We propose the existence of this Source based on the recent tectonic activity of the inner Northern Apennines and on the occurrence of the 14 August 1846 earthquake.
The internal (and western) portion of the chain is traditionally considered an area bearing major extensional tectonics that have affected the pre-existing compressional structures of the inner Northern Apennines. Two extensional stages have been recognized in the Tertiary evolution of the Tuscany region. The first tectonic phase has occurred between the Late Oligocene and Late Tortonian and caused a stretching of 120% or greater. The second phase has occurred between the Late Tortonian and Quaternary time. The main result of the second extensional phase is represented by high-angle normal faults which dissect all previous structures. These faults border the NW-SE trending horst and graben that developed during the late Miocene-Pleistocene (e.g. Carmignani et al., 1994, 2001).
According to this view, most of the regional deformation would be exerted by a few extensional fault systems responsible for the large scale landscape features of the inner Northern Apennines. Instrumental catalogues show a seismic sequence (with max Mw of 4.2) that hit the offshore area located about 15 km west of the Orciano Pisano source in April 1984 (Castello et al., 2006). Out of the sequence, the 22 April 1984 earthquake, showing a normal fault plane solution (Pondrelli et al., 2006) with two conjugate NNW-SSE planes, can help defining the seismotectonics of the area.
In Tuscany, reflection seismic data have revealed a regional reflector referred to as the K-horizon (Batini et al., 1978), a layer of fractured rocks hosting high-pressure fluids that usually coincides with the top of the crustal brittle-ductile transition, which ranges in depth between 4 and 9 km (Cameli et al., 1998).
We propose the following geometrical parameters for the source:
This Source is associated with the 14 August 1846 earthquake (Mw 5.7) reported by CPTI (2004) and having its epicentral area between Orciano Pisano, Lorenzana, and Fauglia. Our source model is well consistent with the highest damage of the earthquake, and with the possible surface expression identified at the base of the eastern slope of the Livorno Hills (Piccardi et al., 2017).
1) What is the average return period of this Source?
2) Is this Source a normal fault?