SOUTHERN CALIFORNIA
SEISMOGRAPHIC NETWORK
A Cooperative Project of
Caltech and the U.S. Geological Survey
September 21, 1995
Weekly Earthquake Report for Southern California
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September 14 - 20, 1995
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 Thursday
morning, September 14, Pacific Daylight Time, to midnight
Wednesday night, September 20, 1995, Pacific Daylight Time. We
recorded 614 earthquakes during the 7-day period covered.
The most excitement for the week was clearly the ML5.8
Ridgecrest quake which happened yesterday afternoon. It was
felt widely as far away as the L.A. area, with a few scattered
inquiries from Las Vegas and even San Diego. Like the similar
M5.4 event on August 17, '95, it had a focal mechanism that
showed a combination of strike-slip and normal (extentional)
faulting. A preliminary look at the data suggests that it may
have been on the same fault as the August quake, rupturing an
adjacent patch of fault to the southeast and slightly shallower.
This activity is simliar to the earthquakes that occurred
in the same region in 1981-1982, which was the last time we saw
significant earthquakes near China Lake. In that sequence, a
"cluster" of earthquakes continued on and off for more than a
year. The earthquakes in that cluster migrated over several
different faults around Indian Wells Valley. This month's
earthquakes are larger than any seen in 1982 (the largest then
was M4.9) and are located to the east of the 1982 events.
We received many questions about whether it was an
"aftershock" of the August event or not. Actually, we could
call the August quake a foreshock of the ML5.8, since it was
smaller, but the term "foreshock" is usually reserved for
earthquakes closer in time (less than 3-5 days). We could also
call the two M5+ events a pair of mainshocks. The point is that
these two quakes are close enough in time and space that they
must be related. The terms foreshock, mainshock and aftershock
are semantic terms created by humans, used to describe a wide
range of possible earthquakes. Given the patterns observed in
this region (i.e., the long cluster of earthquakes in several
locations in 1981-1982), we have decided to call both of these
two events mainshocks, and when needed explain that the first
was a preshock of the second. A "preshock" occurs further back
in time than a foreshock, like the 1992 M6.1 Joshua Tree
"preshock", two months before the M7.3 Landers earthquake.
The M5.8 mainshock of September 20 is generating its own
aftershock sequence. The rate of aftershocks has already begun
to decrease as we expect. Eighteen hours after the mainshock,
we see an aftershock sequence close to the average for
California. Given this, an aftershock larger than than the
largest we have seen so far (a M4.1 on just after midnight on
Thursday morning) would not be surprising.
It is always problematic when a large quake occurs right
before the Weekly Report is issued. We have not finished the
data processing of the aftershocks, so the final weekly total
will be somewhat larger than the 614 included here. We believe,
however, that we have a good handle on everything about M2.5.
Some revisions are likely, however.
The table lists the quakes this week that were M2.0 or
larger in the central part of the coverage area, except for
the Ridgecrest aftershocks since the ML5.8, which must be M2.5
or larger to be listed. 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.
The second figure shows a map view and cross section
of the Ridgrest activity so far. The two stars are the M5+
quakes; the more southeasterly and shallower one is this week's
earthquake.
