COMMENTS

For the Gemona East Source, the causative fault of the 6 May 1976 earthquake, we 
propose a blind thrust geometry with the following parameters:

- the EW strike is chosen according with the general orientation of mapped tectonic 
structures and with the mean orientation of the nodal planes of the focal 
mechanisms of the  6 May 1976 earthquake (see figure "Summary of mechanisms for 
1976 sequence" from Pondrelli et al., 2001); the position of the fault is constrained by 
the epicentral location of the main shock (following Slejko et al. 1989, and Aoudia et 
al., 2000), and by modelling of repeated levelling measurements (Talamo et al., 1978);

- the fault dips 20 towards the north in agreement with the geometry of the blind 
thrusts imaged in the seismic profiles, with the preferred nodal plane of the focal 
mechanisms of the  6 May 1976 earthquake, and with the spatial distribution of the 
aftershocks;

- the rake (90, pure thrusting) follows seismological observations (focal 
mechanisms) (see Figure "Summary of mechanisms for 1976 sequence" from 
Pondrelli et al., 2001);

- the down-dip width (12 km) is based on the thickness of the seismogenic layer 
derived from the distribution of the 1976 Earthquake aftershocks (Finetti et al., 1976), 
and on empirical relationships of rupture area with respect to the magnitude;

- the minimum and maximum depths (1.5 and 6.1 km respectively) follow the 
observations of Cipar (1980), the aftershocks distribution, and general subsurface 
geological observations;

- the length (16 km) is based on scaling with width and on geological and 
seismological observations.

We propose that the causative faults of the  6 May and 15 September 1976, 09:21 
shocks are EW-striking, S-verging blind-thrusts, while the 15 September 1976, 03:15 
source is a transfer fault acting as western segment boundary for the Gemona East 
Source. The location of the hypocentre of the aftershocks shows that the second shock 
of September was generated by a structure deeper than the previous two.

The elevation changes derived from repeated levelling along the IGM geodetic Line 
36 (Talamo et al., 1978; Pondrelli et al., 2001;) show two possible distinct components 
of uplift: a regional part due to the uplift of the Alps, and a coseismic part due to the 
whole 1976-1977 Friuli sequence. The area affected by coseismic uplift along the 
profile falls between the towns of Tricesimo and Venzone, for a total length of about 
20 km. The extent of the uplifted region seems to be larger than that expected to be 
associated with the 6 May 1976 main-shock alone. We therefore hypothesise that the 
geodetic signal is the sum of coseismic deformation induced by the main shock and 
by the strongest aftershocks.

The epicentral locations proposed by Lyon-Caen (1980), Barbano et al. (1985), Slejko et 
al. (1989) and Aoudia et al. (2000) seem to be in a better agreement with the area of 
greatest coseismic deformation, as it is shown in the IGM geodetic Line 36, and with 
the area that experienced the greatest damage.

The 6 May 1976 earthquake seems to have nucleated on the lower-eastern corner of 
the fault plane and propagated westward with a pure reverse movement. The 
position of the maximum damage area slightly shifted westward with respect to the 
epicentral location appears to be in agreement with this hypothesis.

If the "fault scarps" described by Bosi et al (1976) are the real surface expression of the 
fault plane, they should be located at the intersection between the prolongation of the 
fault plane and the surface; instead these features are located directly above
the plane itself. Besides the ruptures follow the roughly E-W trend of the axis of the 
major folds and fall near the hinge of the coseismic uplift shown in the geodetic line. 
On the basis of these considerations the ruptures could be interpreted as due to 
landslides, or to be the expression of extrados deformation (i.e.: associated with 
bending at the top of a rising anticline).

The Gemona East Source falls in the area where the Alpine structures meet the 
Dinaric structures. Some investigators propose an horse-tail splay geometry to link 
this source with the Dinaric fault system, while according to other workers it would 
belong to the Alpine system because of its strike.


OPEN QUESTIONS

1) What are the relationships between the Gemona East Source and the other 
seismogenic sources located eastward and belonging to the Dinaric system 
(particularly the Cividale Source and the fault responsible for the 1998 Bovec 
earthquake)? Are these faults characterised mainly by strike-
slip movements? Or mainly by reverse movements?

2) Are the Gemona East Source and the sources of the other 1976 mainshocks 
linked with an horse-tail splay geometry with the Dinaric system?

3) Were the ruptures described by Bosi et al. (1976) evidence of genuine coseismic 
surface faulting? Or are they the evidence of landslides and or fractures triggered by 
the earthquake?
