Geophys. J . Int. (1997) 129,368-388
Contemporaneous extension and compression in the Northern
Apennines from earthquake fault-plane solutions
A. Frepoli and A. Amato
Istitho Nazionale di Geofisicu, Via di Vigna Murata 605, 00143 Rome, Italy
Accepted 1997 January 8. Received 1997 January 2; in original form 1996 June I
SUMMARY
The Northern Apenninic arc belongs to the deformation zones surrounding the Adriatic
plate, which behaves as a rigid block. It is a NE-verging fold-and-thrust belt, which
developed since the Miocene with the migration of an extensional-compressional pair.
Previous seismological data are roughly in agreement with this deformation picture,
although the outer compressional front was not clearly defined by the few available
solutions. In this work we confirm the existence of two adjacent zones of contemporaneous extension and compression in the Northern Apennines, defining the extent of
these two zones and their interim1 deformation better than was previously done. This
is accomplished through the analysis of 125 new focal mechanisms of earthquakes
(2.6 < Md < 4.8) recorded by the national network of the lstituto Nazionale di Geofisica
in the period 1988-1995. The two deformation zones are clearly separated and lie very
close to each other with a partial overlapping in the Emilian-Tuscan Apennines. Strikeslip events are scattered in most of the study area. Thrust-faulting earthquakes are
located in the internal part of the most recently active thrust front (middle-upper
Pleistocene). There is a general trend of the compressional P-axes of thrust-fault and
strike-slip solutions to be perpendicular to the Apenninic direction in the external zone,
while the tensional axes of the normal-fault and strike-slip solutions in the belt do not
show a uniform orientation. However, clearly evident is the -ENE direction of
extension in the peri-Tyrrhenian region, consistent with the direction of Shmlnin the
Quaternary volcanoes of this region inferred from borehole breakouts and microearthquake fault-plane solutions. We determined the orientation of the stress tensor in
the two main deformation zones by inversion of the 125 fault-plane solutions determined
in this study. We found that the external belt is characterized by NE-SW compression
(olhorizontal, oriented N45"E),while a normal-faulting regime with o3 E-W oriented
is characteristic of the Umbria-Tuscany (back-arc) region.
-
Key words: focal mechanisms, Northern Apennines, seismicity, stress distribution.
INTRODUCTION
The growth of the Apenninic belt falls within the complex
structural environment of the central Mediterranean region,
dominated by the approximately NNW-SSE convergence
between the African and Eurasian plates at a velocity of less
than 1 cm yr-'. The opening of the Tyrrhenian Basin started
with the rifting in an E-W direction of the continental lithosphere of the Corsica-Tuscany block from the Burdigalian
(17 Ma), and then developed towards the SE with the migration
of the Calabrian Arc (Malinverno & Ryan 1986). The lithospheric thinning and the opening of a back-arc basin are associated
with the migration of the foredeep and the subduction of
ancient oceanic lithosphere attached to the continental Adriatic
plate, according to Reutter (1981), Royden, Patacca &
368
Scandone (1987), Amato et ul. (1993) and Royden (1993),
among many others. The slab retreat and the opening of the
Tyrrhenian Basin have driven the eastward migration of the
'extension-compression' system deforming the Meso-Cenozoic
sedimentary unit that forms the Apenninic thrust and fold belt
(Reutter 1981; Malinverno & Ryan 1986; Philip 1987; Royden
et al. 1987; Laubscher 1988).
For the Northern Apennines we can distinguish three main
geodynamic provinces (Bigi et al. 1990) (Fig. I): (1) a backarc basin, which includes the Tyrrhenian Sea and the periTyrrhenian region of the Italian peninsula (dominated by
extension since the upper Miocene); (2) a belt-foredeep region
along the Adriatic margin characterized by crustal shortening
and westward passive subduction of the Adriatic plate (Reutter
1981; Royden et ul. 1987; Selvaggi & Amato 1992; Amato et ul.
0 1997 RAS
Extension and compression in the Northern Apennines
5.---. contour lines of the isobaths
in km of the base of Pliocene
,, - - - _
369
-
Middle Pliocene-Pleistocenethrust front
Early Pliocene thrust front
~
--
vertical, strike-slip and transform
faults (undifferentiatedage)
normal faults (undifferentiated age)
Messinian thrust front
Serravallian to Tortonian (southern Alps) and
Torionian (Apennines) thrust front
Figure 1. Main tectonic features of the Northern Apennines and isobaths of the base of the Pliocene (modified from the 'Structural model of
Italy', Bigi et a / . 1990) and epicentral map of crustal earthquakes (location errors ERH and ERZ less than 8 km) that occurred between 1988 and
1995 (data source: ING bulletin).
1993); (3) a foreland region formed by the great sedimentary
basin of the Padana Plain and the Adriatic Sea, situated
over continental lithosphere. The crustal shortening of the
Apenninic belt occurred in paroxystic phases of 0.1-1 Myr
duration separated by longer periods of breaks (Vai 1987).
This tectonic evolution is also characterized by repeated frontal
retreat. In fact, the most recent (middle-upper Pleistocene)
thrust front is localized in a more internal position, in
agreement with the distribution of the seismic activity, as
discussed later.
A detailed knowledge of the seismicity distribution enables
us to gain a better understanding of the recent tectonic
evolution and the present-day state of stress of the Northern
Apennines. Analysing the seismic activity of the past 10 years
(Fig. l ) , we note that the maximum concentration of events
approximately coincides with the highest elevations. In this
part of the belt the seismogenic brittle crust has a thickness of
about 15-20 km, whereas in the peri-Tyrrhenian region,
characterized by a higher geothermal gradient (up to 60 mW
m-', Mongelli et al. 1989) and positive Bouguer gravity
anomalies, the seismogenic crust is less than 10 km thick.
The existence of subcrustal events as deep as -90 km, which
apparently dip at 40-45" towards the SW, provides evidence
for the location of subducted Adriatic lithosphere beneath the
Northern Apennines (Selvaggi & Amato 1992). In addition,
the tomographic images in the same portion of the Apenninic
region show the existence of a high-velocity body reaching
0 1997 RAS, G J I 129, 368-388
-300 km depth, interpreted as a remnant of former oceanic
or continental lithosphere (Amato et a/. 1993).
A basic tool for the inference of the present state of deformation of a region is the analysis of earthquake fault-plane
solutions. The availability of digital data from the seismic
network of the Istituto Nazionale di Geofisica (ING) since
1988 has allowed us to conduct a systematic investigation of
the seismicity of the Northern Apennines and to gain a more
detailed knowledge of the strain release in this region. In this
paper, we present the fault-plane solutions of 125 earthquakes
that occurred in the Northern Apennines between 1988
and 1995. These original data are compared with previously
published focal mechanisms and with the recent geological
evolution of the thrust belt.
PREVIOUS S T U D I E S
Many focal mechanisms of moderate events with magnitudes
ranging between 4.0 and 5.8 have been determined for the
Northern Apennines. The available fault-plane solutions of the
period 1939-1980, calculated from arrival times and polarities
read from seismic bulletins, are reported by Gasparini,
Iannaccone & Scarpa (1985). Fig. 2 displays the best focal
mechanisms of Gasparini et al. (1985) as selected by Zoback
(1992) for the world stress map (WSM) database (quality B
and C ) , together with three centroid moment tensor (CMT)
solutions of the Parma (1983), Norcia (1979) and Perugia
370
A. Frepoli and A . Amato
Figure 2. Fault-plane solutions from Gasparini et al. (1985) (compressional quadrants in black) selected by Zoback (1992) for the world stress
map (WSM) database (quality B and C). CMT solutions (compressional quadrants in light grey) (from N to S) of the Parma (1983), Perugia
(1984) and Norcia (1979) earthquakes. Fault-plane solutions of the four largest events of the Porto San Giorgio sequence of July 1987 (compressional
quadrants in dark grey) from Riguzzi et al. (1989).
(1984) earthquakes and the focal mechanisms of four events
of the Porto San Giorgio seismic sequence of July 1987
(Riguzzi, Tertulliani & Gasparini 1989). The two latter CMT
solutions show an evident normal-fault regime along the inner
portion of the Apenninic belt, while the 1983 Parma event, as
well as other mechanisms such as the strike-slip solution of
the Ancona (1972) event, suggest the existence of active compression along the outer part of the Apenninic belt (Gasparini
et al. 1985). The Ancona strike-slip solution shows a compressional axis ( P ) with an ENE-WSW orientation, while that
of the Parma event is aligned NNW-SSE. The first-motion
polarity solution for the latter event (Haessler et al. 1988) is
quite different from the CMT solution. The focal mechanism
is still a thrust solution but with an approximately N-S P-axis.
The two more recent seismic sequences of this region, Norcia
and Perugia, have been studied by Deschamps, Iannaccone &
Scarpa (1984) and Haessler et al. (1988), respectively. The
mb = 5.9 Norcia main-shock focal mechanism calculated by
Deschamps et al. (1984) is a normal-fault-plane solution with
a small strike-slip component. The CMT solution for this
event is slightly different, with an almost pure normal-faultplane solution. The tensional axis (T) for both the first-motion
polarities solution and the CMT solution trends ENE-WSW.
The first-motion-polarities focal mechanism of the mb = 5.2
Perugia earthquake shows a normal-fault-plane solution with
a small strike-slip component. The CMT solution in this case
is also slightly different, showing an almost pure normal
solution. The T-axis is oriented in a NNE-SSW direction for
both solutions.
The seismic sequence of Porto San Giorgio of July 1987,
approximately 50 km south of Ancona, was studied by Riguzzi
et al. (1989). This sequence had four events with magnitudes
ranging between 4.0 and 4.5, for which the authors determined
fault-plane solutions, although no indications about the
reliability of these solutions are reported. The four focal
mechanisms are almost pure thrust solutions with the maximum compression axis trending approximately E-W. This
orientation is in agreement with that observed for the Ancona
strike-slip solution.
DATA SELECTION
The 176 earthquakes analysed in this study are localized in
the Northern Apennines from 42" to 45"N and from 9" to
14"E (Fig. 3). Arrival times and polarities were accurately
re-picked from digital seismic waveforms recorded by the
seismic network of the ING (Fig. 4) in the period 1988-1995.
