1
2
Pre-existing cross-structures and active fault segmentation in the northern-central
Apennines (Italy).
3
4
Alberto Pizzi (1) and Fabrizio Galadini (2)
5
6
7
(1) Dipartimento di Scienza della Terra, Campus Universitario, Università “G. D’Annunzio”,
Chieti – pizzi@unich.it
8
(2) Istituto Nazionale di Geofisica e Vulcanologia, Milano – galadini@mi.ingv.it
9
Key words: active faults, segmentation, pre-existing cross-structure, structural barrier,
10
11
northern/central Apennines
Abstract
12
The multideformed axial zone of the Apennines provides a great opportunity to explore the
13
influence of pre-existing cross-structures (inherited from pre-Quaternary tectonic phases) on the
14
segmentation of Quaternary/active seismogenic extensional faults. Detailed geological and
15
structural data and their comparison with seismological data show that although the attitudes
16
(strike and dip) of oblique pre-existing faults are certainly an important factor in determining a
17
segment boundary, the size of the inherited oblique structures seems to be more crucial. Pre-
18
existing cross-structures with lengths ranging from several kilometers to a few tens of kilometers
19
show a twofold behavior. They can act as segment barriers during the rupture of a single fault
20
segment or they can be reactivated as transfer zones inducing the activation of two adjacent
21
segments that belong to the same fault system. Regional basement/crustal oblique pre-existing
22
cross-structures, with lengths ranging from several tens of kilometers to hundreds of kilometers
23
(commonly NNE-striking), may act as “persistent structural barriers” that halt both fault segment
24
and fault system propagation, thus determining their terminations and maximum sizes. In the
25
northern-central Apennines, the NNE-striking Ancona-Anzio, Valnerina, and Ortona-Roccamonfina
26
tectonic lineaments, although having been repeatedly reactivated since the Mesozoic, represent
27
the most important examples of these structures. Moreover, probably due to their misorientation
28
with respect to the present extensional stress field, regional NNE-striking pre-existing structures
1
29
appear to be less likely to produce strong magnitude events (no surface evidence for Quaternary
30
faulting has been found thus far and historical and instrumental seismicity shows only M<6 events).
31
M ~7 event, on the other hand, are more likely to occur along the (N)NW-(S)SE trending normal
32
fault systems. Lastly, we propose a model that can explain the different sizes of fault segments and
33
fault systems on the basis of their location with respect to the “persistent structural barriers” and
34
their spacing. In this view, our results may contribute to a more reasonable assessment of the
35
nature and size of future surface ruptures in the northern-central Apennines, which are of critical
36
importance to estimating seismic hazard.
37
38
1. Introduction
39
Since faults are geometrically and mechanically segmented at a variety of scales (e.g.,
40
Schwartz and Sibson, 1989), analyses aimed at defining fault segmentation have become an
41
important technique for seismic hazard assessment. The key point is to identify persistent
42
segment boundaries, where most or all of the propagating rupture terminates after each event
43
(e.g., Das and Aki, 1977; Aki, 1979; 1984; King, 1986; Sibson, 1987; 1989; Schwartz and
44
Sibson, 1989; Scholz, 1990; Crone and Haller, 1991; Zhang et al., 1991). Among the different
45
types of segment boundaries, the term “structural boundary” identifies the segments bounded
46
by fault branches or intersections with other faults or cross-structures (dePolo et al., 1991;
47
Knuepfer, 1989; McCalpin, 1996). According to Knuepfer (1989), the structures most likely to
48
occur at rupture endpoints on normal faults are “cross-structures,” even if not all structural
49
boundaries are capable of arresting fault ruptures because they can break through several
50
structural boundaries.
51
Other authors have shown that small-scale structural boundaries (less than 1 km) are
52
probably not capable of stopping an earthquake rupture greater than 30 km in length or of
53
magnitude 7 or larger (Sibson, 1987; Crone and Haller, 1991; Zhang et al., 1991). Therefore,
54
the size of a structural boundary with respect to the rupture length or displacement may play an
55
important role in controlling rupture termination. Since the concept of self-similar fault behavior
56
requires a segment boundary of a certain size to arrest a rupture propagation of a certain size
2
57
(e.g., Sibson, 1989), “it is crucial to evaluate the size of structural boundaries that were broken
58
through by earthquake rupture and of those that arrested or significantly impeded earthquake
59
rupture” (i.e., barriers) (Zhang et al., 1999).
60
Although most of these characteristics of segment boundaries have been derived from
61
studies of historical earthquake ruptures (paleoseismological data), Wheeler (1989) stated that
62
even the paleoseismological record is insufficiently long to define a “persistent barrier”, and
63
“long term” geological criteria must be used.
64
According to this statement, we will show how structural geologic criteria can be useful in
65
determining the long-term behavior of seismogenic faults, in particular for areas—like the
66
Apennines of Italy—where a strong connection between “geological structures”, “earthquake
67
ruptures”, and “seismological faults” has already been documented by seismological and
68
paleoseismological studies (e.g., Galadini and Galli, 2000; Chiaraluce et al., 2005). A central
69
question in our discussion is: to what extent have pre-existing cross-structures influenced the
70
propagation and the segmentation of the active extensional faults along the axial zone of the
71
Apennines? We will show examples of Mesozoic basement/crustal cross-faults (i.e., the
72
Ancona-Anzio, Valnerina, and Ortona-Roccamonfina lines) that, although having been
73
repeatedly reactivated during the Neogene emplacement of the Apennine chain (e.g.,
74
Tavarnelli et al., 2004; Butler et al., 2006), have acted as “persistent structural barriers” to the
75
propagation of the Quaternary fault systems and have determined their terminations and size.
76
Lastly, we propose a model that can explain the different sizes of fault segments and fault
77
systems on the basis of their location with respect to the “persistent structural barriers” and
78
their spacing.
79
In our discussion, we use the term “pre-existing cross-structure” to indicate structures (i)
80
inherited from earlier tectonic phases with respect to the Quaternary/active deformation and (ii)
81
“oblique” to the mean orientation of the seismogenic faults. Therefore, the term “structural
82
barrier” is restricted here to imply those Mesozoic to Tertiary oblique structures acting as
83
obstacles to the propagation of the NW-SE Quaternary faults accommodating active NE-SW
84
extension along the axial zone of the Apennines (see below).
3
85
86
2. Structural setting
87
The axial zone of the Umbria-Marche northern Apennines and Abruzzi central Apennines
88
(Fig. 1) is a tectonically active region affected by post-orogenic Quaternary extension (Calamita
89
and Pizzi, 1994; Lavecchia et al., 1994; Ghisetti and Vezzani, 1999; Piccardi et al., 1999;
90
Morewood and Roberts, 2000; Galadini and Galli, 2000; Valensise and Pantosti, 2001).
91
Extensional faulting is expressed at the surface by a set of mainly (N)NW–(S)SE trending, 15 to
92
35 km-long, normal or normal-oblique fault systems (Figs. 2 and 3). The fault systems are
93
usually made up of en–echelon fault segments with lengths ranging from a few km to 15-20 km,
94
mostly steeply dipping towards the SW. Fault slip data measured along Quaternary/active fault
95
planes revealed an ongoing extension driven by a nearly horizontal ca. NE-trending 3-axis
96
(e.g., Calamita and Pizzi, 1994; Lavecchia et al., 1994). Normal faults kinematics is consistent
97
with the focal mechanism solutions of the northern and central Apennines earthquakes which
98
indicate a present T-axis mainly oriented NE-SW (e.g., Frepoli & Amato, 1997).
99
The study area, however, was affected by multiphased contractional and extensional
100
deformation. Quaternary post-orogenic extension is superimposed on a Neogene fold-and-
101
thrust belt developed after the collision of the African and European continental margins (e.g.,
102
Elter, 1975; Patacca and Scandone, 1989; Boccaletti et al., 1990; Carmignani and Kligfield,
103
1990). Thrust faulting, in turn, was preceded by Triassic, Jurassic, Cretaceous-Paleogene, and
104
Miocene extension (Centamore et al., 1971; Castellarin et al., 1978; Decandia, 1982;
105
Montanari et al., 1989; Marchegiani et al., 1999; Scisciani et al., 2002; Butler et al., 2006 and
106
references therein).
107
108
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).
109
The structures striking between NW-SE and NNW-SSE, represent the mean trend of the
110
northeast verging thrust fronts of the Neogene Apennine chain (Fig. 4b). The location of such
111
thrust planes has often been controlled by pre-existing SE trending extensional structures (Fig.
112
4a) that, in some cases, were also inverted (e.g., Scisciani et al., 2002 and references therein).
4
113
Moreover, since the SE trending structures were favorably oriented with respect to the
114
“principal” direction of the Quaternary extension (i.e., ca. NE-trending 3, see Fig. 4c), some of
115
them have been further reactivated as Quaternary normal faults (e.g., Pizzi and Scisciani,
116
2000).
117
Major NNE-SSW and ESE-WNW striking faults instead represent the principal pre-existing
118
cross-structures with respect to the axis of Quaternary extensional faulting. In the study area,
119
the arc-shaped major Neogene thrust fronts at the outer zone of the Apennine belt, the
120
Olevano-Antrodoco-Sibillini Mts. thrust (OAST), the Mt. Cavallo thrust (MCT), and the Sangro-
121
Volturno thrust zone (SVTZ), are characterized by NNE-striking regional dextral oblique thrust
122
ramps with displacements of up to several tens of kilometers and along-strike lengths ranging
123
from tens up to of hundreds of kilometers (Fig. 1). The occurrence of these NNE-striking thrust
124
ramps, in turn, reflects the influence of pre-existing structures, the “Ancona-Anzio”, “Valnerina”,
125
and “Ortona-Roccamonfina” lines, respectively (see Fig. 2). These latter structures have, in
126
fact, been active for a long time, since they strongly affected the Meso-Cenozoic tectono-
127
sedimentary evolution as well as the pattern of the Neogene fold and thrust belt in the northern
128
and central Apennines through episodes of repeated reactivation (e.g., Tavarnelli et al., 2001;
129
2004 and references therein).
130
In particular, the Ancona-Anzio Line is a more than one hundred km long high-angle crustal
131
fault that acted, during the Mesozoic-Early Tertiary as a syn-sedimentary extensional fault
132
separating the Umbria and Marche pelagic domains to the north from the Lazio-Abruzzi
133
carbonate platform domain to the south (Fig. 1) (Castellarin et al., 1978). Therefore, the NNE-
134
striking ramp of the OAST represents the surficial expression of the Ancona-Anzio Line that
135
was reactivated during the Neogene as a high-angle dextral transpressional shear zone (e.g.,
136
Koopman, 1983; Lavecchia, 1985; Finetti et al., 2005; Butler et al., 2006 and references
137
therein).
