Hauksson, Egill and Jones, Lucile M. and Hutton, Kate (1995) The 1994 Northridge earthquake sequence in California: Seismological and tectonic aspects. Journal of Geophysical Research B, 100 (B7). 12,335-12,355. ISSN 0148-0227 http://resolver.caltech.edu/CaltechAUTHORS:20121128-095539910
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The M_w 6.7 Northridge earthquake occurred on January 17, 1994, beneath the San Fernando Valley. Two seismicity clusters, located 25 km to the south and 35 km to the north-northwest, preceded the mainshock by 7 days and 16 hours, respectively. The mainshock hypocenter was relatively deep, at 19 km depth in the lower crust. It had a thrust faulting focal mechanism with a rake of 100° on a fault plane dipping 35° to the south-southwest and striking N75°W. Because the mainshock did not rupture the surface, its association with surficial geological features remains difficult to resolve. Nonetheless, its occurrence reemphasized the seismic hazard of concealed faults associated with the contractional deformation of the Transverse Ranges. The Northridge earthquake is part of the temporal increase in earthquake activity in the Los Angeles area since 1970. The mainshock was followed by an energetic aftershock sequence. Eight aftershocks of M ≥ 5.0 and 48 aftershocks of 4 ≤ M ≤ 5 occurred between January 17 and September 30, 1994. The aftershocks extend over most of the western San Fernando Valley and Santa Susana Mountains. They form a diffuse spatial distribution around the mainshock rupture plane, illuminating a previously unmapped thrust ramp, extending from 7–10 km depth into the lower crust to a depth of 23 km. No flattening of the aftershock distribution is observed near its bottom. At shallow depths, above 7–10 km, the thrust ramp is topped by a dense distribution of aftershock hypocenters bounded by some of the surficial faults. The dip of the ramp increases from east to west. The west side of the aftershock zoae is characterized by a dense, steeply dipping, and north-northeast striking planar cluster of aftershocks that exhibited mostly thrust faulting. These events coincided with the Gillibrand Canyon lateral ramp. Along the east side of the aftershock zone the aftershocks also exhibited primarily thrust faulting focal mechanisms. The focal mechanisms of the aftershocks were dominated by thrust faulting in the large aftershocks, with some strike-slip and normal faulting in the smaller aftershocks. The 1971 San Fernando and the 1994 Northridge earthquakes ruptured partially abutting fault surfaces on opposite sides of a ridge. Both earthquakes accommodated north-south contractional deformation of the Transverse Ranges. The two earthquakes differ primarily in the dip direction of the faults and the depth of faulting. The 1971 north-northeast trend of left-lateral faulting (Chatsworth trend) was not activated in 1994.
|Additional Information:||© 1995 American Geophysical Union. Received November 15, 1994; revised March 3, 1995; accepted March 13, 1995. J. Unruh, J. Scott, and D. Wald provided helpful critical reviews. We are grateful to the seismic analysts of Caltech and the USGS for quick and competent processing of the earthquake data. This research was partially supported by USGS grant 1434-94-G-2440, USGS cooperative agreement 1434-92-A-0960, and NSF grant 94-16119 to Caltech. Southern California Earthquake Center publication 125. Contribution 5463, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena.|
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|Official Citation:||Hauksson, E., L. M. Jones, and K. Hutton (1995), The 1994 Northridge earthquake sequence in California: Seismological and tectonic aspects, J. Geophys. Res., 100(B7), 12,335–12,355, doi:10.1029/95JB00865.|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Ruth Sustaita|
|Deposited On:||28 Nov 2012 18:16|
|Last Modified:||27 Dec 2012 03:04|
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