Creation of a Digital Database for XXth Century Historical Earthquakes Occurred in the Iberian Area

The problems of recovering the seismic information contained in the old seismograms through their digitization and processing by computer methods are discussed. We present the main principles of a simple manual technique for digitization of early seismic records of the Wiechert seismograph. Detrending of the zero-line slope, circular arc removal, smoothing and interpolation treatment of the digital data are made. The accuracy of the digitizing process is assessed and its reliability is tested by a comparison with automatically obtained digital data and their spectral amplitudes. The deconvolution of the seismograph response has allowed us to obtain the time variation of ground motion which is then contrasted with direct measurements of ground motion displacement amplitudes given in the old-time bulletins. We have created a digital database for historical earthquakes which occurred in the Iberian area during the period 1912–1940 and recorded by the Wiechert seismograph at the Geophysical Observatory of Toledo, Spain. It contains the following output data the digitized original records; the geometrically corrected and interpolated data; the time variation of ground motion; maximum amplitudes and corresponding periods; total duration of the seismic oscillations and amplitude spectra. We carry out magnitude estimates and give formulae for magnitude classification based on the signal duration and on the maximum ground displacement amplitude. We also perform seismic moment determinations by spectral analysis of waveforms and propose a new seismic moment-magnitude relation.