COMMENTS

The model selected for the Fucino basin source is a surface-breaking, 
N145-striking, 60-dipping, 30 km long fault characterised by 
pure normal slip. The model agrees with direct observations of the 
coseismic fault scarps formed during the 13 January, 1915 earthquake, 
with the inversion of 1915 coseismic elevation changes, and with 
paleoseismological analyses of the main fault scarps observed in the Fucino 
Plain.

There are inconsistencies among different authors concerning the 
names used for the faults of the Fucino region. Following is a list of 
the different names used for each fault branch, along with the most 
recent references.
- San Benedetto dei Marsi-Gioia dei Marsi Fault (SBGF) (Galadini and
  Galli, 1999) = Serrone Fault (Piccardi et al., 1999) = Celano-Gioia 
  Fault (Michetti et al., 1996);
- Marsicana Highway Fault (MHF) (Galadini and Galli, 1999) = 
  Parasano Fault (Piccardi et al., 1999) = Parasano-Cerchio Fault 
  (Michetti et al., 1996);
- Pescina Railway Station Fault (PRSF) (Galadini and Galli, 1999) 
  = Muricci Fault (Piccardi et al., 1999) = Aielli-Giovenco Fault 
  (Michetti et al., 1996);
- Trasacco Fault and Vallelonga Fault (TF and VLF) (Galadini and 
  Galli, 1999) = Vallelonga Fault (Michetti et al., 1996);
- Velino Fault (VF) (Galadini and Galli, 1999) = Velino Fault and 
  Magnola Fault (Piccardi et al., 1999).

There is still no consensus on whether the active normal faults controlling the 
recent evolution of the Fucino Plain are characterised by pure normal movements 
or if they slip in a largely oblique fashion; different dataset used by 
different authors exhibit contrasting evidence. On average, however, these 
studies seems to suggest that the lateral component of motion is a small 
fraction of the vertical, if any.

SEISMOLOGY: The three seismologcal focal mechanisms of the 13 January 1915 
earthquake published so far used first motion polarities recorded at several 
stations wordlwide, but they are all different. Micci et al. (1975) propose a 
solution characterised by normal faulting along a NE-SW striking plane; 
Gasparini et al. (1985) propose left-lateral strike-slip on a NE-SW striking 
plane; Basili and Valensise (1991) propose left-lateral strike-slip faulting on 
a NW-SE striking plane. As it is pointed out by Basili and Valensise (1991) 
and by Galadini and Galli (1999), all the solutions are poorly constrained due 
to the low quality of the seismic dataset and are in conflict with all the 
available geological, geomorphological and paleoseismological evidence, which 
suggests that the Fucino basin is actively undergoing deformation related to NW-
striking normal faults located at the eastern border of the Plain.

GEODESY: Ward and Valensise (1989) fit the coseismic elevation changes induced 
by the 13 January 1915 earthquake with pure normal slip on a NW-trending fault 
plane, constraining their results with field observations of coseismic fault 
scarps and with kinematic indicators on the bedrock scarp of the Serrone Fault.
These investigators claim that a subordinate lateral component of slip could not 
be resolved due to the poor quality of the dataset, but also that a substantial 
component of strike slip, particularly if left-lateral, is rather unlikely. 
Conversely Amoruso et al. (1998) use a joint inversion of seismic and geodetic 
data to obtain a solution characterised by a strong sinistral strike-slip 
component (rake=3188) on a NW-striking, SW-dipping fault whose surface 
projection corresponds to the Serrone Fault.

PALEOSEISMOLOGY: Michetti et al. (1996) point out that, according to 
contemporary descriptions of the coseismic surface ruptures of the 1915 
earthquake and to additional modern field surveys, nowhere along the fault 
scarps it is possible to detect significant coseismic or Holocene lateral 
displacement. Results of the paleoseismological trenching of the main Holocene 
fault branches found in the Fucino plain, summarised in Galadini and Galli 
(1999), confirm that motion is mainly dip-slip with no lateral component.

