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The slip history of the 1994 Northridge, California, earthquake determined from strong-motion, teleseismic, GPS, and leveling data

Wald, David J. and Heaton, Thomas H. and Hudnut, K. W. (1996) The slip history of the 1994 Northridge, California, earthquake determined from strong-motion, teleseismic, GPS, and leveling data. Bulletin of the Seismological Society of America, 86 (1B). S49-S70. ISSN 0037-1106.

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We present a rupture model of the Northridge earthquake, determined from the joint inversion of near-source strong ground motion recordings, P and SH teleseismic body waves, Global Positioning System (GPS) displacement vectors, and permanent uplift measured along leveling lines. The fault is defined to strike 122° and dip 40° to the south-southwest. The average rake vector is determined to be 101°, and average slip is 1.3 m; the peak slip reaches about 3 m. Our estimate of the seismic moment is 1.3 ± 0.2 × 10^(26) dyne-cm (potency of 0.4 km3). The rupture area is small relative to the overall aftershock dimensions and is approximately 15 km along strike, nearly 20 km in the dip direction, and there is no indication of slip shallower than about 5 to 6 km. The up-dip, strong-motion velocity waveforms are dominated by large S-wave pulses attributed to source directivity and are comprised of at least 2 to 3 distinct arrivals (a few seconds apart). Stations at southern azimuths indicate two main S-wave arrivals separated longer in time (about 4 to 5 sec). These observations are best modeled with a complex distribution of subevents: The initial S-wave arrival comes from an asperity that begins at the hypocenter and extends up-dip and to the north where a second, larger subevent is centered (about 12 km away). The secondary S arrivals at southern azimuths are best fit with additional energy radiation from another high slip region at a depth of 19 km, 8 km west of the hypocenter. The resolving power of the individual data sets is examined by predicting the geodetic (GPS and leveling) displacements with the dislocation model determined from the waveform data, and vice versa, and also by analyzing how well the teleseismic solution predicts the recorded strong motions. The general features of the geodetic displacements are not well predicted from the model determined independently from the strong-motion data; likewise, the slip model determined from geodetic data does not adequately reproduce the strong-motion characteristics. Whereas a particularly smooth slip pattern is sufficient to satisfy the geodetic data, the strong-motion and teleseismic data require a more heterogeneous slip distribution in order to reproduce the velocity amplitudes and frequency content. Although the teleseismic model can adequately reproduce the overall amplitude and frequency content of the strong-motion velocity recordings, it does a poor job of predicting the geodetic data. Consequently, a robust representation of the slip history and heterogeneity requires a combined analysis of these data sets.

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Heaton, Thomas H.0000-0003-3363-2197
Hudnut, K. W.0000-0002-3168-4797
Additional Information:© 1996 Seismological Society of America. Manuscript received 20 January 1995. G. Hawkins of Southern California Edison, M. Trifunac of the University of Southern California, J. Steidl of the University of California at Santa Barbara, and R. Tognazinni, C. Davis, and P. Lahr of the Los Angeles Department of Power and Water generously provided in formation and data from their strong-motion stations. R. Stein of the USGS graciously allowed use of the postearthquake leveling data collected under his direction, with funding support from FEMA. J. Satalich of Caltrans provided all of the preliminary leveling data we required for this analysis. Figure 3 was provided by N. Blaise and Kevin Miller of FEMA. Discussions with D. Dreger, R. Graves, N. Smith, and H. Thio were helpful. Reviews by P. Spudich and D. Jackson improved the original manuscript.
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Federal Emergency Management Agency (FEMA)UNSPECIFIED
Issue or Number:1B
Record Number:CaltechAUTHORS:20130305-093406967
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:37296
Deposited By: Tony Diaz
Deposited On:05 Mar 2013 23:57
Last Modified:03 Oct 2019 04:46

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