Assessing the Fidelity of Seismic Records from Microfilm and Paper Media

Abstract During the analog era of seismology, seismograms were typically written to paper media. The legacy records from this era make up an irreplaceable resource that covers over a century, and many of them remain extant in both their original paper form and as microformat copies. While microformat copies of seismograms drove decades of seismological research, new capabilities have been developed that allow us to revisit the analog data sets and extend application of contemporary analyses to a considerably longer seismological record. Though microformat seismogram copies have been extensively used, we seek to understand possible differences that could arise from using data recovered from such copies compared to using the original seismograms. Identification of these differences provides understanding of the fidelity of information that is recoverable from these different media. We present a case study here, applying a number of quantitative measures to a pair of records (original paper and its 35 mm microfilm copy), and revealing that microfilm-derived time series differ from the original paper in timing and amplitude. Based on a simple analysis of this pair of short-period vertical records, we estimate uncertainties of ±0.1–0.3 for earthquake magnitude (hence, a factor of about 1.3–2.0 in energy) and ±10 km for hypocentral location.