138
In the same way, the NNE-SSW dextral thrust-ramp of the MCT, located in an inner position
139
with respect to the Ancona-Anzio Line, is due to the Neogene reactivation of the northernmost
140
sector of the Valnerina Line (Figs. 1 and 2), a Late Cretaceous-Eocene syn-sedimentary
5
141
normal fault that was reactivated during the Neogene as a regional (ca. 50 km long) high-angle
142
basement structure (Decandia, 1982; Montanari et al., 1989; Calamita and Pierantoni, 1993;
143
Lavecchia, 1985; Alberti, 2000; Tavarnelli et al., 2004).
144
In the southern sector of the study area, another regional, more than one hundred km long,
145
NNE-striking oblique lineament known as the “Ortona-Roccamonfina Line” is traditionally
146
considered as the boundary between the central and the Southern Apennines (Locardi, 1982)
147
(Fig. 2). The SVTZ represents the present expression of this crustal/lithospheric discontinuity
148
and consists of a complex Pliocene dextral fault zone several tens of km long (Figs. 1 and 2)
149
(Locardi, 1982; Patacca et al., 1990; Di Bucci and Tozzi, 1991; Cinque et al., 1993; Ghisetti et
150
al., 1993; Oldow et al, 1993; Ghisetti and Vezzani, 1997).
151
Similarly, but to a lesser extent, structures striking between ESE-WNW to E-W have
152
controlled the boundaries of different sedimentary environments since Mesozoic times. One
153
example in the Abruzzi Apennines is the E-W striking faults along the Maiella massif, which is
154
represented by the sharp boundary between the Cretaceous carbonate platforms and the
155
adjacent slope-transitional areas (Fig. 1) (Rusciadelli, 2005 and references therein). In the
156
Gran Sasso Massif, the Mesozoic-Cenozoic WNW-ESE paleomargin between carbonate
157
platform and basinal areas, probably more than 40 km long, controlled the localization of the
158
ca. 30 km long sinistral oblique lateral ramp at the northern front of the Gran Sasso thrust
159
(Satolli et al., 2005). In the hanging-wall of such basement/crustal thrust ramps (e.g., Finetti et
160
al., 2005), Quaternary extensional fault systems (e.g., the Assergi and the Campo Imperatore
161
fault systems, “AFS” and “CIFS”, respectively; see figure 2) show a parallel ESE-trend (e.g.,
162
Demangeot, 1965; Ghisetti and Vezzani, 1986; Carraro and Giardino, 1992; D’Agostino et al.,
163
1998; Galli et al., 2002) forming the most evident anomaly with respect to the ca. NW-SE mean
164
trend of the Quaternary Apennine extensional belt. Geological and geomorphological data have
165
indicated that some of these normal faults reactivated Meso-Cenozoic pre- and syn-orogenic
166
normal faults (e.g., Calamita et al., 2000b).
167
168
3. Pre-existing cross-faults vs. structural barriers in Apennine literature: a brief review
6
169
The role of pre-existing cross-structures in the seismogenic framework of the Apennines has
170
been evidenced in the seismotectonic zoning drawn for seismic hazard assessment (Scandone
171
and Stucchi, 2000). Valensise and Pantosti (2001) proposed that “transverse structures” can
172
play a twofold role: as segment boundaries (“passive role”) or, alternatively, they can
173
themselves be the sources of both large and small earthquakes. Mostly based on
174
seismological and geological data, recent studies have pointed out the importance of pre-
175
existing cross-structures in the segmentation of the active extensional belt of the Apennines.
176
By the identification of an active fault system in the Molise region, Di Bucci et al. (2002)
177
hypothesized that the boundary between the central and southern Apennines might be
178
regarded as a long-term barrier to the rupture propagation of active faulting. Based on the
179
analysis of structural features, the distribution of aftershocks, and focal mechanisms related to
180
the 1984 Sangro Valley earthquake (Ms 5.8) in the Abruzzi region, Pace et al. (2002)
181
suggested a dual role for the W(NW)-E(SE) trending pre-existing strike-slip fault (“GF” in Fig. 2)
182
in this central Apennine area. Indeed, it behaved as a barrier during the 1984 earthquake, but
183
the geological evidence suggests its long-term behavior is as a transfer fault. Also based on
184
geological observations, Boncio et al. (2004) defined a qualitative segmentation model
185
consisting of major faults separated by kilometric scale structural-geometric complexities
186
considered as probable barriers preventing the propagation of the earthquake ruptures. The
187
presence of structural barriers has also been invoked for the 1997 Colfiorito seismic sequence
188
in the Umbria-Marche northern Apennines. In particular, Chiaraluce et al. (2005) showed that
189
the two main shocks of the Colfiorito sequence originated close to the intersections between
190
active normal faults and a NNE-striking pre-existing dextral transpressional structure inherited
191
from the Neogene contractional tectonic phase. This inherited fault acted as a “lateral barrier”
192
to the rupture propagation and consequently constrained the fault size. Moreover, Collettini et
193
al. (2005) suggested that the elastic stress perturbation around this ca. NNE-striking pre-
194
existing fault promoted its later reactivation as a result of the different amount of slip
195
experienced along the two normal faults responsible for the greater main shocks.
7
196
Although the above-discussed works show that the effects of pre-existing cross-faults on the
197
evolution of active extensional faulting are increasingly being recognized, a structural-
198
geological overview at the regional scale in order to define the geometry, different types, and
199
the behavior of the pre-existing cross-faults is still lacking.
200
201
4. NNE-striking pre-existing cross-structures vs. Quaternary fault segmentation
202
4.1 The Mt. Cavallo thrust (Valnerina Line) and the Colfiorito fault system
203
In the Colfiorito area, the geometry of the Quaternary en-echelon fault segments, their normal
204
kinematics based on the analysis of striated planes, and the relationship between the long-term
205
vertical displacement and the adjacent tectonic depressions were already well defined before
206
the 1997 earthquakes (Pizzi, 1992; Calamita et al., 1994; Cello et al., 1997). The numerous
207
multidisciplinary works which followed the 1997 seismic sequence, based on seismological,
208
paleoseismological, geological, geomorphological, GPS, and DIn-SAR methods, among others,
209
indicated the consistency between the earthquake segments and most of the already mapped
210
Quaternary fault segments (Amato et al., 1998; Galadini et al., 1999; Hunstad et al., 1999;
211
Calamita et al., 2000a; Cattaneo et al., 2000; Salvi et al., 2000; Messina et al., 2002;
212
Hernandez et al., 2004; Chiaraluce et al., 2005; Collettini et al., 2005; Alberti, 2006; Dalla Via et
213
al., 2007; Moro et al., 2007). Due to the peculiar geological-structural setting, the Colfiorito area
214
is one of the best places to perform field investigations on the relationship between the NW-
215
striking active normal faults and the pre-existing (inherited from earlier tectonic phases) NNE-
216
striking faults. Indeed, this crustal volume is affected by numerous high-angle Neogene right-
217
lateral transpressive structures, which are mostly NNE-trending. Among these faults, the Mt.
218
Cavallo thrust (MCT) represents the main regional structure from which several minor branches
219
splay (Fig. 5). This arc-shaped thrust front is sub-parallel to the outer OAST and has a regional
220
oblique ramp that follows the ca. 50 km long Valnerina Line (see Tavarnelli et al., 2004 and
221
references therein). Based on seismological and geological evidence, some authors have
222
already pointed out the role of the Colfiorito-Mt. Pennino transpressive fault (T1 in Fig. 5),
223
which is parallel to the MCT ramp and ca. 10-15 km long. On September 26, 1997, this pre-
8
224
existing cross fault acted as a barrier to rupture propagation, separating the two main shocks
225
(see Fig. 5) (Chiaraluce et al., 2005). Moreover, at the end of the seismic sequence this
226
inherited dextral transpressive structure was reactivated with opposite kinematics, as
227
suggested by sinistral strike-slip and reverse minor events mainly located near this structure
228
(e.g., Collettini et al., 2005; Alberti, 2006 and references therein).
229
We agree with the interpretations of the previous authors about how the pre-existing cross-
230
fault formed a rupture barrier. However, based on the peculiar structural setting of the area, we
231
point out further implications to explain the occurrence of minor sinistral strike-slip and reverse
232
events at the end of the seismic sequence. Based on the anomalous left step-over of the two
233
en-echelon fault segments that slipped on September 26, 1997 (F1 and F2 in Fig. 5), we
234
suggest that this N(NE) pre-existing dextral transpressive structure represents a “geometric
235
non-conservative discontinuity” (King and Yielding, 1983). In the typical right step-overs that
236
characterizes almost all the NW-SE striking en-echelon faults in the northern/central Apennine,
237
the slip vectors of the adjacent segments are coherent with those of the transfer zone and no
238
volume change or new fault(s) are required to accommodate slip (Fig. 6a, c) Therefore, the
239
NNE striking pre-existing structure may act as a “conservative” discontinuity. Conversely, when
240
the oblique inherited structure is located between two propagating segments forming a left
241
step-over, as in the Colfiorito case (Fig. 6b, d), the slip generates local contraction in the
242
transfer zone and subsidiary faulting is required to accommodate the volume decrease
243
(“nonconservative” discontinuity). In this view, we suggest that the subordinate sinistral strike-
244
slip and reverse events during the Colfiorito seismic sequence, which were mainly located
245
along the N(NE) Colfiorito-Mt. Pennino pre-existing secondary structure, were probably
246
associated with the activation/reactivation of minor strike-slip/thrust faults that accommodated
247
local compressional stress generated between the two NW-SE segments activated in this
248
“unfavorable” structural setting.
249
We suggest that under the present stress regime, secondary (e.g., up to 10-15 km long) NNE
250
striking pre-existing cross faults are not “major” seismogenic sources, whereas they can be
251
“locally” reactivated as transfer faults between two main NW-SE en-echelon normal faults if
9
252
they are located in a favorable setting (Fig. 6a, c). We furthermore hypothesize that if two
253
propagating en-echelon segments are separated by a cross-structure that can kinematically act
254
as a transfer fault, it is more likely that the seismic rupture can jump coseismically from one
255
segment to another, thereby generating a larger earthquake. Conversely, in an unfavorable
256
structural setting (Fig. 6b, d), secondary pre-existing cross faults are more likely to act as
257
structural barriers to rupture propagation, preventing a coseismic kinematic link between two
258
propagating en-echelon segments. As suggested by Collettini et al. (2005) for the Colfiorito
259
seismic sequence, however, in the latter case a “later” and “local” reactivation of the pre-
260
existing NNE striking fault can be promoted in order to accommodate the enhanced stress
261
generated between the two NW-SE normal faults activated by the two previous major
262
earthquakes.
