COMMENTS

Based on the analysis of the subsurface data provided by the oil industry (Cassano et al., 
1986; Pieri and  Groppi, 1981) and on geomorphological observations provided by several 
investigators, we propose the  existence of a major blind thrust (Bagnacavallo Source) 
having the following geometrical characteristics:
- the strike is chosen according with the general orientation of mapped tectonic 
structures;
- the fault dips 30 towards the SW, 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 (3 and 6 km respectively) are constrained by 
subsurface geology, by  the symmetry of the anticline and by the general aspect-ratio of 
the anticline-syncline couple;
- the length (10 km) is based on scaling with width and constrained by geomorphological 
observations.

Current catalogues (CPTI, 1999) report a large event in 1688 (Me 5.7) that falls very 
close to the Bagnacavallo  Source, having its epicentral area in Cotignola (Imax=IX). 
We associate this historical earthquake with the  Bagnacavallo Source. Our 
preferred source model is consistent with the largest damage having been  reported 
in Cotignola, Bagnacavallo, Solarolo, Russi and Lugo. A further constraint for the 
southeastern edge of the source could be represented by the occurrence of a large 
aftershock near Russi on 18 April 1688, a  week after the mainshock (reported by 
Boschi et al., 1997)

This source belongs to the Romagna Folds the more internal folds of the Ferrara-
Romagna Arc, that  represents the outermost propagation of the Apennines 
compressional wedge.

The reconstruction of the drainage system made by Gambi (1949) shows progressive 
attraction of the Senio  and Lamone rivers towards each other in an area located around 
the town of Cotignola; downstream this  area the two rivers are progressively shift 
sideways. The analysis of the rivers channel behaviour constrain  the position of the 
syncline and of the anticline.

Ferrari et al (1985) and Boschi et al. (1997) report the occurrence of floods of the Senio 
and Lamone rivers in  the Cotignola area about ten days after the earthquake. These 
accounts are consistent with sudden coseismic  uplift of the anticline and subsidence of 
the syncline driven by the blind thrust fault.

The position of the active anticline, located by means of geomorphological observations, 
is slightly shifted  to the north-east with respect to the geological anticline mapped on 
the basis of geophysical exploration  data. This observation may be the evidence of 
growth of the thrust fault along dip (the top of the fault plane  becomes shallower ).

The Bagnacavallo Source is very well developed in the subsurface, but has a limited 
geomorphic  expression. The frequent and substantial diversions and shifts of the rivers 
flowing around it suggest that  this is a rather fast structure.


OPEN QUESTIONS

1) What is the average return time of the Bagnacavallo Source? Current catalogues 
(CPTI, 1999), which cover a time span of about 1,000 years, report only the large 1688 
event as falling very close to it.

2) What is the seismic behaviour of the Bagnacavallo Source? Does it rupture only in 
large M 6 rather  infrequent events? Or it can also rupture in smaller earthquakes in the 
M range 5-5.5, alternated with long  aseismic periods?
