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GEPHART and FORSYTH's (1984) algorithm for stress inversion of earthquake fault-plane solutions has been applied to a set of ninety intermediate and deep events occurring in the southern Tyrrhenian region between 1976 and 1995. P-and S-wave data from local seismic networks in southern Italy, the Italian National Network and international bulletins, have been used for hypocenter and focal mechanism computations. Stress inversion runs performed after accurate selection and weighting of fault-plane solutions have allowed us to identify stress space variations at a higher level of detail than available from all previous investigations carried out in the study area. The maximum compressive stress has been shown to follow the depth-decreasing dip of the Wadati-Benioff zone, along the entire zone from a depth of 90 km, to the depth of the deepest events (about 500 km). Variations to such a stress pattern have been found, possibly related to mantle dynamics and the complex composition of the subducting structure. The diffused state of down-dip compression suggests that the Tyrrhenian subduction has already evolved to the point where the lower end of the slab has reached high-strength mantle materials, the load of the excess mass is entirely supported from below and most of the subducted slab is under compression. In agreement with the lack of large, shallow thrusting events in the immersion zone, the findings of the present study appear to agree well with geodynamic models assuming a passive subduction process with eastward roll-back of the Ionian lithosphere in the study area. In this context, the depth-decrease of the slab dip may also find a reasonable explanation.

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Summary The Mw 6.0 earthquake that hit central Italy on 24 August 2016 caused an abnormally high level of destruction in the town of Amatrice. In order to clarify the role of site response in causing such a disaster, a series of ambient noise recordings acquired in the aftermath of the event are analysed here to identify site resonance properties from the ratios H/V between horizontal and vertical amplitudes of ground motion. Although the noise data acquisition was limited by the emergency management activities, the use of a new analysis technique, which isolates the contribution of Rayleigh waves to the noise wavefield and averages instantaneous estimates of H/V ratios, provided more stable results compared to the standard Nakamura's technique based on mean spectral ratios. The results demonstrated the occurrence of significant resonance phenomena, but without an obvious correlation with the spatial distribution of damage severity. It is apparent that the damage severity was also influenced by some additional local factors related to building vulnerability. Moreover, the time series analysis revealed seasonal variations in the Rayleigh wave ellipticity curves likely related to the water content changes in the surface deposits and their influence on the Poisson coefficient. Finally, the new method proved capable of recognizing time-varying directions of Rayleigh wave propagation. This capability could be exploited to support other passive seismic methods (e.g. ReMi), whose results reliability is limited by the lack of control on wave direction origin.

The village of Collebrincioni is located right at the center of the most damaged area of the April 6th 2009 L'Aquila earthquake. It is known from historical data that this site had been heavily affected by the 1461 and 1703 seismic events and for that reason remained abandoned for a long time; the area has been settled again at the end of the 18th century with the rebuilding of the present conurbation in a new safer location. After the April 6th 2009 earthquake many evidences of coseismic effects have been found around the Collebrincioni surroundings, even though the macroseismic intensity calculated in the new settlement was found to be moderate (6 MCS). The aim of this work is to verify whether the cause of the migration from the old location can be linked to site effects; for this purpose, several measurements of horizontal to vertical spectral ratio (HVSR) have been carried out at various locations to obtain information on amplification phenomena. To allow a quick evaluation of site seismic response, without requesting any information about stratigraphical and geotechnical properties of the foundation soils, the Nakamura's technique was adopted in this paper. Measurements of HV spectral ratio carried out in the area show relevant differences between the new and the ancient settlements confirming the hypothesis that the abandonment of the old Collebrincioni settlement can be related to the site effects which have amplified the destructive effects of the earthquakes.