On the recovery and analysis of historical seismograms

The analysis of historical seismograms has proven to be a fundamental tool to help with the definition of the seismic risk in specific regions. In- deed, modern quantitative reappraisal of relevant earthquakes that oc- curred before the 1960’s; i.e., prior to both the developments of modern recording instruments and the theoretical progress, has been essential for the assessment of the seismic potential of a source area. However, due to the characteristics (transducing and recording) of the old analog seismographs, the data available are affected by intrinsic uncertainties, and errors can be introduced during the processing of waveform digiti- zation. These drawbacks can seriously influence the quality and relia- bility of an investigation. In general, no standard technique can be applied when dealing with historical seismograms. Thus, specific tests and cross-checks have to be designed to estimate the limits of each spe- cific analysis. Here, we aim to provide an overview of the whole proce- dure while focusing on the most crucial steps, from the seismogram recovery to the application of modern techniques for the retrieval of the seismic source information. We also suggest possible checks for the ro- bustness of the data and for the available instrument characteristics, with a description of the effects of various uncertainties on the results that can be obtained. We thus provide useful indications for the analy- sis of historical seismograms, and also for the correct interpretation of the resulting characteristics of the seismic source.