Magnitude 4.8 near Yorba Linda, CA
Tuesday, September 3, 2002 at 0:08:51 AM (PDT)
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Preliminary Earthquake
Report
Magnitude |
4.8 (updated from M4.6)
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Time |
Tuesday, September 3, 2002 at 7:08:51
(UTC)
Tuesday, September 3, 2002 at 0:08:51 AM (PDT) |
Distance
from |
4 km (3 miles) NNE (32 degrees) of Yorba
Linda, CA
7 km (5 miles) SSW (212 degrees) of Chino Hills, CA
9 km (6 miles) ENE (64 degrees) of Placentia, CA
15 km (9 miles) NE (51 degrees) of Anaheim, CA |
Coordinates |
33 deg. 55.2 min. N (33.919N)
117 deg. 45.9 min. W (117.764W) |
Depth |
7.3 km (4.5 miles) |
Quality |
Fair |
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Preliminary Summary Report - September 3, 2002
A M4.8 (updated from M4.6) mainshock occurred at 00:08
am on 3 September northeast of Yorba Linda in Orange County
at a depth of 10 km. It was preceded by two foreshocks at
09:50pm (ML2.6) and 10:23pm (ML1.5) on Sept 2nd. It was
also followed by 23 aftershocks during the next 9 hours,
with the two largest aftershocks of ML2.8 at 00:15am and
04:28am.
We use seismic records to determine the orientation of the
fault on which an earthquake occurs. The mainshock exhibited
strike-slip faulting (horizontal movement) on a vertical
plane striking N30°W. This mechanism is consistent with
the mainshock being near the Whittier fault, one of the
fastest moving faults (~2 to 3 mm/yr) in the Los Angeles
basin. However, preliminary locations of the aftershocks
appear to form a northeast trend thus suggesting that this
sequence is occurring on a small conjugate fault, adjacent
to the Whittier fault. Alternatively, this sequence is occurring
near a jog in the Whittier fault itself.
This sequence is located along the eastern part of the Los
Angeles basin where the Whittier fault and the buried thrust
faults to the west from a complex zone of deformation. The
1987 ML5.9 Whittier Narrows earthquake occurred near the
north end of this zone. The last previous M4 earthquake
to occur in the greater Los Angeles area was located near
Compton on 28 October 2001. Today’s M4.6 event is
the largest in the Los Angeles metropolitan area since a
M5.1 Northridge aftershock in April 1997.
California Integrated Seismic Network
(CISN): Cooperative Project of Caltech, USGS, CGS, & UCB, Southern California Seismic Network/TriNet
Egill Hauksson and Kate Hutton, Caltech, Pasadena, CA91125,
hauksson@gps.caltech.edu;
Lucy Jones and Doug Given USGS, Pasadena, CA 91106, jones@usgs.gov
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