In addition, for events that occurred in the period May-August
1994, we used arrival times and polarities recorded by stations
of the temporary teleseismic transect of the Northern
Apennines (Fig. 4), carried out in the framework of EC project
GeoModAp (contract EV5V-CT94-0464).
The magnitudes Md of these events range between 2.6 and
4.8. The smaller events (Md between 2.6 and 3.0) are mostly
0 1997 RAS, G J I 129, 368-388
Extension and compression in the Northern Apennines
37 1
Figure 3. Epicentral distribution of the 125 events with focal mechanisms selected in this study. The events are grouped in different geographic
sectors as indicated by the dotted lines (see text for details).
7
9
11
13
15
17
19
47
45
43
41
Figure 4. Station distribution map of the Italian seismic network (only central and northern Italy are shown), and of the temporary teleseismic
transect of the Northern Apennines (May-August 1994).
0 1997 RAS, GJI 129, 368-388
372
A . Frepoli and A. Amato
located in the northern part of the studied region, where the
coverage of the seismic network is denser.
The 176 events were located with the computer program
HYPOINVERSE (Klein 1989) using the gradient velocity structure
reported in Table 1. We have obtained good values of rms
residuals for most analysed earthquakes. Fig. 5 shows the
distribution of rms residuals for the 125 selected events (see
below). Most locations have rms residuals below 0.40 s.
The focal solutions were calculated with the program FPFIT
(Reasenberg & Oppenheimer 1985). From the 176 focal solutions, we selected only those with quality Qf and Q e ual to
9
A and B (see Table 2). Qfreflects the solution prediction misfit
to the polarity data Fj (Fj=O.O represents a perfect fit to the
data, while Fj= 1.0 represents a perfect misfit). Q, summarizes
the three parameter uncertainties As, Ad and Ar (ranges of
perturbations to strike, dip and rake, respectively). 5 1 solutions
with one or both quality factors equal to C were rejected. The
remaining 125 fault-plane solutions (Fig. 6) have quality factors
AA, AB, BA and BB. Only 10 solutions were obtained with
less than eight polarities, but we accepted them because they
are well distributed on the focal sphere. The spacing between
Table 1. Gradient velocity model adopted for
locations and focal mechanism computation.
Top of layer (km)
Velocity (km s - l )
5.2
0.0
6.5
15.0
6.7
25.0
30.0
7.6
8.1
31.0
Figure 5. Distribution of rms residuals for the 125 selected
earthquakes.
Table 2. Fault-plane-solutionquality factors Qr
and Q,. Fj is the solution prediction misfit to
the polarity data. As, Ad and Ar are ranges of
perturbations to the strike, dip and rake,
respectively (see text).
A
B
C
Qf
QP
Fj50.025
0.025 < Fj5 0.1
Fj>O.l
As, Ad, Ar 5 20"
20" to 40"
> 40"
the stations of the seismic network, which is on average around
25-30 km, may introduce errors in location, especially for the
hypocentral depth. Different hypocentral locations for an event
could produce a change in the orientation of the two nodal
planes of the focal solution, although we have generally
observed that the orientations of the principal axes (what we
are mainly concerned with in this paper) do not change
dramatically. In fact, discarding the 51 earthquakes with one
or both quality factors equal to C eliminated the most unstable
solutions and thus guaranteed a more robust data set to work
on. Among the 125 selected earthquakes, only 15 per cent have
Q, = B; the remaining 85 per cent have Q,= A (Table 6), which
means the estimated uncertainties on strike, dip and rake are
less than 20".
Nonetheless, in order to test further the 'tectonic' significance
of the solutions, we selected 12 of the 125 events and analysed
them more carefully. Among these 12 selected events, the only
three earthquakes with hypocentral depths fixed at 5 km
(numbers 16, 23 and 83) are included. The focal solutions
obtained for each event fixing the hypocentral depth at the
deepest and at the shallowest values, considering the vertical
standard error ERZ (see Table 5), are shown in Fig. 7a. We
decided to test the focal mechanisms of the three events with
fixed hypocentral depths, shifting the hypocentre to 15 km and
then to 1 km (Fig. 7a). Multiple solutions have been found for
five earthquakes and are shown in Fig. 7a (Reasenberg &
Oppenheimer 1985). In general, we observe that the earthquakes for which we have a good azimuthal coverage and
many polarity data have more stable solutions (e.g. numbers
23,27,67 and 70 in Fig. 7a), whereas those with large azimuthal
gaps show the largest variations (numbers 54, 83 and 108).
Also these three examples, however, show that one of the two
principal axes (T-axes in all cases) does not change substantially
in trend and plunge (less than -20"). In some other cases
(numbers 5, 16, 84, 87 and 115) we observe that the shift
imposed on the hypocentre determination ( Z fERZ) changes
to some extent the polarity distribution on the focal sphere
(changing the take-off angles for some critical distances), and
consequently the orientation of one of the two nodal planes.
Also in these examples, the tectonic interpretation of such
earthquakes would not change much, at least as far as we base
our interpretation on the distributions of P- and T-axes.
The take-off angles from the source to the receivers depend
on the velocity model adopted for locating the earthquakes.
For this reason we also tested the influence of using different
parametrization of the medium on the stability of the faultplane solutions. We located some of the earthquakes with a
layered velocity model (the one routinely used at ING, see
Table 3) and compared the focal mechanisms obtained with
this model with those computed with the gradient velocity
model shown in Table 1. In general, although the polarity
distribution on the focal sphere changes to some degree (see
for instance the southern sector of the sphere for solution
number 71, and solution number 102 in Fig. 7b), the difference
between the two solutions is small. In some other cases, we
observe larger variations in the geometry of the nodal planes,
but a rather stable orientation of the principal axes (for
instance the P-axis of solution number 47).
FAULT-PLANE-SOLUTION CATEGORIES
To describe the distribution of different styles of deformation
within the studied region, we classify our fault-plane solutions
0 1997 RAS, GJI 129, 368-388
Extension and compression in the Northern Apennines
0 0@ @
373
0
1
a
3
2
4
a
Qa
QJ
@
@
@
Q
0
@
7
6 ++
++
11
9
14
16
aQ
@ @
Q Q
@@
25
26
27
@
@@
13
0
19
ttf +
31
12
17
15
+f+
o +
0
20
T++
21
33
32
22
J:
23
24
0
30
28
@ @ @
34
35
36
P O
37
Figure 6 . Schmidt lower-hemisphere projection of the 125 selected fault-plane solutions of this study. Compression and dilatation polarities are
indicated with crosses and circles, respectively.
according to the stress categories of Zoback (1992) for the
WSM (Table 4), which is based on different plunge ranges for
P- and T-axes. This classification is founded on the assumption
that the earthquake focal mechanisms reflect the state of stress
of the region, implying that the compression and tension axes,
to a first approximation, correspond to the principal stress
axes o1 and 03,respectively. It is worth recalling that for pure
normal- or reverse-faulting focal mechanisms the assumption
that the T- and P-axes correspond with o3 and crl is allowed,
particularly if we look at their horizontal distributions.
The 125 solutions are divided as follows: 27 belong to the
pure normal-fault category (NF), three to the normal-fault
with small strike-slip-component category (NS), 40 are
included in the strike-slip category (SS), nine belong to the
thrust-fault with small strike-slip-component category (TS),
and 20 to the pure thrust-fault category (TF). The remaining
26 focal solutions do not fall into any of these categories.
These solutions are included in the unknown-stress-field category (U). For these events the orientations of the P- and Taxes are not described in the next section. Plunges of the axes
0 1997 RAS, G J I 129, 368-388
of four solutions fall outside the ranges defined in Table 4 with
differences of only one or two degrees. We have decided to
assign these solutions to the appropriate category, in this case
two to the NF category and the other two to the TS one. The
categories assigned to these four solutions are given in brackets
in Table 6.
RESULTS
To simplify the characterization of the Northern Apennine
region in terms of areas with different focal-mechanism categories, we have assembled the T-axes of the solutions belonging
to the N F and NS categories in Fig. 8(a), and the P-axes for
the TS and T F categories in Fig. 8(b). T- and P-axes for SS
solutions are shown in Figs 8(c) and (d), respectively.
The most important result achieved with these new data is
the evident separation between an area characterized by normal
(and strike-slip) faulting and one characterized by reverse (and
strike-slip) faulting (see Figs 8a, 8b, 9a, 9b and 9c). Solutions
for N F and NS are distributed over a large portion of the
374
a 0E3 @
0
f
j
J
a
@
A. Frepoli and A . Amato
+
O
+&+
#
't
46
43
*
47
+
45
44
48
Q
0
+%++
52
49
51
53
54
0Q @ @ @ Q
@ 0@
0
@J- Q
Q 00Q
@ Q
@ 0@
56
57
55
o
58
60
59
+
61
63
66
62
00
68
69
67
s
73O
70
PO
71
++
72
78
75
77 O
74
@
83
a
84
Figure 6. (Continued.)
region under study (Fig. 9a), while the reverse solutions are
confined in a narrow belt running from the Emilian region to
the southern Marche (Figs 8b and 9b), which follows the
curvature of the Northern Apenninic arc. The SS solutions are
spread almost everywhere except in central and southern
Tuscany (Fig. 9b). Events with TF, TS and SS solutions,
indicating compression perpendicular to the arc (Figs 9b and
c), are in an inner position with respect to the most recently
active thrust front (middle-upper Pleistocene) (Vai 1987)
(Fig. 1). The solutions belonging to the U category are mostly
distributed in the northern sector (Fig. 9d).
We grouped focal mechanisms in different geographic sectors
(Fig. 3) following both the distribution of event clusters and
the distribution of fault-plane-solution categories. In the following description the solutions belonging to the U categories are
not considered.
Caorso-Cortemaggiore sector
In this sector four solutions were determined: three are T F
and one is SS. The hypocentral depths are in the upper 8 km
and the magnitudes range between 2.9 and 4.0. All the solutions
have P-axes oriented around a NE-SW direction (Figs 8b
and d).
