COMMENTS

The Mt. Gorzano range shows an outstanding relief at the base of which there is a 30 
km-long normal fault system that accounts for a maximum stratigraphic separation 
of ca. 1500 m, determined from displaced Messinian (5.2-6.7 Ma) bedded terrigenous 
units (see also Calamita et al., 1995). Using these figures a long-term slip-rate of 0.22-
0.29 mm/y is obtained. However, assuming that the displacement may have taken 
place only after the onset of the extensional regime in this region (Middle-Late 
Pliocene-Pleistocene? See for example Ghisetti and Vezzani, 2000), an even faster slip 
rate could be found. A slip-rate of less then 0.1 mm/y could be inferred on the basis of 
the displacement of a Villafranchian surface described by Bachetti et al. (1990) and 
Blumetti et al. (1993), assuming the Villafranchian age to be about 2 Ma. However, 
correlating erosive geomorphic features is usually quite problematic. Bachetti et al. 
(1990) point out that the fault may be still active, having caused tilting of a 39.7 ka old 
palustrine deposits, from which a slip rate of 0.22-0.40 mm/y could be inferred. 
Bachetti et al. (1990) also conjectured that such tilting must have occurred during a 
surface faulting event, suggesting that the fault may have a seismogenic nature, but 
did not provide evidence of the associated fault scarp. Finally, Galadini and Galli 
(2000) report a vertical slip rate of 0.30-0.36 mm/y for the southern part of their 
Laga Fault, which roughly coincides with the Campotosto Source.

The Mt. Gorzano fault system has been split into two fault segments, each one 
representing an individual seismogenic source. The Campotosto Source, which is 
discussed here, is the southernmost segment in the system. Di Filippo and Marcelli 
(1951) first hypothesised that the 5 September 1950 "Gran Sasso" earthquake could be 
associated with the Mt. Gorzano fault by placing the instrumental epicentre in the 
Campotosto basin. However, on the basis of the macroseismic field this epicentre 
seems to be located some 25 km to the east of the fault, suggesting that the earthquake 
and the hypothetical geologic source are not correlated. The only available focal 
mechanism for this earthquake (M=5.5, depth=10km: Gasparini et al., 1985) is not very 
well constrained, however, it shows normal faulting along NNE-trending conjugated 
fault-planes and a tension axis oriented N122,which differs by about 50 from the one 
expected for the Campotosto Source.


OPEN QUESTIONS

1) Is the 5 September 1950 earthquake associated with the Campotosto Source?

2) If so, does this historical earthquake testify to the typical behaviour of this source?

3) If not, does the Mt. Gorzano fault-system contain potential sources for moderate to 
large earthquakes?
