Digitization noise and accelerograph pen offset associated with Japanese accelerograms

abstract Ground-motion accelerograms recorded in Japan between 1956 and 1978 were recently processed. Preprocessing studies of the accelerograms were required that consisted of: (1) evaluation of the long-period digitization noise in the accelerograms, and (2) correction for initial offset of the recording pen in the SMAC accelerograph. Long-period digitization noise present in each accelerogram was identified from the Fourier Amplitude Spectrum of the uncorrected accelerogram. Similar investigations with United States accelerograms recorded on 70-mm film revealed that the noise in both sets of data had similar characteristics. In nearly all cases, the noise spectrum was proportional to the period, which suggested that fatigue and carelessness of the operator during digitization may have been responsible for the observed period dependence. Correction for initial pen offset was necessary for some Japanese accelerograms because the pen arm was not parallel to the direction of movement of the recording paper. To account for pen offset, values of the initial pen rotation were assumed, and the accelerograms were corrected using a simple algorithm. Our best estimate of the true offset was based not only on the offset which best removed the slanted appearance, but also the offset inferred from the original accelerogram.