Emilia sector
This sector also is characterized by T F and SS solutions (three
and two, respectively). The magnitudes of these events range
between 3.0 and 3.7 and the depths are between 8 and 17 km.
P-axes for these solutions have an approximately E-W direction (Figs 8b and d). Only one solution has a maximum
compression axis oriented NNE-SSW (Fig. 8b).
Romagna and Marche sector
In this sector, we do not include the numerous Cesena-Forli
focal mechanisms because they are clustered in a well-defined
area of the Romagna region. We describe this cluster of events
in the next section.
In the Romagna-Marche sector there is a predominance of
T F solutions (eight) over SS (three)and N F (two). The presence
0 1997 RAS, GJI 129, 368-388
Extension and compression in the Northern Apennines
B
87
86
@
Q @
91
0 0
92
CF';
T+
85
+ ++
93
375
88
89
90
@
@
@
94
95
96
0Q @ @ @
@ @ 652 @ @ @
@
98
99
+++
97
101
103
104
105
@o
0
100
106
102
107
10
@
@
Q
@
@ @ 69 Q
(Q
Q
110
111
109
115
116
@
121
122
++ T
+ + *
112
113
118
119
aQ
117
123
114
124
Figure 6. (Continued.)
of these two NF solutions and the nearness of the inner
extensional-regime zone allow us to suppose the existence of
a sudden change in the strain regime or a partial superimposition of the two regimes at a different depth. It is difficult to
demonstrate this last hypothesis because the present spacing
between the stations of the national seismic network does not
allow us to constrain the hypocentral depths for crustal events
well. In general, we can say that the hypocentral depths in this
sector are in the upper 20km, with magnitudes ranging
between 2.9 and 3.5. Generally, the P-axes of both the T F and
SS solutions are oriented around a NNE-SSW direction (Figs
8b and d). Only two T F solutions in the western part of this
sector have P-axes with an average direction around NW-SE
(Fig. 8b).
compute 40 fault-plane solutions, 32 per cent of the entire
data set.
Most of these events occurred in the period from October
15 to December 5, 1993. 17 and 15 events have SS and T F
solutions (Figs 10a and 1la), respectively, while only two have
N F solutions (Fig. 9a) and six belong to the unknown stress
category (Fig. 12). The depths of these events are concentrated
in the upper 15 km, with magnitudes between 2.6 and 3.7.
Generally, the T F events have a larger magnitude than the SS
events. The average magnitude for the T F is 3.3, while for the
SS it is 2.9. Taken together SS and T F solutions have P-axes
with an average orientation around the NE-SW direction
(Figs 10b and llb). The two N F solutions show a T-axis with
an ESE-WNW direction (Fig. 8a).
Cesena-Forli sector
North-western Tuscany and Frignano sector
In the analysed period the Cesena-Forli sector was affected by
a higher seismic activity in comparison with the other regions
of the area under study. For this sector it was possible to
This sector is mostly characterized by extension, with five N F
and nine SS solutions, and no T F earthquakes. Considering
both categories (SS and NF), the T-axes are approximately
0 1997 RAS, GJ1 129, 368-388
376
A . Frepoli and A. Amato
@
T +
z = 1.0
Z = 13.8
z = 3.3
54
Z = 15.0
z = 7.4
Z = 17.5
Z = 11.8
Z = 13.3
@Q
Z = 5.0
Z = 15.0
Q
Be
Z = 8.9
87
z = 17.7
(0 Q
Z = 11.6
a4
Caorso-Cortemagg.
z = 1.0
C.Metallifere
z = 1.0
@
67
83
z = 1.0
P
Z = 9.1
70
Southern Umbria
J:
Z = 5.0
27
Southern Umbria
z = 1.0
Z = 5.0
23
Northern Marche
0
@
@
0
@ '@
a
0 pjj
Z = 6.0
16
O
P+
Z = 9.6
108@
Z = 6.6
115J-f
z = 7.9
Z = 17.4
Emilia
0
Q0
z = 5.9
Cesena-Forli'
z = 9.9
Northern Abruzzi
a
z = 1.0
z = 1.0
Q
T+ +
@
Central Tuscany
Northern Umbria
T++
Z = 11.8
z = 7.4
@
Q
@
z = 1.0
Z = 16.2
Z = 13.2
Umbria
Q
+
Northern Tuscany
Z = 2.6
Figure 7. (a) Focal mechanisms tested for stability shifting the hypocentre considering the vertical standard error ERZ (Table 5). The solutions of
Fig. 6 are given in the left column; in the central and right columns the focal mechanisms (some with multiple solutions) calculated with a fixed
hypocentral depth are shown. (b) Focal mechanisms tested using two different velocity models: solutions calculated with the gradient velocity
model (Table 1j on the left; solutions calculated with the layered velocity model IPO (Table 3) on the right. For each event, numerical order, sector
name (see Table 5 ) and focal depth are indicated.
0 1997 RAS, G J I 129, 368-388
0
(b)
+
47
Z = 17.3
71
0
Extension and compression in the Northern Apennines
Colline Metallifere sector
Romagna
2 = 20.5
Cesena-Forli'
PO
377
++
This portion of central western Tuscany is characterized by
five N F solutions. There is no agreement among the orientation
of the T-axes: two are around a ENE-WSW orientation, two
are in a nearly NW-SE direction and one is in a nearly N-S
direction (Fig. 8a). The hypocentral depths are less than 13 km,
while magnitudes are between 2.9 and 3.8.
Z = 14.4
Romagna
102
Z = 12.9
Z = 7.8
125
@
@
Frignano
Z = 13.2
Z = 8.9
Figure 7. (Continued.)
Table3. Layered velocity model IPO, routinely used for bulletin locations at ING.
Velocity (km s-')
5.O
6.0
8.1
Top of layer (km)
0.0
10.0
30.0
Table 4. Fault-plane categories based on different
plunge ranges for P- and T-axes according to Zoback
(1992) for the WSM.
Plunges of axes
P
pl 2 52"
40"5 pl < 52"
pl <40"
PlI20"
PI< 20"
pl I 35"
FPS categories
T
pl535"
PI< 20"
PlI20"
pl <40"
40" I
pl< 52"
p l 2 52"
NF
NS
ss
ss
TS
TF
oriented in an E-W direction in the middle and lower Arno
valley, while in the Apuane Alps and along the TuscanyEmilia border region the orientation is around NE-SW (Figs
8a and c). In this northern part there are also two N F and
two SS solutions with T-axes oriented NW-SE. These two
SS solutions belong to the Eastern Frignano region, lie near
the external compressional Apenninic belt, and have P-axes
oriented consistently with the P-axis orientations found in this
sector (NE-SW). The magnitudes of these events range
between 2.8 and 4.1, while focal depths are in the first 20 km.
The hypocentral depths of events in the SW portion of this
sector are shallower, between 2 and 6 km.
Umbria-Marche Apenninic belt and the northern Abruzzi
sector
This sector also is characterized by extension. There are 11
and seven N F and SS solutions, respectively (no T F solutions).
The T-axes of these solutions do not show a preferential
orientation (Figs 8a and c). Only four N F and three SS
solutions show an orientation approximately perpendicular to
the trend of the Apennines. Magnitudes are between 3.0 and
4.3, while focal depths are concentrated in the first 20 km.
0 1997 RAS, G J I 129, 368-388
Peri-Tyrrhenian sector (Mt. Amiata, Vulsini and Colli
Albani)
In the Vulsini region there are two NF solutions and one SS.
For all these solutions the T-axis is oriented roughly
ENE-WSW (Figs 8a and c). The N F solution located close to
Mt. Amiata volcano has a NE-SW T-axis. In both regions
focal depths are shallow, between 2 and 6 km. Magnitudes are
between 3.0 and 3.7.
In this sector we included an earthquake that occurred in
Rome on June 12, 1995 (Md=3.8; 12 km depth). Its SS focal
mechanism is very similar to the solution calculated with the
S-wave polarization (Basili et al. 1997), showing a NE-trending
T-axis.
The N F solution of the Colli Albani earthquake of April 23,
1989 (Md=3.7; 4.0 km depth) shows a horizontal tension axis
N80"E (Amato et al. 1994). Both these SS and N F solutions
are in agreement with the general NE-SW extension direction deduced in this sector, and with the regional extensional
stress inferred from breakouts and microearthquake focal
mechanisms in the Quaternary volcano belt (Montone et al.
1995a).
STATE OF STRESS F R O M FOCALMECHANISM INVERSION
The orientations of fault planes and the directions of slip on
these planes observed from a population of focal mechanisms
can be used for the determination of the regional stress tensor.
We have applied the inverse technique of Gephart & Forsyth
(1984) for finding the stress field that is consistent with our
data set of focal mechanisms. The inversion is based on the
assumption that the deviatoric stress tensor is uniform over
the region under study. The inversion determines the four
model parameters (the three principal stress directions and the
)
measure of relative stress magnitude R = ~ J - c T ~ / c T ~ - c T ~that
minimize the misfit of a number of observations from the
predictions of the model. The misfit is given by the measure
of the smallest rotation about an axis of any orientation that
brings one of the two nodal planes with its slip direction and
sense of slip into an orientation that is compatible with the
stress model. These measures of rotation between observation
and model reflect either errors in the data or variations in the
stress field within the region under study. The four model
parameters that most closely match all the observations
are found for the two areas characterized by extension and
compression (Fig. 13).
The inversion result for the extension area gives a stresstensor orientation compatible with a normal-stress regime. The
minimum principal stress axis (c3)is horizontal and close to
the E-W direction, while the maximum principal stress axis
(ol)is vertical. In this inversion 54 focal mechanisms were
used. The relatively high average misfit value (13.8") is probably
378
A. Frepoli and A. Amato
Table 5 . Hypocentral parameters, magnitudes, location azimuthal gaps, location errors and geographic sector indications of the 125 events with
selected fault-plane solutions
Orig. time
Lat
Lon
Z
Md
ERH ERZ Sector
Date
No.
