Source parameters of old earthquakes: Semi-automatic digitization of analog records and seismic moment assessment

Recovering seismic information contained in old analog records could increa se our knowledge of seismic source characteristics and the seismicity of a region. This is particularly important in zones with low to moderate seismi city. To extract the available information, it is necessary to digitize the seism ic records. This is not an easy task especially owing to the generally poor quality of the original records, with illegible or missing parts. However some exceptions were found in the records of a few seismological stations i n Germany, Sweden and France. This paper presents an example of the recovering of source parameters and c an be divided in two parts: the first one presents a simple semi-automatic technique for digitization of old analog seismic records, developed using c ommercial software on a PC; the second part describes the methodology of as sessing the seismic moment, using empirical and theoretical relations, as w ell as the seismic source dimensions. The earthquake selected to illustrate this procedure is the 23 April 1909 B enavente (Portugal) earthquake. This earthquake occurred in the Lower Tagus Valley region and caused great destruction in the meizoseismal area. It is the biggest earthquake that occurred during this century in the central pa rt of the country and its magnitude has been estimated between 6.6 and 7.6. The digitization procedure allowed the recovery of seismic information cont ained in old analog records, in particular, the seismic moment estimation. The results obtained indicated that 7.6 was a very high value for the magni tude of the 23 April 1909 earthquake, suggesting that the magnitude reporte d in the Portuguese catalogue is overestimated. The estimated moment magnit ude is 6.0.