Analysis of current methods of interpreting strong-motion accelerograms

The influence on the calculated velocity and displacement and shock spectra of accelerogram vagaries is examined. The effect of centerline adjustment, small random reading errors, and chart paper distortion are considered in detail. An expression for the variance of the final displacement error, which is caused by accelerogram reading errors, is presented. This expression can be used to determine the reading accuracy required to evaluate the final ground displacement to desired accuracy. The utility of determining ground velocity and displacement is discussed. The effects of accelerogram reading errors on shock spectra is considered. The adequacy of current chart paper speeds and transducer natural frequencies is discussed. It is concluded: (a) that the use of accelerograms to accurately determine net ground displacement is impractical; (b) that reading errors in accelerogram charts, as they are currently being recorded, can cause significant errors in response spectra; (c) that limitations of current seismographs may attenuate the high frequency content in strong-motion seismic shocks or distort low-frequency spectra due to aliasing.