Gap rms
Emilia
6.2
174 0.25 2.1
07:35
44 28 09 56 3.1 3.2
880816
1
Colli Albani
1.5
41 46 12 40 4.0 3.7
200 0.09 1.6
23 :32
890423
2
2.0
Cesena-Forli
139 0.23 1.3
44 08 12 06 7.9 3.1
03 :03
891104
3
Cesena-Forli
44 08 12 06 5.9 3.3
139 0.23 1.4 2.0
03:14
891104
4
Northern Marche
7.8
164 0.42 3.8
43 37 12 27 6.0 3.3
11:30
900126
5
Northern Umbria
2.7
0.29 2.1
43 19 12 17 19.4 3.1
123
05:27
900224
6
Emilia
98 0.28 1.0 2.6
44 42 10 02 9.0 3.7
21 : 38
900411
7
Northern Umbria
2.3
0.36 1.2
103
43 38 12 07 1.6 3.7
22:33
8
900508
C. Metallifere
5.6
188 0.32 3.3
43 18 10 40 2.1 3.5
05 : 52
900624
9
Southern Umbria
5.7
0.45 1.8
42 45 12 45 10.0 3.6
119
02:59
900912
10
Emilia
108 0.28 1.4 3.3
19 : 58
44 56 10 54 17.0 3.1
901016
11
Roma gna
1.2
122 0.22 1.8
44 02 11 44 9.4 3.2
18:43
901124
12
Southern Umbria
124 0.25 1.0 2.1
42 36 12 55 17.5 3.0
20:14
910110
13
Romagna
0.9
173 0.01 2.2
17:55
43 50 12 02 9.6 3.0
910114
14
3.3
Umbria
140 0.40 2.9
42 58 12 51 5.9 3.0
23 :46
910114
15
Southern Umbria
136 0.31 1.0 42 41 12 58 5.0 3.1
03 : 29
910331
16
4.6
Northern Tuscany
103
0.37 1.2
02:32
43 41 10 58 2.0 3.3
910518
17
C. Metallifere
150 0.24 1.0 4.8
43 12 11 02 3.3 3.4
00:12
910530
18
Southern Umbria
3.7
170 0.49 2.6
42 54 12 51 1.6 3.6
15:48
910712
19
1.7 Cesena-Forli
127 0.31 0.9
44 07 12 09 11.2 3.2
02:25
911026
20
Cesena-Forli
2.3
146 0.19 0.8
44 07 12 08 7.8 3.0
14:48
911030
21
CaorseCortemag.
3.1
0.30 0.7
44 57 10 01 4.5 4.0
101
09:31
911031
22
Southern Umbria
151
0.33 1.6
21 :45
42 50 13 21 5.0 3.1
911112
23
3.4 Northern Umbria
1.5
151
0.22
4
3
53
17.7
3.0
20:00
12
02
911122
24
Frignano
4.8
44 22 10 31 5.6 3.6
157 0.34 3.9
12:25
911208
25
Northern Marche
1.6
0.34 3.7
225
22:32
43 38 13 04 9.3 3.4
911215
26
CaorseCortemag.
4.1
98 0.31 1.1
10:12
44 57 09 58 3.3 3.4
920101
27
CaorseCortemag.
3.2
12:30
44 58 09 58 1.3 3.2
90 0.27 0.8
920102
28
Vulsini
2.0
42 41 11 58 5.7 3.0
212
0.08 1.1
22:20
920207
29
4.1 Vulsini
0.10
1.5
42 47 12 03 3.4 3.2
203
23 : 37
920207
30
Vulsini
6.0
195 0.33 5.8
42 44 11 55 1.8 3.2
06: 50
920208
31
Romagna
2.3
0.21 1.3
159
44 14 11 56 6.7 3.1
17:10
920220
32
Cesena-Forli
3.3
44 02 12 12 0.7 3.2
204 0.47 2.0
14:56
920224
33
Romagna
1.2
100 0.27 0.9
44 25 12 01 14.1 3.5
21:23
920403
34
Northern Marche
2.4
200 0.30 3.6
43 39 12 40 3.5 3.1
08:19
920412
35
Frignano
1.0
101
0.13 1.1
44 19 10 50 23.1 3.8
11:59
920417
36
Cesena-Forli
1.6
157 0.15 0.6
44 06 12 10 1.0 3.1
19:23
920510
37
Roma gna
2.3
91
0.12 0.9
44 08 11 31 9.1 3.5
18:13
920527
38
Romagna
5.7
93
0.20 0.9
44 09 11 27 1.8 3.4
18:18
920527
39
Mt. Amiata
3.6
112 0.30 1.4
42 58 11 30 6.5 3.7
09 : 28
920612
40
Frignano
2.0
44 21 10 19 2.1 3.6
144 0.22 1.3
05: 38
920614
41
2.0
Frignano
105
0.30 1.4
44 24 10 18 5.0 3.4
06: 05
920614
42
Romagna
4.2
92 0.40 1.3
44 20 11 31 2.6 3.3
19:43
920702
43
Romagna
82 0.33 1.4 2.9
44 15 11 29 18.3 3.4
10:29
920816
44
Northern Abruzzi
1.1
148 0.13 0.9
42 26 13 21 11.0 3.9
02: 25
920825
45
Romagna
1.5
86
0.35 1.2
43 58 11 55 18.6 3.2
22:18
920829
46
Romagna
105 0.29 1.0 0.8
43 58 11 56 17.3 2.9
22:36
921023
47
Northern Tuscany
7.5
43 45 10 52 5.5 2.8
165 0.34 2.5
02:26
921028
48
0.6
Roma gna
102 0.25 0.9
11:19
44 01 11 55 19.8 3.5
921231
49
Umbria
3.9
109 0.34 1.9
10:51
43 34 12 13 12.4 4.0
930117
50
Cesena-Forli
2.3
192 0.23 1.6
44 05 12 05 9.0 2.9
22: 35
93031 1
51
Cesena-Forli
1.5
140 0.26 0.8
44 10 12 06 5.9 3.0
22: 38
930311
52
C. Metallifere
5.8
160 0.32 2.2
20:48
43 21 10 54 11.6 3.2
930320
53
C. Metallifere
8.4
199
0.30 3.4
43 20 10 54 9.1 3.0
04:59
930321
54
CMetallifere
8.2
0.30 3.5
43 20 10 53 12.5 2.9
201
08:13
930321
55
1.4 Northern Marche
129 0.15 1.2
22:55
43 36 12 36 28.0 3.2
930406
56
Caorso-Cortemag.
3.2
79
0.47 1.1
10:46
44 59 09 55 8.1 2.9
930513
57
Umbria
1.4
83
0.30 0.9
21 : 36
43 06 12 42 8.1 3.7
930604
58
Umbria
1.2
85
0.24 1.2
43 07 12 44 7.2 4.3
19:16
930605
59
5.0 C. Metallifere
43 12 10 46 10.0 3.8
184 0.45 4.5
07:51
930806
60
Southern Umbria
7.0
195 0.13 1.1
42 57 13 17 12.9 3.4
09:12
930806
61
Cesena-Forli
1.6
151
0.28 0.9
44 01 12 10 9.1 3.4
02:43
931015
62
Cesena-Forli
44 07 12 10 3.2 3.0
151
0.36 1.0 3.4
03:OO
931015
63
Cesena-Forli
1.3
0.17 1.0
44 03 12 09 9.7 2.8
23 : 52
189
931015
64
Cesena-Forli
1.2
44 12 12 17 10.3 3.2
110 0.33 1.5
01 : 58
931105
65
0 1997 RAS, GJI 129, 368-388
Extension and compression in the Northern Apennines
Table 5 . (Continued.)
No
Date
Orig. time
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
931105
931106
931107
931108
931109
931109
931109
931112
931113
931113
931113
931114
931128
931205
940202
940209
940309
940313
940325
940408
940413
940430
940516
940612
940616
940630
940705
940711
940711
940712
940719
940720
940721
940722
940722
940727
940730
941010
950211
950530
950530
950530
950530
950612
950614
950723
950823
950824
950825
950914
951010
951013
951120
951207
951214
951226
951227
951227
951228
951231
02:Ol
15:12
23:21
03: 59
13:46
13:52
19:08
01:42
09:23
12:49
16:11
08:43
10:41
15:10
10: 12
22:21
18:23
15:14
04:08
22:20
09:38
06:03
02:02
09:07
16:49
04:23
10:20
21 : 31
21 : 57
19 : 26
18:43
17:41
01:ll
04:08
10:25
20:27
03:16
17:59
04:22
03 : 36
05: 39
11:02
22:11
18:13
05:27
22:38
04:31
17:27
07: 09
14:32
06:54
19:03
02:46
20: 20
15 :26
06:15
19:59
23 :44
01 : 24
21:29
Lat
Lon
Z
Md
Gap
44 07
44 38
44 11
44 04
44 04
44 08
44 08
44 08
44 09
44 07
44 08
44 09
43 46
44 02
44 02
42 42
44 39
42 26
43 31
44 04
44 25
43 25
44 19
44 05
44 17
44 48
44 06
44 07
44 08
44 41
44 10
44 07
44 07
44 06
44 09
44 06
44 06
44 42
44 57
43 26
43 24
44 09
43 25
41 48
44 49
42 43
43 58
44 08
43 50
44 02
44 08
43 24
43 43
42 29
43 26
44 04
44 11
44 08
44 07
44 20
12 10
09 58
12 15
12 09
12 10
12 10
12 10
12 09
12 10
12 09
12 10
12 09
12 49
12 08
12 08
12 14
10 24
13 16
11 21
12 09
10 30
12 19
10 56
10 45
10 51
10 45
12 08
12 10
12 09
10 37
12 15
12 10
12 10
12 10
12 13
12 09
11 37
09 47
09 59
12 39
12 42
12 02
12 44
12 31
09 50
14 02
11 02
10 46
10 26
11 03
10 10
12 38
10 40
13 13
10 39
12 03
12 07
12 04
12 04
10 33
8.2
11.8
13.0
8.6
11.6
14.4
9.6
9.2
15.0
4.5
9.6
13.2
26.0
6.3
8.0
2.7
12.3
5.0
8.9
5.7
18.1
9.6
14.2
11.1
12.7
12.7
1.0
2.5
7.6
14.1
8.5
3.2
2.6
2.1
5.7
5.0
7.8
9.8
7.6
2.0
4.0
7.5
6.6
12.6
8.5
19.2
5.8
9.6
4.5
7.9
2.0
24.9
2.1
7.8
3.4
3.0
3.7
3.2
3.6
3.2
3.1
2.8
2.7
2.6
3.1
2.9
3.3
3.2
3.6
3.5
3.4
3.5
3.3
2.6
2.9
3.1
3.2
3.0
2.9
2.7
3.3
2.7
2.8
2.9
3.0
2.8
2.9
3.1
3.2
3.2
3.5
3.1
3.1
3.8
3.8
3.0
3.2
3.8
3.2
3.7
3.7
4.1
3.5
3.1
4.8
3.2
3.2
3.3
3.3
2.9
3.0
3.5
3.1
4.0
128
149
109
193
159
98
117
142
118
150
117
117
205
184
184
209
108
135
150
191
118
144
87
69
69
134
150
151
114
160
105
151
151
151
106
140
60
134
83
113
106
114
133
188
117
190
99
75
182
108
167
107
156
141
250
186
117
107
118
92
1.0
11.3
12.5
7.4
4.5
8.9
due to the existence of local variations in the stress orientations
within the extension area, which is quite large (-200 x 300 km).
