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Pre-existing cross-structures and active fault segmentation in the northern-central
Apennines (Italy).
Alberto Pizzi (1) and Fabrizio Galadini (2)
(1) Dipartimento di Scienza della Terra, Campus Universitario, Università “G. D’Annunzio”,
Chieti – [email protected]
(2) Istituto Nazionale di Geofisica e Vulcanologia, Milano – [email protected]
Key words: active faults, segmentation, pre-existing cross-structure, structural barrier,
northern/central Apennines
Abstract
The multideformed axial zone of the Apennines provides a great opportunity to explore the
influence of pre-existing cross-structures (inherited from pre-Quaternary tectonic phases) on the
segmentation of Quaternary/active seismogenic extensional faults. Detailed geological and
structural data and their comparison with seismological data show that although the attitudes
(strike and dip) of oblique pre-existing faults are certainly an important factor in determining a
segment boundary, the size of the inherited oblique structures seems to be more crucial. Pre-
existing cross-structures with lengths ranging from several kilometers to a few tens of kilometers
show a twofold behavior. They can act as segment barriers during the rupture of a single fault
segment or they can be reactivated as transfer zones inducing the activation of two adjacent
segments that belong to the same fault system. Regional basement/crustal oblique pre-existing
cross-structures, with lengths ranging from several tens of kilometers to hundreds of kilometers
(commonly NNE-striking), may act as “persistent structural barriers” that halt both fault segment
and fault system propagation, thus determining their terminations and maximum sizes. In the
northern-central Apennines, the NNE-striking Ancona-Anzio, Valnerina, and Ortona-Roccamonfina
tectonic lineaments, although having been repeatedly reactivated since the Mesozoic, represent
the most important examples of these structures. Moreover, probably due to their misorientation
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with respect to the present extensional stress field, regional NNE-striking pre-existing structures
appear to be less likely to produce strong magnitude events (no surface evidence for Quaternary
faulting has been found thus far and historical and instrumental seismicity shows only M<6 events).
M ~7 event, on the other hand, are more likely to occur along the (N)NW-(S)SE trending normal
fault systems. Lastly, we propose a model that can explain the different sizes of fault segments and
fault systems on the basis of their location with respect to the “persistent structural barriers” and
their spacing. In this view, our results may contribute to a more reasonable assessment of the
nature and size of future surface ruptures in the northern-central Apennines, which are of critical
importance to estimating seismic hazard.
1. Introduction
Since faults are geometrically and mechanically segmented at a variety of scales (e.g.,
Schwartz and Sibson, 1989), analyses aimed at defining fault segmentation have become an
important technique for seismic hazard assessment. The key point is to identify persistent
segment boundaries, where most or all of the propagating rupture terminates after each event
(e.g., Das and Aki, 1977; Aki, 1979; 1984; King, 1986; Sibson, 1987; 1989; Schwartz and
Sibson, 1989; Scholz, 1990; Crone and Haller, 1991; Zhang et al., 1991). Among the different
types of segment boundaries, the term “structural boundary” identifies the segments bounded
by fault branches or intersections with other faults or cross-structures (dePolo et al., 1991;
Knuepfer, 1989; McCalpin, 1996). According to Knuepfer (1989), the structures most likely to
occur at rupture endpoints on normal faults are “cross-structures,” even if not all structural
boundaries are capable of arresting fault ruptures because they can break through several
structural boundaries.
Other authors have shown that small-scale structural boundaries (less than 1 km) are
probably not capable of stopping an earthquake rupture greater than 30 km in length or of
magnitude 7 or larger (Sibson, 1987; Crone and Haller, 1991; Zhang et al., 1991). Therefore,
the size of a structural boundary with respect to the rupture length or displacement may play an
important role in controlling rupture termination. Since the concept of self-similar fault behavior
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requires a segment boundary of a certain size to arrest a rupture propagation of a certain size
(e.g., Sibson, 1989), “it is crucial to evaluate the size of structural boundaries that were broken
through by earthquake rupture and of those that arrested or significantly impeded earthquake
rupture” (i.e., barriers) (Zhang et al., 1999).
Although most of these characteristics of segment boundaries have been derived from
studies of historical earthquake ruptures (paleoseismological data), Wheeler (1989) stated that
even the paleoseismological record is insufficiently long to define a “persistent barrier”, and
“long term” geological criteria must be used.
According to this statement, we will show how structural geologic criteria can be useful in
determining the long-term behavior of seismogenic faults, in particular for areas—like the
Apennines of Italy—where a strong connection between “geological structures”, “earthquake
ruptures”, and “seismological faults” has already been documented by seismological and
paleoseismological studies (e.g., Galadini and Galli, 2000; Chiaraluce et al., 2005). A central
question in our discussion is: to what extent have pre-existing cross-structures influenced the
propagation and the segmentation of the active extensional faults along the axial zone of the
Apennines? We will show examples of Mesozoic basement/crustal cross-faults (i.e., the
Ancona-Anzio, Valnerina, and Ortona-Roccamonfina lines) that, although having been
repeatedly reactivated during the Neogene emplacement of the Apennine chain (e.g.,
Tavarnelli et al., 2004; Butler et al., 2006), have acted as “persistent structural barriers” to the
propagation of the Quaternary fault systems and have determined their terminations and size.
Lastly, we propose a model that can explain the different sizes of fault segments and fault
systems on the basis of their location with respect to the “persistent structural barriers” and
their spacing.
In our discussion, we use the term “pre-existing cross-structure” to indicate structures (i)
inherited from earlier tectonic phases with respect to the Quaternary/active deformation and (ii)
“oblique” to the mean orientation of the seismogenic faults. Therefore, the term “structural
barrier” is restricted here to imply those Mesozoic to Tertiary oblique structures acting as
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obstacles to the propagation of the NW-SE Quaternary faults accommodating active NE-SW
extension along the axial zone of the Apennines (see below).
2. Structural setting
The axial zone of the Umbria-Marche northern Apennines and Abruzzi central Apennines
(Fig. 1) is a tectonically active region affected by post-orogenic Quaternary extension (Calamita
and Pizzi, 1994; Lavecchia et al., 1994; Ghisetti and Vezzani, 1999; Piccardi et al., 1999;
Morewood and Roberts, 2000; Galadini and Galli, 2000; Valensise and Pantosti, 2001).
Extensional faulting is expressed at the surface by a set of mainly (N)NW–(S)SE trending, 15 to
35 km-long, normal or normal-oblique fault systems (Figs. 2 and 3). The fault systems are
usually made up of en–echelon fault segments with lengths ranging from a few km to 15-20
km, mostly steeply dipping towards the SW. Fault slip data measured along Quaternary/active
fault planes revealed an ongoing extension driven by a nearly horizontal ca. NE-trending 3-
axis (e.g., Calamita and Pizzi, 1994; Lavecchia et al., 1994). Normal faults kinematics is
consistent with the focal mechanism solutions of the northern and central Apennines
earthquakes which indicate a present T-axis mainly oriented NE-SW (e.g., Frepoli & Amato,
1997).