263
A more complex pattern instead characterizes the intersection zone between the
264
southernmost fault segments of the CFS and the Neogene NNE-striking MCT, (Fig. 5). Based
265
on seismological data from the 1997 Colfiorito seismic sequence, Chiaraluce et al. (2005)
266
suggested the activation of a NW-SE striking fault cutting through the MCT. Our “long term”
267
geological, structural, and geomorphological field study, however, indicates that the
268
southernmost fault segments of the CFS do not cut through the MCT (see Fig. 5). The southern
269
termination of the CFS is instead characterized by i) a rapid decrease in the normal
270
displacement on the fault, ii) a complex fault pattern with sharp changes in the fault orientation
271
(as in the case of the Costa-San Martino segment, which rotates from NW to a strike of ca.
272
N80° approaching the MCT), and iii) an evident fragmentation of the main fault into several
273
short segments (see the area between Sellano and Mt. Cavallo in Fig. 5). Based on these
274
observations, we suggest that the ca. 50 km long Valnerina Line, a pre-existing basement
275
structure reactivated during Neogene compression as an high-angle dextral thrust ramp (i.e.,
276
MCT), acted during Quaternary extension as a “persistent structural barrier”, hindering the
277
southeastward propagation of the CFS.
278
Several seismological aspects corroborate the evidence of the structural control related to the
279
MCT on the seismogenic behavior of the area, including the locations of the main shock that
10
280
struck Sellano on October 14, 1997 (Mw 5.62), about 20 days after the two main shocks in
281
Colfiorito (Fig. 5), and most of the aftershocks that occurred within the hanging-wall of the
282
thrust (Ekström, 1998; Cattaneo et al., 2000; Chiaraluce et al., 2003). In addition, the damage
283
distribution, which is generally considered to reflect the seismogenic processes (e.g., Gasperini
284
et al., 1999; Sirovich and Pettenati, 2004), is located in the hanging-wall of the thrust (see also
285
Figs. 9 A-B for the damage distribution associated to the September 26, 1997 Colfiorito
286
mainshocks).
287
Considering these roles of pre-existing cross-structures, we can assume that secondary NNE
288
striking pre-existing cross-structures, with lengths of up to 10-15 km and probably confined
289
within the sedimentary cover (e.g., Barchi and Mirabella, 2008), may have a twofold role: as
290
transfer faults and/or as structural barriers between individual segments of the same
291
extensional fault system. In contrast, NNE striking basement faults with lengths of several tens
292
of km, which have long histories, perform the role of “persistent structural barriers” to the
293
propagating Quaternary extensional fault systems.
294
295
4.2 The Olevano-Antrodoco-Sibillini Mts. thrust ramp (Ancona-Anzio Line)
296
Two major (N)NW-(S)SE striking systems of active extensional faults can be observed in the
297
hanging-wall of the outermost sector of the OAST-sheet: the Norcia–Mt. Fema fault system
298
(NFFS) and the Mt. Vettore-Mt. Bove fault system (VBFS) (Fig. 2). The activity of these
299
systems has been indicated by geomorphological and paleoseismological studies (Calamita et
300
al., 1982; 1992; Brozzetti and Lavecchia, 1994; Calamita and Pizzi, 1994; Blumetti, 1995;
301
Coltorti and Farabollini, 1995; Cello et al., 1998; Galadini and Galli, 2003; Galli et al., 2005).
302
These studies have indicated that the faults have Holocene displacements. Based on
303
paleoseismological data, historical surface faulting occurred during the January 14, 1703
304
earthquake (moment magnitude derived from the macroseismic data: Maw 6.81 in
305
Working Group CPTI, 2004) and possibly during the September 19, 1979 earthquake (Maw 5.9
306
in Working Group CPTI, 2004) along the NFFS (Cello et al., 1998; Galli et al., 2005). In
11
307
contrast, the latest surface faulting event for the VBFS is not more recent than the 6 th-7th
308
century AD (Galadini and Galli, 2003).
309
Structural geological and geomorphological data suggest that both fault systems are
310
characterized by southern terminations in the area close to the NNE-striking OAST-crustal
311
ramp (Fig. 7).
312
In particular, the amount of displacement related to the Quaternary activity of the ca. 30 km
313
long VBFS abruptly decreases near its intersection with the OAST-ramp (Pizzi and Scisciani,
314
2000). Here, the SE termination of the fault system is made up of two segments. The tip of the
315
eastern segment, although covered by Quaternary slope deposits (Fig. 8), is not present as far
316
as 1.5 km SE from the intersection with the OAST trace, where the outcropping strata of the
317
Messinian sandstones (OAST footwall unit) are not displaced by the fault. The strike of the
318
western segment clearly deflects parallel to the trace of the OAST and the displacement
319
progressively dies out ca. 3 km to the south (Figs. 7 and 8).
320
The ca. 35 km long NFFS includes at least three fault segments with right en-echelon step-
321
overs (Fig. 2). Both northern and southern tips of the system are located close to the
322
intersection with two major pre-existing NNE-SSW cross-structures (i.e., the MCT and the
323
OAST ramps). The northern termination of the NFFS is located close to the MCT, since the
324
northernmost NFFS segment (i.e., Mt. Fema) does not cut through this structure at the surface
325
(Fig. 2). In this view, the NNE-SSW dextral lateral ramp of the aforementioned thrust forms a
326
“persistent structural barrier” for both SE-ward propagation of the CFS and NW-ward
327
propagation of the NFFS. Moreover, the southern end of the Mt. Fema segment is constrained
328
by the intersection with a minor N(NE)-striking contractional structure (Visso thrust, see
329
location in Fig. 1). Much further to the south, the longest Norcia fault segment extends for
330
about 15-20 km in a ca. NW-SE direction (Figs. 2 and 7), bounding a large intermontane
331
tectonic basin (Norcia basin). Close to the NNE-striking OAST ramp, the normal displacement
332
of the fault decreases very rapidly (Pizzi and Scisciani, 2000), and the fault strike deflects
333
almost parallel to the thrust ramp and does not displace it (Fig. 7). It is probable that part of the
334
displacement is transferred to several minor branches
of the Mt. Alvagnano westernmost
12
335
segment, which in turn terminates against the OAST ramp (Fig. 7). Indeed, the normal fault
336
located in the footwall of the OAST south of the Mt. Alvagnano segment (Mt. Prato fault, see
337
Fig. 7), which has already been interpreted as a syn-orogenic (Neogene) normal fault (Alberti
338
et al., 1996; Tavarnelli et al., 2004), does not show evidence for Late Quaternary activity.
339
Therefore, as in the case of the Colfiorito fault system, long-term geological data suggests
340
that secondary NNE striking pre-existing contractional structures can represent the boundaries
341
of single segments of the same extensional fault system. Similar major regional NNE-trending
342
pre-existing contractional cross-structures (i.e., MCT and OAST ramps), representing the
343
Neogene inversion of Mesozoic basement/crustal extensional faults (i.e., Valnerina and
344
Ancona-Anzio lines, respectively), can stop the propagation of the entire fault system, acting as
345
“persistent structural barriers”.
346
This evidence can also be derived from the distribution and the characteristics of the
347
historical seismicity. Indeed, apart from the VBFS, to which no historical events can be
348
associated (Galadini and Galli, 2003), the NFFS has produced numerous destructive
349
earthquakes since the Middle Ages. The historical earthquakes of this area support the
350
hypothesis of segmentation (Fig. 9; Galadini et al., 1999). The largest earthquake occurred on
351
January 14, 1703, and the associated damage, distributed throughout the whole sector
352
affected by the NFFS, suggests that the entire fault system activated synchronously (Galadini
353
et al., 1999), producing a large magnitude (Maw 6.8) earthquake. In contrast, the damage
354
distributions of the lower magnitude earthquakes (1328, 1599, 1859, 1730, 1979) suggest the
355
activation of single segments of the fault system (Galadini et al., 1999). This means that in
356
some cases the segment activation is limited by barriers represented by the secondary pre-
357
existing cross-faults. This occurs for earthquakes with M 5.5-6.0. In contrast, in other cases
358
these secondary structures do not represent barriers; the entire fault system activates and the
359
dimension of the larger seismogenic source, generally consistent with earthquake magnitudes
360
up to 7.0, is limited only by the main NNE pre-existing cross-structures (persistent structural
361
barriers).
13
362
The reason why secondary pre-existing cross-faults sometime act as barriers and sometime
363
do not likely depends on the energy release associated with a given earthquake. In other
364
words, the examples presented above indicate that pre-existing cross-structures with lengths
365
less than 30-40 km, possibly confined within the sedimentary cover, are probably not capable
366
of stopping an earthquake rupture with magnitude ~ 7. If this hypothesis is correct, then the
367
release of energy capable of overcoming the secondary barriers occurs with recurrence
368
intervals larger than 1,000 years. This represents the recurrence interval for Apennine
369
seismogenic sources in the case of large magnitude earthquakes (e.g., Galadini and Galli,
370
2000 and references therein).
371
372
4.3 The Sangro-Volturno thrust zone (Ortona-Roccamonfina Line, ORL)
373
Original field data in the Maiella area provides new insights about another “persistent
374
structural barrier”, the NNE-striking ORL, which is traditionally considered to be the boundary
375
between the central and the southern Apennines (Figs. 1 and 2), (Locardi, 1982; Patacca et al.,
376
1990).
377
The Maiella massif is the outermost anticline of the central Apennine fold-and-thrust belt in
378
the Abruzzi region, involving Mesozoic-Cenozoic carbonate rocks, at the surface (e.g., Patacca
379
and Scandone, 1989). The thrust activity responsible for the growth of the Maiella anticline
380
probably lasted until the Late Pliocene (e.g., Casnedi et al., 1981; Ghisetti and Vezzani, 1983;
381
Patacca et al., 1991; Scisciani et al., 2002; Pizzi, 2003).The NNE-SSW orientation of the
382
southern forelimb of the Maiella fold, defining its arc-shaped geometry in plan view, is strongly
383
controlled by the ORL, whose present expression is given by a complex dextral oblique-slip
384
crustal fault zone of Late Pliocene age, the SVTZ (Figs. 1 and 2). The Maiella area, however,
385
has been considered to be the epicentral area of two major historical earthquakes, which
386
occurred in 1706 (Maw = 6.60 in Working Group CPTI, 2004) and 1933 (Maw = 5.7 in Working
387
Group CPTI, 2004).
388
New geological field data indicate evidence of Late Pleistocene-Holocene activity along a ca.
