Defining seismogenic sources from historical earthquake felt reports

TLDR

Intensity data provide rich information to constrain seismic source characteristics, including that source orientation is reflected in damage pattern elongation and can be represented by a rectangle approximating the fault or crustal source projection. The study presents a method using macroseismic intensity data to determine the location, size, and orientation of large historical earthquake sources. The method models a seismic source as an oriented rectangle whose dimensions are derived from moment magnitude via empirical relationships. Applying the method to all M > 5.5 earthquakes in the central and southern Apennines over four centuries yielded results consistent with instrumental, geological, and geodynamic evidence, with stable, statistically significant, and predominantly Appennines‑parallel source orientations matching the region’s NE‑SW extension.

Abstract

Abstract We present a method that uses macroseismic intensity data to assess the location, physical dimensions, and orientation of the source of large historical earthquakes. Intensity data contain a great deal of information that can be used to constrain the essential characteristics of the seismic source. In particular, both the seismological theory and its practice suggest that the orientation of the source of significant earthquakes is reflected in the elongation of the associated damage pattern. A plausible and easily manageable way of describing a seismic source is by representing it as an oriented “rectangle,” the length and width of which are obtained from moment magnitude through empirical relationships. This rectangle is meant to represent either 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. The systematic application of this method to all the M > 5.5 earthquakes that occurred in the central and southern Apennines (Italy) in the past four centuries returned encouraging results that 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 that in most cases can be shown to be statistically significant. In particular, the resulting pattern of source orientations is rather homogeneous, showing a consistent Appennines-parallel trend that agrees well with the NE-SW extension style of deformation active in the central and southern portions of the Italian peninsula.