SOUTHERN CALIFORNIA
SEISMOGRAPHIC NETWORK
A Cooperative Project of
Caltech and the U.S. Geological Survey
August 15, 1996
Weekly Earthquake Report for Southern California
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August 8 - 14, 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 Thursday
morning, August 8, Universal Time (ie. GMT), to midnight Wednesday
night, August 14, Universal Time. In local time, the period of
coverage is from 5 p.m., August 7, Pacific Daylight Time, to 5 p.m.,
August 14, Pacific Daylight Time. We detected and processed
285 earthquakes during the seven-day period covered.
Although the total number of quakes this week was not
higher than what we've seen recently, and there were only three
events over M3, there were four events felt.
The largest and most widely felt was a M4.3 in the Mojave
Desert on Tuesday evening. It had a strike-slip focal mechanism.
It was felt as far away as the Los Angeles area.
We decided to call this earthquake an aftershock to the
Landers event, but the decision was not clear cut. The usual
criteria for an aftershock this long after the mainshock is that
it occurs within the "original" aftershock zone, which is the area
that saw an increase within the first few days. In the case of
Landers that triggered activity as far away as Yellowstone, we
further restrict the zone to be within one fault length. The
Landers fault is 90 km long, so that means within 90 km of the
main fault trace. Within that 90 km zone, many faults produced
aftershocks and many did not. The region that produced this
week's M4.5 event is east of the main fault and did NOT rupture
in the Landers mainshock. It did have a M5.4 aftershock one week
after the mainshock that had its own aftershock sequence. The
confusing point is that aftershocks in that region stopped in
1992, and, since then, we have had only three earthquakes above
M2.5 in that spot. This made us wonder if this event is a "true"
aftershock, but, because a previous M5.4 aftershock was at the
same place, we said yes. The reality is that all earthquakes
change the probability of all other earthquakes, and the size of
the change depends on distance and time. By naming one event an
aftershock and another event not an aftershock we force a division
in a continuous distribution.
Other earthquakes felt this week include a M2.9 near Indio
on Saturday, and a M2.9 and a M2.2 near Inglewood on Monday
afternoon. The Indio quake was at the southern end of the Landers
aftershock zone.
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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8/7 7:43 pm 33 10.7 115 35.9 2.0 2 mi. ENE of Obsidian
Butte
8/7 11:09 pm 34 56.0 116 45.5 2.1 15 mi. ENE of Barstow
8/8 6:30 am 33 10.6 115 35.9 2.1 2 mi. E of Obsidian
Butte
8/8 9:27 am 35 6.0 117 29.6 2.0 26 mi. E of California City
8/8 10:02 am 35 5.9 117 29.5 2.0 "
8/8 10:03 am 35 5.9 117 29.6 2.3 "
8/8 10:40 am 32 44.5 116 2.8 2.0 3 mi. W of Ocotillo
8/8 5:43 pm 34 24.0 116 28.5 2.7 19 mi. N of Yucca Valley
8/8 6:52 pm 34 36.0 116 17.4 2.2 32 mi. N of Joshua Tree
8/8 7:39 pm 35 6.0 117 29.7 2.9 26 mi. E of California City
8/8 8:37 pm 33 15.6 116 17.2 3.3 4 mi. E of Borrego Springs
8/9 7:38 am 34 21.2 119 16.9 2.3 5 mi. NNE of Ventura
8/9 8:37 am 34 55.0 116 55.0 2.0 6 mi. ENE of Barstow
8/9 6:08 pm 34 36.0 116 17.3 2.4 31 mi. N of Joshua Tree
8/9 6:16 pm 33 59.9 118 6.5 2.1 4 mi. N of Downey
8/10 4:12 am 34 28.9 119 22.8 2.6 7 mi. WNW of Ojai
8/10 5:52 am 35 47.7 117 39.7 2.4 11 mi. N of Ridgecrest
8/10 6:20 am 33 40.3 118 4.3 2.4 2 mi. WSW of Bolsa Chica
Beach
8/10 12:33 pm 33 44.4 116 9.9 2.9 3 mi. ENE of Indio; FELT
8/10 12:34 pm 33 44.4 116 9.5 2.1 "
8/10 12:35 pm 33 44.2 116 9.4 2.2 "
8/10 2:03 pm 32 52.5 115 34.4 2.1 5 mi. NNW of El Centro
8/10 5:31 pm 35 5.4 117 28.9 2.1 27 mi. E of California City
8/10 10:29 pm 35 47.8 116 5.9 2.1 15 mi. SE of Shoshone
8/10 11:29 pm 33 10.8 115 36.1 2.3 2 mi. ENE of Obsidian Butte
8/11 2:26 am 33 10.6 115 36.2 2.5 "
8/11 6:56 am 35 43.9 117 37.5 2.2 7 mi. NNE of Ridgecrest
8/11 3:57 pm 33 15.6 116 12.4 2.1 9 mi. NNW of Ocotillo Wells
8/11 6:49 pm 33 15.6 116 12.3 2.3 "
8/11 8:47 pm 33 15.8 116 17.0 2.0 5 mi. E of Borrego Springs
8/12 1:14 am 34 4.0 116 47.4 2.3 3 mi. SE of Mt. San
Gorgonio
8/12 10:57 am 33 10.5 115 36.5 2.9 1 mi. ENE of Obsidian Butte
8/12 11:36 am 35 56.5 117 13.9 2.1 14 mi. NNE of Trona
8/12 1:17 pm 34 35.3 116 17.4 2.0 31 mi. N of Joshua Tree
8/12 1:25 pm 33 58.8 118 19.9 2.9 1 mi. NNW of Inglewood
FELT
8/12 1:39 pm 33 59.8 118 20.9 2.2 " FELT
8/12 2:03 pm 34 1.0 116 19.0 2.2 8 mi. S of Joshua Tree
8/12 2:28 pm 33 10.8 115 36.2 2.9 2 mi. ENE of Obsidian Butte
8/13 8:40 am 34 38.2 116 30.5 2.1 28 mi. ENE of Lucerne
Valley
8/13 10:29 am 34 28.2 116 30.6 2.0 24 mi. N of Yucca Valley
8/13 1:04 pm 33 10.3 115 36.4 2.2 1 mi. E of Obsidian Butte
8/13 1:12 pm 33 10.5 115 36.6 2.4 "
8/13 1:43 pm 33 31.8 118 2.0 2.3 8 mi. SW of Newport Beach
8/13 2:19 pm 33 10.5 115 36.7 2.2 1 mi. ENE of Obsidian Butte
8/13 3:30 pm 34 27.3 116 30.1 2.2 23 mi. N of Yucca Valley
8/13 5:42 pm 32 49.1 115 37.3 2.2 4 mi. WNW of El Centro
8/13 8:05 pm 34 36.0 116 16.8 4.3 32 mi. N of Joshua Tree
WIDELY FELT
8/13 10:01 pm 34 36.0 116 17.0 2.2 "
8/14 12:22 am 34 35.7 116 16.6 2.2 "
8/14 12:22 am 34 6.9 116 56.6 2.0 6 mi. W of Mt. San
Gorgonio
8/14 6:00 am 34 36.2 116 17.0 2.2 32 mi. N of Joshua Tree
8/14 8:04 am 32 37.7 116 6.9 2.4 10 mi. SW of Ocotillo
8/14 10:27 am 35 44.9 117 28.4 3.3 5 mi. WSW of Trona
8/14 11: 2 am 34 35.9 116 17.3 2.1 31 mi. N of Joshua Tree
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