COMMENTS

Based on geomorphological observations provided by several investigators and on the 
analysis of the subsurface data provided by the oil industry (Cassano et al., 1986; Pieri 
and Groppi, 1981), we propose the existence of a major blind thrust (Orzinuovi Source) 
having the following geometrical characteristics:

- the strike is chosen according with the general orientation of  mapped tectonic 
structures (N266);
- the fault dips 30 towards the North, in agreement with  subsurface evidence and 
based on the characteristic distance  between the synclinal and anticlinal axes;
- the rake is assumed to be 90 (pure thrusting) based on strike  and on general 
geodynamic considerations;
- the down-dip width (6 km) is based on the characteristic distance  between the 
synclinal and anticlinal axes and on the assumptions  made concerning the minimum 
and maximum faulting depth;
- the minimum and maximum depth (2 and 5 km respectively) are  constrained by 
subsurface geology, by the symmetry of the  anticline and by the general aspect-ratio of 
the anticline-syncline pair;
- the length (10 km) is based on geomorphological observations  and constrained by 
scaling with width.

Current catalogues (CPTI, 1999) report a large event in 1802 (Me 5.7) that falls in the 
upper Oglio River valley. This is the only event that can be associated with the source. 
Calculations made with the method proposed by Gasperini et al. (1999) obtained a 
source slightly smaller than that proposed on the basis of geological and 
geomorphological observations alone, and located few km to the north. However the 
strike of the two sources is the same, and is in agrrement with that of the local tectonic 
structures.

This source belongs to the Lombardian thrust system, the southernmost 
compressional front of the Alps.

Boschi et al. [1997] report that the water flow of the Oglio River in the Soncino area 
stopped for a while after the earthquake. This account is consistent with sudden 
coseismic uplift of the anticline and subsidence of the syncline driven by the blind 
thrust fault.

The Orzinuovi Source is developed in the subsurface, but has no clear geomorphic 
expression. The presence of the Casalmorano paleo-channel south of the diversion of 
the Oglio River can be the geomorphic evidence of the long-term deformation 
produced by the Orzinuovi Source.


OPEN QUESTIONS

1) What is the average return time of the Orzinuovi Source? Current catalogues (CPTI, 
1999) which cover a time span of about 2,000 years, report only a large event in 1802 
that could be associated with the Orzinuovi Source.

2) What is the seismic behaviour of the Orzinuovi Source? Does it rupture only with 
events in the magnitude range 5.5-6.0, or it can generate also larger rather infrequent 
events?

3) Does the Orzinuovi Source belong to a system of south-verging blind thrusts?
