CREATION OF A DIGITAL DATABASE FOR 20TH CENTURY HISTORICAL EARTHQUAKES OCCURRED IN THE IBERIAN AREA

The problems of recovering the seismic information contained in the ol d seismograms through their digitization and processing by computer me thods are discussed. We present the main principles of a simple manual technique for digitization of early seismic records of the Wiechert s eismograph. Detrending of the zero-line slope, circular are removal. s moothing 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 t heir spectral amplitudes. The deconvolution of the seismograph respons e has allowed us to obtain the time variation of ground motion which i s then contrasted with direct measurements of ground motion displaceme nt amplitudes given in the old-time bulletins. We have created a digit al database for historical earthquakes which occurred in the Iberian a rea during the period 1912-1940 and recorded by the Wiechert seismogra ph at the Geophysical Observatory of Toledo, Spain. It contains the fo llowing 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 s eismic oscillations and amplitude spectra. We carry out magnitude esti mates and give formulae for magnitude classification based on the sign al duration and on the maximum ground displacement amplitude. We also perform seismic moment determinations by spectral analysis of waveform s and propose a new seismic moment-magnitude relation.