389
20 km long fault system (the Maiella fault system, MFS), which extends from the southern
14
390
Morrone massif to the southern portion of the carbonate relief represented by the Maiella
391
massif and Mt. Porrara (Figs. 2 and 10). In particular, the southern part of the MFS is
392
composed of a complex set of fault segments (i.e., the Palena fault, “PF” and the Western
393
Porrara fault, “WPF”) that displace late Quaternary deposits. These segments represent the
394
outermost (easternmost) active extensional structures of the central Apennines. The Western
395
Porrara fault locally reactivates the southernmost segment of the Messinian-Pliocene
396
Caramanico fault (Fig. 10), whereas the Palena fault reactivates (with normal dip-slip
397
kinematics) a pre-existing WNW-ESE left-lateral oblique fault associated with the emplacement
398
of the Maiella fold and thrust. On the whole, the present tectonic regime has been responsible
399
for the reactivation of favorably oriented pre-existing Miocene-Pliocene contractional and
400
extensional structures capable of accommodating the active (N)NE-extension.
401
Field mapping at the southern ends of the Western Porrara and Palena faults instead shows
402
that the fault planes stop abruptly in correspondence of the intersection with the regional NNE-
403
striking pre-existing ORL. Similar to the southern end of the Colfiorito and NFFS, part of the
404
displacement of the MFS is probably transferred to several minor branches (i.e.,
405
Pescocostanzo, Cinquemiglia, and Aremogna faults), which in turn terminate against the SVTZ
406
(Fig. 10).
407
We interpret this geometric and kinematic evidence as being due to the Quaternary role of
408
the pre-existing crustal ORL as a “persistent structural barrier” hindering the SE-propagation of
409
the MFS. This hypothesis is also in agreement with that proposed by Di Bucci et al. (2002) for
410
the Isernia area (Molise), about 50 km further to the SSW. In addition, the authors observed the
411
lack of continuity of the active fault system across the boundary between the central and the
412
southern Apennines (i.e., ORL) as well as a different style of Quaternary faulting (mostly SW-
413
dipping faults in the central Apennines, and NE-dipping faults in the Molise area). In the Maiella
414
area, however, subsurface data precludes the occurrence of active normal faulting further east
415
of the ORL (i.e., toward the outer zones of the buried frontal Apennine chain and in the Apulian
416
foreland). Moreover, a few kilometers east of the Maiella mountain front, recent geological and
417
morphological data provide evidence for active contractional deformation with the occurrence
15
418
of a growing anticline (Pizzi et al., 2007). Therefore, we suggest that in the study area the
419
NNE-SSW striking ORL presently acts not only as a “persistent structural barrier” hindering the
420
southward growth of the MFS but also represents a major boundary of two distinct tectonic
421
regimes: an area experiencing active extensional faulting located to the west of the ORL and
422
an area characterized by contractional and strike-slip deformation to the east.
423
5. ESE-striking pre-existing structures
424
In the northern-central Apennines, major E(SE)-striking cross-structures commonly
425
correspond to both extensional pre-orogenic faults and Neogene contractional (mostly sinistral
426
transpressive) faults. Field structural mapping showed that Quaternary reactivation of these
427
pre-existing structures, with transtensive (dextral oblique) and/or dip-slip kinematics, is more
428
frequent than on the NNE-striking ones, since they are more favorably oriented with respect to
429
the principal direction of active extension (i.e., α < 45°-50°, see Fig. 4c) (Calamita and Pizzi,
430
1994; Galadini, 1999; Piccardi et al., 1999). However, similar to the NNE striking pre-existing
431
cross-structures, their seismogenic behavior seems to be determined by their size.
432
Pre-existing cross-structures striking between ESE-WNW and E-W with lengths ranging from
433
hundreds of meters to a few kilometers represent limited heterogeneities that can be cut or
434
reactivated during Quaternary normal faulting, producing local bending along propagating NW-
435
SE striking fault segments.
436
Based on the analysis of structural features, the distribution of the aftershocks, and the focal
437
mechanisms related to the Ms 5.8 Sangro Valley earthquake of May 1984, Pace et al. (2002)
438
suggested a twofold role for the W(NW)-E(SE) striking pre-existing Mt. Greco fault (“GF” in Fig.
439
2). This ca. 8 km long inherited Neogene sinistral transpressive structure behaved as a
440
conservative barrier, halting the propagation of the 1984 earthquake rupturing along the NNW–
441
SSE Barrea normal fault (“BF” in Fig. 2). In contrast, the long-term geological and structural
442
data indicate that it has also acted as a reactivated dextral normal oblique transfer fault (Pace
443
et al., 2002).
444
Regional ESE-striking pre-existing Mesozoic extensional structures at least 40 km long,
445
probably controlled the location of the ca. 30 km long northern oblique thrust front of the Gran
16
446
Sasso Massif. In the hanging-wall of this Neogene crustal thrust (Finetti et al., 2005), the
447
Assergi “AFS” and Campo Imperatore “CIFS” active fault systems consist of reactivated ESE
448
pre-existing contractional and extensional en-echelon fault segments up to 15 km long (Fig. 2)
449
(e.g., Ghisetti and Vezzani, 1986; Carraro and Giardino, 1992; D’Agostino et al., 1998;
450
Calamita et al., 2000b; Galli et al., 2002). Indeed, the trends of these extensional structures
451
represent the major anomaly with respect to the NW-SE trend of the Quaternary Apennine
452
extensional belt. North of the Gran Sasso thrust, in the footwall block, the active extensional
453
Laga faults system, LFS (Fig. 2) follows the regional NW-SE trend. Although the southern
454
termination of the LFS is very close to the Gran Sasso chain normal faults, geological and
455
geomorphological data suggest that a kinematic continuity between these structures is lacking.
456
Therefore, also in this case, the lack of continuity between two aligned and propagating
457
Quaternary fault systems correlates with the occurrence of a regional Mesozoic cross-fault.
458
Hence, this more than 40 km long basement-involved structure can be considered as a
459
“persistent structural barrier” during post-orogenic Quaternary extension.
460
461
6. Size of pre-existing cross-structures vs. size of Quaternary fault segments
462
The structural cases described above suggest that the intersections between active normal
463
faults and pre-existing (from Mesozoic to Neogene) cross-structures acting as barriers to fault
464
propagation can play a major role in the long-term segmentation of the northern/central
465
Apennines extensional system. Although the attitude (strike and dip) of the oblique pre-existing
466
faults is certainly an important factor in determining segment boundaries, the size of the
467
inherited oblique structures seems to be more crucial.
468
As reported in the previous sections, the single seismogenic segments in the northern/central
469
Apennines are characterized by lengths ranging between a few km and 10-15 km, while the
470
fault systems can reach lengths of up to 30-35 km. Hence, based on the empirical relationships
471
between fault length and earthquake magnitude (e.g., Wells and Coppersmith, 1994), we can
472
roughly associate M 5.5-6.0 earthquakes with the activation of a single fault segment and M ~ 7
473
events to the activation of an entire fault system.
17
474
As a result, evaluating the ability of pre-existing cross-structure to stop a propagating fault is
475
crucial in fault segmentation analysis and, hence, in seismic hazard assessment. Since our
476
observations thus far suggest that Quaternary faults of a certain size require pre-existing cross-
477
structures of a certain size to arrest their propagation, we try to roughly correlate the size
478
relationships observed in this study. In particular, considering the length of the NNE- and ESE-
479
trending pre-existing cross-structures (PL) with respect to the mean length (i.e., 10 km) of the
480
extensional fault segment (FL), we note that (see Tab.1):
481
1) ( PL < FL) - pre-existing cross-structures with lengths ranging from hundreds of meters to a
482
few kilometers represent limited heterogeneities that can be cut or partially/entirely reactivated
483
during normal faulting, producing local bending along the propagating NW-SE fault segment;
484
2) (PL ≈ FL) - pre-existing cross-structures with lengths ranging from several kilometers to a
485
few tens of kilometers show a twofold behavior. They can act as segment barriers during the
486
rupture of a single fault segment or they can be reactivated as transfer zones inducing the
487
activation of two adjacent segments of the same fault system. The behavior of the faults
488
responsible for the 1984 and 1997 earthquakes (Pace et al., 2002; Chiaraluce et al., 2005)
489
probably results from this type of geometric and kinematic relationship;
490
3) (PL » FL) - regional basement/crustal oblique pre-existing cross-structures with lengths
491
ranging between several tens of kilometers and hundreds of kilometers (commonly NNE-
492
striking) may act as “persistent structural barriers” halting both fault segment and fault system
493
propagation, thus defining the maximum size of a fault system.
494
Apennines, the NNE-striking Valnerina Line, and particularly the Ancona-Anzio and the Ortona-
495
Roccamonfina tectonic crustal lineaments, represent the most important examples of these
496
structures.
In the northern-central
497
498
7. The structural barrier model for fault segmentation
499
Considering these kinds of structural relationships, it may be possible to define the relative
500
length of the single segments or the maximum length of the entire fault system. As for a single
501
segment within a fault system, its maximum length is probably controlled by the distance from
18
502
the “persistent structural barriers”. Fault segments whose “center” (i.e., usually the place where
503
the maximum displacement occurs) is more than 10-15 km from the main segment boundaries
504
generally represent the longer Apennine fault segments (e.g., the Norcia fault segment in the
505
NFFS, the Assergi fault segment in the AFS, and the Fucino fault segment within the Fucino
506
fault system (FFS); see Fig. 11).
507
In contrast, fault segments with their “center” less than 10 km from the principal cross-
508
structures are generally shorter and may display a more dispersed and complex pattern.
509
Branching at fault terminations could be a result of the deformation in areas where seismogenic
510
faults intersect a pre-existing structural barrier (e.g., the Sellano fault segment in the CFS, the
511
Mt. Alvagnano fault segment in the NFFS, the Aremogna and Cinquemiglia fault segments; see
512
Fig. 11).
513
We suggest, therefore, that fault segments which originate far (i.e., more than 10 km) from
514
the structural barriers can grow radially without early interference with these segment
515
boundaries, and therefore can become larger than the fault segments whose growth started
516
close to a “persistent structural barrier” (i.e., distance < 10 km).
517
Furthermore, considering the length of an entire fault system, we suggest that if “persistent
518
structural barriers” are able to control the termination of a fault segment, then the spacing
519
between two successive “persistent structural barriers” directly constrains the maximum length
520
of the fault system. As an example, the narrower spacing between the Valnerina Line and
521
Ancona-Anzio line structural barriers than that between the Ancona-Anzio line and the Ortona-
522
Roccamonfina Line may have constrained the shorter length of the NFFS with respect to the
523
Fucino fault system (FFS) (Fig. 11).
