SOUTHERN CALIFORNIA
SEISMOGRAPHIC NETWORK
A Cooperative Project of
Caltech and the U.S. Geological Survey
October 23, 1997
Weekly Earthquake Report for Southern California
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October 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 626-395-6327. For daily
updates, call our Earthquake Information Hotline: 626-395-6977.
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If you receive this Report by FAX and if you have recently had
a change of AREA CODE, please notify us ASAP. We have no other
way to know which fax numbers need to be changed!! Actually, if
your area code is 818 and it HASN'T changed, it would be helpful
to know that, too.
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This week's Report covers the time period from midnight Thursday
morning, October 16, Universal Time (ie. UTC), to midnight
Wednesday night, October 22, Universal Time. In local time,
the period of coverage is from 5 p.m., October 15, Pacific
Daylight Time, to 5 p.m., October 22, Pacific Daylight Time. We
detected 216 earthquakes during the seven-day period covered.
These Weekly Reports should not be used for archival
purposes. Further data processing often changes the magnitudes
slightly and, especially during times of high seismicity,
sometimes changes the estimates of total seismicity. For the
best earthquake catalog currently available, see:
http://scec.gps.caltech.edu/catalog-search.html
There were four M3+ events this week. The largest was
offshore, between Santa Barbara Is. and Catalina. It occurred
on Saturday afternoon, and had a magnitude of 3.4. We received
no inquiries. Due to the sparsity of seismic stations in the
area (confined to the islands), the focal mechanism is difficult
to determine.
There were two quake felt. The first was a M3.0 North-
ridge aftershock last Thursday morning. It was felt in the
San Fernando Valley. Unlike most Northridge aftershocks,
this one had a strike-slip mechanism. It was relatively
shallow (4 kilometers depth) and probably represents deformation
of the hanging wall above the mainshock's fault.
A M3.0 Landers aftershock occurred early Wednesday
morning; it was felt in the Yucca Valley and Landers area.
Unlike most Landers aftershocks, but similar to some of the
background activity at this location, this event had a normal
mechanism.
There was one more M3+ event, in the early hours of
last Thursday. It was a M3.1 deep (12 kilometers) below
the San Gorgonio Pass, near Cabazon. We received no inquiries
about that one either, so we presume it was not felt. This
quake also had a normal focal mechanism.
Following is our routine quarterly summary of the
seismic activity. Note that, due to the fact that it had a
significantly smaller mainshock (Mw6.7 compared to Mw7.3),
the Northridge seismicity rate is now much lower than the
Landers seismicity rate, even though the Northridge quake
happened more recently.
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
1977 3,699 39 3,276 54 3,193 31
Northridge
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1/17/94 - 9/30/97
All events 14,402 (198 of them this quarter)
3.0 - 3.9 409 (4 of them this quarter)
4.0 - 4.9 52 (none this quarter)
5.0 and up 11 (none this quarter)
Landers/Big Bear
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4/23/92 - 9/30/97
All events 67,535 (689 of them this quarter)
3.0 - 3.9 1,632 (1 of them this quarter)
4.0 - 4.9 171 (2 of them this quarter)
4.1 on Sept. 19
4.4 on Sept. 28
5.0 and up 23 (none this quarter)
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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10/15 8:49 pm 34 15.1 117 40.4 2.2 2 mi. SSW of Mt. Baldy
10/16 2:05 am 32 52.7 115 41.7 2.5 10 mi. NW of El Centro
10/16 2:37 am 34 1.2 116 44.9 3.1 7 mi. NNE of Cabazon
10/16 5:59 am 33 53.5 119 36.9 2.3 11 mi. SSW of Santa Cruz
Is. (E end)
10/16 7:17 am 35 47.5 117 38.7 2.0 11 mi. N of Ridgecrest
10/16 9:02 am 34 13.4 118 37.2 3.0 1 mi. NNW of Canoga Park
FELT
10/17 7:43 pm 32 19.4 115 11.9 2.8 29 mi. SE of Calexico
10/17 10:09 pm 33 11.3 115 34.0 2.3 4 mi. SW of Niland
10/18 6:19 am 33 21.0 116 26.2 2.1 7 mi. NNW of Borrego
Springs
10/18 1:57 pm 33 25.1 118 44.6 3.4 18 mi. ESE of Santa
Barbara Is.
10/18 9:18 pm 33 48.5 119 17.4 2.3 20 mi. SE of Santa Cruz
Is. (E end)
10/18 9:20 pm 33 56.1 116 1.4 2.0 14 mi. S of Twentynine
Palms
10/18 9:38 pm 34 59.2 116 57.2 2.7 7 mi. NNE of Barstow
10/19 12:37 am 33 37.9 116 42.5 2.1 5 mi. NNW of Anza
10/21 4:15 am 33 15.8 116 18.3 2.1 3 mi. E of Borrego
Springs
10/21 9:48 am 33 11.5 115 36.2 2.0 2 mi. NE of Obsidian
Butte
10/22 7:09 am 34 18.1 116 27.1 3.0 12 mi. N of Yucca Valley
FELT
10/22 8:42 am 33 58.3 116 35.1 2.0 5 mi. S of Morongo Valley
10/22 11:14 am 32 43.0 118 4.9 2.5 16 mi. ESE of San
Clemente Is. (SE tip)
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