From earthquake intensities to earthquake sources: extending the contribution of historical seismology to seismotectonic studies

The epicentral locations and magnitudes of the events reported in the Catalogue of Strong Italian Earthquakes are obtained from intensity data through a standardized and established algorithm. However, we contend that the dense and homogeneously collected data sets presented in this catalogue can also be used to assess the location, physical dimensions and orientation of the earthquake source on purely historical grounds. The method we describe is of special value for older earthquakes and for all events that fall in areas where the understanding of faulting and tectonics is limited. At the end of the calculations the seismic source is represented as an oriented "rectangle", the length and width of which are obtained from moment magnitude through empirical relationships. This rectangle is meant to represent the actual surface projection of the seismogenic fault or, at least, the projection of the portion of the Earth crust where a given seismic source is likely to be located. Sources derived through this procedure can then be juxtaposed to sources derived from instrumental and geological data for constructing fault segmentation and earthquake recurrence models and for highlighting linear gaps in the global seismic release. To test the method we applied it systematically to all M > 5.5 earthquakes that occurred in the Central and Southern Apennines in the past four centuries. The results are encouraging and compare well with existing instrumental, direct geological and geodynamic evidence. The method is quite stable for different choices of the algorithm parameters and provides elongation directions which in most cases can be shown to be statistically significant. The resulting pattern of source locations and orientations is homogeneous, showing a consistent Apennines-parallel trend that agrees well with the NE-SW tectonic extension style of the central and southern portions of the Italian peninsula