524
525
8. Discussion: nature and behavior of the persistent structural barriers
526
Our geological, structural, and geomorphological field evidence indicates that no (Late)
527
Quaternary “surface faulting” can be associated with the reactivation of the NNE striking
528
persistent structural barriers. Furthermore, we showed that the areas surrounding these
529
barriers are generally characterized by distributed secondary faulting (Fig. 12). These
19
530
observations, however, do not mean that the more unfaulted rock volume along these regional
531
structures experienced a smaller amount of Quaternary deformation, but rather implies that
532
extension may have been accommodated by different mechanisms. Secondary faulting, blind-
533
faulting, and/or creep processes associated with low-magnitude and diffuse seismicity could
534
accommodate the deficit of deformation along the barriers after a seismic event or during the
535
interseismic period (e.g., Sibson, 1989). As the growing Quaternary faults interact with the
536
structural barriers, a large amount of energy is probably dissipated along the pre-existing fault
537
zone so that the fault cannot overcome and displace the barriers. Therefore, the fault
538
propagation is delayed and slows and a series of small segments activates, which also may
539
represent pre-existing structures, sometimes with trends which are not ideal for the present
540
tectonic regime (Fig. 12). These possible mechanisms were corroborated by Chiaraluce et al.
541
(2005) and Alberti (2006) for the 1997 Colfiorito earthquakes.
542
We suggest, therefore, that persistent structural barriers are not able to generate
543
earthquakes of M > 5.6-6.0 (i.e., threshold value for extensional surface faulting earthquake;
544
e.g., Wells and Coppersmith, 1994; Pavlides and Caputo, 2004) under the present day stress
545
regime. The spatial distribution of the seismicity in the northern/central Apennines strongly
546
supports this hypothesis. It is characterized by more frequent low-to-moderate magnitude
547
events (M not exceeding 6.0) in these zones, while the main historical events are generally
548
located far from them (e.g. Working Group CPTI, 2004).
549
In other words, assuming that a Coulomb frictional rheology (i.e., that there are many pre-
550
existing faults, some of which will be in ideal orientations with respect to the present stress
551
field; e.g., Sibson, 1987; Ranalli, 2000) is reasonable for the multideformed brittle upper crust in
552
the Apennines, this can explain why largely misoriented (NNE striking) pre-existing structures
553
with respect to the present extensional stress field (i.e.,  > 45-50°, see Fig. 4c) are very
554
rarely–if ever–reactivated. However, the mechanisms by which long-lived (repeatedly
555
reactivated) basement/crustal structures can stop a propagating Quaternary extensional fault
556
are still unclear. These relationships between the behavior of pre-existing cross-structures with
557
respect to their sizes (length, and hence amount of displacement), however, strongly suggest
20
558
that the nature and thickness of the associated fault rocks could play a critical role.
559
Unfortunately, basement fault rocks are not exposed in the study area, so our observations
560
were limited to the surficial expressions of these faults, which are generally related to their last
561
reactivation during the Neogene deformation of the Apennine belt. Therefore, no direct
562
information was available about the critical parameters such as the dip of the faults at depth,
563
widths of the fault zone, nature of the fault rocks, and associated values of friction coefficient
564
and pore fluid pressure. Dedicated geological and geophysical studies are thus still necessary
565
in the key zones where faults of the extensional belt intersect pre-existing cross-structures in
566
order to develop more refined hypotheses on the relationship between segmentation and the
567
presence of persistent structural barriers. For example, a key aspect will be understanding if
568
the mechanical properties of the fault rock associated with the basement/crustal barriers can be
569
considered as “weak” or “strong”, i.e., which kind of structural behavior can be expected (e.g.,
570
Tavarnelli et al., 2001).
571
572
9. Conclusions
573
The multideformed axial zone of the Apennines provides a great opportunity to explore the
574
influence of pre-existing cross-structures (inherited from pre-Quaternary tectonic phases) on
575
the propagation and segmentation of Quaternary/active seismogenic extensional faults. In the
576
Umbria-Marche northern Apennines and the Abruzzi central Apennines, two principal trends
577
(NNE-SSW and ESE–WNW) characterize the orientation of the major pre-existing structures
578
oblique to the mean NW-SE trend of the Quaternary faults.
579
Our “long-term” geological-structural study, compared with the available seismological data,
580
showed that although the attitude (strike and dip) of pre-existing cross-faults is certainly an
581
important factor in determining a segment boundary, the size of the inherited oblique structures
582
seems to be more crucial.
583
- Pre-existing cross-structures with lengths ranging from several kilometers to a few tens of
584
kilometers show a twofold behavior. They can act as segment barriers during the rupture of
585
single fault segments, i.e., during seismic events with M≤ 5.5-6.0 (considering the Apennines
21
586
seismotectonic framework), or they can be reactivated as transfer faults inducing the activation
587
of two NW-SE adjacent segments that belong to the same fault system.
588
- Regional basement/crustal oblique pre-existing cross-structures, with lengths ranging from
589
several tens of kilometers to hundreds of kilometers (commonly NNE-striking) may act as
590
“persistent structural barriers”, halting both fault segment and fault system propagation – i.e.,
591
they are able to stop a rupture produced by a ca. M7 earthquake (corresponding to the
592
maximum earthquake magnitude recorded in the Apennines). Thus, in most cases, the location
593
and spacing of such “persistent structural barriers” can reasonably be used to determine the
594
terminations and sizes of active fault segments and fault systems and hence their expected
595
maximum magnitude.
596
The NNE-striking Ancona-Anzio, Valnerina, and Ortona-Roccamonfina tectonic lineaments,
597
although having been repeatedly reactivated since the Mesozoic, represent the most important
598
examples of these “persistent structural barriers”.
599
600
The reason for their present-day inactivity is due to their misorientation with respect to the
principal stress axes during Quaternary post-orogenic extension.
601
Field studies have indicated no evidence for large (Late) Quaternary surface faulting
602
associated with the reactivation of these structures, and the structural fault dataset clearly
603
shows that pre-existing cross-structures with misorientation angles greater than 45-50° (with
604
respect to the NW-SE ideally oriented normal faults) are not suitable to be reactivated.
605
This evidence, moreover, is strongly supported by the historical and instrumental seismicity,
606
which shows more frequent low-to-moderate magnitude events (M not exceeding 6.0)
607
distributed along these NNE striking persistent structural barriers, while the main historical
608
events are generally located far from them (e.g., Working Group CPTI, 2004).
609
The mechanisms by which pre-existing cross-structures can arrest a propagating normal
610
fault, however, are still not clear. The evidence that pre-existing cross-structures have different
611
behaviors as a function of their size (and hence displacement) strongly suggests that the
612
nature and thickness of the associated fault rocks could play a critical role.
22
613
Dedicated geological and geophysical studies are thus still necessary in the key zones where
614
faults of the Apennines extensional belt intersect pre-existing cross-structures. The importance
615
in recognizing and classifying the different structural barriers is evident, considering the impact
616
that the geometry and the length of the active fault systems have in the seismic zonation and
617
evaluation of the seismogenic potential. Indeed, the correct location of barriers in such
618
applications will constrain the maximum expected magnitude based on the length of the
619
surficial fault expression.
620
621
622
623
624
625
626
627
REFERENCES
Aki, K., 1979. Characterization of barriers on an earthquake fault. Journal of Geophysical Research
84, 6140–6148.
Aki, K., 1984. Asperities, barriers, characteristic earthquakes, and strong motion prediction. Journal
of Geophysical Research 89, 5867–5872.
628
Alberti, M., 2000. Along-strike variations of thrusts in the Spoleto-Valnerina area: genesis and
629
influence on the evolution of the Umbria-Marche Apennines. Bollettino della Società Geologica
630
Italiana 119, 655-666.
631
Alberti, M., 2006. Spatial variations in the similarity of earthquake populations: The case of the
632
1997 Colfiorito–Sellano (northern Apennines, Italy) seismic sequence. Tectonophysics 421,
633
231–250.
634
Alberti, M., Decandia, F.A., Tavarnelli, E., 1996. Modes of propagation of the compressional
635
deformation in the Umbria-Marche Apennines. Memorie della Società Geologica Italiana 51,
636
71-82.
637
Amato, A., Azzara, R., Chiarabba, C., Cimini, G.B., Cocco, M., Di Bona, M., Margheriti, L., Mazza,
638
S., Mele, F., Selvaggi, G., Basili, A., Boschi, E., Courboulex, F., Deschamps, A., Gaffet, S.,
639
Bittarelli, G., Chiaraluce, L., Piccinini, D., Ripepe, M., 1998. The 1997 Umbria–Marche, Italy,
23
640
earthquake sequence: a first look at the main shocks and aftershocks. Geophysical Research
641
Letters 25, 2861–2864.
642
643
Barchi, M.R., Mirabella, F., 2008. The 1997–98 Umbria–Marche earthquake sequence:
“Geological” vs. “seismological” faults. Tectonophysics, doi:10.1016/j.tecto.2008.09.013.
644
Bigi, G., Cosentino, D., Parotto, M., Sartori, R., Scandone, P., 1992. Structural Model of Italy scale
645
1: 500.000. Progetto Finalizzato Geodinamica. CNR-GNDT. Roma.Blumetti, A.M., 1995.
646
Neotectonic investigation and evidence of paleoseismicity in the epicentral area of the
647
January–February 1703, central Italy, earthquakes. In: Serva, L., Slemmons, B., (Eds.),
648
Perspectives in Paleoseismology. Special Publication-Association of Engineering Geologists 6,
649
pp. 83– 100.
650
Boccaletti, M., Ciaranfi, N., Cosentino, D., Deiana, G., Gelati, R. Lentini, F. Massari, F. Moratti, G.,
651
Pescatore,
652
paleogeographic reconstruction of the peri-Tyrrhenian area during the Neogene. In: Boccaletti,
653
M., Moratti, G. (Eds.), Neogene paleogeography of the perityrrhenian area. Palaeogeography,
654
Palaeoclimatology, Palaeoecology, 77, pp. 41–50.
T.,
Ricci
Lucchi,
F.,
Tortorici,
G.,
1990.
Palinspastic
restoration
and
655
Boncio, P., Pace, B., Lavecchia, G., 2004. Defining a model of 3D seismogenic sources for
656
Seismic Hazard Assessment applications: The case of central Apennines (Italy). Journal of
657
Seismology 8, 407–425.
658
659
660
661
662
663
Brozzetti, F., Lavecchia, G., 1994. Seismicity and related extensional stress field; the case of the
Norcia seismic zone (central Italy). Annales Tectonicae 8 , 36– 57.
Butler, R., Tavarnelli, E., Grasso, M., 2006. Tectonic inversion and structural inheritance in
mountain belts. Journal of Structural Geology 28, 1893-1908.
Calamita, F., Pierantoni, P.P., 1993. Il sovrascorrimento di M.Cavallo-M.Primo (Appennino UmbroMarchigiano). Bollettino della Società Geologica Italiana 112 (3-4), 825-835.