The study area, however, was affected by multiphased contractional and extensional
deformation. Quaternary post-orogenic extension is superimposed on a Neogene fold-and-
thrust belt developed after the collision of the African and European continental margins (e.g.,
Elter, 1975; Patacca and Scandone, 1989; Boccaletti et al., 1990; Carmignani and Kligfield,
1990). Thrust faulting, in turn, was preceded by Triassic, Jurassic, Cretaceous-Paleogene, and
Miocene extension (Centamore et al., 1971; Castellarin et al., 1978; Decandia, 1982;
Montanari et al., 1989; Marchegiani et al., 1999; Scisciani et al., 2002; Butler et al., 2006 and
references therein).
The three major structural trends that make up the present structural framework, striking NE,
NNE and E(SE), result from these phases of deformation (Fig. 4).
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The structures striking between NW-SE and NNW-SSE, represent the mean trend of the
northeast verging thrust fronts of the Neogene Apennine chain (Fig. 4b). The location of such
thrust planes has often been controlled by pre-existing SE trending extensional structures (Fig.
4a) that, in some cases, were also inverted (e.g., Scisciani et al., 2002 and references therein).
Moreover, since the SE trending structures were favorably oriented with respect to the
“principal” direction of the Quaternary extension (i.e., ca. NE-trending 3, see Fig. 4c), some of
them have been further reactivated as Quaternary normal faults (e.g., Pizzi and Scisciani,
2000).
Major NNE-SSW and ESE-WNW striking faults instead represent the principal pre-existing
cross-structures with respect to the axis of Quaternary extensional faulting. In the study area,
the arc-shaped major Neogene thrust fronts at the outer zone of the Apennine belt, the
Olevano-Antrodoco-Sibillini Mts. thrust (OAST), the Mt. Cavallo thrust (MCT), and the Sangro-
Volturno thrust zone (SVTZ), are characterized by NNE-striking regional dextral oblique thrust
ramps with displacements of up to several tens of kilometers and along-strike lengths ranging
from tens up to of hundreds of kilometers (Fig. 1). The occurrence of these NNE-striking thrust
ramps, in turn, reflects the influence of pre-existing structures, the “Ancona-Anzio”, “Valnerina”,
and “Ortona-Roccamonfina” lines, respectively (see Fig. 2). These latter structures have, in
fact, been active for a long time, since they strongly affected the Meso-Cenozoic tectono-
sedimentary evolution as well as the pattern of the Neogene fold and thrust belt in the northern
and central Apennines through episodes of repeated reactivation (e.g., Tavarnelli et al., 2001;
2004 and references therein).
In particular, the Ancona-Anzio Line is a more than one hundred km long high-angle crustal
fault that acted, during the Mesozoic-Early Tertiary as a syn-sedimentary extensional fault
separating the Umbria and Marche pelagic domains to the north from the Lazio-Abruzzi
carbonate platform domain to the south (Fig. 1) (Castellarin et al., 1978). Therefore, the NNE-
striking ramp of the OAST represents the surficial expression of the Ancona-Anzio Line that
was reactivated during the Neogene as a high-angle dextral transpressional shear zone (e.g.,
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Koopman, 1983; Lavecchia, 1985; Finetti et al., 2005; Butler et al., 2006 and references
therein).
In the same way, the NNE-SSW dextral thrust-ramp of the MCT, located in an inner position
with respect to the Ancona-Anzio Line, is due to the Neogene reactivation of the northernmost
sector of the Valnerina Line (Figs. 1 and 2), a Late Cretaceous-Eocene syn-sedimentary
normal fault that was reactivated during the Neogene as a regional (ca. 50 km long) high-angle
basement structure (Decandia, 1982; Montanari et al., 1989; Calamita and Pierantoni, 1993;
Lavecchia, 1985; Alberti, 2000; Tavarnelli et al., 2004).
In the southern sector of the study area, another regional, more than one hundred km long,
NNE-striking oblique lineament known as the “Ortona-Roccamonfina Line” is traditionally
considered as the boundary between the central and the Southern Apennines (Locardi, 1982)
(Fig. 2). The SVTZ represents the present expression of this crustal/lithospheric discontinuity
and consists of a complex Pliocene dextral fault zone several tens of km long (Figs. 1 and 2)
(Locardi, 1982; Patacca et al., 1990; Di Bucci and Tozzi, 1991; Cinque et al., 1993; Ghisetti et
al., 1993; Oldow et al, 1993; Ghisetti and Vezzani, 1997).
Similarly, but to a lesser extent, structures striking between ESE-WNW to E-W have
controlled the boundaries of different sedimentary environments since Mesozoic times. One
example in the Abruzzi Apennines is the E-W striking faults along the Maiella massif, which is
represented by the sharp boundary between the Cretaceous carbonate platforms and the
adjacent slope-transitional areas (Fig. 1) (Rusciadelli, 2005 and references therein). In the
Gran Sasso Massif, the Mesozoic-Cenozoic WNW-ESE paleomargin between carbonate
platform and basinal areas, probably more than 40 km long, controlled the localization of the
ca. 30 km long sinistral oblique lateral ramp at the northern front of the Gran Sasso thrust
(Satolli et al., 2005). In the hanging-wall of such basement/crustal thrust ramps (e.g., Finetti et
al., 2005), Quaternary extensional fault systems (e.g., the Assergi and the Campo Imperatore
fault systems, “AFS” and “CIFS”, respectively; see figure 2) show a parallel ESE-trend (e.g.,
Demangeot, 1965; Ghisetti and Vezzani, 1986; Carraro and Giardino, 1992; D’Agostino et al.,
1998; Galli et al., 2002) forming the most evident anomaly with respect to the ca. NW-SE mean
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trend of the Quaternary Apennine extensional belt. Geological and geomorphological data have
indicated that some of these normal faults reactivated Meso-Cenozoic pre- and syn-orogenic
normal faults (e.g., Calamita et al., 2000b).
3. Pre-existing cross-faults vs. structural barriers in Apennine literature: a brief review
The role of pre-existing cross-structures in the seismogenic framework of the Apennines has
been evidenced in the seismotectonic zoning drawn for seismic hazard assessment (Scandone
and Stucchi, 2000). Valensise and Pantosti (2001) proposed that “transverse structures” can
play a twofold role: as segment boundaries (“passive role”) or, alternatively, they can
themselves be the sources of both large and small earthquakes. Mostly based on
seismological and geological data, recent studies have pointed out the importance of pre-
existing cross-structures in the segmentation of the active extensional belt of the Apennines.
