Characterization of the seismogenic source of the great 1905 Calabria (southern Italy) earthquake from environmental effects

Abstract

The september 8, 1905 Calabria (Southern Italy) earthquake belongs to a peculiar family of highly destructive (I0=XI) seismic events, mostly occurred at the dawning of the instrumental seismology, for which location, geometry and size of the source are still substantially unconstrained. During the century elapsed since the earthquake, previous Authors identified three different epicenters that are more than 50 km apart and calculated magnitudes between M≤6.2 and M=7.9; even higher uncertainties were found when the geometry of the earthquake source was estimated. In this study, we strictly constrain the magnitude, location and kinematics of the 1905 earthquake through the analysis of the remarkable environmental effects produced by the event (162 deeply reviewed observations at 95 different localities throughout the Calabria region). The data used in our analysis include ground effects (landslides, rock falls and lateral spreads), hydrological changes (streamflow variations, liquefaction, rise of water temperature and turbidity) and light phenomena. To better define the magnitude of the event we use a number of empirical relations between seismic parameters and distribution of ground effects and hydrological changes. In order to provide constraints to the location of the event and to the geometry of the source, we reproduce the coseismic static strain associated to different possible 1905 causative faults and compare its pattern to the documented streamflow changes. From the analysis of the seismically-induced environmental changes we find that: 1) the 1905 earthquake had a magnitude M=6.7÷6.8; 2) the event occurred in an offshore area west of the epicenters proposed by the historical seismic Catalogs; 3) it most likely occurred along a 100°N oriented normal fault with a left-lateral component. This solution is consistent with the seismotectonic setting of the area. Our approach shows that the seismically-induced environmental effects are a significant footmark of an earthquake, and that their analysis is an important tool to better characterize seismic events when reliable seismological and geological data are lacking.