Table 1
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Date Time N Lat. W Long. Mag
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9/14 12:29 am 32 48.4 115 42.2 2.9 8 mi. W of El Centro
9/14 3:59 am 35 48.0 117 38.2 3.4 12 mi. N of Ridgecrest
9/14 11:59 am 33 18.7 118 18.0 2.3 2 mi. SSE of Avalon,
Catalina Island
9/14 12:09 pm 34 22.3 116 28.4 2.0 17 mi. N of Yucca Valley
9/14 2:45 pm 35 45.7 117 38.7 2.1 9 mi. N of Ridgecrest
9/14 3:33 pm 33 34.3 117 57.0 2.3 3 mi. SSW of Newport Beach
9/14 5:06 pm 35 44.8 117 38.2 2.5 8 mi. N of Ridgecrest
9/14 5:57 pm 34 44.3 120 2.5 2.1 11 mi. NNE of Solvang
9/14 9:14 pm 34 44.6 116 42.8 2.0 20 mi. ESE of Barstow
9/15 12:22 am 35 47.3 117 38.5 2.1 11 mi. N of Ridgecrest
9/15 4:29 am 35 46.4 117 39.2 2.0 10 mi. N of Ridgecrest
9/15 9:43 pm 34 2.5 116 19.2 2.1 6 mi. S of Joshua Tree
9/15 9:56 pm 34 9.6 116 25.8 2.3 3 mi. NNE of Yucca Valley
9/16 12:53 am 33 10.6 115 39.2 2.9 1 mi. WNW of Obsidian
Butte
9/16 5:53 am 32 2.2 116 18.7 2.1 51 mi. SSW of Ocotillo
9/16 9:54 am 33 58.8 116 40.2 2.0 6 mi. SW of Morongo Valley
9/16 10:48 am 34 1.9 116 19.2 2.4 7 mi. S of Joshua Tree
9/16 12:26 pm 32 1.0 116 15.1 2.7 51 mi. SSW of Ocotillo
9/16 2:08 pm 35 45.7 117 38.7 2.6 9 mi. N of Ridgecrest
9/16 3:16 pm 35 47.2 117 39.3 2.1 11 mi. N of Ridgecrest
9/16 3:22 pm 35 45.5 117 38.6 2.1 9 mi. N of Ridgecrest
9/16 4:13 pm 35 47.0 117 39.4 2.3 11 mi. N of Ridgecrest
9/16 5:54 pm 33 57.3 116 58.3 2.1 1 mi. N of Beaumont
9/16 6:34 pm 32 24.9 115 24.9 2.8 18 mi. SSE of Calexico
9/16 7:48 pm 32 27.2 115 25.6 3.2 15 mi. SSE of Calexico
9/16 8:07 pm 34 48.4 118 58.0 2.0 1 mi. SW of Frazier Park
9/16 10:35 pm 33 51.7 117 3.1 2.2 6 mi. SW of Beaumont
9/17 12:21 am 33 18.5 116 18.5 2.5 5 mi. NE of Borrego
Springs
9/17 3:55 am 33 54.6 116 17.5 2.2 14 mi. NNW of Indio
9/17 9:20 am 33 56.4 116 20.1 2.2 9 mi. E of Desert Hot
Springs
9/17 10:59 am 35 7.2 118 33.3 2.0 6 mi. W of Tehachapi
9/17 10:59 am 32 7.4 116 39.8 2.5 36 mi. SE of San Ysidro
9/17 11:35 am 34 22.1 118 40.2 2.0 6 mi. NNE of Simi Valley
9/17 12:30 pm 32 18.5 115 13.6 2.9 29 mi. SSE of Calexico
9/17 5:28 pm 35 47.7 117 37.9 2.0 12 mi. N of Ridgecrest
9/18 3:58 am 33 3.5 116 30.0 2.2 5 mi. ESE of Julian
9/18 5:58 am 35 45.7 117 38.8 2.4 9 mi. N of Ridgecrest
9/18 2:37 pm 35 46.5 117 38.8 2.0 10 mi. N of Ridgecrest
9/18 3:06 pm 35 47.4 117 39.5 2.1 11 mi. N of Ridgecrest
9/18 3:38 pm 35 47.4 117 39.2 2.2 "
9/18 11:26 pm 35 46.8 117 38.2 2.3 "
9/19 12:41 am 33 1.3 117 45.4 2.1 24 mi. WSW of Oceanside