STRUCTURAL GEOLOGY: according to Piccardi et al. (1999), the most recent 
generation of slickensides seen on the bedrock fault scarps of the Serrone, 
Parasano and Ventrino normal faults, have a dextral component of slip and are 
superimposed on an older set that instead shows sinistral motion. According to 
their interpretation the right-lateral component of motion is due to 
counterclockwise rotation of the fault-bounded blocks induced by a 
Regional left-lateral shear zone, the surface expression of which is 
represented by the NNW-trending Ovindoli and Sangro-Giovenco faults.
In contrast Galadini and Galli (1999) contend that the kinematic indicators 
found along the bedrock scarp of the Serrone Fault, that is the southeastern 
continuation of the San Benedetto-Gioia Fault, are mainly dip-slip.

GEOMORPHOLOGY: Piccardi (1995) reports that streams running across the scarp at 
the base of the Ventrino Fault are offset right-laterally; the slip vector he 
calculates using the morphological indicators is consistent with that obtained 
by means of structural analysis. No geomorphic offsets are reported along the 
San Benedetto-Gioia and Marsicana Highway faults. In fact, there is no 
consensus on the present activity of the Ventrino Fault, which based on
previous fault compilations Galadini and Galli (1999) consider to be an 
Early Pleistocene fault.
An explanation for the contrasting indications arising from structural, 
geomorphological and paleoseismological observations, could be that the 
kinematic indicators found on bedrock scarps are not related to the presently 
active tectonic regime, and that the bedrock fault scarps have been exhumed by 
erosional processes. The formation of a white ribbon of fresh limestone fault 
plane reported formed along the southwestern slope of Mt. Serrone after the 1915 
earthquake is suggestive of coseismic reactivation with prevalent dip-slip 
motion, although it is clearly difficult to fully discriminate between tectonic 
and gravitational movements.

Two kinematic models have been proposed to explain the recent evolution of the 
Fucino region. Both of them involve left-lateral displacement on NW- to NNW-
striking faults. The model proposed by Galadini (1999) is based on 
structural, geomorphological and stratigraphic data and predicts strike-slip or 
oblique-slip left-lateral movements on faults striking N130 to N160, and pure 
normal movements on faults striking N125 to E-W. According to this worker 
the strike-slip deformation accommodates extension on faults whose geometry is 
inherited and that are not perpendicular to the current extension direction. 
Galadini and Galli (1999) calculate a NE-SW extension rate across the entire 
Fucino ranging between 0.6 and 1.0 mm/yr; they assume pure normal slip on the 
NW-trending fault branches. This calculation is in agreement with that made for 
the Ovindoli-Pezza Fault by Pantosti et al. (1996), both for direction and for 
slip rate. 

In contrast, the model presented by Piccardi et al. (1999) envisions 
the recent tectonic evolution of the area as governed by a deep-seated sinistral 
shear zone. According to these workers the N2010E directed extension, 
operating at a rate ranging between 1 and 3 mm/yr in the Fucino area, suggests a 
left-lateral component of motion between Adria and Tyrrhenia along the central 
Apennines. In this view the NNW-striking structures, such as the Ovindoli and 
Sangro-Giovenco faults, characterised by a more or less strong left-lateral 
oblique component of motion, are the main shallow expression of the deep shear 
zone, while the NW-striking faults terminating on them with a horsetail geometry 
are secondary features accommodating the left-lateral shearing.

The strike of the Marsicana Highway and the San Benedetto-Gioia normal faults is 
very similar, within few degrees, to that of the faults belonging to the Upper 
Sangro Fault Zone and of the Ovindoli Fault, that are considered to have left-
lateral oblique and strike-slip kinematics. Detailed morphological and 
microtopographic analyses of the central part of the Ovindoli-Pezza Fault 
(D'Addezio et al., 1996) clearly show that, despite the abrupt change in strike 
of this fault with respect to the general Apennines trend (up to 30), the 
kinematics remain prevalently normal, with a maximum horizontal movement not 
exceeding 30% of the vertical displacement. Trenching along the same 
branch (D'Addezio et al., 1996) demonstrates that the two most recent 
paleoearthquakes were characterised by an age of occurrence and extent of 
deformation similar to those observed for the two most recent surface faulting 
events identified along the Piano di Pezza segment, and hence that the two 
faults slipped together. The study of the Ovindoli-Pezza Fault stresses the fact 
that we should not expect significant changes in the direction of motion along 
faults characterised by slight changes in strike; and also that all 
the faults found in the Fucino region presumably accommodate NE-SW oriented 
extension, similarly to several other large seismogenic faults along the 
Appennines and in agreement with the indications from seismological data.

