The Serchio Valley, commonly referred to as Garfagnana, is an active tectonic area that corresponds to a ca. 40 km-long, very well expressed physiographical feature. We interpret the recent evolution of this major tectonic feature as controlled by the activity of two large normal faults aligned along the axis of the Serchio Valley.
The northernmost fault is interpreted as the source of the destructive 7 September 1920 earthquake. Its length and width are constrained by the characteristics of this historical earthquake. The strike obtained from intensity data has been slightly rotated northward to match the orientation of the main valley floor. The proposed source is a moderately blind normal fault dipping towards the northeast and producing progressive lowering of the valley floor with respect to the Alpi Apuane (to the west) and to the crest of the Apennines (to the east). This configuration is somehow confirmed by unpublished seismic lines obtained by AGIP in the adjacent Lunigiana basin. The lines show the existence of a 40°-50° northeast-dipping master fault that would represent the northwestward prolongation of the Altotiberina Fault well imaged in northern Umbria.
The entire valley is bounded to the north by the Sarzana-Equi Terme Line and to the south by the Viareggio-Val di Lima-Bologna Line, which also appear to form the main boundaries of the block corresponding to the Alpi Apuane. Two additional important transverse lineaments, the Secchia Line and the Massa-Mt. Cervarola Line, subdivide the Serchio Valley into two nearly equal (17-20 km) portions separated by the small Mt. Perpoli ridge. All of these lineaments are marked by anomalous thermal springs, which shows that these are important lithospheric discontinuities rather than shallow-rooted features generated during the latest compressional tectonic phase. In its turn, this circumstance suggests that these lineaments represent major segment boundaries that are not likely to be violated during a large earthquake. The distribution of damage in 1920 and several other lines of evidence (location of major aftershocks, occurrence of historical earthquakes roughly coinciding and aligned with the Mt. Perpoli ridge) also evident surface breaks are reported for this fault. Possible evidence for coseismic faulting associated with the 1920 earthquake is reported for Minucciano, but these ruptures seem to accommodate passive reactivation of an old lineament rather than testifying to the Holocene activity of a youthful normal fault. Many faults in bedrock are exposed on both sides of the valley, but no conclusive evidence of Holocene faulting is seen throughout the area.
The fault configuration envisioned for this source comprises a typical case of "mimicking", where present-day faulting generates strains that emphasise the already strong topographic imprint left by an older tectonic regime. In other words, this Source is producing bowl-shaped deformation of a trough that existed prior to the onset of present-day extension and that was eventually filled by lacustrine sedimentation. These lake deposits are presently being eroded away as a result of breaching of the Serchio River at Chiusa di Calavorno (near the junction with the river Lima), probably due to an increased erosional power caused by significant Middle and Late Pleistocene regional uplift.
1) Is there any undetected direct surface evidence for the 1920 earthquake? What is the role of the Minucciano Fault?
2) Is the case of Garfagnana another example of a youthful normal fault that is working its way against a well-established post-compressional landscape?
3) Can the slight departure between the geological and intensity-based solutions for this source be explained in terms of northward directivity of rupture in 1920?
4) Does the location of the 11 April 1837 earthquake mark the northern end of this Source?
Bartolini and Bortolotti (1971)
The paper is concerned with the continental deposits of the northern Garfagnana. The thickness of the continental deposits reaches about 200m. The author distinguishes two main phases: i) an upper Pliocene (lower Villafranchian) lacustrine phase, dated on the basis of the faunae and florae of the Barga and Castel Nuovo deposits which indicate a closed environment with few detritic apports; ii) a second phase characterised by graveles and intercalated sands with detrital supplies from the surrounding reliefs. Above these deposits, some Quaternary alluvial terraces are also present.
The first and second phases deposits (lucustrine, gravels and sands) show two tilting events towards W and SW with values ranging between 10° and 15° for the lowermost lacustrine deposits and with values from 0° to 15° for the gravels and sands deposits. These values are also in accordance with the values of tilting of the first-order terrace surfaces of the Serchio river which dip towards WSW.
A paleogeographic reconstruction of the Barga valley is made. The oldest deposits of the basin overlaying the substratum are upper-Pliocene clay deposits. The clays are overlain by Pliocene sands and gravels which in some cases overlay directly the substratum. Some Quaternary fluvial terraces are also mapped. On the basis of the age of the lacustrine deposits (clays) and of their tilting towards SW, the formation of the Serchio graben is dated middle-upper Pliocene and the master fault is supposed to dip towards NE. Other authors consider the master fault as dipping towards the West and the Est-dipping fault as antithetic (Eva et al., 1978; Boccaletti and Coli, 1985).
Boncio et al. (2000)
They use surface and subsurface geological data, as well as an analysis of the instrumental seismicity to describe the geometry and the possible seismogenic characteristics of the Alto Tiberina Fault (ATF). The ATF is a low-angle, NE-dipping normal fault that can be mapped for a length of more than 70 km, and belongs to the regional NE-dipping Etrurian normal fault system (EFS). The EFS runs from northern Tuscany to southern Umbria, and its individual fault segment are associated with the main extensional basins of the northern Apennines. The authors suggest that the EFS controls active extension and seismic release all along this stretch of the Apennines.