36 4.0 117 39.0 2.8 "
11/28 3:29 am 36 5.4 117 39.4 3.2 "
11/28 3:44 am 36 6.0 117 38.7 2.5 "
11/28 6:46 am 36 3.8 117 38.5 2.7 "
11/28 6:48 am 35 48.2 117 37.9 2.0 12 mi. N of Ridgecrest
11/28 8:37 am 33 44.0 118 2.7 2.1 3 mi. N of Bolsa Chica
Beach
11/28 12:21 pm 36 5.3 117 39.5 2.5 16 mi. E of Coso Junction
11/28 1:06 pm 36 5.5 117 39.0 2.5 "
11/28 1:47 pm 36 5.2 117 39.1 3.5 "
11/28 1:49 pm 36 5.4 117 39.1 2.5 "
11/28 2:03 pm 36 34.7 117 53.5 2.4 9 mi. E of Lone Pine
11/28 2:05 pm 36 5.4 117 39.1 3.7 16 mi. E of Coso Junction
11/28 2:51 pm 36 4.7 117 39.9 2.6 "
11/28 3:10 pm 36 5.3 117 39.5 2.6 "
11/28 3:39 pm 36 34.5 117 53.1 2.7 10 mi. E of Lone Pine
11/28 3:45 pm 36 5.0 117 38.8 2.8 16 mi. E of Coso Junction
11/28 7:15 pm 36 3.8 117 39.0 2.8 "
11/29 12:57 am 36 7.1 117 37.7 2.5 "
11/29 4:21 am 36 4.2 117 38.4 2.5 "
11/29 5:42 am 36 4.5 117 38.0 2.7 "
11/29 10:01 am 36 5.1 117 38.5 2.5 "
11/29 10:14 am 34 17.7 116 11.8 2.2 12 mi. NNE of Joshua Tree
11/29 10:34 am 36 5.0 117 39.2 2.6 16 mi. E of Coso Junction
11/29 1:31 pm 36 5.4 117 39.3 3.1 "
11/29 3:44 pm 32 45.9 115 26.8 2.2 6 mi. ESE of El Centro
11/29 7:46 pm 36 5.4 117 38.3 2.6 16 mi. E of Coso Junction
11/30 8:28 am 36 5.4 117 39.3 2.7 "
11/30 9:54 am 33 21.2 116 17.9 2.1 8 mi. NNE of Borrego
Springs
11/30 10:46 am 36 5.2 117 38.9 2.5 16 mi. E of Coso Junction
11/30 11:03 am 36 4.0 117 38.8 2.8 "
11/30 12:15 pm 36 5.6 117 39.3 2.8 "
11/30 2:11 pm 36 5.6 117 39.1 3.6 "
12/1 11:42 pm 33 57.5 116 19.4 2.1 10 mi. E of Desert Hot
Springs
12/1 7:29 am 34 35.7 116 37.0 2.3 21 mi. ENE of Lucerne
Valley
12/1 7:30 am 34 35.4 116 37.1 3.1 "
12/1 10:45 am 36 5.4 117 39.4 2.8 16 mi. E of Coso Junction
12/1 12:25 pm 34 34.4 116 32.8 2.2 24 mi. ENE of Lucerne
Valley
12/1 2:04 pm 36 3.9 117 38.4 2.6 16 mi. E of Coso Junction
12/1 6:13 pm 36 4.6 117 38.2 2.5 "
12/1 7:47 pm 33 38.5 116 43.3 2.4 6 mi. NNW of Anza
12/1 11:18 pm 36 5.7 117 39.2 2.5 16 mi. E of Coso Junction
12/2 5:55 am 34 20.4 116 27.9 2.0 15 mi. N of Yucca Valley
12/2 6:02 am 34 20.5 116 28.0 2.1 "
12/2 10:52 am 34 19.4 118 32.3 2.2 3 mi. S of Newhall
12/2 10:59 am 34 7.0 116 59.6 2.1 6 mi. NNE of Yucaipa
12/2 2:15 pm 33 14.2 116 46.5 2.5 Under Lake Henshaw
12/2 8:28 pm 36 5.1 117 38.1 2.9 16 mi. E of Coso Junction
12/2 8:28 pm 36 4.9 117 38.2 2.8 "
12/2 10:34 pm 36 5.6 117 39.4 2.8 "
12/3 5:36 am 34 16.4 118 30.9 2.0 Under Granada Hills
12/3 10:00 am 36 5.5 117 39.3 2.9 16 mi. E of Coso Junction
12/3 10:36 am 36 5.7 117 39.1 2.7 "
12/3 12:44 pm 36 3.8 117 39.1 3.3 "
12/4 1:18 am 34 27.0 116 30.8 2.5 23 mi. N of Yucca Valley
12/4 4:38 am 35 43.3 117 37.2 2.2 7 mi. NNE of Ridgecrest
12/4 4:55 am 36 4.2 117 39.1 2.6 16 mi. E of Coso Junction
12/4 3:15 pm 34 28.5 119 21.8 2.2 6 mi. WNW of Ojai
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