By the identification of an active fault system in the Molise region, Di Bucci et al. (2002)
hypothesized that the boundary between the central and southern Apennines might be
regarded as a long-term barrier to the rupture propagation of active faulting. Based on the
analysis of structural features, the distribution of aftershocks, and focal mechanisms related to
the 1984 Sangro Valley earthquake (Ms 5.8) in the Abruzzi region, Pace et al. (2002)
suggested a dual role for the W(NW)-E(SE) trending pre-existing strike-slip fault (“GF” in Fig. 2)
in this central Apennine area. Indeed, it behaved as a barrier during the 1984 earthquake, but
the geological evidence suggests its long-term behavior is as a transfer fault. Also based on
geological observations, Boncio et al. (2004) defined a qualitative segmentation model
consisting of major faults separated by kilometric scale structural-geometric complexities
considered as probable barriers preventing the propagation of the earthquake ruptures. The
presence of structural barriers has also been invoked for the 1997 Colfiorito seismic sequence
in the Umbria-Marche northern Apennines. In particular, Chiaraluce et al. (2005) showed that
the two main shocks of the Colfiorito sequence originated close to the intersections between
active normal faults and a NNE-striking pre-existing dextral transpressional structure inherited
from the Neogene contractional tectonic phase. This inherited fault acted as a “lateral barrier”
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to the rupture propagation and consequently constrained the fault size. Moreover, Collettini et
al. (2005) suggested that the elastic stress perturbation around this ca. NNE-striking pre-
existing fault promoted its later reactivation as a result of the different amount of slip
experienced along the two normal faults responsible for the greater main shocks.
Although the above-discussed works show that the effects of pre-existing cross-faults on the
evolution of active extensional faulting are increasingly being recognized, a structural-
geological overview at the regional scale in order to define the geometry, different types, and
the behavior of the pre-existing cross-faults is still lacking.
4. NNE-striking pre-existing cross-structures vs. Quaternary fault segmentation
4.1 The Mt. Cavallo thrust (Valnerina Line) and the Colfiorito fault system
In the Colfiorito area, the geometry of the Quaternary en-echelon fault segments, their normal
kinematics based on the analysis of striated planes, and the relationship between the long-term
vertical displacement and the adjacent tectonic depressions were already well defined before
the 1997 earthquakes (Pizzi, 1992; Calamita et al., 1994; Cello et al., 1997). The numerous
multidisciplinary works which followed the 1997 seismic sequence, based on seismological,
paleoseismological, geological, geomorphological, GPS, and DIn-SAR methods, among others,
indicated the consistency between the earthquake segments and most of the already mapped
Quaternary fault segments (Amato et al., 1998; Galadini et al., 1999; Hunstad et al., 1999;
Calamita et al., 2000a; Cattaneo et al., 2000; Salvi et al., 2000; Messina et al., 2002;
Hernandez et al., 2004; Chiaraluce et al., 2005; Collettini et al., 2005; Alberti, 2006; Dalla Via et
al., 2007; Moro et al., 2007). Due to the peculiar geological-structural setting, the Colfiorito area
is one of the best places to perform field investigations on the relationship between the NW-
striking active normal faults and the pre-existing (inherited from earlier tectonic phases) NNE-
striking faults. Indeed, this crustal volume is affected by numerous high-angle Neogene right-
lateral transpressive structures, which are mostly NNE-trending. Among these faults, the Mt.
Cavallo thrust (MCT) represents the main regional structure from which several minor branches
splay (Fig. 5). This arc-shaped thrust front is sub-parallel to the outer OAST and has a regional
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oblique ramp that follows the ca. 50 km long Valnerina Line (see Tavarnelli et al., 2004 and
references therein). Based on seismological and geological evidence, some authors have
already pointed out the role of the Colfiorito-Mt. Pennino transpressive fault (T1 in Fig. 5),
which is parallel to the MCT ramp and ca. 10-15 km long. On September 26, 1997, this pre-
existing cross fault acted as a barrier to rupture propagation, separating the two main shocks
(see Fig. 5) (Chiaraluce et al., 2005). Moreover, at the end of the seismic sequence this
inherited dextral transpressive structure was reactivated with opposite kinematics, as
suggested by sinistral strike-slip and reverse minor events mainly located near this structure
(e.g., Collettini et al., 2005; Alberti, 2006 and references therein).
We agree with the interpretations of the previous authors about how the pre-existing cross-
fault formed a rupture barrier. However, based on the peculiar structural setting of the area, we
point out further implications to explain the occurrence of minor sinistral strike-slip and reverse
events at the end of the seismic sequence. Based on the anomalous left step-over of the two
en-echelon fault segments that slipped on September 26, 1997 (F1 and F2 in Fig. 5), we
suggest that this N(NE) pre-existing dextral transpressive structure represents a “geometric
non-conservative discontinuity” (King and Yielding, 1983). In the typical right step-overs that
characterizes almost all the NW-SE striking en-echelon faults in the northern/central Apennine,
the slip vectors of the adjacent segments are coherent with those of the transfer zone and no
volume change or new fault(s) are required to accommodate slip (Fig. 6a, c) Therefore, the
NNE striking pre-existing structure may act as a “conservative” discontinuity. Conversely, when
the oblique inherited structure is located between two propagating segments forming a left
step-over, as in the Colfiorito case (Fig. 6b, d), the slip generates local contraction in the
transfer zone and subsidiary faulting is required to accommodate the volume decrease
(“nonconservative” discontinuity). In this view, we suggest that the subordinate sinistral strike-
slip and reverse events during the Colfiorito seismic sequence, which were mainly located
along the N(NE) Colfiorito-Mt. Pennino pre-existing secondary structure, were probably
associated with the activation/reactivation of minor strike-slip/thrust faults that accommodated
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local compressional stress generated between the two NW-SE segments activated in this
“unfavorable” structural setting.
We suggest that under the present stress regime, secondary (e.g., up to 10-15 km long) NNE
striking pre-existing cross faults are not “major” seismogenic sources, whereas they can be
“locally” reactivated as transfer faults between two main NW-SE en-echelon normal faults if
they are located in a favorable setting (Fig. 6a, c). We furthermore hypothesize that if two
propagating en-echelon segments are separated by a cross-structure that can kinematically act
as a transfer fault, it is more likely that the seismic rupture can jump coseismically from one
segment to another, thereby generating a larger earthquake. Conversely, in an unfavorable
structural setting (Fig. 6b, d), secondary pre-existing cross faults are more likely to act as
structural barriers to rupture propagation, preventing a coseismic kinematic link between two
propagating en-echelon segments. As suggested by Collettini et al. (2005) for the Colfiorito
seismic sequence, however, in the latter case a “later” and “local” reactivation of the pre-
existing NNE striking fault can be promoted in order to accommodate the enhanced stress
generated between the two NW-SE normal faults activated by the two previous major
earthquakes.
A more complex pattern instead characterizes the intersection zone between the
southernmost fault segments of the CFS and the Neogene NNE-striking MCT, (Fig. 5). Based
on seismological data from the 1997 Colfiorito seismic sequence, Chiaraluce et al. (2005)
suggested the activation of a NW-SE striking fault cutting through the MCT. Our “long term”
geological, structural, and geomorphological field study, however, indicates that the
southernmost fault segments of the CFS do not cut through the MCT (see Fig. 5). The southern
termination of the CFS is instead characterized by i) a rapid decrease in the normal
displacement on the fault, ii) a complex fault pattern with sharp changes in the fault orientation
(as in the case of the Costa-San Martino segment, which rotates from NW to a strike of ca.
