New geological and morpho-structural surveys are integrated with seismological and seismic reflection data in order to define the active and potentially seismogenic sources of the Lunigiana and Garfagnana extensional basins (Tuscany, Northern Italy). The seismogenic role of the E-W-striking transfer fault between the Lunigiana and Garfagnana basins, located at the northern termination of the Apuane metamorphic core, and its possible association with a number of historical earthquakes are also discussed

Bernini, M.

Boncio, P.

Brozzetti, F.

Di Naccio, D.

Eva, E.

Lavecchia, G.

Solarino, S.

Tinari, P.

Torelli, L.

Earth-prints Repository

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A multidisciplinary approach to the seismotectonics of the Lunigiana and 
Garfagnana extensional basins (Northern Tuscany, Italy) 
 
F. BROZZETTI 
(1)
, P. BONCIO 
(1)
, D. DI NACCIO 
(1)
, G. LAVECCHIA 
(1)
, D.P. TINARI 
(1)
, L. TORELLI
(2), 
M. BERNINI
(2)
, E. EVA 
(3)
 AND S. SOLARINO
(3)
. 
 
(1)
 GeoSisLab – Dipartimento di Scienze della Terra, Università di Chieti-Pescara. 
(2)
 Dipartimento di Scienze della Terra, Università degli Studi di Parma. 
(3) Istituto Nazionale di Geofisica e Vulcanologia, CNT, c/o DipTeRis - Università degli Studi 
di Genova. 
 
New geological and morpho-structural surveys are integrated with seismological and 
seismic reflection data in order to define the active and potentially seismogenic sources of 
the Lunigiana and Garfagnana extensional basins (Tuscany, Northern Italy). The 
seismogenic role of the E-W-striking transfer fault between the Lunigiana and Garfagnana 
basins, located at the northern termination of the Apuane metamorphic core, and its 
possible association with a number of historical earthquakes are also discussed. 
The Lunigiana and Garfagnana basins are two NW-SE-striking asymmetric grabens, 
originated in the hanging wall of regional low-angle detachment faults, sloping gradually 
beneath the Apennine chain. These faults are referable to the northern termination of the 
Etrurian Fault System (EFS, BONCIO et alii, 2000), a Plio-Pleistocene en échelon fault- 
array made of east-dipping low-angle normal faults. South of the Garfagnana, the EFS 
includes the Mugello, the Casentino and, more to the south, the Tiber grabens. 
The Lunigiana and Garfagnana grabens are bounded inward by an east-dipping normal 
fault set and outward by a west-dipping fault set. Each set consists of several normal fault 
alignments rooting on the detachment at increasing depths, from 1 to 2.5 sec TWT, from 
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W to E. In the Lunigiana, the depth geometry of the detachment and of the main high-
angle normal faults has been precisely defined through a detailed interpretation of three 
seismic lines (see also CAMURRI et alii, 2001 and ARGNANI et alii, 2003). The detachment 
fault crops out along the eastern border of the Mt. Picchiara-Mt. Cornoviglio horst and 
deepens under the chain down to the depths of 4.5-5 sec TWT (~12-15 km). 
The field survey of the faults showing evidence of Quaternary activity has been carried out 
with particular attention, in order to detect their dip-angle and segmentation pattern. The 
outcrop expression of the two conjugate sets is similar, but the east-dipping faults show, 
on average, lower dip-angles (i) (30°<i<60°, increasing from W to E) and higher cumulative 
displacement (D) values (D>4 km) than the west-dipping ones (50°<i<70°, D~2.5 km). The 
kinematic analysis on major faults shows that the more recent movements are normal or 
slightly oblique and are coherent with a tensional stress field characterised by a sub-
horizontal NE-SW-trending sigma 3 axis. 
In the Lunigiana, the east-dipping Mulazzo and Olivola faults (the outermost of the east-
dipping set) and the Groppodalosio and Compione faults (the outermost of the west-
dipping set) show the strongest geomorphic signature and are the most likely active faults. 
This is also confirmed by the geomorphic analysis carried out integrating aerial photograph 
interpretation and medium resolution DTM analysis. 
An E-W tectonic lineament, detectable in aerial photographs and well evident in seismic 
line, is present between the Lunigiana and Garfagnana grabens, from Tendola to Equi 
Terme and Gramolazzo. This alignment, here named “North Apuane Fault Zone”, may be 
interpreted as an active right-lateral transfer fault zone between the two grabens. 
In the Garfagnana, the NW-SE striking Casciana-Sillicano e Bolognana-Gioviano east-
dipping faults show morpho-structural evidence of Late Quaternary faulting and can be 
interpreted as active and potentially seismogenic structures rooting at depth of 3-4 km on 
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the basal detachment. Along the eastern side, the major antithetical Quaternary faults crop 
out along the M. Prato-Colle Uccelliera-M. Mosca ridge. 
Our seismotectonic analysis integrates the geological and morpho-structural data with the 
available instrumental and historical earthquake catalogues. The instrumental dataset 
consists of the earthquakes recorded by the Lunigiana-Garfagnana local Seismic Network 
from 1999 to 2006. About 500 earthquakes (0.4≤Ml≤4.2) have been accurately located by 
using a probabilistic non-linear program which computes the maximum hypocentral 
probability through a 3D research algorithm (LOMAX et alii, 2000). The obtained errors for 
the hypocentral locations and the related residuals are small if compared with those 
coming from “standard” procedures. In the Lunigiana, the seismicity concentrates east of 
the M. Picchiara - M. Cornoviglio horst (Pontremoli and Aulla grabens). The Garfagnana 
area shows very low background seismicity, with few epicentres concentrated east of the 
Apuane Alps. On SW-NE oriented vertical sections, the hypocentres mainly locate within 
the upper 15-20 km and affect progressively thicker crustal volumes moving from W to E. 
In correspondence of the Pontremoli and Aulla grabens, earthquakes occur at depths 
shallower than 5 km, whereas under the Apennine ridge they reach 20 km depths. The 
depth-distribution is also accompanied by a concentration of the hypocentres within a 
narrow volume dipping at ~30° eastward. This east-dipping geometry of the seismicity is 
particularly evident across the Aulla basin. An integrated and revised database of 
earthquake focal mechanisms from 1939 to 2006 indicates the existence of a 30 km-wide 
belt, extending from the western border of the Lunigiana-Garfagnana grabens to the 
Apennine chain, characterized by prevailing extensional mechanisms (average N40°-E 
stretching direction), in agreement with geological data. On the whole, instrumental 
seismicity is consistent with the present activity of the basal detachment and of the 
associated high-angle structures, at least as the Lunigiana area concerns. The integrated 
analysis of geological and seismological data, including historical earthquakes (e.g. 
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February 14, 1834, I=VIII-IX M~5.6; May 7, 1481, I=VIII-IX M~5.8 earthquakes), suggests 
that the more active and potentially seismogenic structures are the Mulazzo and Olivola-
Soliera faults (NE-dipping) and the Groppodalosio and Compione-Comano faults (SW-
dipping). Other SW-dipping faults showing evidence of Late Quaternary activity, are not 
considered seismogenic due to their shallow decollement depths (~2 km).  
Another subject of considerable importance is the seismogenic role played by the North 
Apuane Fault Zone (NAFZ). Our interpretation associates a primary seismogenic role to 
the NAFZ, as suggested by the October 1995 earthquake (M = 4.9) and its focal 
mechanism. We associate to the NAFZ also the April 11, 1837 earthquake (I=IX-X, M~5.7) 
plus a number of small-magnitude earthquakes (4.5≤M≤5.2) occurred in 1767, 1902, 1928 
and 1962. The September 7, 1920 earthquake, which is the largest event of the Lunigiana-
Garfagnana area (I=IX-X, M~6.5), could be associated to a complex rupture involving the 
entire Casciana-Sillicana fault plus the eastern portion of the NAFZ. 
 
