COMMENTS

Based on the analysis of repeated levelling data and on geomorphological observations 
provided by several investigators (e.g. Castaldini and Panizza, 1988; De Martini et al., 
1997, 1998; Burrato et al., 1999), we propose the existence of a major blind thrust 
(Mantova Source) having the following geometrical characteristics:
- the E-W strike is chosen according to the orientation of the major diversion of the 
Mincio River near the locality of Grazie;
- the fault dips 30 towards the N, based on the characteristic distance between the 
synclinal and anticlinal axes;
- the rake is assumed to be 90 (pure thrusting) based 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 pair;
- the length (10 km) is based on scaling with width and constrained by 
geomorphological observations.

Given the estimated width and length, the fault is capable of generating a M 6+ 
earthquake that would have its strongest effects in the region between the Po river and 
the Alps piedmont. Current catalogues (CPTI, 1999) report a large event in 1117 (Me 
6.6) that falls in the general region of the Mantova Source, and for which a causative 
source has not been confidently located yet (although this Database proposes it may 
have been generated by the Adige Plain Source, to which the reader may refer). 
However, both the location and the magnitude of this event are highly uncertain due 
to the very limited number of data available. No other historical events can be 
associated with the Mantova Source except for the small 6 July 1693 earthquake 
(Me=5.0) near Goito.

The diversion of the Mincio River, which according to Veggiani (1974) occurred in the 
VIII  century B.C., can be explained by uplift of the area south of the Lakes of Mantova 
and relative subsidence of the sector to the north. This pattern of deformation is in 
agreement with that expected for an EW-trending, N-dipping, low-angle blind thrust. 
However, as it is proposed by De Martini et al. (1997, 1998), the same pattern of 
deformation would be produced by an EW-trending, S-dipping, high-angle reverse 
fault; in this case the relative position of the syncline-anticline  would be reversed (the 
syncline would be in front of the fault, instead of being at its back). We favour the 
south-verging blind thrust hypothesis, since its low-angle geometry seems more 
compatible with an out-of-sequence propagating compressional front. This geometry is 
also in agreement with that hypothesised for the nearby Adige Plain Source (south-
verging, low-angle blind thrust).

The position of the Mantova and Adige Plain sources and their blind thrust geometry 
imply the presence of an active front of the Alps  farther south than the currently 
mapped subsurface compressional fronts. Regional cross-sections and subsurface oil 
industry data (Pieri and Groppi, 1981; Cassano et al., 1986) show that north of the Po 
River the sediments filling the plain have a uniform S-sloping homoclinal geometry. 
Only few high-angle, S-verging reverse faults, generally sealed by Pliocene sediments, 
break the continuity of the homocline. No major subsurface tectonic structures are 
shown in the vicinity of the Mantova Source. Hence the presence of this source is 
hypothesised largely on the basis of the geomorphic and geodetic anomalies.


OPEN QUESTIONS

1) There exists a fundamental question concerning the seismogenic potential of this 
source. Does it rupture in large M 6+ rather infrequent events? Or in smaller more 
frequent earthquakes in the M range 5-5.5? Or is it totally aseismic?

2) Can the Mantova Source be associated with any historical earthquakes, or does it 
represent a seismic gap?

3) Is this source a high-angle S-dipping reverse fault, or a low-angle N-dipping thrust 
fault?

4) Can the diversion of the Mincio River be due only to the capture by a left tributary 
of the Po River?