N80° approaching the MCT), and iii) an evident fragmentation of the main fault into several
short segments (see the area between Sellano and Mt. Cavallo in Fig. 5). Based on these
observations, we suggest that the ca. 50 km long Valnerina Line, a pre-existing basement
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structure reactivated during Neogene compression as an high-angle dextral thrust ramp (i.e.,
MCT), acted during Quaternary extension as a “persistent structural barrier”, hindering the
southeastward propagation of the CFS.
Several seismological aspects corroborate the evidence of the structural control related to the
MCT on the seismogenic behavior of the area, including the locations of the main shock that
struck Sellano on October 14, 1997 (Mw 5.62), about 20 days after the two main shocks in
Colfiorito (Fig. 5), and most of the aftershocks that occurred within the hanging-wall of the
thrust (Ekström, 1998; Cattaneo et al., 2000; Chiaraluce et al., 2003). In addition, the damage
distribution, which is generally considered to reflect the seismogenic processes (e.g., Gasperini
et al., 1999; Sirovich and Pettenati, 2004), is located in the hanging-wall of the thrust (see also
Figs. 9 A-B for the damage distribution associated to the September 26, 1997 Colfiorito
mainshocks).
Considering these roles of pre-existing cross-structures, we can assume that secondary NNE
striking pre-existing cross-structures, with lengths of up to 10-15 km and probably confined
within the sedimentary cover (e.g., Barchi and Mirabella, 2008), may have a twofold role: as
transfer faults and/or as structural barriers between individual segments of the same
extensional fault system. In contrast, NNE striking basement faults with lengths of several tens
of km, which have long histories, perform the role of “persistent structural barriers” to the
propagating Quaternary extensional fault systems.
4.2 The Olevano-Antrodoco-Sibillini Mts. thrust ramp (Ancona-Anzio Line)
Two major (N)NW-(S)SE striking systems of active extensional faults can be observed in the
hanging-wall of the outermost sector of the OAST-sheet: the Norcia–Mt. Fema fault system
(NFFS) and the Mt. Vettore-Mt. Bove fault system (VBFS) (Fig. 2). The activity of these
systems has been indicated by geomorphological and paleoseismological studies (Calamita et
al., 1982; 1992; Brozzetti and Lavecchia, 1994; Calamita and Pizzi, 1994; Blumetti, 1995;
Coltorti and Farabollini, 1995; Cello et al., 1998; Galadini and Galli, 2003; Galli et al., 2005).
These studies have indicated that the faults have Holocene displacements. Based on
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paleoseismological data, historical surface faulting occurred during the January 14, 1703
earthquake (moment magnitude derived from the macroseismic data: Maw 6.81 in
Working Group CPTI, 2004) and possibly during the September 19, 1979 earthquake (Maw 5.9
in Working Group CPTI, 2004) along the NFFS (Cello et al., 1998; Galli et al., 2005). In
contrast, the latest surface faulting event for the VBFS is not more recent than the 6th-7th
century AD (Galadini and Galli, 2003).
Structural geological and geomorphological data suggest that both fault systems are
characterized by southern terminations in the area close to the NNE-striking OAST-crustal
ramp (Fig. 7).
In particular, the amount of displacement related to the Quaternary activity of the ca. 30 km
long VBFS abruptly decreases near its intersection with the OAST-ramp (Pizzi and Scisciani,
2000). Here, the SE termination of the fault system is made up of two segments. The tip of the
eastern segment, although covered by Quaternary slope deposits (Fig. 8), is not present as far
as 1.5 km SE from the intersection with the OAST trace, where the outcropping strata of the
Messinian sandstones (OAST footwall unit) are not displaced by the fault. The strike of the
western segment clearly deflects parallel to the trace of the OAST and the displacement
progressively dies out ca. 3 km to the south (Figs. 7 and 8).
The ca. 35 km long NFFS includes at least three fault segments with right en-echelon step-
overs (Fig. 2). Both northern and southern tips of the system are located close to the
intersection with two major pre-existing NNE-SSW cross-structures (i.e., the MCT and the
OAST ramps). The northern termination of the NFFS is located close to the MCT, since the
northernmost NFFS segment (i.e., Mt. Fema) does not cut through this structure at the surface
(Fig. 2). In this view, the NNE-SSW dextral lateral ramp of the aforementioned thrust forms a
“persistent structural barrier” for both SE-ward propagation of the CFS and NW-ward
propagation of the NFFS. Moreover, the southern end of the Mt. Fema segment is constrained
by the intersection with a minor N(NE)-striking contractional structure (Visso thrust, see
location in Fig. 1). Much further to the south, the longest Norcia fault segment extends for
about 15-20 km in a ca. NW-SE direction (Figs. 2 and 7), bounding a large intermontane
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tectonic basin (Norcia basin). Close to the NNE-striking OAST ramp, the normal displacement
of the fault decreases very rapidly (Pizzi and Scisciani, 2000), and the fault strike deflects
almost parallel to the thrust ramp and does not displace it (Fig. 7). It is probable that part of the
displacement is transferred to several minor branches of the Mt. Alvagnano westernmost
segment, which in turn terminates against the OAST ramp (Fig. 7). Indeed, the normal fault
located in the footwall of the OAST south of the Mt. Alvagnano segment (Mt. Prato fault, see
Fig. 7), which has already been interpreted as a syn-orogenic (Neogene) normal fault (Alberti
et al., 1996; Tavarnelli et al., 2004), does not show evidence for Late Quaternary activity.
Therefore, as in the case of the Colfiorito fault system, long-term geological data suggests
that secondary NNE striking pre-existing contractional structures can represent the boundaries
of single segments of the same extensional fault system. Similar major regional NNE-trending
pre-existing contractional cross-structures (i.e., MCT and OAST ramps), representing the
Neogene inversion of Mesozoic basement/crustal extensional faults (i.e., Valnerina and
Ancona-Anzio lines, respectively), can stop the propagation of the entire fault system, acting as
“persistent structural barriers”.
This evidence can also be derived from the distribution and the characteristics of the
historical seismicity. Indeed, apart from the VBFS, to which no historical events can be
associated (Galadini and Galli, 2003), the NFFS has produced numerous destructive
earthquakes since the Middle Ages. The historical earthquakes of this area support the
hypothesis of segmentation (Fig. 9; Galadini et al., 1999). The largest earthquake occurred on
January 14, 1703, and the associated damage, distributed throughout the whole sector
affected by the NFFS, suggests that the entire fault system activated synchronously (Galadini
et al., 1999), producing a large magnitude (Maw 6.8) earthquake. In contrast, the damage
distributions of the lower magnitude earthquakes (1328, 1599, 1859, 1730, 1979) suggest the
activation of single segments of the fault system (Galadini et al., 1999). This means that in
some cases the segment activation is limited by barriers represented by the secondary pre-
existing cross-faults. This occurs for earthquakes with M 5.5-6.0. In contrast, in other cases
these secondary structures do not represent barriers; the entire fault system activates and the
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dimension of the larger seismogenic source, generally consistent with earthquake magnitudes
up to 7.0, is limited only by the main NNE pre-existing cross-structures (persistent structural
barriers).
