COMMENTS

For the Gemona West Source, the causative fault of the 15 September 1976, 09:21 
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 (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 30 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, 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 (7 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 magnitude;

- the minimum and maximum depths (6.5 and 10.0 km respectively) follow the 
observations of Cipar (1980), the aftershocks distribution, and general subsurface 
geological observations;

- the length (8.5 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.


OPEN QUESTIONS

1) What are the relationships between the Gemona West Source and the Gemona 
North and East sources?

2) What are its relationships with the Dinaric  system?
