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Synthesis of Strong Ground Motion of the Northridge Earthquake: Comparison with Observed Damage Distribution in the San Fernando Valley

by A.S. Papageorgiou

This article presents research resulting from NCEER's program in seismic hazards and ground motion. A companion article appeared in the January issue of the Bulletin (Volume 10, No. 1) regarding the use of a "2.5-D" model in interpreting strong ground motion caused by the Loma Prieta earthquake. This article compares strong ground motion generated by the Northridge earthquake to damage patterns in the affected area. Comments and questions should be directed to Apostolos Papageorgiou, Rensselaer Polytechnic Institute, at (518) 276-6331; email: papaga@socrates.eng.rpi.edu.

The Northridge, California earthquake of January 17, 1994, (moment magnitude MW = 6.7), centered beneath the densely populated San Fernando Valley, severely impacted people and manmade structures in the Los Angeles area. It produced the strongest ground motions ever instrumentally recorded in an urban setting and the greatest financial losses ($13 billion to $20 billion) from a natural disaster in the United States since 1906 (Scientists of USGS and SCEC, 1994).

The rupture occurred on a blind thrust fault (i.e., a buried thrust fault that does not extend to the surface) dipping 42o to the southwest on a fault plane striking about 112o clockwise from the north (figure 1). Faulting initiated at a depth of about 19 km beneath the northwestern San Fernando Valley and propagated updip to a depth of about 7 km.

The Northridge earthquake generated an extraordinary collection of strongmotion data (more than 200 permanently deployed accelerographs recorded the motion). Furthermore, a significant number of freefield records were obtained at sites within 25 km of the source.

Wald et al. (1996) performed a systematic inversion of these near source records (along with teleseismic and General Positioning System (GPS) and leveling data) and obtained the slip history of the main event (i.e. when, how much, for how long and in what direction each point of the fault slipped during the event).

The slip model that Wald et al. (1996) inferred was used in this project to simulate strong ground motion over a dense grid of points of dimensions 50 km x 50 km, centered on the San Fernando Valley (Zhang and Papageorgiou, 1995). The model is a fault plane embedded in a layered viscoelastic halfspace (i.e., identical to the model used by Wald et al., 1996, to infer the slip history of the event). The discrete wavenumber method was used to represent the source wavefield (Bouchon and Aki, 1977; Bouchon, 1979) and a recursive algorithm in terms of generalized transmission and reflection coefficients (Luco and Apsel, 1983) to propagate the wavefield through the stack of layers. The simulated strong ground motion time histories include displacements, velocities and various measures of strain. Particle trajectories and snapshots of the seismic wave propagation (figure 2, figure 3 and figure 4) clearly display the directions along which seismic energy was preferentially radiated. In the case of the Northridge earthquake, as a result of directivity and the radiation pattern of the source mechanism (e.g., Aki and Richards, 1980), the most intense ground motions were observed north of the epicenter and towards the updip direction, as is evident in figures 2, 3 and 4. The intensity of ground motion due to directivity and radiation pattern was one of the major factors that contributed to the collapse of the Antelope Valley Freeway overpass onto the Golden State Freeway south of Newhall (figure 1). The same effects also explain the severe ground motions recorded in the vicinity of Granada Hills and Sylmar.

The comparison of the distribution of building damage (red tagged buildings) in the San Fernando Valley (OES, 1995) with one measure of the intensity of ground motion (peak value of the northsouth component of velocity) is shown in the last panel of figure 4. Although one must be careful in drawing immediate conclusions from such a comparison (density and type of construction, among other factors, must be taken into account), it is evident that the high concentration of damage at Sherman Oaks (figure 1) cannot be explained by the synthetic motions that were generated using a fault model embedded in a layered halfspace. Explanation of the concentrated damage at Sherman Oaks most likely requires a more complex model (e.g., a model accounting for basin edge effects) than the one used in the present study.

As part of this study (Zhang and Papageorgiou, 1996), various measures of strain were also compared with the distribution of damage to the water distribution system of pipelines (not shown here) in an attempt to clarify the extent to which wave propagation contributed to the observed damage.

Finally, from figures 2, 3 and 4 it is evident that if the fault plane of the 1994 event had been dipping to the north (instead of to the south), the intensity of ground motion in communities such as Santa Monica, Beverly Hills, Hollywood and downtown Los Angeles would have been far greater and the effects of the earthquake far more devastating.

Acknowledgments

The author would like to express his gratitude to Drs. R.T. Eguchi and Neil Blais for kindly providing the building and pipeline damage data that are used in the present study for comparison with the synthetic motions, and Dr. D.J. Wald for providing the slip history and selected recorded data of the event.

References

Aki, K., and Richards, P.G., (1980), Quantitative Seismology, Theory and Methods, New York: W.H. Freeman and Co.

Bouchon, M., and Aki, K., (1977), "Discrete Wavenumber Representation of Seismic Source Wavefields," Bulletin of the Seismological Society of America, Vol. 67, pp. 259277.

Bouchon, M. (1979), "Discrete Wave Number Representation of Elastic Wave Fields in ThreeSpace Dimensions," Journal of Geophysical Research, Vol. 84, pp. 36093614.

Luco, J.E. and Apsel, R.J., (1983), "On the Green's Function for a Layered Halfspace, Part I," Bulletin of the Seismological Society of America, Vol. 73, pp. 909929.

OES, 1995, "The Northridge Earthquake of January 17, 1994: Report of Data Collection and Analysis, Part A: Damage and Inventory Data," Prepared by EQE International, Inc. and the Geographic Information Systems Group of the Governor's Office of Emergency Services for the Governor's Office of Emergency Services of the State of California.

Scientists of the U.S. Geological Survey and the Southern California Earthquake Center, (1994), "The Magnitude 6.7 Northridge, California, Earthquake of 17 January 1994," Science, Vol. 266, pp. 389397.

Wald, D.J., Heaton, T.H. and Hudnut, K.W., (1996), "The Slip History of the 1994 Northridge, California, Earthquake Determined from StrongMotion, Teleseismic, GPS, and Leveling Data," Bulletin of the Seismological Society of America, Vol. 86, No. 1B, pp. S49S70.

Zhang, B. and Papageorgiou, A.S., (1995), "Synthesis of Strong Ground Motion of the 1994 Northridge, California, Earthquake Based On an Inferred Slip History: Comparison with Observed Damage Distribution in the San Fernando Valley," EDS, Transactions, AGU 1995 Fall Meeting, Vol. 76, No. 46 (Supplement), pp. F358.

Zhang, B. and Papageorgiou, A.S., (1996), "Estimation of the Differential Ground Motions Induced in the Near Field by the 1994 Northridge, California, Earthquake," Seismological Research Letters, Vol. 67, No. 2, pp. 63.

Some of the material reported herein is based upon work supported in whole or in part by the National Science Foundation, the State of New York, the U.S. Department of Transportation, the Federal Highway Administration, the Federal Emergency Management Agency and other sponsors. Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of NCEER or its sponsors.

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