Insights into the geometry and faulting style of the causative faults of the M6.7 1805 and M6.7 1930 earthquakes in the Southern Apennines (Italy) from coseismic hydrological changes

Abstract

I study the coseismic hydrological changes associated with two M6.7 earthquakes that occurred in 1805 and in 1930 in the Southern Apennines of Italy to provide additional constraints on the geometry and mechanism of their causative faults. The two earthquakes originated remarkable datasets of 84 and 52 observations of hydrological changes. I discounted those observations potentially affected by the geomorphological, climatic and methodological contamination of data. Then, I compared the distribution of the streamflow changes with the coseismic strain field induced by a number of seismogenic sources proposed for the two earthquakes. A first important outcome of this study is that it provides evidence against some of the seismogenic structures previously associated with the 1805 and 1930 earthquakes. In addition, for the 1805 earthquake, I find that ~300°- striking, NE-dipping, ∼30–35 km-long seismogenic structures consisting of four to five surface-rupturing normal fault segments best explain (with almost 75% consistency) the distribution of the observed hydrological data. However, I also find that almost 100% of the observations are consistent with the deformation imposed by the three southernmost segments and suggest that most of the seismic release during the 1805 event was provided by those segments only. For the 1930 earthquake, I find that the pattern of the coseismic hydrological changes is best fitted by the only fault characterized by ~E-W trending, oblique slipping, NE-dipping, and a shallow top of fault. The depth of the top of the fault plays an important role in the generation of more pronounced fields of deformation, even for those faults that do not rupture up to the free surface, as in the 1930 case. This study confirms that the hydrological signatures of earthquake strain provide supplemental constraints in the estimating geometry and faulting style of major historical earthquakes that produced large and documented amounts of hydrological data.