Influence of Soil-Structure Interation on Motions Recorded at the Jensen Filtration Plant during Northridge Earthquake & Aftershocks & its Implications for Nonlinear Site R

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The strong ground motions recorded at the Jensen Filtration Plant (JFP) during the 1994 Northridge earthquake sequence (mainshock and aftershocks) provide an opportunity to study soil nonlinearity. Two stations close together at the JFP recorded the earthquake motions; one station was situated over moderately stiff soil, the other on very stiff soil. The characters of the two recordings, which were quite different, suggest nonlinear soil response at the moderately stiff soil site. Unfortunately the two buildings where the motions were recorded were not identical in construction; this raises the issue whether soil-structure interaction (SSI) affected the motions at one or both stations.The United States Geological Survey (USGS) maintains, retrieves, processes and disseminates the strong motion data recorded at JFP. USGS researchers have studied these records for nonlinear site response, and their findings suggest that SSI effects may be affecting the results. The principal investigator recently concluded a research program involving dynamic SSI investigations of both JFP buildings: the 3-story main administrative building (JMB) and the much smaller 1-story generator building (JGB).This action is to continue the program with the following tasks: (1) analyze the dynamic test data for inertial SSI effects, (2) analyze kinematic interaction (high frequency filtering by the building foundation) based on data recorded at other sites during the Northridge and other earthquakes, (3) analyze motions recorded simultaneously at the JMB and JGB stations and at new JMB and JGB free-field stations recently installed by the USGS, (4) construct SSI analytical models for the JMB and JGB stations, (5) correct the JMB and JGB strong motion records for SSI, (6) reassess nonlinear site response (in cooperation with USGS researchers), and (7) prepare a technical report followed by journal publications.The ultimate objective of this investigation is to further the understanding of nonlinear site response during weak, moderate, and very strong ground shaking. The results are expected to provide additional insights into nonlinear site response, and its consideration in future refinements of attenuation and site-response models of earthquake strong ground motions.