Discussion
Debate is ongoing concerning the numbers, geometries and actual positions of active normal faults in the Italian Apennines (compare the fault maps of Valensise and Pantosti 2001, Roberts and Michetti 2004, Papanikoloau and Roberts 2007, Galadini and Galli 2000, Guidoboni et al. 2007, Ithaca Project 2007). This, in the view of the authors, has in part been due to the difficulty in visualising regional patterns of deformation from observations of geomorphic features that need to be viewed at a metre-scale spatial resolution; Google Earth now allows such visualisation and quantification of the active fault geometries (Figure 1) and (Figure 6). It is the opinion of the author that such visualisation is the way to conclude such debates with agreement needed in the near future on a final active fault map. However, it is clear from the relatively discontinuous nature of the scarps seen with Google Earth, and the relatively continuous throw and kinematic variations along the faults (Figure 6) that one cannot simply use Google Earth to map such faults without detailed fieldwork. In particular, the kinematic data are extremely useful in helping define fault lengths, and such data can only be collected at outcrop at present (Roberts 1996, 2006, Roberts and Michetti 2004, Papanikolaou and Roberts 2007).
It is important to correctly identify the numbers, geometries and actual positions of active normal faults in the Italian Apennines and elsewhere, because these are the seismogenic sources that certainly control the locations of future devastating earthquakes, and possibly the maximum magnitudes of expected earthquakes (Wells and Coppersmith 1994, Pace et al. 2002). Through comparison of multi-seismic-cycle strain-rates defined by slip across fault scarps and single-seismic-cycle strain rates from instrumental seismicity and geodesy (e.g. Roberts 2006), one may be able to constrain the stress-loading and strain-release patterns that govern earthquake recurrence, through improved understanding of the mechanics of the seismic cycle; visualisation of active fault geometries is a key waypoint on the journey to this goal.