The reason why secondary pre-existing cross-faults sometime act as barriers and sometime
do not likely depends on the energy release associated with a given earthquake. In other
words, the examples presented above indicate that pre-existing cross-structures with lengths
less than 30-40 km, possibly confined within the sedimentary cover, are probably not capable
of stopping an earthquake rupture with magnitude ~ 7. If this hypothesis is correct, then the
release of energy capable of overcoming the secondary barriers occurs with recurrence
intervals larger than 1,000 years. This represents the recurrence interval for Apennine
seismogenic sources in the case of large magnitude earthquakes (e.g., Galadini and Galli,
2000 and references therein).
4.3 The Sangro-Volturno thrust zone (Ortona-Roccamonfina Line, ORL)
Original field data in the Maiella area provides new insights about another “persistent
structural barrier”, the NNE-striking ORL, which is traditionally considered to be the boundary
between the central and the southern Apennines (Figs. 1 and 2), (Locardi, 1982; Patacca et al.,
1990).
The Maiella massif is the outermost anticline of the central Apennine fold-and-thrust belt in
the Abruzzi region, involving Mesozoic-Cenozoic carbonate rocks, at the surface (e.g., Patacca
and Scandone, 1989). The thrust activity responsible for the growth of the Maiella anticline
probably lasted until the Late Pliocene (e.g., Casnedi et al., 1981; Ghisetti and Vezzani, 1983;
Patacca et al., 1991; Scisciani et al., 2002; Pizzi, 2003).The NNE-SSW orientation of the
southern forelimb of the Maiella fold, defining its arc-shaped geometry in plan view, is strongly
controlled by the ORL, whose present expression is given by a complex dextral oblique-slip
crustal fault zone of Late Pliocene age, the SVTZ (Figs. 1 and 2). The Maiella area, however,
has been considered to be the epicentral area of two major historical earthquakes, which
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occurred in 1706 (Maw = 6.60 in Working Group CPTI, 2004) and 1933 (Maw = 5.7 in Working
Group CPTI, 2004).
New geological field data indicate evidence of Late Pleistocene-Holocene activity along a ca.
20 km long fault system (the Maiella fault system, MFS), which extends from the southern
Morrone massif to the southern portion of the carbonate relief represented by the Maiella
massif and Mt. Porrara (Figs. 2 and 10). In particular, the southern part of the MFS is
composed of a complex set of fault segments (i.e., the Palena fault, “PF” and the Western
Porrara fault, “WPF”) that displace late Quaternary deposits. These segments represent the
outermost (easternmost) active extensional structures of the central Apennines. The Western
Porrara fault locally reactivates the southernmost segment of the Messinian-Pliocene
Caramanico fault (Fig. 10), whereas the Palena fault reactivates (with normal dip-slip
kinematics) a pre-existing WNW-ESE left-lateral oblique fault associated with the emplacement
of the Maiella fold and thrust. On the whole, the present tectonic regime has been responsible
for the reactivation of favorably oriented pre-existing Miocene-Pliocene contractional and
extensional structures capable of accommodating the active (N)NE-extension.
Field mapping at the southern ends of the Western Porrara and Palena faults instead shows
that the fault planes stop abruptly in correspondence of the intersection with the regional NNE-
striking pre-existing ORL. Similar to the southern end of the Colfiorito and NFFS, part of the
displacement of the MFS is probably transferred to several minor branches (i.e.,
Pescocostanzo, Cinquemiglia, and Aremogna faults), which in turn terminate against the SVTZ
(Fig. 10).
We interpret this geometric and kinematic evidence as being due to the Quaternary role of
the pre-existing crustal ORL as a “persistent structural barrier” hindering the SE-propagation of
the MFS. This hypothesis is also in agreement with that proposed by Di Bucci et al. (2002) for
the Isernia area (Molise), about 50 km further to the SSW. In addition, the authors observed the
lack of continuity of the active fault system across the boundary between the central and the
southern Apennines (i.e., ORL) as well as a different style of Quaternary faulting (mostly SW-
dipping faults in the central Apennines, and NE-dipping faults in the Molise area). In the Maiella
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area, however, subsurface data precludes the occurrence of active normal faulting further east
of the ORL (i.e., toward the outer zones of the buried frontal Apennine chain and in the Apulian
foreland). Moreover, a few kilometers east of the Maiella mountain front, recent geological and
morphological data provide evidence for active contractional deformation with the occurrence
of a growing anticline (Pizzi et al., 2007). Therefore, we suggest that in the study area the
NNE-SSW striking ORL presently acts not only as a “persistent structural barrier” hindering the
southward growth of the MFS but also represents a major boundary of two distinct tectonic
regimes: an area experiencing active extensional faulting located to the west of the ORL and
an area characterized by contractional and strike-slip deformation to the east.
5. ESE-striking pre-existing structures
In the northern-central Apennines, major E(SE)-striking cross-structures commonly
correspond to both extensional pre-orogenic faults and Neogene contractional (mostly sinistral
transpressive) faults. Field structural mapping showed that Quaternary reactivation of these
pre-existing structures, with transtensive (dextral oblique) and/or dip-slip kinematics, is more
frequent than on the NNE-striking ones, since they are more favorably oriented with respect to
the principal direction of active extension (i.e., α < 45°-50°, see Fig. 4c) (Calamita and Pizzi,
1994; Galadini, 1999; Piccardi et al., 1999). However, similar to the NNE striking pre-existing
cross-structures, their seismogenic behavior seems to be determined by their size.
Pre-existing cross-structures striking between ESE-WNW and E-W with lengths ranging from
hundreds of meters to a few kilometers represent limited heterogeneities that can be cut or
reactivated during Quaternary normal faulting, producing local bending along propagating NW-
SE striking fault segments.
Based on the analysis of structural features, the distribution of the aftershocks, and the focal
mechanisms related to the Ms 5.8 Sangro Valley earthquake of May 1984, Pace et al. (2002)
suggested a twofold role for the W(NW)-E(SE) striking pre-existing Mt. Greco fault (“GF” in Fig.
2). This ca. 8 km long inherited Neogene sinistral transpressive structure behaved as a
conservative barrier, halting the propagation of the 1984 earthquake rupturing along the NNW–
SSE Barrea normal fault (“BF” in Fig. 2). In contrast, the long-term geological and structural
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data indicate that it has also acted as a reactivated dextral normal oblique transfer fault (Pace
et al., 2002).
Regional ESE-striking pre-existing Mesozoic extensional structures at least 40 km long,
probably controlled the location of the ca. 30 km long northern oblique thrust front of the Gran
Sasso Massif. In the hanging-wall of this Neogene crustal thrust (Finetti et al., 2005), the
Assergi “AFS” and Campo Imperatore “CIFS” active fault systems consist of reactivated ESE
pre-existing contractional and extensional en-echelon fault segments up to 15 km long (Fig. 2)
(e.g., Ghisetti and Vezzani, 1986; Carraro and Giardino, 1992; D’Agostino et al., 1998;
Calamita et al., 2000b; Galli et al., 2002). Indeed, the trends of these extensional structures
represent the major anomaly with respect to the NW-SE trend of the Quaternary Apennine
extensional belt. North of the Gran Sasso thrust, in the footwall block, the active extensional
Laga faults system, LFS (Fig. 2) follows the regional NW-SE trend. Although the southern
termination of the LFS is very close to the Gran Sasso chain normal faults, geological and
geomorphological data suggest that a kinematic continuity between these structures is lacking.
