The geometry of this Source is based on geological and geomorphologic observations on the Ufita River valley and surrounding areas, and on the re-analysis of the 1732 Irpinia earthquake macroseismic field. Further constraints come also from recent studies that focused on the nearby Source, responsible for the 1930 Irpinia earthquake.
This Source is a N-dipping fault with a normal kinematics with a right-lateral sense of motion. Together with the nearby Sources located further to the east, responsible for the 1851 earthquake near the Vulture volcano, this Source belongs to an ESE-WNW trending tectonic lineament that disrupts the seeming continuity of the extensional seismogenic belt that runs along the crest of the Southern Apennines.
The Ufita River valley falls in a portion of the Southern Apennines seismogenic belt that was hit several times by destructive events (namely, the 1688, 1702, 1732, 1930, and 1962 earthquakes). The clustering of earthquakes distinguishes this area from nearby regions, where the destructive events are seemingly aligned along the belt (i.e. in a NW-SE direction). On the contrary, the mesoseismal area of some of these earthquakes is shifted to the east of the chain, suggesting that in this area two quite different seismogenic faulting types may coexist - if in a spatially complex arrangement.
The resulting E-W trending tectonic lineament, to which these Sources belong, is characterized by right-lateral kinematics and may be an inherited crustal discontinuity, reactivated nowadays under the NNW directed Adria plate motion. This geometry and kinematics is similar to those of the best known Molise-Mattinata-Gondola shear zone, located more to the N, and associated with the 2002 Molise seismic sequence. In this context, the three seismogenic sources may be shallow segments of a flower structure branching from the deep shear zone.
Moving westward along this lineament, the kinematic interaction with the extensional belt (with NE-SW oriented minimum stress) should increase inducing oblique slip on the nearly E-W trending seismogenic sources.
The 1732 Irpinia earthquake produced the maximum damages in the Ufita River valley and in the Baronia ridge. The macroseismic field of this earthquake is characterized by two areas of intensity maxima. These are the Ufita Valley to the north, and to the south the towns of Lioni and Teora close to the northern portion of the 1980 earthquake seismogenic source. The latter area was already hit by the 1694 Irpinia earthquake. When considering only the northern area, it has a characteristic elongation in a ca. E-W direction, in that resembling the macroseismic field of the 1930 Irpinia earthquake, which occurred about 30 km to the east.
A re-evaluation of digitized seismograms of the 1930 earthquake led to a stable solution showing a principal plane striking almost WNW-ESE and an oblique component, so that the source is characterised by normal kinematics with a right-lateral sense of motion (Pino et al., 2008).
The source of the 1732 earthquake was traditionally considered to be a NW-SE trending (i.e. parallel to the Ufita River valley strike) normal fault, located either on the eastern valley shoulder or along the western one.
In particular, a set of triangular facets recognised along the NE side of the Ufita River valley for a distance of more than 10 km were considered as the surface expression of a NW-SE striking, SW-dipping active normal fault (Brancaccio et al., 1981; 1984; Basso et al., 1996).
These 100-m-high facets, developed on Lower Pliocene soft rocks, are very gently inclined (less than 10°), and according to these Authors would delineate a 22 km long fault characterized by a Late Quaternary vertical slip rate of 0.2 mm/yr (Cinque et al., 2000). It was also hypothesised to be the seismogenic source of the 1732 earthquake (Meletti et al., 2000).
The solution adopted in DISS is based on multidisciplinary observations and studies, including morphotectonic analysis and geophysical surveys.
The morphotectonic analysis highlighted the existence of an E-W striking morphological lineament, recognized using interpretation of remotely sensed imagery, and locally consistent with field-mapped faults and fractures. This lineament runs from the Ufita River valley eastwards and is connected with similar features mapped in the epicentral area of the 1930 and 1851 earthquakes.
The geophysical studies, that include several shallow and deep ERT profiles (Giocoli et al., 2008), probed the geometry of the Quaternary alluvial and colluvial bodies across a morphological scarp running at the foot of the NE slope of the Ufita Valley and the depositional setting of the deeper sector of the Ufita River valley. The ERT profiles highlighted a shallow bedrock at the foot of the triangular facets, constrained by field observations of bedrock outcrops in the Ufita River thalweg.
These triangular facets, traditionally considered as the expression of an active normal fault, are characterized by a very low angle frontal slope that is in contrast with the presumed tectonic origin, and may have been generated by fluvial processes of the right tributaries of the Ufita River.
From a geological point of view, the hypothesized normal fault should produce a pair of subsiding and uplifting areas of dimensions scaled to the fault size. Besides, using the average height of the triangular facets (about 100 m) as a proxy for the footwall vertical uplift, the related total subsidence should be 3/4 times that value (i.e. in the order of about 3/400 m) and the area that records at least 100 m of subsidence should be several kilometers wide.
Following these observations, a deep depositional basin filled with lacustrine and fluvial sediments in the hanging-wall of the normal fault, and a wedge of recent deposits opening towards the fault itself were expected to be found. Instead, none of these subsurface geometries were observed in the shallow ERT (Giocoli et al., 2008).
Moreover, the geological results of the deep ERT show that the dimension of the associated basin (4 km wide and 5 km long) is not scaled to the size of the hypothesized fault.
Following these observations, we believe that the triangular facets and the scarplet at their foot may be geomorphic features generated by the linear erosion process of the longitudinal Ufita River and of its right tributaries.
1) What is the structural relationship (if any) among the causative sources of the 1930, the 1732, the 1694 and the 1851 earthquakes?
2) What are the roles (if any) of the buried Apulian platform and of the deep-seated basement on the occurrence and/or the development of deeply-rooted large seismogenic sources in this sector of the Southern Apennines?
