Investigation of Long-Period Ground Motion in the NorthridgeEarthquake of January 17, 1994.

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9413273 Iwan The Northridge earthquake of January 17, 1994 yielded what may prove to be the most valuable set of strong motion data ever obtained from an earthquake. These data include the largest horizontal ground motion ever recorded. The damage associated with these large ground motions is well documented. In some of the near-field accelerograms, particularly the Sylmar station, there is evidence of a significant long period displacement pulse in the ground motion. The existence of such motion has recently been the subject of great concern as it relates to the design of base isolated and high-rise structures. These structures are particularly susceptible to large input displacements. There is a recognized need to better understand the nature of this long period motion and, ultimately, to be able to model and predict is occurrence. Present routine techniques for the processing of strong motion data tend to remove or distort the long-period information from recorded accelerograms. However, under a recent NSF exploratory grant, techniques have been developed to recover much of the long-period information in accelerograms. This is accomplished by nontraditional baseline correction and signal processing that is based on an improved understanding of the physics of the ground motion and the nature of the recording instruments. The techniques have been verified for ground motions from the Landers earthquake. The goal of this research program is to apply the recently developed data processing techniques to a suite of earthquake records from the Northridge earthquake. The ultimate goal is to provide a basis for guidelines that may be used to characterize the long-period content of earthquake motion for structural design. This exploratory investigation addresses the following questions: 1. What is the nature of the long period (displacement) content of ground motion from this type of thrust earthquake? 2. How does this vary with distance from th e epicenter and orientation with respect to the fault plain. 3. How is the long-period content (peak displacement) related to the short-period content (peak acceleration)? 4. What generalizations can be made to future events of this, or another, type? 5. Is there a correlation between the observed long-period ground motion and structural damage? The research is based on digital data obtained from the California Division of Mines and Geology and the U.S. Geological Survey. ***