Therefore, also in this case, the lack of continuity between two aligned and propagating
Quaternary fault systems correlates with the occurrence of a regional Mesozoic cross-fault.
Hence, this more than 40 km long basement-involved structure can be considered as a
“persistent structural barrier” during post-orogenic Quaternary extension.
6. Size of pre-existing cross-structures vs. size of Quaternary fault segments
The structural cases described above suggest that the intersections between active normal
faults and pre-existing (from Mesozoic to Neogene) cross-structures acting as barriers to fault
propagation can play a major role in the long-term segmentation of the northern/central
Apennines extensional system. Although the attitude (strike and dip) of the oblique pre-existing
faults is certainly an important factor in determining segment boundaries, the size of the
inherited oblique structures seems to be more crucial.
As reported in the previous sections, the single seismogenic segments in the northern/central
Apennines are characterized by lengths ranging between a few km and 10-15 km, while the
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fault systems can reach lengths of up to 30-35 km. Hence, based on the empirical relationships
between fault length and earthquake magnitude (e.g., Wells and Coppersmith, 1994), we can
roughly associate M 5.5-6.0 earthquakes with the activation of a single fault segment and M ~ 7
events to the activation of an entire fault system.
As a result, evaluating the ability of pre-existing cross-structure to stop a propagating fault is
crucial in fault segmentation analysis and, hence, in seismic hazard assessment. Since our
observations thus far suggest that Quaternary faults of a certain size require pre-existing cross-
structures of a certain size to arrest their propagation, we try to roughly correlate the size
relationships observed in this study. In particular, considering the length of the NNE- and ESE-
trending pre-existing cross-structures (PL) with respect to the mean length (i.e., 10 km) of the
extensional fault segment (FL), we note that (see Tab.1):
1) ( PL < FL) - pre-existing cross-structures with lengths ranging from hundreds of meters to a
few kilometers represent limited heterogeneities that can be cut or partially/entirely reactivated
during normal faulting, producing local bending along the propagating NW-SE fault segment;
2) (PL ≈ FL) - pre-existing cross-structures with lengths ranging from several kilometers to a
few tens of kilometers show a twofold behavior. They can act as segment barriers during the
rupture of a single fault segment or they can be reactivated as transfer zones inducing the
activation of two adjacent segments of the same fault system. The behavior of the faults
responsible for the 1984 and 1997 earthquakes (Pace et al., 2002; Chiaraluce et al., 2005)
probably results from this type of geometric and kinematic relationship;
3) (PL » FL) - regional basement/crustal oblique pre-existing cross-structures with lengths
ranging between several tens of kilometers and hundreds of kilometers (commonly NNE-
striking) may act as “persistent structural barriers” halting both fault segment and fault system
propagation, thus defining the maximum size of a fault system. In the northern-central
Apennines, the NNE-striking Valnerina Line, and particularly the Ancona-Anzio and the Ortona-
Roccamonfina tectonic crustal lineaments, represent the most important examples of these
structures.
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7. The structural barrier model for fault segmentation
Considering these kinds of structural relationships, it may be possible to define the relative
length of the single segments or the maximum length of the entire fault system. As for a single
segment within a fault system, its maximum length is probably controlled by the distance from
the “persistent structural barriers”. Fault segments whose “center” (i.e., usually the place where
the maximum displacement occurs) is more than 10-15 km from the main segment boundaries
generally represent the longer Apennine fault segments (e.g., the Norcia fault segment in the
NFFS, the Assergi fault segment in the AFS, and the Fucino fault segment within the Fucino
fault system (FFS); see Fig. 11).
In contrast, fault segments with their “center” less than 10 km from the principal cross-
structures are generally shorter and may display a more dispersed and complex pattern.
Branching at fault terminations could be a result of the deformation in areas where seismogenic
faults intersect a pre-existing structural barrier (e.g., the Sellano fault segment in the CFS, the
Mt. Alvagnano fault segment in the NFFS, the Aremogna and Cinquemiglia fault segments; see
Fig. 11).
We suggest, therefore, that fault segments which originate far (i.e., more than 10 km) from
the structural barriers can grow radially without early interference with these segment
boundaries, and therefore can become larger than the fault segments whose growth started
close to a “persistent structural barrier” (i.e., distance < 10 km).
Furthermore, considering the length of an entire fault system, we suggest that if “persistent
structural barriers” are able to control the termination of a fault segment, then the spacing
between two successive “persistent structural barriers” directly constrains the maximum length
of the fault system. As an example, the narrower spacing between the Valnerina Line and
Ancona-Anzio line structural barriers than that between the Ancona-Anzio line and the Ortona-
Roccamonfina Line may have constrained the shorter length of the NFFS with respect to the
Fucino fault system (FFS) (Fig. 11).
8. Discussion: nature and behavior of the persistent structural barriers
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Our geological, structural, and geomorphological field evidence indicates that no (Late)
Quaternary “surface faulting” can be associated with the reactivation of the NNE striking
persistent structural barriers. Furthermore, we showed that the areas surrounding these
barriers are generally characterized by distributed secondary faulting (Fig. 12). These
observations, however, do not mean that the more unfaulted rock volume along these regional
structures experienced a smaller amount of Quaternary deformation, but rather implies that
extension may have been accommodated by different mechanisms. Secondary faulting, blind-
faulting, and/or creep processes associated with low-magnitude and diffuse seismicity could
accommodate the deficit of deformation along the barriers after a seismic event or during the
interseismic period (e.g., Sibson, 1989). As the growing Quaternary faults interact with the
structural barriers, a large amount of energy is probably dissipated along the pre-existing fault
zone so that the fault cannot overcome and displace the barriers. Therefore, the fault
propagation is delayed and slows and a series of small segments activates, which also may
represent pre-existing structures, sometimes with trends which are not ideal for the present
tectonic regime (Fig. 12). These possible mechanisms were corroborated by Chiaraluce et al.
(2005) and Alberti (2006) for the 1997 Colfiorito earthquakes.
We suggest, therefore, that persistent structural barriers are not able to generate
earthquakes of M > 5.6-6.0 (i.e., threshold value for extensional surface faulting earthquake;
e.g., Wells and Coppersmith, 1994; Pavlides and Caputo, 2004) under the present day stress
regime. The spatial distribution of the seismicity in the northern/central Apennines strongly
supports this hypothesis. It is characterized by more frequent low-to-moderate magnitude
events (M not exceeding 6.0) in these zones, while the main historical events are generally
located far from them (e.g. Working Group CPTI, 2004).
