COMMENTS

The Serchio Valley, commonly referred to as Garfagnana, is an active tectonic area 
that corresponds with a ~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 southernmost fault, which is included in the Database as Garfagnana south, 
lacks of a large historical earthquake associated. Its length and width are 
constrained by similarity with the adjacent Garfagnana North Source and based on
structural constraints. 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This configuration is somehow confirmed by unpublished seismic lines obtained by 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 support this hypothesis. 

Unlike its northern counterpart, the Garfagnana South Source does possess relatively 
convincing surface evidence. Between the Mt. Perpoli ridge and Chiusa di Calavorno, 
along most of the presumed length of the fault segment (12 km) the Serchio River 
and the most recent basin-fill appear to record progressive tilting towards the 
southwest. Mild but consistent southwestward tilting affects also Late Pliocene (Early 
Pleistocene?) lacustrine and lignite beds forming the basin fill. The southwestern 
flank of the valley is surprisingly straight and exhibits the typical features of an active 
front (triangular facets, very incised valleys, low sinuosity). Similarly to the rest of 
Garfagnana, many faults in bedrock are exposed on the northeastern side of the 
valley, but the extent of their activity (possibly as antithetic faults) is difficult to 
assess and their freshness is likely to result simply from erosional rejuvenation. 
In summary, no conclusive direct but only indirect (geomorphic and long-term) 
evidence of Holocene faulting is available for this fault. 

The length of this source appears to be very tightly constrained by the Mt. Perpoli 
ridge to the north, which is the locus of several small historical earthquakes, and to 
the south by the intersection with the Val di Lima, a major regional lineament that 
separates very different geologic and topographic domains and is marked by 
frequent instrumental seismicity. Nearly no seismicity, both historical and 
instrumental, is reported from the inner portions of the segment, but some 
background earthquakes occur along the surface projection of the lower edge of the 
fault plane.

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, the 
Garfagnana North 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. 


OPEN QUESTIONS

1) Is tilting of the Lower Serchio Valley sufficient evidence for the Late Pleistocene 
and Holocene activity of this fault? Could some direct evidence (a fault scarp) be 
found along the rather inaccessible mountain front to the southwest of the valley?

2) What is the recurrence interval of this fault? Has it really been quiescent 
throughout the entire known history? 

3) Is the case of Garfagnana another example of a youthful normal fault that is 
working its way against a well-established post-compressional landscape?

4) What is the meaning of the concentration of background seismicity all around this 
source?
