December 15, 1994
Weekly Earthquake Report for Southern California
------------------------------------------------
December 8 - 14, 1994
Prepared by: Kate Hutton, Seismological Laboratory
(kate@bombay.gps.caltech.edu)
Lucy Jones, U.S. Geological Survey
California Institute of Technology
This document is a commentary on current seismic activity. The
earthquakes discussed have been detected and processed
automatically. All epicenters and magnitudes have been reviewed
to exclude obvious blunders; however, they must still be
considered preliminary. 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, December 8, Pacific Standard Time, to midnight Wednesday
night, December 14, Pacific Standard Time. We recorded 343 earth-
quakes of M1.0 or larger during the 7-day period covered.
Although the total number of earthquakes this week was
about the same as last week, the proportion of M3+ quakes was
slightly higher than usual. We had six of them, at various
locations. Only two of them were aftershocks: the M3.3 quake on
Saturday morning, a Landers aftershock, and the M3.2 quake on
Tuesday morning, a Big Bear aftershock. The first was felt in
Barstow and the second one in the Big Bear Lake area.
The other four M3+ events were scattered: a M3.6 on Friday
evening near Ojai, which was felt; a M3.4 early Sunday morning
near Marina del Rey, which was felt; a M3.6 on Monday morning in
the Imperial Valley; and a M3.7 near Ridgecrest, also on Monday
morning. None of these quakes were in places that surprised us,
and none were part of extensive swarms. Both the Ojai and Marina
del Rey quakes occurred on approximately east-striking faults.
Several systems of such faults are evident in both areas. The
San Cayatano and Oak Ridge systems both go through the Ojai area.
Several researchers have inferred blind thrust faults in the
Marina area, based on the sedimentary geology and on the presence
of small events (like this week's) with thrust focal mechanisms.
The Imperial Valley quake was near Obsidian Butte, the
site of hundreds on M3+ earthquakes in the last decade. The
Ridgecrest quake was also in a perennial hot spot. Both these
quakes had approximately strike-slip mechanisms, as expected for
those locations.
The fact that these particular quakes all happened in one
week is undoubtably explained by statistical fluctuation. Random
events, such as coin flips and earthquakes, sometimes show "runs",
which the human mind almost always perceives as significant.
Actually, the total number of non-aftershocks has been
a slightly lower than average this year. We normally expect to see
about 100 independent M3+ quakes each year in our coverage area.
This year we've only had 65. So the overall seismicity is only
higher than normal because there are two major aftershock sequences
going on.
The table 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
-------
Date Time N Lat. W Long. Mag
-------------------------------------------------------------
12/8 5:50 am 35 49.7 116 49.4 2.0 27 mi. SSE of Telescope Peak,
Panamint Range
12/8 8:38 pm 34 10.2 116 26.2 2.3 3 mi. N of Yucca Valley
12/8 8:56 pm 34 57.0 116 55.8 2.7 6 mi. NE of Barstow
12/8 10:01 pm 34 21.6 118 38.9 2.2 5 mi. SW of Magic Mtn.
12/9 3:50 pm 34 58.6 116 57.3 2.1 7 mi. NNE of Barstow
12/9 4:09 pm 35 5.4 118 17.6 2.7 7 mi. WNW of Mojave
12/9 6:34 pm 34 26.8 119 20.1 2.0 5 mi. W of Ojai
12/9 6:34 pm 34 26.6 119 20.6 3.6 " FELT
12/10 12:05 am 33 15.5 115 39.1 2.0 7 mi. SE of Bombay Beach
12/10 12:20 am 34 5.7 116 24.0 2.0 3 mi. ESE of Yucca Valley
12/10 3:04 am 33 53.5 118 42.8 2.1 9 mi. SE of Pt. Dume
12/10 9:54 am 34 58.8 116 57.1 3.3 7 mi. NNE of Barstow; FELT
12/10 4:06 pm 34 16.6 118 40.3 2.3 2 mi. E of Simi Valley
12/10 4:08 pm 34 0.7 116 19.2 2.8 10 mi. SE of Yucca Valley
12/10 7:42 pm 34 26.5 119 20.7 2.3 5 mi. W of Ojai
12/10 9:12 pm 34 30.0 116 30.9 2.3 26 mi. N of Yucca Valley
12/10 10:44 pm 34 28.3 117 50.9 2.1 15 mi. WNW of Wrightwood
12/11 12:38 am 34 46.6 116 17.9 2.0 36 mi. SSW of Baker
12/11 1:40 am 35 52.2 117 48.7 2.3 14 mi. SSE of Coso Junction
12/11 2:48 am 33 59.3 118 26.1 3.4 1 mi. NE of Marina del Rey
FELT
12/11 8:15 am 34 37.5 116 33.5 2.0 25 mi. ENE of Lucerne Valley
12/11 8:59 am 34 2.0 117 13.2 2.0 2 mi. WSW of Redlands
12/11 11:07 am 34 22.0 116 27.8 2.1 17 mi. N of Yucca Valley
12/11 1:31 pm 34 49.8 116 36.0 2.3 24 mi. E of Barstow
12/11 2:23 pm 32 50.0 117 39.5 2.2 23 mi. W of La Jolla
12/11 6:18 pm 34 23.8 116 28.0 2.3 19 mi. N of Yucca Valley
12/11 8:37 pm 34 46.9 116 19.6 2.1 36 mi. SSW of Baker
12/11 8:58 pm 34 23.5 118 38.6 2.1 3 mi. SW of Magic Mtn.
