Seismicity at the border between Central and Southern Apennines (Italy): Re-evaluation of the early 1984 instrumental earthquake

Abstract

We re-evaluate the 1984 Abruzzo-Lazio Earthquake on the basis of original seismological data discussed in light of previous interpretations from other authors. This sequence, characterized by two distinct mainshocks (Ms=5.8 and Ms=5.2; NEIS) having low spatial and temporal separation, developed at the border between Central and Southern Apennines. The sequence originated in a narrow area, adjacent to the main NW–SE structures belonging to the Apenninic Chain, crossed by fault segments with different orientation. The spatiotemporal evolution of the seismicity, the focal mechanisms of some aftershocks, never obtained before, and waveform analysis suggest that the sequence developed in several stages. The beginning of the two main stages was marked by two events (Ms=5.8 and Ms=5.2), and the entire sequence was strongly controlled by the structural heterogeneity in the medium involved in the stress release process. The ruptures nucleated on a ENE–WSW striking fault segment belonging to the NNE-striking Ortona-Roccamonfina tectonic line and propagated towards ENE. The presence of the NW–SE structures belonging to the Apennine Chain and their geometry acted as a barrier to the spread of the aftershocks northeastward. As a consequence, a local concentration of static stress in the area enclosed between the northern edge of the rupture segment of the first mainshock and the NW-striking structures triggered the Ms=5.2 event on a W–E pre-existing fault segment. In turn, the static stress changes due to the second mainshock activated adjacent NE–SW and NW– SE fault segments. The NW-striking structures belonging to the Apennines acted as a structural barrier, halting the propagation of the ruptures nucleating on a fault segment that belongs to the NNE-striking Ortona- Roccamonfina tectonic line.