Seismic expression of active extensional faults in northern Umbria (Central Italy)

Seismic expression of active extensional faults in northern

Umbria (Central Italy)

Cristiano Collettini, Massimiliano Barchi*, Cristina Pauselli, Costanzo Federico,Giampaolo Pialli

Dipartimento di Scienze della Terra, UniversitaÁ di Perugia, P.za UniversitaÁ, 06100 Perugia, Italy

Abstract

This paper deals with the geometry and kinematics of the active normal faults in northern Umbria,

and their relationship with the seismicity observed in the area. In particular, we illustrate the

contribution of seismic re¯ection data (a network of seismic pro®les, NNW±SSE and WSW±ENE

trending) in constraining at depth the geometry of the di�erent active fault systems and their reciprocal

spatial relationships. The main normal fault in the area is the Alto Tiberina fault, NNW trending and

ENE dipping, producing a displacement of about 5 km, and generating a continental basin (Val

Tiberina basin), in®lled by up to 1500 m with Upper Pliocene±Quaternary deposits. The fault has a

staircase trajectory, and can be traced on the seismic pro®les to a depth of about 13 km. A set of WSW-

dipping, antithetic faults can be recognised on the pro®les, the most important of which is the Gubbio

fault, bordering an extensional Quaternary basin and interpreted as an active fault based on geological,

geomorphologic and seismological evidence. The epicentral distribution of the main historical

earthquakes is strictly parallel to the general trend of the normal faults. The focal mechanisms of the

major earthquakes show a strong similarity with the attitude of the extensional faults, mapped at the

surface and recognised on the seismic pro®les. These observations demonstrate the connection between

seismicity in the area and the activity of the normal faults. Moreover, the distribution of the

instrumental seismicity suggests the activity of the Alto Tiberina fault as the basal detachment for the

extensional tectonics of the area. Finally, the action of the Alto Tiberina fault was simulated using two

dimensional ®nite element modelling: a close correspondence between the concentration of shear stresses

in the model and the distribution of the present earthquakes was obtained. # 1999 Elsevier Science Ltd.

All rights reserved.

Journal of Geodynamics 29 (2000) 309±321

0264-3707/00/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.

PII: S0264-3707(99)00059-9

* Corresponding author. Fax: +39-75-585-3203.

E-mail address: [email protected] (M. Barchi).

1. Introduction

The backbone of Central Italy consists of the northern Apennines orogene, a typical thrust

and fold belt, NE verging, involving progressively shallower rocks from the inner to the outer

part. In fact, as we look at di�erent sets of geological and geophysical data, they all underline

the presence of two di�erent crustal sectors. In the external (eastern) sector (Adriatic Domain)

the geometry of the thrust belt and of the related foredeep basins is still preserved: the crust is

characterised by normal thickness (30±35 km), and we observe negative gravity anomaly (ÿ80

mgal; Decandia et al., 1998; Scarascia et al., 1998), and low heat ¯ow values (30 mW/m2 on

average). Moreover, Pliocene±Quaternary basins have a compressional character (foredeep

and/or piggy-back basins).

In the internal (western) sector (Tuscan Domain) on the contrary, the compressional belt is

disrupted by important extensional deformations: we observe a signi®cantly thinned crust (20±

25 km), high positive gravity anomaly (with a large wavelength) and high values of heat ¯ow

(more than 90 mW/m2 Mongelli and Zito, 1991). The Pliocene±Quaternary sedimentation

occurs in graben and half-graben, bordered by normal faults. Finally, the crustal seismicity

shows a distinctive extensional character.

Despite all this geological and geophysical evidence, the role of extensional deformations in

the structural setting of the Tuscan Domain has been neglected for a long time, and even

recently some authors (Boccaletti et al., 1991) have considered the western part of the northern

Apennines as being currently a�ected by compressional tectonics.

Among the ®rst researchers only Signorini (1946) underlined the surface geology expression

of west-dipping and east-dipping normal master faults, superimposed on compressive structures

and responsible for the formation of the Neogene basins. Only in the last two decades, inspired

by the research works carried out on Basin and Range and on other extensional provinces in

the world, has the geology of the Tuscan sector been reinterpreted, showing, with di�erent

models, the role of extensional tectonics in the evolution of the northern Apennines (Lavecchia

et al., 1984, 1989; Carmignani et al., 1994; Baldi et al., 1994; Liotta and Salvatorini, 1994).