664
Calamita, F., Pizzi, A., 1994. Recent and active extensional tectonics in the southern umbro-
665
marchean Apennines (central Italy). Memorie della Società Geologica Italiana 48, 541–548.
24
666
Calamita, F., Pizzi, A., Roscioni, M., 1992. I "fasci" di faglie recenti ed attive di M. Vettore-M. Bove
667
e di M. Castello-M. Cardosa (Appennino umbro-marchigiano). Studi Geologici Camerti, spec.
668
vol. 92/1, 81-95.
669
Calamita, F., Coltorti, M., Deiana, G., Dramis, F., Pambianchi, G., 1982. Neotectonic evolution and
670
geomorphology of the Cascia and Norcia depression (Umbria–Marche Apennine). Geografia
671
Fisica e Dinamica Quaternaria 5, 263– 276.
672
Calamita, F., Coltorti, M., Farabollini, P., Pizzi, A., 1994. Le faglie normali quaternarie nella dorsale
673
appenninica umbro-marchigiana: proposta di un modello di tettonica di inversione. Studi
674
Geologici Camerti, spec. vol. CROP18, 211-225.
675
Calamita, F., Coltorti, M., Piccinini, D., Pierantoni, P.P., Pizzi, A., Ripepe, M., Scisciani, V., Turco,
676
E., 2000a. Quaternary faults and seismicity in the Umbro-Marchean Apennines (central Italy).
677
Journal of Geodynamics 29, 245–264.
678
Calamita, F., Pizzi, A., Scisciani, V., De Girolamo C., Coltorti, M., Pieruccini, P., Turco, E., 2000b.
679
Caratterizzazione delle faglie quaternarie nella dorsale appenninica umbro-marchigiana-
680
abruzzese. In: Galadini, F., Meletti, C., Rebez, A. (Eds.), Le Ricerche del GNDT Nel Campo
681
Della Pericolosità Sismica (1996–1999). CNR-Gruppo Nazionale per la Difesa dai Terremoti,
682
Roma, pp.157–169.
683
684
685
686
687
688
689
690
Carmignani, L., Kligfield, R., 1990. Crustal extension in the northern Apennines: the transition from
compression to extension in the Alpi Apuane core complex. Tectonics 9, 1275–1303.
Carraro, F., Giardino, M., 1992. Geological evidence of recent fault evolution. Examples from
Campo Imperatore (L'Aquila-central Apennines). Il Quaternario 5, 181-200.
Casnedi, R., Crescenti, U., D'amato, C., Moscardini, F., Rossi, U., 1981. Il Plio-Pleistocene del
sottosuolo molisano. Geologica Romana 20, 1-42.
Castellarin, A., Colacicchi, R., Praturlon, A., 1978. Fasi distensive, trascorrenze e sovrascorrimenti
lungo la linea Ancona-Anzio dal Lias al Pliocene. Geologica Romana 17, 161–189.
691
Cattaneo, M., Augliera, P., De Luca, G., Gorini, A., Govoni, A., Marcucci, S., Michelini, A.,
692
Monachesi, G., Spallarossa, D., Troiani, L., XGUMS, 2000. The 1997 Umbria–Marche (Italy)
25
693
earthquake sequence: analysis of the data recorded by the local and temporary networks.
694
Journal of Seismology 4, 401–414.
695
696
Cello, G., Mazzoli, S., Tondi, E., 1998. The crustal fault structure responsible for the 1703
earthquake sequence of central Italy. Journal of Geodynamics 26, 443–460.
697
Cello, G., Mazzoli, S., Tondi, E., Turco, E., 1997. Active tectonics in the central Apennines and
698
possible implications for seismic hazard analysis in peninsular Italy. Tectonophysics 272, 43–
699
68.
700
Centamore, E., Chiocchini, M., Deiana, G., Micarelli, A., Pieruccini, U., 1971. Contributo alla
701
conoscenza del Giurassico dell'Appenino umbro-marchigiano. Studi Geologici Camerti 1, 70-
702
90.
703
Chiaraluce, L., Ellsworth, W.L., Chiarabba, C., Cocco, M., 2003. Imaging the complexity of an
704
active normal fault system: the 1997 Colfiorito (central Italy) case study. Journal of Geophysical
705
Research 108 (B6), 2294. doi:10.1029/2002JB002166.
706
Chiaraluce, L., Barchi, M., Collettini, C., Mirabella, F., Pucci, S., 2005. Connecting seismically
707
active normal faults with Quaternary geological structures in a complex extensional
708
environment: the Colfiorito 1997 case history (northern Apennines, Italy). Tectonics 24,
709
TC1002. doi:10.1029/2004TC001627.
710
Cinque, A., Patacca, E., Scandone, P., Tozzi, M., 1993. Quaternary kinematic evolution of the
711
Southern Apennines. Relationships between surface geological features and deep lithospheric
712
structures. Annali di Geofisica 36, 249–260.
713
Collettini, C., Chiaraluce, L., Pucci, S., Barchi, M.R., Cocco, M., 2005. Looking at fault reactivation
714
matching structural geology and seismological data. Journal of Structural Geology 27, 937–
715
942.
716
717
Coltorti, M., Farabollini, P., 1995. Quaternary evolution of the Castelluccio di Norcia basin (UmbroMarchean Apennines, Italy). Il Quaternario 8, 149–166.
718
Crone, A., Haller, K.M., 1991. Segmentation and the coseismic behaviour of Basin and Range
719
normal faults: examples from east-central Idaho and southwestern Montana, U.S.A. Journal of
720
Structural Geology 13, 151–164.
26
721
D’Agostino, N., Chamot-Rooke, N., Funiciello, R., Jolivet, L., Speranza, F., 1998. The role of
722
preexisting thrust faults and topography on the styles of extension in the Gran Sasso range
723
(central Italy). Tectonophysics 292, 229–254.
724
Dalla Via, G., Crippa, B., Toraldo Serra, E. M., Giacomuzzi, G., Saladini, R., 2007. Exploitation of
725
high-density DInSAR data points of the Umbria-Marche (Italy) 1997 seismic sequence for fault
726
characteristics. Geophysical Research Letters 34, L17301, doi:10.1029/2007GL030718.
727
728
729
730
731
732
Das, S., Aki K., 1977. Fault plane with barriers: A versatile earthquake model. Journal of
Geophysical Research 82, 5658–5670.
Decandia, F.A., 1982. Geologia dei Monti di Spoleto (Prov. di Perugia). Bollettino della Società
Geologica Italiana 101, 291-315.
Demangeot, J., 1965. Geomorphologie des Abruzzes Adriatiques. Centre Recherche et
Documentation Cartographiques, Memoires et Documents, Paris, 403 pp.
733
dePolo, C.M., Clark, D.G., Slemmons, D.B., Ramallie, A., 1991. Historical Basin and Range
734
Province surface faulting and fault segmentation. Journal of Structural Geology 13, 123–136.
735
Di Bucci, D., Tozzi, M., 1991. La linea Ortona-Roccamonfina: Revisione dei dati esistenti e nuovi
736
contributi per il settore settentrionale (media Valle del Sangro). Studi Geologici Camerti,
737
1991/2, 397–406.
738
739
Di Bucci, D., Corrado, S., Naso, G., 2002. Active faults at the boundary between Central and
Southern Apennines (Isernia, Italy). Tectonophisics 359, 47-63.
740
Ekström, G., Morelli, A., Boschi, E., Dziewonski, A.M., 1998. Moment tensor analysis of the central
741
Italy earthquake sequence of September–October 1997. Geophysical Research Letters 25,
742
1971–1974.
743
Elter, P., Giglia, G., Tongiorgi, M., Trevisan, L., 1975. Tensional and compressional areas in recent
744
(Tortonian to Present) evolution of north Apennines. Bollettino di Geofisica Teorica ed
745
Applicata 17, 3–18.
746
Finetti, I.R., Calamita, F., Crescenti, U., Del Ben, A., Forlin, E., Pipan, M., Rusciadelli G., Scisciani,
747
V., 2005. Crustal Geological Section across Central Italy from the Corsica Basin to the Adriatic
27
748
Sea based on Geological and CROP Seismic Data. In: Finetti, I.R. (Ed.), CROP Project: Deep
749
Seismic Exploration of the Central Mediterranean and Italy Elsevier, Chapter 9.
750
751
752
753
754
755
Galadini, F., 1999. Pleistocene change in the central Apennine fault kinematics, a key to decipher
active tectonics in central Italy. Tectonics 18, 877-894.
Galadini, F., Galli, P., 2000. Active tectonics in the Central Apennines (Italy)—Input data for
Seismic Hazard Assessment. Natural Hazards 22, 225–270.
Galadini, F., Galli, P., 2003. Paleoseismology of silent faults in the central Apennines (Italy): the
Mt. Vettore and Laga Mts. Faults. Annals of Geophysics 46, 815–836.
756
Galadini, F., Galli, P., Leschiutta, I., Monachesi, G., Stucchi, M., 1999. Active tectonics and
757
seismicity in the area of the 1997 earthquake sequence in central Italy: a short review, Journal
758
of Seismology 2, 1–9.
759
760
Galli, P., Galadini, F., Calzoni, F., 2005. Surface faulting in Norcia (central Italy): a
“paleoseismological perspective”. Tectonophysics 403, 117-130.
761
Galli, P., Galadini, F., Moro, M., Giraudi, C., 2002. New paleoseismological data from the Gran
762
Sasso d'Italia area (central Apennines). Geophysical Research Letters 29(7), 1134,
763
doi:10.1029/2001GL013292.
764
765
Gasperini, P., Bernardini, F., Valensise, G., Boschi, E., 1999. Defining seismogenic sources from
historical earthquake felt reports. Bulletin of the Seismological Society of America 89, 94-110.
766
Ghisetti, F., Vezzani, L., 1983. Deformazioni pellicolari pioceniche e plioceniche nei domini
767
strutturali esterni dell’Appennino centro-meridionale (Maiella ed arco Morrone-Gran Sasso).
768
Memorie della Società Geologica Italiana 26, 563-577.
769
770
771
772
773
774
Ghisetti, F., Vezzani, L., 1986. Carta geologica del Gruppo M. Siella-M.Camicia-M. Prena-M.
Brancastello (Gran Sasso d'Italia, Abruzzo). S.EL.CA., Firenze, Scale 1: 15,000.
Ghisetti, F., Vezzani, L., 1997. Interfering paths of deformation and development of arcs in the foldand-thrust belt of the central Apennines(Italy). Tectonics, 16, 523-536.
Ghisetti, F., Vezzani, L., 1999. Depth and modes of Pliocene-Pleistocene crustal extension of the
Apennines (Italy). Terra Nova, 11, 67-72.