The stress regime in the compressional belt is characterized
by a maximum principal stress (al)oriented N45"E, and
0 1997 RAS, G J I 129, 368-388
rms
0.31
0.29
0.34
0.27
0.28
0.22
0.30
0.30
0.19
0.24
0.29
0.23
0.34
0.30
0.32
0.42
0.26
0.38
0.45
0.33
0.28
0.15
0.30
0.32
0.37
0.40
0.36
0.45
0.30
0.31
0.49
0.41
0.39
0.44
0.50
0.37
0.24
0.23
0.48
0.47
0.24
0.27
0.41
0.27
0.26
0.42
0.28
0.23
0.32
0.34
0.28
0.33
0.40
0.42
0.30
0.21
0.28
0.30
0.35
0.40
ERH
0.8
3.6
1.2
1.1
1.2
0.6
0.7
0.8
0.8
0.8
0.7
0.9
4.2
1.2
1.4
4.7
1.0
1.6
2.3
1.7
1.5
1.1
1.0
0.6
1.0
1.9
0.8
1.1
0.7
2.1
2.4
1.3
1.0
1.1
1.3
0.9
0.6
1.0
1.1
2.5
0.8
0.8
4.7
0.9
0.7
3.8
1.0
0.7
1.5
1.2
1.2
1.6
1.9
1.6
6.3
1.5
1.2
0.7
0.8
0.9
ERZ
1.3
5.9
1.2
1.7
1.7
1.2
1.2
2.0
2.6
1.7
1.5
1.9
1.4
2.3
2.2
8.2
2.5
~
8.5
2.7
1.8
2.2
1.1
1.3
1.2
5.1
1.2
2.9
1.5
3.1
2.0
2.7
2.2
2.3
1.9
1.8
1.1
1.8
3.6
8.3
2.6
2.1
9.6
1.1
1.9
3.7
3.9
1.3
2.4
5.3
1.9
2.1
3.6
3.5
5.2
2.1
1.0
1.3
1.5
0.9
319
Sector
Cesena-Forli
Emilia
Cesena-Forli
Cesena-Forli
Cesena-Forli
Cesena-Forli
Cesena-Forli
Cesena-Forli
Cesena-ForIi
Cesena-Forli
Cesena-Forli
Cesena-Forli
Northern Marche
Cesena-Forli
Cesena-Forli
Vulsini
Emilia
Northern Abruzzi
Central Tuscany
Cesena-Forli
Frignano
Northern Umbria
Frignano
Northern Tuscany
Frignano
Emilia
Cesena-Forli
Cesena-Forli
Cesena-Forli
Emilia
Cesena-Forli
Cesena-Forli
Cesena-Forli
Cesena-Forli
Cesena-Forli
Cesena-Forli
Romagna
Emilia
Caorso-Cortemag.
Umbria
Umbria
Cesena-Forli
Umbria
Rome
Emilia
Southern Marche
Northern Tuscany
Northern Tuscany
Northern Tuscany
Northern Tuscany
Northern Tuscany
Umbria
Northern Tuscany
Northern Abruzzi
C. Metallifere
Cesena-Forli
Cesena-Forli
Cesena-Forli
Cesena-Forli
Frignano
oblique a2 and c3 (Fig. 13). The data in this sector are more
consistent with this stress model. In fact, although the number
of focal mechanisms is higher (71 events) than in the extension
area, the average misfit is smaller (8.3"). For both inversions
380
A. Frepoli and A. Amato
Table 6. Strikes, dips and rakes of the first nodal planes dipping to the east going clockwise from the north, azimuths and plunges
axes, fault-plane-solution categories, number of polarities and quality factors for the 125 selected focal mechanisms.
No
Date
Orig. time
Str.
Dip
T-axes
Pol. num.
Rake
P-axes
FPS categ
azim
plun.
azim. plun
1
880816
07: 35
95
85
246
42
120
33
11
- 120
U
2
0
890423
23 : 32
255
45
20
348
82
82
- 100
NF
3
03 :03
891 104
170
90
20
129
9
20
30
- 30
ss
4
891104
23 5
85
9
10
279
12
03: 14
3
- 170
ss
5
900126
20
211
11:30
55
140
11
358
30
40
TF
900224
171
13
73
05 : 27
28
210
80
13
150
ss
,6
7
900411
5
70
300
21 : 38
16
71
35
337
- 150
ss
8
62
247
6
22 : 33
45
95
19
900508
NF
350
- 50
9
54
20
35
180
14
900624
05 : 52
27 1
48
- 160
U
10
55
8
10
45
02:59
155
22
- 140
NF
9009 12
263
11
166
30
55
19 : 58
40
14
901016
17
10
ss
266
12
18:43
901124
68
63
35
13
265
9
- 60
NF
190
13
56
337
26
25
120
20: 14
11
9101 10
NF
- 140
198
14
55
77
215
17:55
9101 14
9
69
T’F
7
100
207
15
25 1
35
120
23 :46
70
9
9101 14
5
30
ss
157
16
75
1
150
03 : 29
11
910331
45
315
NF
52
- 70
17
24
910518
02:32
2
155
11
75
288
196
20
ss
18
28
46
190
12
910530
40
00: 12
80
166
120
U
19
45
910712
44
15:48
68
3
245
9
- 60
NF
144
20
90
210
45
12
911026
180
0
02 :25
90
TF
29
21
21
14:48
165
80
9
911030
299
20
6
ss
206
22
41
270
70
19
911031
136
09:31
10
140
236
TS
23
34
55
10
8
911112
235
21 : 45
- 90
55
55
NF
24
44
65
90
20 :00
8
911122
74
44
235
- 100
U
25
38
150
20
12:25
13
911208
32
44
- 170
U
250
26
66
195
30
22 : 32
9
911215
258
TF
17
36
60
27
51
240
920101
10: 12
35
9
139
150
22
18
u (TS)
28
38
260
40
920102
142
12: 30
10
27
170
28
U
29
920207
22 : 20
13
90
35
248
68
- 60
25
15
NF
30
15
255
30
23 : 37
920207
82
73
6
NF
-100
28 1
31
920208
32
170
90
137
06: 50
32
7
- 50
22
U
32
19
145
75
920220
116
17: 10
4
44
9
- 50
NS
33
20
170
90
14: 56
920224
7
129
20
- 30
30
ss
34
34
345
80
21 : 23
920403
123
18
20
40
19
ss
35
35
100
35
920412
219
08: 19
10
35
340
0
U
36
21
50
920417
11 : 59
20
115
343
- 170
U
33
238
37
46
19:23
165
85
920510
324
36
13
70
182
U
38
14
58
18: 13
185
75
920527
18
100
TF
29
176
39
36
18: 18
210
85
920527
64
6
130
28
178
U
40
6
09 :28
185
65
920612
218
- 140
NS
45
314
17
41
18
105
35
920614
250
05 : 38
14
57
11
- 40
NF
42
20
145
25
920614
06 :05
317
69
124
9
NF
- 80
43
64
175
25
920702
120
19 :43
11
22
332
120
TF
44
64
20
9208 16
10:29
121
14
105
80
25
292
TF
45
2
140
920825
02 :25
292
50
15
60
26
- 50
NF
46
40
35
920829
22: 18
95
341
U
30
22 1
13
170
47
75
921023
90
224
128
9
22:36
31
12
-150
ss
48
80
130
0
921028
02 : 26
14
265
355
-10
ss
8
49
40
11:19
115
921231
346
27
16
232
-170
U
38
50
9301 17
40
165
252
5
8
10: 51
NF
-120
22
69
51
15
65
930311
105
57
22: 35
12
269
60
TF
31
52
30
235
93031 1
132
47
22: 38
5
14
29
160
U
53
75
55
930320
77
24
20:48
22
- 120
20 1
50
u “F)
54
50
120
930321
140
04: 59
232
0
13
67
- 120
NF
55
50
120
930321
134
08: 13
3
12
235
-110
NF
74
56
65
150
1
930406
22: 55
112
21
- 30
11
38
ss
57
70
170
930513
20
10:46
301
34
9
10
ss
7
58
55
120
930604
17
21 : 36
346
246
20
- 170
ss
30
59
930605
65
135
19: 16
356
4
- 160
264
14
31
ss
60
15
165
07: 51
930806
34
36
22
240
- 150
50
u “F)
61
09: 12
930806
70
170
136
10
14
41
36
- 40
NS
62
35
205
02 :43
931015
305
66
13
51
30
22
u tTS)
63
03 :00
931015
85
100
139
- 150
9
16
24
237
ss
64
50
105
23 : 52
931015
8
344
24 1
39
160
15
ss
of P- and TQr
QP
A
A
A
A
A
B
B
A
B
B
A
A
A
A
A
B
A
A
B
B
B
A
A
A
A
A
A
A
A
A
A
A
A
B
B
B
B
A
A
A
A
A
A
B
A
B
A
B
A
B
B
A
B
B
A
A
A
A
B
A
B
B
A
A
A
A
B
A
A
A
A
A
A
A
A
A
B
A
A
B
A
A
A
A
B
A
B
A
A
B
A
B
A
A
B
A
A
A
A
A
A
A
B
B
B
A
A
A
A
A
A
B
A
A
A
A
A
A
A
A
B
A
A
A
A
A
B
A
0 1997 RAS, GJI 129, 368-388
Extension and compression in the Northern Apennines
Table 6. (Continued.)
