COMMENTS

For the Gemona North Source, the causative fault of the 15 September 1976, 03:15 
earthquake, we propose a blind thrust geometry with the following parameters: 

- the strike (N230) is chosen according with the general orientation of mapped 
tectonic structures and with the mean orientation of the nodal planes of the focal 
mechanism of the earthquake calculated by Cipar [1980] (see Figure "Summary of 
mechanisms for 1976 sequence" from Pondrelli et al., 2001); the position of the fault is 
constrained by the epicentral location and by modelling of repeated levelling 
measurements (Talamo et al., 1978);

- the fault dips 60 towards the northwest in agreement with the preferred nodal 
plane of the focal mechanisms;

- the rake (46, reverse slip with a strong left-lateral component) follows 
seismological observations (focal mechanisms) (see Figure "Summary of 
mechanisms for 1976 sequence" from Pondrelli et al., 2001);

- the down-dip width (6 km) is based on empirical relationships of rupture area with 
respect to the magnitude;

- the minimum and maximum depths (1.0 and 6.2 km respectively) follow the 
observations of Cipar (1980), the aftershocks distribution, and general subsurface 
geological observations;

- the length (8 km) is based on scaling with width and on geological and seismological 
observations.

The geometry we propose for the Gemona North Source is mainly based on the focal 
mechanism computed by Cipar (1980), and on general geologic considerations.

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.


OPEN QUESTIONS

1) What are the relationships between the Gemona North Source and the Gemona 
East and West sources? What are its relationships with the Dinaric  system?

2) Based on focal mechanisms, some investigators propose for the 15 September 1976, 
03:15 earthquake a source geometry similar to that of the other two sources (i.e.: 
nearly EW-striking, S-verging thrust). What is the real geometry of this source?