28
775
776
Ghisetti, F., Vezzani, L., Follador, U., 1993. Transpressioni destre nelle zone esterne
dell’Appennino Centrale. Geologica Romana 29, 73–95.
777
Hernandez, B., Cocco, M., Cotton, F., Stramondo, S., Scotti, O., Courboulex, F., Campillo, M.,
778
2004. Rupture history of the 1997 Umbria-Marche (Central Italy) main shocks from the
779
inversion of GPS, DInSAR and near field strong motion data. Annals of Geophysics 47, 1355-
780
1376.
781
Hunstad, I., Anzidei, M., Cocco, M., Baldi, P., Galvani, A., Pesci, A., 1999. Modelling the coseismic
782
displacement during the Umbria-Marche earthquake. Geophysical Journal International 139,
783
283-295.
784
King, G.C.P., 1986. Speculations on the geometry of the initiation and termination processes of
785
earthquake rupture and its relation to morphology and geological structure. Pure and Applied
786
Geophysics 124 (3), 567–585.
787
King, G.C.P., Yielding, G., 1983. The evolution of a thrust fault system: processes of rupture
788
initiation, propagation and termination in the 1980 El Asman (Algeria) earthquake. Geophysical
789
Journal of the Royal Astronomical Society 77, 915–933.
790
Knuepfer, P.L.K., 1989. Implications of the characteristics of end-points of historical surface fault
791
ruptures for the nature of fault segmentation. U.S. Geol. Surv. Open-File Rep. 89-315, 193–
792
228.
793
794
795
796
Koopman, A., 1983. Detachment tectonics in the central Apennines, Italy. Geologica Ultraiectina
30, 1–155.
Lavecchia, G., 1985. Il sovrascorrimento dei Monti Sibillini: analisi cinematica e strutturale.
Bollettino della Societa Geologica Italiana 104, 161–194.
797
Lavecchia, G., Brozzetti, F., Barchi, M., Menichetti, M., Keller, J.V.A., 1994. Seismotectonic zoning
798
in east-central Italy deduced from an analysis of the Neogene to present deformations and
799
related stress fields. Geological Society of America Bulletin 106 (9), 1107–1120.
800
801
Locardi, E., 1982. Individuazione di strutture sismogenetiche dall’esame dell’evoluzione vulcanotettonica dell’Appennino e del Tirreno. Memorie della Società Geologica Italiana 34, 569–596.
29
802
Marchegiani, L., Bertotti, G., Cello, G. Deiana, G. Mazzoli S., Tondi , E., 1999. Pre-orogenic
803
tectonics in the Umbria–Marche sector of the Afro–Adriatic continental margin. Tectonophysics
804
315 , 123–143.
805
McCalpin, J.P., 1996. Paleoseismology. Academic Press, San Diego, 588 pp.
806
Messina, P., Galadini, F., Galli, P., Sposato, A., 2002. Quaternary basin evolution and present
807
tectonic regime in the area of the 1997-98 Umbria-Marche seismic sequence (central Italy).
808
Geomorphology 42, 97-116.
809
Montanari, A., Chan, L.S., Alvarez, W., 1989. Synsedimentary tectonics in the Late Cretaceous-
810
Early Tertiary pelagic basin of the Northern Apennines. In: Controls on carbonate platform and
811
basin development. Soc. Econ. Paleont. Mineral. 44, pp. 379-399, Spec. Publ.
812
Morewood, N.C., Roberts, G.P., 2000. The geometry, kinematics and rates of deformation within
813
an en échelon normal fault segment boundary, central Italy. Journal of Structural Geology 22,
814
1027– 1047.
815
Moro, M., Saroli, M., Salvi, S., Stramondo S., Doumaz, F., 2007. The relationship between seismic
816
deformation and deep-seated gravitational movements during the 1997 Umbria-Marche
817
(Central Italy) earthquakes. Geomorphology 89, 297-307.
818
Oldow, J.S.D, D'Argenio, B., Ferranti, L., Pappone, G., Marsella, G., Sacchi, E., 1993. Large-scale
819
longitudinal extension in the Southern Apennines contractional belt, Italy. Geology 21, 1123–
820
1126.
821
Pace, B., Boncio, P., Lavecchia, G., 2002. The 1984 Abruzzo earthquake (Italy): an example of
822
seismogenic process controlled by interaction between differently-oriented sinkinematic faults.
823
Tectonophysics 350, 237–254.
824
Patacca, E., Scandone, P., 1989. Post-Tortonian mountain building in the Apennines; the role of
825
the passive sinking of a relic lithospheric slab, In: Boriani, A., Bonafede, M., Piccardo, G. B.,
826
Vai, G.B. (Eds.), The Lithosphere in Italy. Advances in Earth Science Research, Accademia
827
Nazionale dei Lincei, Rome, 79, pp. 46-65.
828
Patacca, E., Sartori, R., Scandone, P., 1990. Tyrrhenian basin and Apenninic arcs: kinematic
829
relations since late Tortonian times. Memorie della Società Geologica Italiana 45, 425–451.
30
830
Patacca, E., Scandone, P., Bellatalla, M., Perilli, N., Santini U., 1991. La zona di giunzione tra
831
l'arco appenninico settentrionale e l'arco appenninico meridionale nell'Abruzzo e nel Molise.
832
Studi Geologici Camerti, Vol. Spec. 1991/2, CROP 11, 417-441.
833
834
Pavlides, S. Caputo, R., 2004. Magnitude versus faults' surface parameters: quantitative
relationships from the Aegean Region, Tectonophysics, 380, 159-188.
835
Piccardi, L., Gaudemer, Y., Tapponier, P., Boccaletti, M., 1999. Active oblique extension in the
836
central Apennines (Italy): evidence from the Fucino region. Geophysical Journal International
837
139, 499– 530.
838
839
840
841
Pizzi, A., 1992. Faglie recenti ed attive e origine delle depressioni tettoniche. Esempi
dall'Appennino umbro-marchigiano. PhD Thesis, Università degli Studi della Calabria, 172 pp.
Pizzi, A., 2003. Plio-Quaternary uplift rates in the outer zone of the central Apennine fold-andthrust belt, Italy. Quaternary International 101-102C, 229-237.
842
Pizzi, A., Scisciani, V., 2000. Methods for determining the Pleistocene-Holocene component of
843
displacement on active faults reactivating pre-Quaternary structures: examples from the central
844
Apennines (Italy). Journal of Geodynamics 29, 445–457.
845
Pizzi, A., Pomposo, G., Scisciani, V., Galadini, F., 2007. Evidenze geologiche e morfologiche di
846
strutture in crescita nell’area periadriatica abruzzese colpita dal terremoto del 1881. In: abstract
847
volume, “Dieci anni dopo il terremoto dell'Umbria-Marche: stato delle conoscenze sulla
848
sismogenesi in Italia” Workshop, Camerino, 26-27 giugno 2007.
849
850
Ranalli, G., 2000. Rheology of the crust and its role in tectonic reactivation. Journal of
Geodynamics 30, 3–15.
851
Rusciadelli, G., 2005. The Maiella escarpment (Apulia platform, Italy): geology and modeling of an
852
Upper Cretaceous scalloped erosional platform margin. Bollettino della Società Geologica
853
Italiana124 (3), 661-673.
854
Salvi, S., Stramondo, S., Cocco, M., Tesauro, M., Hunstad, I., Anzidei, M., Briole, P., Baldi, P.,
855
Sansosti, E., Lanari, R., Doumaz, F., Pesci, A. Galvani, A., 2000. Modeling Coseismic
856
Displacements resulting from SAR Interferometry and GPS measurements during the 1997
857
Umbria-Marche seismic sequence. Journal of Seismology 4, 479-499.
31
858
Satolli, S., Speranza, F., Calamita, F., 2005. Paleomagnetism of the Gran Sasso range salient
859
(central Apennines, Italy): Pattern of orogenic rotations due to translation of a massive
860
carbonate indenter. Tectonics 24, 1-22.
861
Scandone, P., Stucchi, M., 2000. La zonazione sismogenetica ZS4 come strumento per la
862
valutazione della pericolosità sismica. In: Galadini, F., Meletti, C., Rebez, A. (Eds.), Le ricerche
863
del GNDT nel campo della pericolosità sismica (1996-1999). CNR-Gruppo Nazionale per la
864
Difesa dai Terremoti, Roma, pp. 3-14.
865
866
867
868
Scholz, C.H., 1990. The Mechanics of Earthquakes and Faulting. Cambridge Univ. Press,
Cambridge, 439 pp.
Schwartz, D.P., Sibson, R.H. (Eds.), 1989. Fault segmentation and controls of rupture initiation and
termination. United States Geological Survey Open-File Report, vol. 89-315, pp. 1–447.
869
Scisciani, V., Tavarnelli, E., Calamita, F., 2002. The interaction of extensional and contractional
870
deformations in the outer zones of the central Apennines, Italy. Journal of Structural Geology
871
24, 1647–1658.
872
873
874
875
Sibson, R.H., 1987. Effects of fault heterogeneity on rupture propagation. U.S. Geol. Surv. OpenFile Rep. 87-673, 362–273.
Sibson, R.H., 1989. Earthquake faulting as a structural process. Journal of Structural Geology 11,
1–14.
876
Sirovich, L., Pettenati, F., 2004. Source inversion of intensity patterns of earthquakes: A
877
destructive shock in 1936 in northeast Italy. Journal of Geophysical Research 109, B10309,
878
doi:1029/2003JB002919.
879
Tavarnelli, E., Decandia, F.A., Renda, P., Tramutoli, M., Guegen, E., Alberti, M., 2001. Repeated
880
reactivation in the Apennine-Maghrebide system, Italy: a possible example of fault-zone
881
weakening? In: Holdsworth, R.E:, Strachan, R.A., Magloughlin, J.F., Knipe, R.J., (Eds.), The
882
nature and Tectonic Significance of Fault Zone Weakening. Geological Society, London 186,
883
pp. 273-286, Special Publications.
884
Tavarnelli, E., Butler, R.W.H., Decandia, F.A., Calamita, F., Grasso, M., Alvarez, W., Renda, P.
885
2004. Implications of fault reactivation and structural inheritance in the Cenozoic tectonic
32
886
evolution of Italy. In: Crescenti, U., D'Offizi, S., Merlini, S., Sacchi, R. (Eds.), The Geology of
887
Italy. Societa Geologica Italiana, pp. 201-214, Special Volume.
888
889
Valensise, G., Pantosti, D., 2001. The investigation of potential earthquake sources in peninsular
Italy: a review. Journal of Seismology 5, 287– 306.