No.
Date
Orig. time
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
I15
116
117
118
119
120
121
122
123
124
125
931105
931 105
931106
93 1107
931108
931109
931109
931 109
931112
931113
931113
931113
931114
931 128
931205
940202
940209
940309
9403 13
940325
940408
940413
940430
9405 16
940612
940616
940630
940705
94071 1
94071 1
9407 12
9407 19
940720
940721
940722
940722
940727
940730
941010
950211
950530
950530
950530
950530
950612
950614
950723
950823
950824
950825
950914
951010
951013
951120
951207
951214
951226
951227
951227
951228
951231
01 : 58
02 :01
15: 12
23 :21
03 : 59
13:46
13:52
19 :08
01:42
09 : 23
12:49
16:11
08 :43
10:41
15: 10
10: 12
22:21
18:23
15: 14
04 :08
22 :20
09:38
06 :03
02 :02
09 :07
16 : 49
04:23
10:20
21 : 31
21 : 57
19:26
18 :43
17:41
01:11
04 :08
10:25
20 :27
03 : 16
17:59
04: 22
03 : 36
05 : 39
11:02
22: 11
18: 13
05 :27
22:38
04:31
17:27
07 :09
14:32
06:54
19 :03
02 : 46
20: 20
15:26
06: 15
19:59
23 :44
01 : 24
21 : 29
0 1997 RAS, G J l 129, 368-388
Str.
95
205
105
195
225
220
250
70
185
295
185
275
185
110
170
23.5
280
75
135
260
I80
25
150
165
135
250
100
240
10
205
140
205
125
245
180
30
175
225
130
165
185
190
175
205
85
210
190
165
160
235
275
170
170
105
175
170
40
200
105
175
245
Dip
45
50
35
75
55
45
55
35
50
60
85
80
85
35
70
45
85
55
75
25
80
45
85
90
45
65
35
75
85
55
5
50
65
50
55
75
50
70
30
60
60
65
80
40
90
45
40
60
80
65
25
80
80
80
50
55
80
25
70
60
55
Rake
- 180
60
110
50
40
40
80
110
20
-50
30
120
30
0
40
80
30
50
30
- 90
30
- 140
- 120
- 30
- 170
160
170
40
- 10
20
40
- 10
170
80
50
10
- 10
140
- 160
30
-50
- 20
- 40
- 10
10
100
60
- 70
- 150
170
- 100
170
160
- 170
- 40
-110
40
80
-110
40
- 170
P-axes
azim.
220
225
270
223
77
73
257
235
48
168
224
25 1
224
350
203
62
319
102
173
260
218
133
301
25
255
28
226
274
235
57
6
81
259
252
207
254
51
191
232
22
58
60
36
87
129
22
30
25
296
9
295
305
128
239
60
299
74
27
256
207
11
plun.
30
0
11
19
1
10
9
11
15
55
16
28
16
35
10
0
16
2
9
70
13
55
42
20
35
4
30
14
10
11
41
33
10
4
2
3
33
10
47
2
55
31
34
48
7
0
8
68
28
10
69
0
6
14
53
71
18
20
60
2
30
T-axes
azim
329
317
44
335
345
328
35
9
305
267
322
125
322
229
303
328
57
195
269
79
316
238
175
124
4
120
346
16
325
328
194
336
354
12
300
163
306
91
359
290
157
328
140
333
220
116
277
150
198
104
102
35
35
329
321
184
178
220
120
300
111
plun.
30
67
73
44
51
55
77
73
39
6
24
46
24
35
41
82
24
58
31
19
28
10
33
20
24
31
40
38
3
37
48
21
24
81
58
17
21
41
29
41
6
4
18
27
7
82
69
12
13
24
20
14
21
0
6
7
34
69
22
48
17
FPS categ.
Pol. num.
Qf
U
TF
TF
TS
TS
TF
TF
TF
11
16
9
20
18
18
13
12
12
13
9
14
12
22
13
18
16
18
11
14
6
10
15
11
13
9
6
15
12
12
8
10
13
10
11
12
14
18
6
13
12
20
6
8
13
8
19
16
18
11
10
20
13
11
7
9
8
12
18
12
22
A
A
A
A
B
A
A
A
A
A
A
A
A
B
B
ss
NF
ss
U
ss
U
TS
TF
ss
TF
ss
NF
ss
NF
U
ss
U
ss
U
ss
ss
ss
U
U
ss
TF
TF
ss
U
TS
U
TS
NF
ss
ss
U
ss
TF
TF
NF
ss
ss
NF
ss
ss
ss
NF
NF
ss
TF
NF
TS
ss
A
B
A
A
A
A
A
B
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
B
B
A
B
A
A
A
A
A
A
38 1
QP
A
A
A
A
A
A
A
A
A
A
B
A
A
A
A
A
A
A
A
A
B
A
A
A
A
B
A
A
A
A
A
A
A
A
A
B
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
B
A
A
A
A
A
A
382
A . Frepoli and A . Amato
Figure 8. Distributions of P - and T-axes of the 125 selected fault-plane solutions. (a) T-axes of normal-fault solutions (NF-NS categories);
(b) P-axes of thrust-fault solutions (TF-TS); (c) T-axes of strike-slip solutions (SS); (d) P-axes of strike-slip solutions (SS).
the measure of the relative stress magnitude R is close to 0.5,
i.e. the three principal stresses are different from one another.
A more detailed study based on the same stress-inversion
technique applied to smaller subregions for the entire Italian
peninsula is presently in progress.
DISCUSSION
The Northern Apennines belong to the deforming belts that
surround the Adriatic plate. The Adriatic plate is made of
continental lithosphere with thickness increasing from 70 km
to the north of the Gargano Ridge area to more than 110 km
to the south of this area (Calcagnile & Panza 1982). This
plate behaves as a rigid, almost aseismic block, surrounded by
highly seismically active regions (Apennines, southern Alps,
Dinarides) in which seismicity is concentrated in 50,630 km
wide belts.
From slip directions of the strongest earthquakes of a 21 year
period, Anderson & Jackson (1987) proposed a counterclockwise rotation of the Adriatic plate around a pole in northern
Italy. According to Anderson & Jackson, this rotation could
-
explain the present-day deformation around the Adriatic plate,
with N-S compression in the Eastern Alps, NE-SW compression in the southern Dinarides, and parallel extension
along the Apennines. This rotation involves greater deformation in the areas more distant from the rotation pole, i.e.
the southern Dinarides and the southern Apennines, consistent
with the observations from the distribution of large earthquakes. The seismic deformation rates calculated for both the
compression in the Dinarides and the extension in the central
southern Apennines are around 2.5 mm yr-' for a short period
of observation (21 years; Anderson & Jackson 1987) and are
similar to those calculated by Jackson & McKenzie (1988)
for a longer period (70 years) using stronger earthquakes
(2.1mmyr-' in the Apennines and 1.5mmyr-' in the
Dinarides). Pondrelli, Morelli & Boschi (1995) find similar
results using strong earthquakes of the last 16 years with CMT
solutions. All these works are based on moment summation
of the largest earthquakes that occurred in the Apennines in
this century, and consider the Apennines as a continuous belt
characterized only by extension perpendicular to its length.
Jackson & McKenzie (1988) also remark that it is difficult to
0 1997 RAS, GJI 129, 368-388
Extension and compression in the Northern Apennines
383
Figure 9. Focal mechanisms of this study divided into different stress categories according to Zoback (1992): (a) normal-fault solutions (NF-NS
categories); (b) thrust-fault solutions (TF-TS); (c) strike-slip solutions (SS); (d) unknown-category fault-plane solutions (U). Thrust, strike-slip and
unknown category solutions of the Cesena-Forli sector (indicated with a dashed box) are shown in Figs 10, 11 and 12. The numbers in the first
column of Table 5 are indicated next to each focal solution.
estimate the percentage of aseismic deformation for all the
surrounding areas of the Adriatic plate. Such information is
likely to come from geodetic observations, but due to the low
deformation rates of our region it will take several years before
reliable measurements are available from space geodesy.
Contradictory results were found from VLBI and SLR data
(Ward 1994; Noomen et al. 1996). Ongoing work on old
triangulation data will probably constrain the total strain of
the Apennines better (Hunstad & England 1997).
Taking into account both the instrumental and the historical
seismicity from 1650, Westaway (1992) estimated the average
extension and compression rates in the Northern Apennines
as 0.26 mm yr-' for both, while for the southern Apennines,
characterized only by extension, he estimated a much higher
deformation rate with a maximum of 5 mm yr-' near 41"N.
The rotation of the Adriatic plate can give an explanation
for the different rates of deformation in the Northern and
southern Apennines, but does not explain the existence of a
clear separation between the northern and the southern arcs,
-
0 1997 RAS, GJI 129, 368-388
evident from geological data (Patacca & Scandone 1989),
crustal earthquake distribution (Cocco et al. 1993), subcrustal
seismicity (Selvaggi & Amato 1992) and deep structures
(Amato et al. 1993).