The Deep Seismic Re¯ection Pro®le CROP 03 provides a geophysical image of the crustal

setting of the Tuscan Domain: the lower crust is thinned and ¯attened by ductile ¯ow, while

the upper crust is a�ected by a set of east-dipping normal faults (Barchi et al., 1998b). The

extensional faults began to act in the westernmost sector (Corsica basin) in the Lower

Miocene, disrupting the inner part of the orogene (Barchi et al., 1998a). Then the extension

moved eastward, reaching the Umbria region (i.e. the central part of the peninsula) during the

Upper Pliocene (Val Tiberina basin).

The Umbria region can be regarded as a transitional area between the Tuscan and the

Adriatic domains: here, extensional tectonics are now active, but the process of crustal thinning

is not yet completed. In this paper we focus on this region, using seismic re¯ection data to

describe the geometry and kinematics of the Alto Tiberina fault, which we interpret as the

easternmost, east-dipping, extensional shear zone a�ecting the northern Apennines.

Subsequently, we relate the location and geometry of this master fault to the surface geology

evidence of active faulting and to the distribution of seismicity, in order to characterise the

seismotectonic setting of the region. From a methodological point of view, we wish to

underline the interdisciplinary nature of the studies in recent and active tectonics, involving

C. Collettini et al. / Journal of Geodynamics 29 (2000) 309±321310

several geological and geophysical methods of analysis. In this paper we refer mainly to seismic

re¯ection data, but also describe the surface geology evidences as well as the seismological

data; a numerical modelling approach for studying the fault dynamics is also reported.

2. The Alto Tiberina fault

The Umbria region is a�ected by Pliocene±Quaternary extensional tectonics, which give rise

to continental basins, N15082208 trending (Fig. 1). The most important among them is the

Fig. 1. Schematic structural map of the investigated area. 1. Plio-Quaternary basins: (a) Gubbio, (b) Gualdo

Tadino, (c) Col®orito, (d) Norcia, (e) Cascia, (f) Castelluccio. 2. Normal faults. 3. Thrusts. 4. Seismic lines. 5.

Epicenters and focal mechanisms of the main earthquakes of the area (after Harvard CMT database).

C. Collettini et al. / Journal of Geodynamics 29 (2000) 309±321 311

Val Tiberina basin, whose main branch extends from Sansepolcro, through Perugia, and

stretches into Umbra Valley as far south as Spoleto, with an axial continuity of more than 100

km. The sedimentary sequence in®lling the Val Tiberina basin consists of continental, ¯uvial

and lacustrine deposits, Upper Pliocene to Quaternary in age (Ambrosetti et al., 1978, 1987,

1995; Cattuto et al., 1995). South of Perugia the basin splits into two branches: west of the

Monti Martani chain, the Tiber basin is in®lled by older sediments, Middle Pliocene in age

(Basilici, 1997). To the east of the Val Tiberina basin, a series of minor, intramountain basins

a�ects the compressional structures of the Apennines. The most important (Gubbio, Gualdo

Tadino, Col®orito, Norcia, Cascia and Castelluccio, see Fig. 1) have a sub-polygonal shape

and relatively little longitudinal continuity, but are aligned in a general NNW±SSE pattern:

they are bordered by NNW±SSE trending normal faults, and possibly by minor, normal to

transtensional features, oriented transversally to the major faults.

The WSW-dipping normal faults, bordering the intramountain basins, are commonly

interpreted as active faults, based on seismological, geological and geomorphologic data: recent

literature is available, documenting the activity of the Gubbio fault (Menichetti and Minelli,

1991) and of the Norcia fault (Brozzetti and Lavecchia, 1994). In a more general view, the

most important historical earthquakes on the region are located along a strip that coincides

with the alignment of the intramountain basins.

On the other hand, Cello et al. (1997) consider the normal faults as second order structures,

driven by a set of N±S trending, left-lateral, strike-slip master faults, in a general

compressional environment, with the maximum compression axis oriented NNW±SSE.

In this paper we analyse in detail the geometry of the extensional deformations in northern

Umbria: the study area is located between the towns of Perugia, CittaÁ di Castello, Gualdo

Tadino and Gubbio (Fig. 1). From a lithological point of view, this structural province

(Umbria Pre-Apennines, sensu Deiana and Pialli, 1994) is characterised by the outcropping of

the Miocene turbidites of the Marnoso Arenacea Fm., which represents the uppermost unit of

a litho-structural sequence, including, from top to bottom, the Meso-Cenozoic carbonatic

multilayer, the Triassic evaporites and a Permian±Triassic phyllitic basement. The carbonates

outcrop along the culminations of the two major anticlines in the area, NW±SE trending: the

Gubbio and Assisi anticlines.

The most important extensional basins in the study area are the Val Tiberina basin, to the

west, and the intramountain basins of Gubbio and Gualdo Tadino, to the east.