9/19 1:49 am 34 14.8 118 41.2 2.0 2 mi. SSE of Simi Valley
9/19 7:49 am 35 22.2 118 30.9 2.6 17 mi. NNW of Tehachapi
9/19 10:22 am 34 9.8 116 25.5 2.0 3 mi. NNE of Yucca Valley
9/19 12:34 pm 34 31.7 118 46.7 2.5 9 mi. W of Castaic Lake dam
9/19 1:10 pm 35 47.8 117 37.9 2.4 12 mi. N of Ridgecrest
9/19 1:20 pm 35 47.2 117 39.8 2.5 11 mi. N of Ridgecrest
9/19 1:23 pm 32 58.2 117 48.0 2.1 28 mi. WSW of Oceanside
9/19 3:24 pm 34 59.0 116 58.7 2.0 6 mi. NNE of Barstow
9/19 6:39 pm 32 37.4 115 54.9 2.6 8 mi. SSE of Ocotillo
9/20 12:13 am 34 15.1 116 26.2 2.3 9 mi. N of Yucca Valley
9/20 1:21 am 34 15.2 116 26.0 2.0 "
9/20 1:48 am 34 20.7 116 28.1 2.0 15 mi. N of Yucca Valley
9/20 3:03 am 34 0.9 117 0.8 2.2 1 mi. SE of Yucaipa
9/20 3:44 am 35 46.0 117 38.9 2.3 10 mi. N of Ridgecrest
9/20 9:38 am 35 47.0 117 38.0 2.0 11 mi. N of Ridgecrest
9/20 10:54 am 35 47.5 117 39.1 2.0 "
9/20 4:27 pm 35 45.6 117 38.3 5.8 9 mi. N of Ridgecrest
VERY WIDELY FELT
9/20 4:30 pm 35 43.8 117 36.7 3.7 8 mi. NNE of Ridgecrest
9/20 4:32 pm 35 45.3 117 37.7 3.9 9 mi. NNE of Ridgecrest
9/20 4:37 pm 35 46.4 117 39.8 3.6 10 mi. N of Ridgecrest
9/20 4:53 pm 35 45.3 117 39.2 3.1 "
9/20 4:55 pm 35 46.2 117 39.4 2.8 10 mi. N of Ridgecrest
9/20 4:55 pm 35 46.4 117 39.1 2.9 "
9/20 4:56 pm 35 46.8 117 39.9 3.9 "
9/20 5:16 pm 35 46.5 117 38.4 3.4 "
9/20 5:31 pm 35 45.1 117 39.7 3.2 8 mi. N of Ridgecrest
9/20 5:33 pm 35 46.5 117 39.7 2.5 10 mi. N of Ridgecrest
9/20 5:45 pm 35 45.1 117 39.0 2.7 8 mi. N of Ridgecrest
9/20 5:45 pm 35 45.1 117 38.9 2.6 9 mi. N of Ridgecrest
9/20 5:46 pm 35 46.7 117 38.9 2.7 10 mi. N of Ridgecrest
9/20 5:46 pm 35 46.6 117 39.0 2.8 "
9/20 6:06 pm 35 44.8 117 38.0 2.9 8 mi. NNE of Ridgecrest
9/20 6:13 pm 35 45.0 117 38.3 3.0 8 mi. N of Ridgecrest
9/20 7:07 pm 35 45.8 117 39.0 2.5 9 mi. N of Ridgecrest
9/20 7:13 pm 35 44.7 117 38.6 2.9 8 mi. N of Ridgecrest
9/20 7:16 pm 35 46.1 117 39.0 3.1 10 mi. N of Ridgecrest
9/20 7:17 pm 35 46.8 117 39.3 2.5 "
9/20 7:48 pm 35 44.5 117 37.7 2.7 8 mi. NNE of Ridgecrest
9/20 8:09 pm 35 46.0 117 39.0 3.4 10 mi. N of Ridgecrest
9/20 8:30 pm 35 46.2 117 39.2 2.7 "
9/20 8:51 pm 35 45.6 117 38.2 2.6 9 mi. N of Ridgecrest
9/20 8:56 pm 35 44.2 117 37.6 2.5 8 mi. NNE of Ridgecrest
9/20 9:23 pm 35 45.8 117 39.3 3.1 9 mi. N of Ridgecrest
9/20 9:28 pm 35 43.7 117 37.6 2.5 7 mi. NNE of Ridgecrest
9/20 9:50 pm 35 43.8 117 37.5 2.6 "
9/20 10:10 pm 35 46.0 117 38.7 3.1 10 mi. N of Ridgecrest
9/20 10:27 pm 35 46.2 117 39.3 2.7 "
9/20 10:57 pm 35 45.3 117 38.9 2.7 9 mi. N of Ridgecrest
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