Fidelity of Seismic Data on Microformats

Throughout the history of seismology, ground motion records have been stored in many different forms and formats. Today, seismograms from the analog recording era spanning roughly a century are most commonly found on two types of media, the original paper, and the microform versions, mainly either microfiche and microfilm. These legacy data provide unique scientific potential such as studies of rare earthquakes and slow changes over the last century. However, before the potential of the legacy analog data can be tapped, the critical tasks of scanning and digitization must be completed in order to transform the deteriorating paper or film into digital time series data. One might assume that seismic data on microformats can be recovered with similar fidelity to that available from the original paper records. Here, though, we present a detailed comparison of digitized time series derived from the original paper and its microform version. The two sets of seismograms differ in certain aspects, which can compromise the timing accuracy and distort the waveforms. These differences arise from the images recovered from the microform, which are more limited in resolution and may contain significant distortions. Our observations offer caveats to scientists seeking to extend research across the analog and digital eras, as well as to managers of facilities where seismograms are stored and archived. Although overlap exists between microform and paper archives, perceptions and assumptions that, with microform copies available, the original seismograms are redundant and disposable need to be reevaluated.