Reevaluation of Historic Earthquakes in Spain

In recent years considerable effort has been expended to find new accounts of earthquake effects in Spain. With this information a reevaluation of intensities has been performed. These new data have allowed us to recalculate the seismic hazard maps of Spain which are included in recent seismic codes (Mezcua and Martin, 1993; Rueda and Mezcua, 2001; NCSR, 2O02). The limitation of single intensity values for unambiguous assignment of the strength of shocks is widely known ( e.g. , Reiter, 1990). In particular, the use of maximum intensity values may produce misleading results. This fact may eventually be overcome by the use of magnitude values deduced from the whole set of intensity values for all historical events. There are numerous studies on the conversion of single maximum intensity values or isoseismal areas to different magnitude scales, e.g. , Nuttli (1973), Street and Turcotte (1977), and Johnston (1996a), including Mezcua (1982) for use in the Iberian Peninsula. The correlations obtained consider the magnitude scale that is in use in the area under study. In the case of Spain the mbLg magnitude was used for the relationship. However, it is well known this magnitude definition saturates in the upper range (beyond magnitude 6). This saturation is due, as in other magnitude definitions, to the types and frequencies of the waves used in the magnitude calculation. In recent years many earthquake hazard studies have been performed using moment magnitude, which does not suffer from saturation problems. Another important problem in hazard studies is poor knowledge of the locations of historical events. This uncertainty is translated to the source model needed in any hazard calculation. For that reason, any effort in improving such parameters will improve the hazard values obtained in a specific site. To obtain a relationship between the moment magnitude …