890
Wells, D.L., Coppersmith, K.J., 1994. New empirical relationships among magnitude, rupture
891
length, rupture width, rutpure area, and surface displacement. Bulletin of the Seismological
892
Society of America 84, 974–1002.
893
894
895
896
Wheeler, R.L., 1989. Persistent segment boundaries on Basin and Range normal faults. U.S. Geol.
Surv. Open-File Rep. 89-315, 432–444.
Working Group CPTI, 2004. Catalogo Parametrico dei Terremoti Italiani, vers. 2004 (CPT104), INGV
Bologna, http://emidius.mi.ingv.it/CPTI.
897
Zhang, P., Slemmons, D.B., Mao, F., 1991. Geometric pattern, rupture termination, and fault
898
segmentation of the Dixie Valley–Pleasant Valley active normal fault system, Nevada, USA.
899
Journal of Structural Geology 13, 165–176.
900
Zhang, P., Mao, F., Slemmons, D.B., 1999. Rupture terminations and size of segment boundaries
901
from historical earthquake ruptures in the Basin and Range Province. Tectonophysics 308, 37–
902
52.
903
904
CAPTIONS
905
Fig. 1 - Simplified geologic map of the Umbria-Marche northern Apennines and the Abruzzi
906
central Apennines. MCT, Mt. Cavallo thrust; OAST, Olevano-Antrodoco-Sibillini Mts. thrust;
907
SVTZ, Sangro-Volturno thrust zone modified from Bigi et al. (1992). Extensional faults (see Fig.
908
2) are not shown for clarity.
909
Fig. 2 - Simplified structural map of the axial zone of the Umbria-Marche northern Apennines
910
and the Abruzzi central Apennines showing the geometric relationships between the
911
Quaternary/active extensional faults and the main NNE dextral (and ESE sinistral) transpressive
912
thrust
913
basement/crustal tectonic lineaments with long histories (e.g., Valnerina, Ancona-Anzio, and
ramps.
The
latter
structures
represent
the
Neogene
surface
expression
of
33
914
Ortona-Roccamonfina lines; see text for explanation). Main Quaternary extensional faults: CFS,
915
Colfiorito fault system; VBFS, Mt. Vettore–Mt. Bove fault system; NFFS, Norcia–Mt. Fema fault
916
system; LFS, Laga fault system; AFS, Assergi fault system; CIFS, Campo Imperatore fault
917
system; GF, Mt. Greco fault; BF, Barrea fault. Main Neogene contractional structures: MCT, Mt.
918
Cavallo thrust; OAST, Olevano-Antrodoco-Sibillini Mts. thrust; SVTZ, Sangro-Volturno thrust
919
zone; grey star indicates the epicenter of the 1984 Sangro Valley earthquake (Ms 5.8).
920
Fig. 3 – Rose diagrams showing the frequency distribution of measured Quaternary fault
921
strikes. The large data set indicates a ca. NW-SE mean strike of the fault planes, in agreement
922
with the main axis of Quaternary extension oriented ca. NE-SW. However, faults misoriented up
923
to 45-50° with respect to the mean trend are still statistically represented. The relatively high
924
frequency of ESE-striking faults in the Abruzzi central Apennines (right) is probably controlled by
925
reactivation of locally frequent pre-existing (Mesozoic-Cenozoic) structures (see, Fig. 4) that are
926
abundant in this area (e.g., faults in the Gran Sasso area, Fig. 2). Misoriented faults in the
927
Umbria-Marche region (left) show a more symmetric pattern. It is noteworthy that NNE-striking
928
faults are very scarcely, if at all, represented.
929
Fig. 4 – Kinematic history of the three major structural trends—NNE, E(SE) and SE—that
930
control the present structural framework of the northern-central Apennines. a) From the Mesozoic
931
phases of rifting to the Miocene syn-orogenic foreland flexuring, kinematics that were mainly
932
normal were associated with these faults. b) During Apennines Neogene compression, the ca.
933
NE-SW oriented horizontal σ1 favoured the reactivation of the NNE and E(SE) faults with dextral
934
and sinistral strike-slip/transpressive kinematics, respectively. The frontal thrusts commonly
935
strike SE. c) During post-orogenic Quaternary extension, characterized by a (main) horizontal ca.
936
NE-trending 3, pre-existing SE-striking faults are in ideal orientations to be reactivated. Our fault
937
dataset (see Fig. 3), however, shows that faults misoriented up to ca. 50° with respect to the NW-
938
SE ideally oriented fault (shadow area), are likely to be reactivated. Therefore, reactivation of
939
E(SE) pre-existing structures (i.e.,  < 50°) is likely to occur (it is noteworthy that reactivated ESE
940
fault segments along the AFS and CIFS in the Gran Sasso area show lengths up to 15 km, see
34
941
Fig. 2). These faults commonly show normal to dextral-oblique kinematics, while largely
942
misoriented NNE-striking pre-existing structures ( > 50°) are very rarely reactivated.
943
Fig. 5 - Simplified structural map of the Colfiorito area (Umbria-Marche northern Apennines)
944
showing the “long term” geometric relationships between the CFS (Colfiorito fault system) and
945
the N(NE) striking Neogene dextral transpressive structures, and their relation with the “short
946
term” seismological data associated with the two main shocks that occurred on 26 September
947
1997 in the Colfiorito area. Fault data modified from Pizzi (1992), Calamita and Pizzi (1994),
948
Cello et al. (1997), Galadini et al. (1999), Calamita et al. (2000) and Chiaraluce et al. (2005). Map
949
location in Fig. 2.
950
Fig. 6 - Modes of possible kinematic interaction between a NNE striking pre-existing high-angle
951
cross-structure (e.g., Neogene dextral transpressive fault) and two approaching en-echelon
952
Quaternary fault segments. In the case of a right stepover of the two en-echelon segments (a
953
and c), the slip can be transferred smoothly (no volume change or new faults must be created)
954
and the NNE-striking inherited structure may locally be inverted as a sinistral oblique transfer
955
fault (conservative barrier). Conversely, when the two extensional segments show a left stepover
956
(b and d), the slip generates local contraction at the transfer zone (shadow area) and subsidiary
957
faulting is required to accommodate the volume decrease (“nonconservative” discontinuity).
958
Fig. 7– Simplified structural map of the Umbria-Marche northern Apennines showing the
959
geometric relationships between the southern terminations of the Quaternary extensional fault
960
systems (VBFS and NFFS) and the Neogene Olevano-Antrodoco-Sibillini Mts. dextral thrust
961
ramp (OAST). Fault data modified from Pizzi (1992), Calamita and Pizzi (1994), Lavecchia et al.
962
(1994), Cello et al. (1997) and Galadini and Galli (2000). Map location in Fig. 2.
963
Fig. 8 – View of the Mt. Vettore (Umbria-Marche northern Apennines) showing the geometric
964
relationships between the Quaternary extensional faults at the southern terminations of the Mt.
965
Vettore-Mt. Bove fault system (VBFS) and the Neogene Olevano-Antrodoco-Sibillini Mts. dextral
966
thrust ramp (OAST) (location in Fig. 7).
967
Fig. 9 - Fault and historical earthquakes in the area of the Mt. Cavallo thrust (MCT) and
968
Olevano-Antrodoco-Sibillini Mts. thrust (OAST). The main normal fault systems are reported:
35
969
CFS, Colfiorito fault system; NFFS, Norcia-Mt. Fema fault system; VBFS, Mt. Vettore-Mt. Bove
970
fault system. The damage distribution of Mw 5.7 earthquakes was derived from INGV-DBMI04 at
971
http://emidius.mi.ingv.it/DBMI04/. The intensity in the legend refers to the MCS scale. A) General
972
view of the area. Note i) the correspondence between the damage distribution and the normal
973
fault systems to which the earthquakes are associated, and ii) the location of the earthquake
974
damage and, therefore, of the causative sources (of which the reported fault systems represent
975
the surficial expressions) in the hanging-walls of the major thrusts; B) Detail of the Norcia area.
976
Note that the Mw 6 earthquakes can be associated with the activation of minor sources
977
corresponding to single segments of the NFFS; C) Detail of the Norcia area. Note that the
978
damage distribution of the Jan. 14, 1703 earthquake is consistent with the activation of the entire
979
NFFS. Map location in Fig. 2.
980
Fig. 10 – Simplified structural map of the Maiella-Mt. Porrara area showing the geometric
981
relationships between the Quaternary extensional fault systems at the southern boundary of the
982
Abruzzi central Apennines and the Pliocene Sangro-Volturno thrust zone (SVTZ). Fault data
983
modified from Galadini and Galli (2000). Map location in Fig. 2.
984
Fig. 11 – Proposed model to explain the different size of fault segments and fault systems on
985
the basis of i) their location with respect to the pre-existing cross-structures acting as “persistent
986
structural barriers”; ii) the spacing of the “persistent structural barriers”. See text for explanation.
987
CFS: Colfiorito fault system; VBFS: Mt. Vettore–Mt. Bove fault system; NFFS: Norcia–Mt. Fema
988
fault system; LFS: Laga fault system; AFS: Assergi fault system; CIFS: Campo Imperatore fault
989
system; FFS: Fucino fault system; ACFS: Aremogna and Cinquemiglia fault segments; MCT: Mt.
990
Cavallo thrust; OAST: Olevano-Antrodoco-Sibillini Mts. thrust; SVTZ: Sangro-Volturno thrust
991
zone.
992
Fig. 12 – Schematic block diagram of the complex geometric pattern at the intersection of a
993
NW-SE Quaternary extensional fault system with a regional NNE-SSW pre-existing cross-
994
structure, as observed in the study area (not to scale). a) NNE-SSW striking inherited
995
basement/crustal structure reactivated during the Neogene as a dextral thrust-ramp and acting
996
as a “persistent structural barrier” to the propagation of the Quaternary extensional faults; b)
36
997
major NW-SE striking Quaternary extensional fault segment; c) secondary NW-SE striking fault
998
segments; d) secondary fault segment oriented nearly parallel to the trace of the thrust ramp; e)
999
secondary fault segment oriented at a high angle to the trace of the thrust ramp; f) secondary
1000
fault segment displacing a splay of the major thrust ramp; g) splay of the major thrust ramp; h)
1001
trace of the major Neogene thrust ramp; i) fault segments at the northwestern termination of a
1002
fault system located in the footwall of the major thrust ramp; l) trace of a NW-SE striking thrust
1003
displaced by Quaternary normal faults.
1004
1005
Tab.1 - Role played by pre-existing cross-structures during the propagation of Quaternary
1006
extensional faults. PL: length of the pre-existing cross-structure; FL: mean length of the
1007
Quaternary fault segment in the study area; : misorientation angle between the strike of the pre-
1008
existing cross-structure and the NW-SE ideally oriented Quaternary extensional fault.
1009
37