Information Extracted from Strong-Motion Records Using Computer Calculations

Although useful information can be extracted from strong-motion earthquake records by visually examining time-history plots of the acceleration and by performing simple calculations by hand, it is now a routine procedure to extract further information by computer processing and analysis of the records. The emphasis here will be on two major tools used in this routine analysis, response spectra and Fourier amplitude spectra. In addition, a technique called system identification will be discussed which can be used in special studies of recorded motions of structures such as buildings, bridges and dams. For computer processing, the data must be prepared in a digital form corresponding to sampled values of the acceleration measured at the instrument's location. These digitized values normally correspond to the motion sampled at equal time steps. In the case of a digital accelerograph they are produced directly by the instrument, but in analog accelerographs a special digitizing procedure must be applied to the original records which are produced as traces on film. Once the digitized data have been input to the computer, a calibration of the data is performed, followed by corrections for possible baseline distortion and for the frequency response of the recording instrument. These corrected accelerations are then in effect integrated to produce velocity and displacement time histories. The next step is usually to calculate and plot the response spectra and Fourier amplitude spectra