COMMENTS

The summaries of the main studies may also include studies regarding the description 
of features of a larger area than the single source. This is indeed the case of the Gubbio 
South Source, an active segment of a major tectonic structure herein referred as the 
main Gubbio Fault.

The surface expression of the main Gubbio Fault is about 14 km-long (Barnaba, 1958), 
while geophysical data show that at depth this structure has a total length of about 30 
km (Barchi et al., 1999). It is antithetic to the Altotiberina Fault (Barchi et al., 1999) 
and has a listric geometry (Menichetti et al., 1991), probably due to the reactivation of
an ancient thrust plane at depth (Boncio et al., 2000a). This main structure presents a 
complex geometry characterised by bending, change of the dip and progressive 
southeast deepening of the depth of the intersection with the Altotiberina Fault (Barchi 
at al., 2000).

We suspect that some of the lateral discontinuities could play the role of geometric 
barriers in the propagation of earthquake rupture. Geomorphic and geologic evidence 
for transverse (NE-SW trending) features may also support the segmentation 
hypothesis of the main Gubbio Fault. These lines of evidence include:
- regional watershed flexures (Cencetti, 1988);
- direction of river captures (Cattuto, 1973);
- local watershed orientation in the Gubbio basin;
- discontinuities of geological compressive structures (Menichetti et al., 1986) inherited 
from the extensional regime and discontinuities of the surface of the Altotiberina Fault 
(Barchi et al., 1999).

This source corresponds to the 29 April 1984, M 5.2 mainshock of the Gubbio 
earthquake sequence. The recorded aftershock activity, concentrated mainly between 7 
and 3 km, is confined to the deepest part of the branch line of the Gubbio Fault and 
appears to lie on a SW dipping plane. The southernmost group of aftershocks (Haessler 
et al., 1988) delineates a 9 km-long trend and at depth it coincides with the deepest part 
of the main Gubbio Fault. The CMT focal mechanism of the 1984 mainshock 
(Dziewonski et al., 1985) suggests a normal solution for a SW low-angle dipping plane. 
This source seems to be related to the deepest part of the structure, which is why the 
position of the surface expression of the main Gubbio Fault and the projection at the 
surface of this source do not overlap. The source is bound to the South by the Fossato 
di Vico-Valle dell'Esino transverse tectonic lineament, to the North by a lateral change 
in the geometry of the main Gubbio Fault and by a step between adjacent aftershock 
clusters.


OPEN QUESTIONS

1) Could the Gubbio fault be entirely or partially activated, producing earthquakes of 
M~6.0, or is the 1984 Gubbio earthquake representative of the largest expected event?

2) Are the tectonic structures affecting Quaternary deposits of the Gubbio basin 
(Selvaggi et al., 1989; Boncio et al, 1996) the evidence that the coseismic rupture could 
occasionally reach the surface?

3) Could the hypothesised rupture directivity and the distribution of the aftershocks in 
two clusters suggest a transfer of the stress from the Gubbio South Source to the Gubbio 
Middle Source, suggesting that the latter structure could generate the next earthquake 
in the area?

4) It is known that the depocentre of the Gubbio basin has moved from a near fault 
position (GE.MI.NA., 1962; Menichetti et al, 1991) on the west side of the basin. Could 
such migration indicate a variation in the style of the fault activity due to a rupture 
confined to the deepest part of the seismogenic structure?
