Intensity-based source inversion of the destructive earthquake of 1694 in the southern Apennines, Italy

We inverted the regional pattern of intensities of the catastrophic earthquake of 1694 in the southern Apennines and determined the geometrical and kinematic characteristics of its source, including the double-couple orientation (strike angle 299 degrees 10 degrees, dip 54 degrees 12 degrees, and rake 309 degrees 11 degrees; pure dip-slip solution not precluded). The objective nonlinear inversion was performed using the simple 11 parameter kinematic-function model (KF) with a niching genetic algorithm technique. The similarity between the pattern of the field intensities of the 1694 earthquake and the synthetic pattern is striking. This result is supported by the fact that our inversion technique was verified in the study area, where it was able to again find the source of the M-s 6.9 Irpinia 1980 earthquake, known from instruments and from field surveys of the rupture. This type of inversion enables researchers to exploit the extensive intensity data from Italy and other countries to extend the knowledge of seismotectonic activity to preinstrumental times. New paleoseismological evidence and even published descriptions of the fault rupture by witnesses of the 1694 earthquake were found, confirming our results. We also present and apply a new algorithm, of general interest, to calculate disorientations between double-couples via standard linear algebra. The 3-D rotations that bring the pairwise orthogonal unit vectors that were estimated by our intensity inversion into the instrumental triples of the 1980 Irpinia earthquake, assumed as benchmarks, provide verification of our algorithm.