REFERENCES 
ARGNANI A., G. BARBACINI, BERNINI M, CAMURRI F., GHIELMI M., PAPANI G., RIZZINI F., 
ROGLEDI S. E TORELLI L. (2003). Gravity tectonics driven by quaternary uplift in the Northern 
apennines: insight from the La Spezia-Reggio Emilia geo-transect. Quaternay International 
101-102, 13-26. 
BONCIO P., BROZZETTI F. E LAVECCHIA G. (2000) - Architecture and seismotectonics of a 
regional Low-Angle Normal Fault zone in Central Italy. Tectonics, 19, 1038-1055. 
CAMURRI F., ARGNANI A., BERNINI M., PAPANI G., ROGLEDI S. E TORELLI L. (2001) – The 
basement of the NW Apennines: Interpretation of reflection seismics and geodynamic 
implications. GEOITALIA 2001, 3° Forum Italiano di Scienze della Terra, Chieti 5-8 
Settembre 2001, Fist Riassunti, 50-51. 
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LOMAX A., VIRIEUX J. E THIERRY-BERGE C.(2000). Probabilistic earthquake location in 3D 
and layered models, in Advances in Seismic Event Location, pp. 101–134, eds Thurber, 
C.H. & Rabinowitz, N., Kluwer Academic, Amsterdam. 
 


A multidisciplinary approach to the seismotectonics of the Lunigiana and Garfagnana extensional basins (Northern Tuscany, Italy)

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A multidisciplinary approach to the seismotectonics of the Lunigiana and Garfagnana extensional basins (Northern Tuscany, Italy)

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