It is well known from chronological and analytical studies
of deformation through seismic-reflection profiles and geological data (Castellarin et al. 1986; Vai 1987) that the Northern
Apennines outer margin is characterized by recent compression, while the inner-belt portion is in extension. In
addition to this difference in style of deformation, other
elements discriminate the Northern from the southern
Apennines. Apart from the Calabrian Arc, the southern
Apennines are characterized by the lack of a clear high-velocity
anomaly in the upper -300 km, due to either a slab detachment (Spakman, van der Lee & van der Hilst 1993) or a
thinner, less rigid subducted lithosphere. Only below -300 km
depth is there evidence of a high-velocity body from tomography, while the Northern Apennines are characterized by a
remnant of subducted lithosphere in the upper 300 km accord-
384
A . Frepoli and A . Amato
20
-
20'
15'
15'
10'
10'
5'
5'
441-
44'
55,
12-
5'
10'
IS
20'
k/
I
12'
55'
25
5'
12-
5'
10'
15'
20'
15'
20'
25'
1
Cesena
55'
10'
25'
(c)
Figure 10. (a) Cesena-Forli sector strike-slip solutions; (b) P-axis directions; (c) 7'-axis directions. The numbers in the first column of Table 5 are
indicated next to each focal solution.
ing to Amato et al. (1993), whereas Spakman et al. (1993)
predicted a detachment in the upper 200 km in this region.
Moreover, subcrustal events are located only beneath the
Northern Apennines down to 90 km (Selvaggi & Amato 1992).
These differences between the two Apenninic arcs could be
connected with a lithospheric tear located in the central
Apennines.
The seismological data presented in this paper confirm that
the Northern Apenninic arc is undergoing a markedly different
evolution than the southern Apennines, and provides further
evidence of active compression along its external front. Some
previously published focal mechanisms, like the 1972 Ancona
earthquake strike-slip solution (Gasparini et al. 1985), the
thrust solutions of the 1983 Parma event (Haessler et al. 1988)
and of the 1987 Porto San Giorgio sequence (Riguzzi et al.
1989),show axes of maximum compression ( P )roughly perpendicular to the Apenninic belt. However, the sparsity of these
data did not allow the definition of the areal extent of the
compressional front.
From our results we observe two regions that are clearly
separated. One is characterized by compression along the outer
margin of the belt, and is flanked along the inner part by
another region, which is dominated by extension. There is also
a partial superimposition of the two deformation zones in the
eastern portion of the Emilian-Tuscan Apennines. From the
distribution of our data we note that there is no evidence of
an almost aseismic zone, as proposed by Lavecchia et aE. (1994).
The thrust and strike-slip solutions of the compressional
zone show maximum compression axes ( P ) generally oriented
perpendicular to the trend of the Apennines (Figs 8b, 8d). In
the analysed period we observe a concentration of events with
thrust-fault and strike-slip solutions in the Cesena-Forli sector
(Figs 10, 11 and 12).
The active compression depicted by our data (Figs 8b and d)
in the easternmost sector of the Northern Apenninic belt
apparently affects the entire brittle upper crust. Hypocentral
depths of the earthquakes are generally above 15-20km,
although they are not well constrained by the present national
network, hence no inference can be made on the stress distribution with depth, and on whether the active compression is
confined within the subducting Adriatic plate, or in the
overthrusting Apenninic crustal sheets, or in both. Along the
0 1997 RAS, GJI 129, 368-388
Extension and compression in the Northern Apennines
foredeep of the Northern Apenninic arc the Adriatic plate is
located at a depth of 7-8 km. It seems likely, therefore, that
the seismicity here also affects the underlying Adriatic plate.
Montone et al. (1992) and Montone et al. (1995b) report
results of borehole breakouts along the outer front at a depth
of 1-5 km, showing directions of Shmx perpendicular to the
thrust front, at least in the northernmost sector of the arc,
between latitudes 43"N and 45"N. This is in very good
agreement with the results of this study, and suggests that the
stress distribution is constant with depth. According to Kruse
& Royden (1994) the Adriatic plate was flexing beneath the
Apennines until middle-upper Pliocene times, and was probably uplifting during the Pleistocene due to diminished forces
acting on the subducted slab at depth. According to our results,
the compressional front is still active, implying that the shallow
processes (crustal compression) associated with the deep structures (subducting or sinking slab) are continuing today.
Another indication that confirms these results on the compression in the easternmost part of the Northern Apennines
is given by the study of recent compressional structures in
the Adriatic foreland, observed through seismic-reflection
profiles (Argnani & Frugoni 1997). The authors identified
compressional structures, elongated in an approximately
WNW-ESE direction from the central Adriatic Sea to the
Northern Apennines. From Mt. Conero (Fig. 1) to the west
these structures underlie the Northern Apenninic thrust fronts,
corresponding with a relatively higher seismicity with respect
to the off-shore part of the compressional structures.
In the internal extensional area most of the normal-fault
and strike-slip solutions show minimum compression axes (T)
oriented between E-W and NE-SW (Figs 8a and c). In some
sectors such as the Colline Metallifere and the Umbria-Marche
Apennine, the fault-plane solutions show T-axes with variable
orientations. It is possible for the latter sector that this
inconsistency of axial orientations is connected to a transition
zone between extension to the west and compression to the
east. For the Colline Metallifere, the distribution of 7'-axes
indicates radial extension, possibly due to magmatic processes
active in this region, but the poor network coverage in this
area also suggests caution in interpreting these results.
In the superimposition zone, where we observe both reverse
and normal faulting, we cannot discriminate whether the stress
varies with depth due to the previously described unreliability
of the focal depths typical of locations with regional networks.
Lavecchia et al. (1994) do not take into account this problem
in their seismotectonic zoning. The 'foothills seismic zone' that
they observe between a more internal zone under extension
and the external compressional zone along the Adriatic coast
is characterized, according to these authors, by shallow extension for events between 3 and 7 km depth (no fault-plane
solutions are indicated) and deep compression between 15 and
25 km depth, constrained only by two poorly resolved focal
mechanisms.
From neither the seismicity distribution (Fig. 1) nor the
analysis of our fault-plane solutions do we observe the presence
of transverse elements that disconnect the continuity of the
extensional and compressional zone in the Northern
Apennines, as it is described in the seismotectonic zoning of
Scandone et al. (1990). Some of the clustered distributions of
earthquakes that are visible in the seismicity maps of this
region (Fig. l), with an elongation (NE-SW) perpendicular to
the Apenninic belt, may be an artefact of the network geometry.
We verified that the location errors for events located at the
-
I
55'
10'
5'
12"
-
15'
20
25'
1
I
F55
12'
5'
LO'
is
20
2s
(b>
Figure 11. (a) Cesena-Forli sector thrust solutions; (b) P-axis directions. The numbers in the first column of Table 5 are indicated next
to each focal solution.
15'
I
\
Forli'O
5
@--
76
55'
I
12.
5
10'
15'
20'
2s
Figure 12. Cesena-Forli sector unknown category solutions. The
numbers in the first column of Table5 are indicated next to each
focal solution.
0 1997 RAS, G J I 129,368-388
385
386
A . Frepoli and A . Amato
Gr :82 1356 (pl. I az.)
b r :5 I 4 5 (pl. I az.)
01: 60 I 3 0 6
03:301138
R
:0.4
misfit : 8.3'
No. : 71
R :0.5
misfit : 13.8'
No. : 54
Extension zone:
northern Tuscany
peri-Tyrrhenian zone
Umbria-northern Abruui
(all data)
Compression zone:
Caorso-Emilia
Romagna-Cesena-Marche
(all data)
Figure 13. Stress inversion results for the extension and compression zones. The three principal stress axis directions (plunge and azimuth), the
measure of relative stress magnitudes R, the measure of misfit and the number of fault-plane solutions used for the inversion are indicated.
external boundary of the arc are at a maximum in this direction
(NE-SW) due to a lack of stations in the Adriatic region.
The N F solutions of this study are only concentrated in the
inner belt and along the peri-Tyrrhenian region (Fig. 8a). This
confirms that the Northern Apenninic belt and back-arc region
are undergoing extension, as previously proposed by Gasparini
et al. (1985) based on a few fault-plane solutions of older
events (period 1905-1980), and also suggested by the solutions
of the two recent strongest earthquakes of this area, such as
the 1979 Norcia one (Deschamps et al. 1984) and the 1984
Perugia one (Haessler et al. 1988). Both the first-motion and
the CMT solutions have a tension axis (T) with a NE-SW or
NNE-SSW direction for the Perugia event, while for the
Norcia event the T-axis is ENE-WSW oriented. The rotation
of the extensional axis between these two solutions may reflect
the bending of the tectonic features in this sector of the
Northern Apennines, whose trend changes from NW-SE to
N-S (Fig. 1). This different structural orientation is linked to
the presence of a less pronounced advancement of the outer
thrust front in the southern sector, and to Plio-Pleistocene
clockwise rotation of the foredeep in this southern portion of
the Northern Apennines (Speranza, Sagnotti & Mattei 1997).
In the peri-Tyrrhenian region our results indicate normalfault and strike-slip solutions with minimum compression axes
( T ) in an ENE-WSW direction (Figs 8a and c). The present
extensional regime along the coastal Tyrrhenian region of the
Northern central Apennines is also indicated by borehole breakout analysis and by stress directions inferred from inversion of
microearthquake (A4<4) focal mechanisms (Montone et al.
1995a). Also in this region, the stress directions deduced from
the seismicity are in general in good agreement with those of
the breakout analysis, and indicate a NE to ENE extension
direction. The Tuscania event (M=4.6), which occurred in 1971
and was studied by Gasparini et al. (1985), also shows a normalfault-plane solution with its T-axis oriented NE-SW (Fig. 2 ) .
CONCLUSIONS
The determination of 125 new focal solutions of earthquakes
that occurred in the Northern Apennines from 1988 to 1995
0 1997 RAS, GJI 129, 368-388
Extension and compression in the Northern Apennines
( M < 5) have allowed us to improve our knowledge of the state
of strain within this region. The main result of this study is
the division of the region into two areas with different styles
of deformation: the inner area of the Apenninic belt, which is
characterized by extension, and the outer margin of the belt,
which shows evidence of active compression. In the extensional
area the T-axes of normal and strike-slip solutions do not
have a homogeneous orientation, while in the compressional
area the P-axes of thrust and strike-slip solutions have a
dominant NE-SW direction. The epicentres of the compressional events are in an inner position with respect to the
more recent thrust front of the Northern Apennines. The stresstensor orientations computed for the compressional and extensional zones shows that the maximum principal stress axis in
the external front is horizontal and NE-SW oriented, while in
the internal region the minimum principal stress axis is horizontal and oriented approximately E-W. Considering the
Neogene-Quaternary tectonics of the Northern Apenninic arc
and the persistence of the extensionxompression pair throughout its evolution, our results suggest that the process driving
this mechanism, most probably the slab roll-back, is still
continuing today.
