COMMENTS

The main dispute about the source of the 1857 earthquake concerns the sense 
of dip and hence the exact location of the fault trace at the surface. The 
source included in the Database consists of a NE-dipping, ca. 23 km-long "blind" 
or "hidden" normal fault. This choice is based on the fact that the NE-dipping 
fault fits the overall pattern of recent geomorphic features better than a SW-
dipping surface fault. In fact this source geometry produces a maximum of 
subsidence close to the southwestern flank of the valley, inducing attraction 
and aggradation of the drainage along that side. On the contrary, a SW-dipping 
high-angle fault, that was inferred mainly on the basis of the identification of 
triangular facets near Marsico Nuovo, would produce a maximum of 
subsidence closer to the intersection of the fault with the surface, resulting in 
widespread back-tilting of all slope deposits and erosional surfaces. This would 
also be in contrast with the observation that in the area around Marsico Vetere 
two generations of slope deposits (of Lower Pleistocene and Lower-Middle 
Pleistocene age respectively) are deformed and back-tilted, while the youngest 
slope deposits (Upper Pleistocene-Holocene in age) are undisturbed and lie in 
stratigraphic contact with the younger deposits (Di Niro and Giano, 1995).

Benedetti et al. (1998) hypothesise that the long-term offset of the Val d'Agri 
fault system is represented by the difference in elevation between the flat 
planation surfaces topping the mountain range to the northeast of the high 
Agri valley and the basin floor itself. The offset would be greater near the 
northern end of the fault system close to Marsico Nuovo (less than 800 m), and 
would decrease southward. As a result of this observation, these workers state 
that at least part of the present-day relief is due to the ongoing slip on the 
normal fault system. 

As everywhere along the Apennines, the present day topography is the result 
of the sum of pre-existing topography and of the cumulated long-term 
deformation related to the presently active fault.  The topography that existed 
prior to the inception of the seismogenic fault is not known, and hence it is 
hard to know whether the planation surfaces and the basin floor were part of 
an old flat landscape, or else if topographic contrasts existed already. The 
first hypothesis is supported by the work of Di Niro and Giano (1995) who, on 
the basis of geomorphological observations, correlate the "Paleosuperficie 
sommitale" (corresponding to the flat planation surfaces of Benedetti et al. 
(1998)) to the higher paleosurfaces of the Maddalena Mts. (Santangelo, 1991) 
and to those found on the southern margin of Mt. Marzano (Amato and 
Cinque, 1992), which are tentatively dated Pliocene-Lower Pleistocene. 

The 1857 earthquake consisted of two separated shocks, with the second 
stronger and of longer duration than the first one (Boschi et al., 1995). The 
most damaged areas were the high Agri Valley between Marsico Nuovo and 
Montemurro, and the northern termination of the Vallo di Diano near the 
village of Polla (I=X), west of the high Agri Valley. According to Benedetti et 
al., this anomalous distribution of damage could be explained with the 
activation of both the NW-SE trending, SW dipping Agri Valley normal fault 
system and the Vallo di Diano normal fault system, reaching together a length 
of about 60 km. This would increase substantially the magnitude expected for 
this source. If both shocks occurred on the Val d'Agri fault system alone the 
expected magnitude would be close to M7 and more consistent with the 
magnitude observed for other Apennine sources. Felt reports for the foreshock 
indicate damage only in the village of Grumento Nova and Montemurro 
(southern part of the high Agri Valley) (Boschi et al., 1995). These data, if 
not interpreted as local effects, seem to suggest that the foreshock may be 
located on the same fault plane as the second shock, or on an adjacent segment 
to the south, but certainly do not suggest the activation of the Vallo di Diano 
fault system.


OPEN QUESTIONS

1) Is the lack of clear surface faulting produced by this event a consequence of 
the fact that this source is "hidden", similarly to the source of the 1980 Irpinia 
earthquake? 

2) Are the triangular facets seen near Marsico Nuovo evidence of an active 
tectonic feature, though not necessarily a primary one, or do they represent a 
case of morphological convergence because of the steep downhill attitude of 
the bedding? And is the 2,5 m-high scarplet described by Benedetti et al. (1998) 
truly a tectonic feature associated with the 1857 earthquake? Why only that 
small remnant has been preserved? Could it represent sympathetic slip on an 
antithetic fault?

3) Did the large foreshock of the 1857 earthquake rupture a separate but 
adjacent source of the Agri Fault system? If so, is the Vallo di Diano fault 
system responsible for this shock (as hypothesised by Benedetti et al. (1998)? 
Did the foreshock trigger the second and larger shock on the main Val d'Agri 
seismogenic fault?
