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Northridge Earthquake - Preliminary Leveling Results

In addition to GPS measurements, precise leveling was performed by several agencies. This work is a preliminary result of ongoing work for the USGS's Mission Assignment, funded by the Federal Emergency Management Agency (FEMA). Dr. Ross Stein of the USGS in Menlo Park is the Project Chief for this study. Leveling data from before and after the earthquake were collected by the California Department of Transportation (Caltrans) and the National Geodetic Survey (NGS). The preliminary analysis we have performed to date is a simple comparison of pre-earthquake and post- earthquake adjusted elevations. Further work will allow refinements of this initial result.

Although the leveling data used in this plot are preliminary, a paper has been published in the Bulletin of the Seismological Society of America by D.J. Wald, T.H. Heaton and K.W. Hudnut that presents these data as part of an inversion of seismological and geodetic data. For those interested in using this initial analysis of the leveling data in other investigations of the earthquake, we are now providing this leveling data set. [More recently, a USGS Open-File report has been written to describe these data in detail]

The map below is a summary showing these preliminary leveling data, with individual benchmarks color-coded to indicate the amount of uplift at each point, as indicated in the map legend.


Click on the image to send it to your external viewer (56 kb JPEG file)

GPS measurements were modeled by computer to image slip that occurred on the fault plane that ruptured in the Northridge earthquake. This slip image was then used to compute the surface deformation field that the earthquake caused. The above map depicts contours of vertical uplift calculated from this variable-slip model. Overlain on these contours are the observed vertical deformation as determined by precise leveling.

The uplift pattern formed an elongated (and lopsided) dome over the part of the fault that experienced the most slip in the earthquake. This is represented here by the concentric oval contour pattern. The leveling data are generally consistent with the pattern of uplift we modeled based on the earlier GPS results. One implication of these results is that the vertical deformation caused by the earthquake may have impacted the urban infrastructure, such as water and sewage systems' gradients. This point is developed further on the 'tilt' page.

For more information, contact Ken Hudnut at hudnut@seismo.gps.caltech.edu

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