ACKNOWLEDGMENTS
We thank John Gephart who provided the computer code
relating to the stress-inversion problem. We also thank two
anonymous referees for their critical reviews. Helpful comments
were also provided by Andrea Argnani, Salvatore Barba,
Claudio Chiarabba, Massimo Cocco, Massimo Di Bona,
Francesco Frugoni, Paola Montone and Giulio Selvaggi.
This work was partially funded by EC project GeoModAp
(contract EV5V-CT94-0464). We are grateful to Enzo Boschi
for encouraging this study. One of the authors (AF) dedicates
this work to Guglielmo and Lucia.
REFERENCES
Amato. A., Alessandrini, B., Cimini, G.B., Frepoli, A. & Selvaggi, G.,
1993. Active and remnant subducted slabs beneath Italy: evidence
from seismic tomography and seismicity, Ann. G K O ~ S36,
. , 201-214.
Amato, A., Chiarabba, C., Cocco, M., Di Bona, M. & Selvaggi, G.,
1994. The 1989-1990 seismic swarm in the Alban Hills volcanic
area, Central Italy, J. Vok. Geotherm. RKS.,61, 225-237.
Anderson, H. & Jackson, J., 1987. Active tectonics of the Adriatic
region; Geophys. J. R. astr. Soc., 91, 937-983.
Argnani, A. & Frugoni, F., 1997. Foreland deformation in the Central
Adriatic and its bearing on the evolution of the Northern Apennine,
Ann. G K O ~ Ssubmitted.
.,
Basili, A., Cantore, L., Cocco, M., Frepoli, A,, Margheriti, L., Nostro,
C. & Selvaggi, G., 1997. The June 12, 1995, microearthquake
39, 1167-1175.
sequence in the city of Rome, Ann. GKU~S.,
Bigi, G., Cosentino, D., Parotto, M., Sartori, R. & Scandone, P., 1990.
Structural model of Italy, 1:500000, PFG-CNR, Quad. Ric. Sci., 114.
Calcagnile, G. & Panza, G.F., 1982. The main characteristics of the
lithosphere-asthenosphere system in Italy and surrounding regions,
Pageoph, 119, 865-879.
Castellarin, A,, Eva, C., Giglia, G. & Vai, G.B., 1986. Analisi strutturale
del Fronte Appenninico Padano, Giorn. Geol., 47, 47-76.
Cocco, M., Selvaggi, G., Di Bona, M. & Basili, A,, 1993. Recent seismic
activity and earthquake occurrence along the Apennines, in Recent
Evolution and Seismicity of the Mediterranean Region, pp. 295-312,
eds Boschi, E., Mantovani, E. & Morelli, A,, Kluwer Academic, the
Netherlands.
0 1997 RAS, GJI 129, 368-388
387
Deschamps, A., Iannaccone, G. & Scarpa, R., 1984. The Umbrian
earthquake (Italy) of 19 September 1979, Ann. Geophys., 2, 29-36.
Gasparini, C., Iannaccone, G. & Scarpa, R., 1985. Fault-plane solutions
and seismicity of the Italian Peninsula, Tectonophysics, 117, 59-78.
Gephart, J. & Forsyth, W., 1984. An improved method for determining
the regional stress tensor using earthquake focal mechanism data:
Application to the San Fernando earthquake sequence, J. geophys.
Res., 89, 9305-9320.
Haessler, H. et al., 1988. The Perugia (Italy) earthquake of 29 April
1984 a microearthquake survey, Bull. seism. Soc. Am., 78, 1948-1964.
Hunstad, I. & England, P., 1997. Estimates of strain from old Italian
triangulation network, European Geophys. Soc. X X I l General
Assembly, Vienna, 21-25 (abstract).
Jackson, J. & McKenzie, D., 1988. The relationship between plate
motions and seismic moment tensors, and the rates of active
deformation in the Mediterranean and Middle East, Geophys. J. R .
astr. Soc., 93, 45-73.
Klein, F.W., 1989. Hypoinverse, a program for Vax computers to solve
for earthquake location and magnitude; USGS. Open file rept,
89-314.
Kruse, S.E. & Royden, L.H., 1994. Bending and unbending of an
elastic lithosphere: the Cenozoic history of the Apennine and
Dinaride foredeep basins, Tectonics. 13, 278-302.
Laubscher, H.P., 1988. The arcs of the Alps and the Northern
Apennines: an updated view, Tectonophysics, 146, 67-78.
Lavecchia, G., Brozzetti, F., Barchi, M., Menichetti, M. & Keller,
J.V.A., 1994. Seismotectonic zoning in east-central Italy deduced
from an analysis of the Neogene to present deformations and related
stress fields, Geol. SOC.Am. Bull., 106, 1107-1120.
Malinverno, A. & Ryan, W.B.F., 1986. Extension in the Tyrrhenian
Sea and shortening in the Apennines as result of arc migration
driven by sinking of the lithosphere, Tectonics, 5 , 227-245.
Mongelli, F., Zito, G., Ciaranfi, N. & Pieri, P., 1989. Interpretation of
heat flow density of the Apennine chain, Italy, Tectonophysics,
164, 267-280.
Montone, P., Amato, A., Chiulli, R. & Funiciello, R., 1992. Metodologie
per la determinazione del campo di stress attuale da dati di
perforazioni profonde, Proc. 11th Mtng Gruppo Nazionak di
Geofsica della Terra Solida, pp. 337-348, CNR, Rome, Italy.
Montone, P., Amato, A., Chiarabba, C., Buonasorte, G. & Fiordelisi,
A., 1995a. Evidence of active extension in Quaternary volcanoes of
Central Italy from breakout analysis and seismicity, Geophys. RKS.
Lett., 22, 1909-1912.
Montone, P., Amato, A,, Frepoli, A. & Cesaro, M., 1995b. Active stress
and recent geologic evolution of the Italian region, EOS, Trans. Am.
Geophys. Un., 76, F622 (abstract).
Noomen, R., Springer, T.A., Ambrosius, B.A.C., Herzberger, K.,
Kuijper, D.C., Mets, G.J., Overgaauw, B. & Wakker, K.F., 1996.
Crustal deformations in the Mediterranean area computed from
SLR and GPS observations, J. Geodyn., 21, 73-96.
Patacca, E. & Scandone, P., 1989. Post-Tortonian mountain building
in the Apennines: the role of the passive sinking of a relic lithospheric
slab, In The Lithosphere in Italy: Aduances in Earth Science Research,
pp. 157-176, eds Boriani, A,, Bonafede, M., Piccardo, G.B. & Vai
G.B., Accademia Nazionale dei Lincei, Mid-Term Conference, Rome
5-6 May, 1987.
Philip, H., 1987. Plio-Quaternary evolution of the stress field in
Mediterranean zones of subduction and collision, Ann. Geophys.,
5B,301-320.
Pondrelli, S., Morelli, A. & Boschi, E., 1995. Seismic deformation in
the Mediterranean area estimated by moment tensor summation,
Geophys. J. Int., 122, 938-952.
Reasenberg, P. & Oppenheimer, D., 1985. FPFIT, FPPLOT and
FPPAGE: FORTRAN computer programs for calculating and
displaying earthquake fault-plane solutions, USGS Open f i l e Rept,
85-739.
Reutter, K.J., 1981. A trench-forearc model for the Northern
Apennines, in Sedimentary Basins of Mediterranean Margins, ed.
388
A . Frepoli and A. Amato
Wezel, F.C., pp. 433-443, CNR Italian Project of Oceanography,
Tecnoprint, Bologna.
Riguzzi, F., Tertnlliani, A. & Gasparini, C., 1989. Study of the seismic
sequence of Porto San Giorgio: Marche 3 July 1987, I1 Nuouo
Cimento, 12C, 4,453-466.
Royden, L., 1993. The tectonic expression slab pull at continental
convergent boundaries, Tectonics, 12, 303-325.
Royden, L., Patacca, E. €2 Scandone, P., 1987. Segmentation and
configuration of subducted lithosphere in Italy: an important
cbntrol on thrust-belt and foredeep-basin evolution, Geology, 15,
714-717.
Scandone, P., Patacca, E., Meletti, C., Bellatalla, M., Perilli, N. &
Santini, U., 1990. Struttura geologica, evoluzione cinematica e
schema sismotettonico della penisola italiana, Proc. G N D T
Conference, Pisa, 1, 119-135.
Selvaggi, G. & Amato, A., 1992. Subcrustal earthquakes in the
Northern Apennines (Italy): evidence for a still active subduction?,
Geophys. Res. Lett., 19, 2127-2130.
Spakman, W., van der Lee, S. & van der Hilst, R., 1993. Travel-time
tomography of the European-Mediterranean mantle down to
1400 km, Phys. Earth planet Inter., 79, 3-74.
Speranza, F., Sagnotti, L. & Mattei, M., 1997. Tectonics of the UmbriaMarche-Romagna Arc (central northern Apennines, Italy): New
paleomagnetic constraints, J. geophys. Rex, in press.
Vai, G.B., 1987. Migrazione complessa del sistema fronte deformativoavanfossa-cercine periferico: jl caso dell’Appennino settentrionale,
Mem. Sac. Geol. It., 38, 95-105.
Ward, S.N., 1994. Constraints on the seismotectonics of the central
Mediterranean from very long baseline interferometry, Geophys.
J. Int., 117, 441-452.
Westaway, R., 1992. Seismic moment summation for historical earthquakes in Italy: Tectonic implications; J. geophys. Res., 97,
15437-15 464.
Zoback, M.L., 1992. First- and second-order patterns of stress in the
lithosphere: The world stress map project, J. geophys. Rex, 97,
11703-1 1728.
0 1997 RAS, GJI 129, 368-388