Co-Seismic Displacements of the 1994 Northridge, California, Earthquake
The 17 January 1994 Northridge, California, earthquake significantly deformed the Earth's crust in the epicentral region. Displacements of 66 survey stations determined from Global Positioning System (GPS) observations collected before and after the earthquake show that individual stations were uplifted by up to 417 ± 5 mm and displaced horizontally by up to 216 ± 3 mm. Using these displacements, we estimate parameters of a uniform-slip model. Fault geometry and slip are estimated independent of seismological information, using Monte Carlo optimization techniques that minimize the model residuals. The plane that best fits the geodetic data lies 1 to 2 km above the plane indicated by aftershock seismicity. Modeling for distributed slip on a coplanar, yet larger model fault indicates that a high-slip patch occurred up-dip and northwest of the mainshock hypocenter and that less than 1 m of slip occurred in the uppermost 5 km of the crust. This finding is consistent with the lack of clear surface rupture and with the notion that the intersection with the fault that ruptured in 1971 formed the up-dip terminus of slip in the Northridge earthquake. Displacements predicted by either of these simple models explain most of the variance in the data within 50 km of the epicenter. On average, however, the scatter of the residuals is twice the data uncertainties, and in some areas, there is significant systematic misfit to either model. The co-seismic contributions of aftershocks are insufficient to explain this mismatch, indicating that the source geometry is more complicated than a single rectangular plane.
Additional Information© 1996 Seismological Society of America. Manuscript received 17 January 1995. The GPS data used in this study were, in large part, collected through the cooperative efforts of several agencies and institutions. The National Geodetic Survey, Caltrans District 7 and Headquarters, Los Angeles County, and City of Los Angeles were the main surveying agencies that provided us with crucial assistance in GPS surveys conducted both prior to and following the Northridge earthquake. Without the work of these surveying agencies, only a portion of the results obtained in this study would have been possible. Of particular importance were the help of D. D'Onofrio, L. Fenske, R. Packard, R. Reader, and J. Satalich. The leveling results we discuss resulted from work by these agencies, in part supported by a FEMA mission assignment to the USGS under the direction of R. Stein. Also, the various field, logistical, and other efforts of many of our colleagues after the earthquake were greatly appreciated. We thank the following in particular: D. Agnew, J. Behr, E. Calais, K. Clark, M. Cline, G. Franklin, X. Ge, W. K. Gross, G. Hamilton, K. Hurst, D. Jackson, H. Johnson, S. Larsen, G. Lyzenga, D. Potter, M. Smith, J. Sutton, K. Stark, C. Stiffler, F. Webb, J. Zhang, and J. Zumberge. Much of the software used in producing, modeling, and displaying our results is freely available, and we particularly thank the developers of DISL, FONDA, GINV, GAMIT, GIPSY, GLOBK, and GMT who happen not to be co-authors of the present article: G. Blewitt, D. Dong, K. Feigl, K. Hurst, D. Jefferson, S. Larsen, W. Smith, F. Webb, P. Wessel, and J. Zumberge among many others. Though our published results were obtained using the GAMIT/GLOBK software, earlier analyses using FONDA and GIPSY/OASIS-II contributed significantly to the results. M. Murakami kindly provided a preprint of his article on the SAR interferometry results. We thank reviewers R. Stein, J. Mori, T. Heaton, and an anonymous reviewer for their suggestions. The National Earthquake Hazard Reduction Program (NEHRP) supported much of this work, both through the direct support of the USGS, NSF, and FEMA and indirectly through SCEC contracts with M/T, UCLA, and UCSD; portions of the research described in this work were carded out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA; research at UCSD is funded by Grants NASA NAG 5-1917, USGS 1434-92-G2196, NSF EAR 92 08447, and NSF EAR-9416338, as well as through SCEC. SCEC Contribution Number 153.
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