Assessment of the Effects of Ground Motion Modification on Ground Motions and Seismic Response of Geotechnical Systems.

Abstract

Dynamic analyses are typically required as part of the seismic design of critical infrastructure. Recorded ground motions from earthquake events with characteristics that are consistent to the design earthquake event are typically selected and used as input to perform dynamic analyses. However, for certain regions or design earthquakes, limited appropriate recorded ground motions are available for selection. An alternative approach is ground motion modification, which is executed either with time or frequency domain techniques. Ground motion modification alters the intensity and frequency content of the acceleration time history of a seed ground motion so that its acceleration response spectrum matches a target response spectrum. Despite this benefit, ground motion modification has been criticized for producing ground motions with unrealistic amounts of energy or unrealistic time histories. However, there is limited research on the effects of ground motion modification on the seed motion to support or discourage its use in practice. This research investigates the impact of time and frequency domain modification techniques on ground motion characteristics and time histories and on the response metrics considered in subsequent site response analyses. The effects of modification on three sets of approximately 100 motions selected for three different earthquake scenarios and for each scenario matched to five alternative target spectra are examined along with the effects of modification on site response analyses when these modified motions are used as input to two sites. One of the major findings of this research is that the effects of time domain and frequency domain modification on the characteristics of the seed motion are strongly correlated to the mismatch between the acceleration response spectrum of the original motion and the target response spectrum. This finding allows the engineer to predict the ground motion characteristics of the resulting modified ground motion prior to performing spectral matching and use that information to select the most appropriate ground motions and technique for modification. The effects of modification on the responses for geotechnical dynamic analyses appear to be related to the impact of modification on the ground motion characteristics.