Michetti et al. (1996) identify two pre-1915 surface faulting events through 
trenching along the San Benedetto-Gioia fault. They hypothesise the existence of 
the youngest of the two (Event A of Low Middle Ages age) by interpreting the 
attitude of some sedimentary units as due to the presence of a scarp that could 
be either an erosional feature (i.e. a paleoshoreline of the old Fucino lake) or 
a coseismic fault scarp. They rule out the first hypothesis because of the 
absence of shoreline deposits commonly found elsewhere in the Fucino Plain. 
In contrast, Galadini and Galli (1999) point out that there is no evidence of a 
Low Middle Ages event in any of the trenches excavated across the same fault 
branch, and hence interpret the deposition of the sedimentary sequence as being 
influenced by an erosional scarp rather than by a fault scarp.

If the Low Middle Ages earthquake did occur, we would have seen three 
earthquakes in the past 1,500 years, with a mean recurrence time of about 500 
years, much shorter than the average suggested by older events. A large 
earthquake (M=6.5-7.0) in the Fucino plain would also have produced in the city 
of Rome intensity effects similar to those generated by the 1915 event; in 
contrast current historical catalogues do not report any event during the 
interval between 800 A.D. and 1394 A.D., indicated by Michetti et al. (1996) as 
the most probable time span of occurrence of the postulated Low Middle Ages 
event. If that event really occurred, then the Fucino basin seismogenic source 
should be regarded as capable of generating large earthquakes with a recurrence 
interval much shorter than that expected for most other sources of the Italian 
peninsula.


OPEN QUESTIONS

1) What is the role played by the fults affecting the eastern Fucino margin 
(Marsicana Highway and San Benedetto-Gioia faults)? Galadini and Galli (1999) 
regard them as the main branches of a single NW-striking, SW-dipping seismogenic 
fault characterised by pure normal movement. According to them these faults slip 
always simultaneously with a quasi-periodic Holocene recurrence time and 
characteristic displacement per event; conversely the Trasacco and Luco dei 
Marsi fault branches would move only passively. Piccardi et al. (1999) assign to 
each of them a vertical slip rate, a recurrence time and a maximum expected 
magnitude, therefore considering each of them as a potential seismogenic source.

2) Are all the active faults identified in the Fucino Plain area individual 
seismogenic sources that can generate independent earthquakes? Or are they only 
the shallow expression of a single deep seismogenic source?

3) What are the relationships among the Fucino seismogenic source and its 
northern (the Ovindoli-Pezza Fault) and southern (the Sangro Valley Fault zone) 
counterparts? Are these three distinct seismogenic sources always generating 
earthquakes separately? Paleoseismological studies show that the age of 
occurence of paleoearthquakes along the Fucino and Ovindoli-Pezza faults cannot 
be correlated; we lack seismological and paleoseismological information for the 
area south of the Fucino, even if slope deposits younger than 27,000 years are 
reported displaced by NW-trending faults near Pescasseroli (Galadini et 
al., 1999); in this view the lack of historical seismicity points to the 
presence of a potential seismic gap in the Upper Sangro River Valley.

4) Has the Fucino basin seismogenic source a characteristic behaviour? The data 
obtained from trenching at several sites across different fault branches are not 
conclusive. Only at one site on the Marsicana Highway Fault it was possible to 
observe a costant displacement per event and calculate a quasi-periodic 
recurrence time.

5) When did the penultimate displacement event occur? Galadini and Galli (1996) 
hypothesise that it may have occurred either in 508 A.D. or 618 A.D., while 
Michetti et al. (1996) suggest that it may have occurred in 801 A.D..