In other words, assuming that a Coulomb frictional rheology (i.e., that there are many pre-
existing faults, some of which will be in ideal orientations with respect to the present stress
field; e.g., Sibson, 1987; Ranalli, 2000) is reasonable for the multideformed brittle upper crust
in the Apennines, this can explain why largely misoriented (NNE striking) pre-existing
structures with respect to the present extensional stress field (i.e., > 45-50°, see Fig. 4c) are
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very rarely–if ever–reactivated. However, the mechanisms by which long-lived (repeatedly
reactivated) basement/crustal structures can stop a propagating Quaternary extensional fault
are still unclear. These relationships between the behavior of pre-existing cross-structures with
respect to their sizes (length, and hence amount of displacement), however, strongly suggest
that the nature and thickness of the associated fault rocks could play a critical role.
Unfortunately, basement fault rocks are not exposed in the study area, so our observations
were limited to the surficial expressions of these faults, which are generally related to their last
reactivation during the Neogene deformation of the Apennine belt. Therefore, no direct
information was available about the critical parameters such as the dip of the faults at depth,
widths of the fault zone, nature of the fault rocks, and associated values of friction coefficient
and pore fluid pressure. Dedicated geological and geophysical studies are thus still necessary
in the key zones where faults of the extensional belt intersect pre-existing cross-structures in
order to develop more refined hypotheses on the relationship between segmentation and the
presence of persistent structural barriers. For example, a key aspect will be understanding if
the mechanical properties of the fault rock associated with the basement/crustal barriers can be
considered as “weak” or “strong”, i.e., which kind of structural behavior can be expected (e.g.,
Tavarnelli et al., 2001).
9. Conclusions
The multideformed axial zone of the Apennines provides a great opportunity to explore the
influence of pre-existing cross-structures (inherited from pre-Quaternary tectonic phases) on
the propagation and segmentation of Quaternary/active seismogenic extensional faults. In the
Umbria-Marche northern Apennines and the Abruzzi central Apennines, two principal trends
(NNE-SSW and ESE–WNW) characterize the orientation of the major pre-existing structures
oblique to the mean NW-SE trend of the Quaternary faults.
Our “long-term” geological-structural study, compared with the available seismological data,
showed that although the attitude (strike and dip) of pre-existing cross-faults is certainly an
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important factor in determining a segment boundary, the size of the inherited oblique structures
seems to be more crucial.
- Pre-existing cross-structures with lengths ranging from several kilometers to a few tens of
kilometers show a twofold behavior. They can act as segment barriers during the rupture of
single fault segments, i.e., during seismic events with M≤ 5.5-6.0 (considering the Apennines
seismotectonic framework), or they can be reactivated as transfer faults inducing the activation
of two NW-SE adjacent segments that belong to the same fault system.
- Regional basement/crustal oblique pre-existing cross-structures, with lengths ranging from
several tens of kilometers to hundreds of kilometers (commonly NNE-striking) may act as
“persistent structural barriers”, halting both fault segment and fault system propagation – i.e.,
they are able to stop a rupture produced by a ca. M7 earthquake (corresponding to the
maximum earthquake magnitude recorded in the Apennines). Thus, in most cases, the location
and spacing of such “persistent structural barriers” can reasonably be used to determine the
terminations and sizes of active fault segments and fault systems and hence their expected
maximum magnitude.
The NNE-striking Ancona-Anzio, Valnerina, and Ortona-Roccamonfina tectonic lineaments,
although having been repeatedly reactivated since the Mesozoic, represent the most important
examples of these “persistent structural barriers”.
The reason for their present-day inactivity is due to their misorientation with respect to the
principal stress axes during Quaternary post-orogenic extension.
Field studies have indicated no evidence for large (Late) Quaternary surface faulting
associated with the reactivation of these structures, and the structural fault dataset clearly
shows that pre-existing cross-structures with misorientation angles greater than 45-50° (with
respect to the NW-SE ideally oriented normal faults) are not suitable to be reactivated.
This evidence, moreover, is strongly supported by the historical and instrumental seismicity,
which shows more frequent low-to-moderate magnitude events (M not exceeding 6.0)
distributed along these NNE striking persistent structural barriers, while the main historical
events are generally located far from them (e.g., Working Group CPTI, 2004).
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The mechanisms by which pre-existing cross-structures can arrest a propagating normal
fault, however, are still not clear. The evidence that pre-existing cross-structures have different
behaviors as a function of their size (and hence displacement) strongly suggests that the
nature and thickness of the associated fault rocks could play a critical role.
Dedicated geological and geophysical studies are thus still necessary in the key zones where
faults of the Apennines extensional belt intersect pre-existing cross-structures. The importance
in recognizing and classifying the different structural barriers is evident, considering the impact
that the geometry and the length of the active fault systems have in the seismic zonation and
evaluation of the seismogenic potential. Indeed, the correct location of barriers in such
applications will constrain the maximum expected magnitude based on the length of the
surficial fault expression.
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CAPTIONS
Fig. 1 - Simplified geologic map of the Umbria-Marche northern Apennines and the Abruzzi
central Apennines. MCT, Mt. Cavallo thrust; OAST, Olevano-Antrodoco-Sibillini Mts. thrust;
SVTZ, Sangro-Volturno thrust zone modified from Bigi et al. (1992). Extensional faults (see Fig.
2) are not shown for clarity.
Fig. 2 - Simplified structural map of the axial zone of the Umbria-Marche northern Apennines
and the Abruzzi central Apennines showing the geometric relationships between the
Quaternary/active extensional faults and the main NNE dextral (and ESE sinistral) transpressive
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thrust ramps. The latter structures represent the Neogene surface expression of
basement/crustal tectonic lineaments with long histories (e.g., Valnerina, Ancona-Anzio, and
Ortona-Roccamonfina lines; see text for explanation). Main Quaternary extensional faults: CFS,
Colfiorito fault system; VBFS, Mt. Vettore–Mt. Bove fault system; NFFS, Norcia–Mt. Fema fault
system; LFS, Laga fault system; AFS, Assergi fault system; CIFS, Campo Imperatore fault
system; GF, Mt. Greco fault; BF, Barrea fault. Main Neogene contractional structures: MCT, Mt.
Cavallo thrust; OAST, Olevano-Antrodoco-Sibillini Mts. thrust; SVTZ, Sangro-Volturno thrust
zone; grey star indicates the epicenter of the 1984 Sangro Valley earthquake (Ms 5.8).
Fig. 3 – Rose diagrams showing the frequency distribution of measured Quaternary fault
strikes. The large data set indicates a ca. NW-SE mean strike of the fault planes, in agreement
with the main axis of Quaternary extension oriented ca. NE-SW. However, faults misoriented up
to 45-50° with respect to the mean trend are still statistically represented. The relatively high
frequency of ESE-striking faults in the Abruzzi central Apennines (right) is probably controlled by
reactivation of locally frequent pre-existing (Mesozoic-Cenozoic) structures (see, Fig. 4) that are
abundant in this area (e.g., faults in the Gran Sasso area, Fig. 2). Misoriented faults in the
Umbria-Marche region (left) show a more symmetric pattern. It is noteworthy that NNE-striking
faults are very scarcely, if at all, represented.