Brancaccio et al. (1981)
They analyse the geomorphology of the Baronia, a NW-trending 15 km-long Cenozoic ridge bounded to the SE and to the SW by the Ufita River and to the NE by the Fiumarella Creek. They find that the SW side of the Baronia shows two erosional surfaces about 10 km-long, one at the top of the relief and the other between 500 and 600 m a.s.l. The lower surface is interpreted as a glacis; it is connected to the actual thalweg of the Ufita River by a NW-SE, 12 km-long alignment of 100-150 m-high faceted spurs interpreted by the authors as evidence for inactive normal fault with the west-side down. The authors infer Holocene and historical seismic activity of this fault on the basis of:
1) the well preserved fault slope in contrast to the high erodible deposits; 2) some small fluvial terraces observed in the valleys of the north tributaries of the Ufita river; 3) the north tributaries abrupt angle junctions; 4) some small scarps at the base of the fault slope; 5) the location and the NW trend of the mesoseismal areas of the 1688, 1702, and 1732 A.D. earthquakes.
Ciaranfi et al., (1983)
The Neotectonic Map of Southern Italy Interval IV-V (time-interval 0.7 Myr-0.018 Myr and 0.018 Myr-today) shows a NE-dipping, active normal fault, located along the upper Ufita River Valley and a SW-dipping, likely active normal fault, located along the lower Ufita River Valley even though not specifically defined as potential seismogenic sources.
Pantosti and Valensise (1988; 1989)
Within their fault segmentation model for the Southern Apennines named "Southern Apennines Fault", obtained by merging macroseismic observations and interpretation of field evidences, they propose the Ufita Valley Fault as a NW-striking, NE-dipping 30 km-long blind seismogenic source. They support their hypothesis observing that 1) the Ufita River drainage network has a regular pattern across the damage areas whereas it has a dendritic form in the surrounding areas;2) the Ufita River tributaries are in erosion while the Ufita River is aggrading; 3) the main damaged area of the 1732 earthquake is included between Apice to the north and Guardia dei Lombardi to the south for a distance of 30 km, has a NW-strike, and an asymmetrical distribution suggesting a NE dipping fault plane; 4) similarity with Irpinia Fault.
Alessio et al. (1993)
On the basis of macroseismic and instrumental investigations identified four seismogenic areas marked by a characteristic seismic behaviour: the Southern Abruzzo area, the Molise area, the Beneventano area, and the Campania-Lucania area. The Beneventano seismogenic area includes the 1125, 1688, 1702, 1732 A.D. earthquakes. From the analysis of the recent and current seismic activity in this zone, a swarm-type activity with sequences of comparable magnitude concentrated in time and space is observed.
Basso et al. (1996a; b) and Basso et al. (1997)
Re-analyzing in detail the geomorphology of the Ufita Valley area (from Grottaminarda up to Vallata) confirm the presence of two erosional surfaces and of the NW-SE fault first described by Brancaccio et al. (1981). They also describe six fluvial terraces and a set of NE-SW striking faults.The highest erosional surface is at 800-1100 m a.s.l.; the lowest surface is at 550-700 m a.s.l., it occurs only along the north side of the Ufita River, 150 m above its present-day thalweg, and is interpreted as glacis. In agreement with Brancaccio et al. (1981) the NW-SE striking 12 km-long alignment of 100-150 m-high faceted spurs connecting the lower surface to the present thalweg of the Ufita River is interpreted as evidence for an active SW-dipping normal fault called Ufita River Fault. The faceted spurs have an average dip of 15¡ to the SW. On the basis of stratigraphic correlation a Late Pliocene-Early Pleistocene and an Early Pleistocene-Middle Pleistocene age are assigned respectively to the highest and the lowest surfaces. They refer to Middle Pleistocene the main reactivation of the Ufita River Fault and, in agreement with Brancaccio et al. (1981), suggest the fault was active during Holocene and historical time. They observe that Ufita River Fault is antithetic to the Southern Apennines Fault proposed by Valensise and Pantosti (1988. The six fluvial terraces they describe are located between 3 and 120 m above the present-day thalweg. The I-II order terraces are located only on the faceted spurs between 50 and 100 m while the III-VI order terraces occur along both sides of the river. The I-II order terraces are partly offset by the Ufita River Fault. The NE-SW faults they describe are: 1) the Parolise-Grottaminarda line, active between Middle Pliocene and Quaternary with a normal movement; 2) the left-lateral strike-slip Bagnoli Irpino-Torrente Calaggio line, active between the Pliocene and Middle Pleistocene; 3) the Ufita Valley Normal Fault, that is a late Pleistocene reactivation of the Parolise-Grottaminarda line.
Giocoli et al. (2008)
The Authors present the result of several Electrical Resistivity Tomography (ERT) surveys, carried out to study the subsurface structural and sedimentary settings of the upper Ufita River valley, and to evaluate their efficiency to distinguish the geological boundary between shallow Quaternary sedimentary deposits and clayey bedrock characterized by moderate resistivity contrast.
They describe five shallow ERTs carried out during this study across a morphological scarp running at the foot of the northeastern slope of the valley. This valley shoulder is characterized by a set of triangular facets, that some authors associated to the presence of a SW-dipping normal fault. However, the tomographies showed the geometrical relationships of alluvial and slope deposits, having a maximum thickness of 30-40 m, and do not showing any growth relationships with the supposed fault. The shallow ERTs highlighted also the morphology of the bedrock, that is very shallow and do not form any basin in front of the mountain front.
The Authors also present the tomographic results of one 3560m-long deep ERT carried out across the deeper part of the Ufita Valley with an investigation depth of about 170 m. The deep resistivity result highlighted the complex alluvial setting, characterized by alternating fine grained lacustrine deposits and coarser gravelly fluvial sediments.