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 northernmost fault, which is included in the Database as Garfagnana 
north, 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 (17) 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somehow confirmed by unpublished seismic lines obtained by AGIP in the 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 vident 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, 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 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 (17 in strike, 5 km in absolute location) 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?