Fig. 4 – Kinematic history of the three major structural trends—NNE, E(SE) and SE—that
control the present structural framework of the northern-central Apennines. a) From the Mesozoic
phases of rifting to the Miocene syn-orogenic foreland flexuring, kinematics that were mainly
normal were associated with these faults. b) During Apennines Neogene compression, the ca.
NE-SW oriented horizontal σ1 favoured the reactivation of the NNE and E(SE) faults with dextral
and sinistral strike-slip/transpressive kinematics, respectively. The frontal thrusts commonly
strike SE. c) During post-orogenic Quaternary extension, characterized by a (main) horizontal ca.
NE-trending 3, pre-existing SE-striking faults are in ideal orientations to be reactivated. Our fault
dataset (see Fig. 3), however, shows that faults misoriented up to ca. 50° with respect to the NW-
SE ideally oriented fault (shadow area), are likely to be reactivated. Therefore, reactivation of
E(SE) pre-existing structures (i.e., < 50°) is likely to occur (it is noteworthy that reactivated ESE
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fault segments along the AFS and CIFS in the Gran Sasso area show lengths up to 15 km, see
Fig. 2). These faults commonly show normal to dextral-oblique kinematics, while largely
misoriented NNE-striking pre-existing structures ( > 50°) are very rarely reactivated.
Fig. 5 - Simplified structural map of the Colfiorito area (Umbria-Marche northern Apennines)
showing the “long term” geometric relationships between the CFS (Colfiorito fault system) and
the N(NE) striking Neogene dextral transpressive structures, and their relation with the “short
term” seismological data associated with the two main shocks that occurred on 26 September
1997 in the Colfiorito area. Fault data modified from Pizzi (1992), Calamita and Pizzi (1994),
Cello et al. (1997), Galadini et al. (1999), Calamita et al. (2000) and Chiaraluce et al. (2005). Map
location in Fig. 2.
Fig. 6 - Modes of possible kinematic interaction between a NNE striking pre-existing high-angle
cross-structure (e.g., Neogene dextral transpressive fault) and two approaching en-echelon
Quaternary fault segments. In the case of a right stepover of the two en-echelon segments (a
and c), the slip can be transferred smoothly (no volume change or new faults must be created)
and the NNE-striking inherited structure may locally be inverted as a sinistral oblique transfer
fault (conservative barrier). Conversely, when the two extensional segments show a left stepover
(b and d), the slip generates local contraction at the transfer zone (shadow area) and subsidiary
faulting is required to accommodate the volume decrease (“nonconservative” discontinuity).
Fig. 7– Simplified structural map of the Umbria-Marche northern Apennines showing the
geometric relationships between the southern terminations of the Quaternary extensional fault
systems (VBFS and NFFS) and the Neogene Olevano-Antrodoco-Sibillini Mts. dextral thrust
ramp (OAST). Fault data modified from Pizzi (1992), Calamita and Pizzi (1994), Lavecchia et al.
(1994), Cello et al. (1997) and Galadini and Galli (2000). Map location in Fig. 2.
Fig. 8 – View of the Mt. Vettore (Umbria-Marche northern Apennines) showing the geometric
relationships between the Quaternary extensional faults at the southern terminations of the Mt.
Vettore-Mt. Bove fault system (VBFS) and the Neogene Olevano-Antrodoco-Sibillini Mts. dextral
thrust ramp (OAST) (location in Fig. 7).
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Fig. 9 - Fault and historical earthquakes in the area of the Mt. Cavallo thrust (MCT) and
Olevano-Antrodoco-Sibillini Mts. thrust (OAST). The main normal fault systems are reported:
CFS, Colfiorito fault system; NFFS, Norcia-Mt. Fema fault system; VBFS, Mt. Vettore-Mt. Bove
fault system. The damage distribution of Mw 5.7 earthquakes was derived from INGV-DBMI04 at
http://emidius.mi.ingv.it/DBMI04/. The intensity in the legend refers to the MCS scale. A) General
view of the area. Note i) the correspondence between the damage distribution and the normal
fault systems to which the earthquakes are associated, and ii) the location of the earthquake
damage and, therefore, of the causative sources (of which the reported fault systems represent
the surficial expressions) in the hanging-walls of the major thrusts; B) Detail of the Norcia area.
Note that the Mw 6 earthquakes can be associated with the activation of minor sources
corresponding to single segments of the NFFS; C) Detail of the Norcia area. Note that the
damage distribution of the Jan. 14, 1703 earthquake is consistent with the activation of the entire
NFFS. Map location in Fig. 2.
Fig. 10 – Simplified structural map of the Maiella-Mt. Porrara area showing the geometric
relationships between the Quaternary extensional fault systems at the southern boundary of the
Abruzzi central Apennines and the Pliocene Sangro-Volturno thrust zone (SVTZ). Fault data
modified from Galadini and Galli (2000). Map location in Fig. 2.
Fig. 11 – Proposed model to explain the different size of fault segments and fault systems on
the basis of i) their location with respect to the pre-existing cross-structures acting as “persistent
structural barriers”; ii) the spacing of the “persistent structural barriers”. See text for explanation.
CFS: Colfiorito fault system; VBFS: Mt. Vettore–Mt. Bove fault system; NFFS: Norcia–Mt. Fema
fault system; LFS: Laga fault system; AFS: Assergi fault system; CIFS: Campo Imperatore fault
system; FFS: Fucino fault system; ACFS: Aremogna and Cinquemiglia fault segments; MCT: Mt.
Cavallo thrust; OAST: Olevano-Antrodoco-Sibillini Mts. thrust; SVTZ: Sangro-Volturno thrust
zone.
Fig. 12 – Schematic block diagram of the complex geometric pattern at the intersection of a
NW-SE Quaternary extensional fault system with a regional NNE-SSW pre-existing cross-
structure, as observed in the study area (not to scale). a) NNE-SSW striking inherited
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basement/crustal structure reactivated during the Neogene as a dextral thrust-ramp and acting
as a “persistent structural barrier” to the propagation of the Quaternary extensional faults; b)
major NW-SE striking Quaternary extensional fault segment; c) secondary NW-SE striking fault
segments; d) secondary fault segment oriented nearly parallel to the trace of the thrust ramp; e)
secondary fault segment oriented at a high angle to the trace of the thrust ramp; f) secondary
fault segment displacing a splay of the major thrust ramp; g) splay of the major thrust ramp; h)
trace of the major Neogene thrust ramp; i) fault segments at the northwestern termination of a
fault system located in the footwall of the major thrust ramp; l) trace of a NW-SE striking thrust
displaced by Quaternary normal faults.
Tab.1 - Role played by pre-existing cross-structures during the propagation of Quaternary
extensional faults. PL: length of the pre-existing cross-structure; FL: mean length of the
Quaternary fault segment in the study area; : misorientation angle between the strike of the pre-
existing cross-structure and the NW-SE ideally oriented Quaternary extensional fault.
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