12/11 9:04 pm 32 2.3 116 21.6 2.9 52 mi. SSW of Ocotillo
12/11 9:50 pm 35 56.5 120 29.2 2.9 4 mi. NW of Parkfield
12/11 10:06 pm 34 47.6 116 18.3 2.5 35 mi. SSW of Baker
12/11 10:07 pm 34 2.0 118 4.1 2.0 Whittier Narrows Recreation
Area
12/11 11:24 pm 34 15.6 116 24.8 2.0 9 mi. N of Yucca Valley
12/12 1:37 am 34 26.1 119 20.0 2.2 4 mi. W of Ojai
12/12 1:37 am 32 37.0 115 54.8 2.0 9 mi. SSE of Ocotillo
12/12 3:41 am 33 58.1 117 14.7 2.3 3 mi. NE of Moreno Valley
12/12 5:05 am 33 11.6 115 34.6 3.6 Obsidian Butte area, 4 mi.
SW of Niland
12/12 5:09 am 33 10.7 115 33.2 2.1 "
12/12 5:15 am 33 11.6 115 34.5 2.8 "
12/12 6:04 am 33 11.4 115 34.3 2.0 "
12/12 7:50 am 34 47.8 116 18.3 2.5 34 mi. SSW of Baker
12/12 9:49 am 34 8.4 116 25.8 2.8 1 mi. NNE of Yucca Valley
FELT
12/12 11:48 am 35 41.9 117 39.5 3.7 5 mi. N of Ridgecrest
FELT
12/12 11:49 am 35 41.8 117 39.3 2.5 "
12/12 1:18 pm 35 41.9 117 39.6 2.5 "
12/12 5:24 pm 34 12.7 118 35.8 2.0 Under Canoga Park
12/12 8:25 pm 33 45.9 117 54.4 2.4 3 mi. S of Disneyland
12/12 8:44 pm 34 20.8 116 28.0 2.0 15 mi. N of Yucca Valley
12/12 11:18 pm 33 53.0 119 26.5 2.0 11 mi. SSE of Santa Cruz Is.
12/13 12:40 am 34 47.0 116 18.2 2.0 35 mi. SSW of Baker
12/13 3:07 am 33 54.4 116 46.0 2.3 1 mi. SE of Cabazon
12/13 5:05 am 34 22.2 116 28.0 2.1 17 mi. N of Yucca Valley
12/13 8:43 am 34 37.3 116 32.2 2.3 26 mi. ENE of Lucerne Valley
12/13 9:51 am 34 7.6 116 53.0 3.2 3 mi. WNW of Mt. San Gorgonio
FELT
12/13 3:53 pm 34 23.8 118 37.7 2.0 2 mi. SW of Magic Mtn.
12/13 6:52 pm 34 12.3 116 52.4 2.0 3 mi. SE of town of Big Bear
Lake
12/13 7:31 pm 35 5.3 119 5.6 2.2 9 mi. WNW of the I5/99
interchange
12/13 7:51 pm 33 54.8 119 38.0 2.3 10 mi. SW of Santa Cruz Is.
12/14 4:09 pm 34 20.0 116 28.1 2.5 14 mi. N of Yucca Valley
12/14 4:49 pm 34 25.6 116 30.9 2.3 21 mi. N of Yucca Valley
-------------------------------------------------------------
Figure 1. A map of southern California showing the earthquakes
recorded during the past week by the Caltech/USGS Seismic
Network. Major faults are marked, as well as the metropolitan
areas of Los Angeles (L.A.), Palm Springs (P.S.), San Diego
(S.D.), and Santa Barbara (S.B.). The circles denote the
earthquakes, the size of the circle indicating the magnitude.