West of the Val Tiberina, in the Perugia Massifs area, ®eld mapping demonstrates the

presence of two main sets of NE-dipping normal faults: a low angle set (208 to 358 dip) and a

high angle set (458 to 708 dip, Brozzetti, 1995). The low angle set is generally displaced by the

high angle one and has been interpreted as belonging to an initially high angle set,

progressively tilted eastward (Brozzetti, 1995). The mountain ridges of the area have been

considered as fault bounded blocks, forming a `domino-like' structure, detached on a gently

eastward dipping horizon.

The extensional character of this area also results from the deep wells of Perugia 2 and San

Donato. These wells drilled the tectonic contact between the Miocene turbidites (Marnoso-

Arenacea Fm.) and a thick sequence of Upper Triassic evaporites (Anidriti di Burano Fm.)

(Anelli et al., 1994). On a larger scale, the CROP 03 seismic re¯ection pro®le (Fig. 2) shows

that the Val Tiberina basin is geometrically and genetically related to an east-dipping normal


Fig. 2. Seismic pro®le `A' (CROP03), and its geological interpretation. 1. Neoautochtonous continental deposits; 2.

Miocene turbidites; 3. Mesozoic±Tertiary carbonates; 4. Triassic evaporites; 5. basement s.l.; 6. thrusts; 7. normal

faults. The trace of the pro®le is reported in Fig. 1.


fault (Alto Tiberina fault), that can be drawn to a depth of 5.0 s T.W.T (two way time, here

and after depths will always be considered in T.W.T.), i.e. 12±14 km. The same fault can be

observed on many other seismic pro®les throughout the region: its ascertained longitudinal

continuity is about 60 km. Seismic data are discussed in detail in the next paragraph.

Boncio et al. (1998) interpret the domino-like fault system mapped at surface in the Perugia

Massifs area as splays of the Alto Tiberina fault. The break-away zone of the master fault is

located to the west of the present-day position of the Val Tiberina basin, therefore, the

beginning of the fault activity might be older than the Pleistocene continental deposits in®lling

the basin.

3. Seismic re¯ection pro®les

We interpreted a set of longitudinal (NNW±SSE) and transversal (WSW±ENE) commercial

seismic pro®les, kindly provided by AGIP, in order to ascertain the lateral continuity and the

regional attitude of the Altotiberina fault. The location of the pro®les can be inferred from the

isochronous map in Fig. 4.

The analysed pro®les show some common characteristics:

1. a good general resolution of the seismic re¯ections in the study area, probably favoured by

the outcropping of the turbidites of the Marnoso Arenacea formation;

2. a relatively homogeneous, easily recognisable sequence of seismic markers, which can be

related, from top to bottom, to: the bottom of the Miocene Turbidites (Bisciaro Fm.); a

marly interval embedded in the carbonate sequence (Marne a Fucoidi Fm.); the top of the

evaporites; and the top of the phyllitic basement;

3. the di�erent attitude of the seismic re¯ections, steep and bent, with short wavelength, in the

upper part; smooth and continuous in the lower section: this characteristic underlines the

disharmonic deformation of the Turbidites, with respect to the underlying carbonates and

evaporites;

4. the seismic expression of the Alto Tiberina fault, marked by a strong alignment of

re¯ections and con®rmed by the sharp interruption of the seismic sequence described in 2,

detectable in the fault hanging wall.

Fig. 3. Line drawing of the seismic pro®le `C', crossing the Val Tiberina and the Gubbio structures. 1. thrusts; 2.

normal faults; 3. Fucoidi Marls; 4. top evaporites; 5. top basement. The trace of the pro®le is reported in Fig. 1.


Two examples of the seismic pro®les are shown in Fig. 2 (section A) and Fig. 3 (section C).

The traces of the sections are in Fig. 1.

Section A (Fig. 2) is part of the CROP03 deep seismic pro®le (Barchi et al., 1998a), which

crosses the Val Tiberina basin between the towns of CittaÁ di Castello and Sansepolcro. In this

pro®le the Alto Tiberina fault shows a staircase trajectory: we observe a steep attitude in the

upper part, beneath the Val Tiberina, evolving eastward into a ¯at shear zone and ®nally into

a gently east-dipping segment in the lowermost part (5.0 s., about 13 km in depth), where it

reaches the axial zone of the Apennines. The slight bending of the stratigraphic markers in the

Fig. 4. Isochronous map of the Alto Tiberina fault. Crosses indicate the measure points utilised for contouring.


hanging wall, dipping towards the fault plane, is likely to be related to the curvi-planar shape

of the Alto Tiberina fault. It is important to note, however, that the change in the apparent

dip of the fault is partially connected to the lateral variability in the interval velocities of the

rocks and is signi®cantly smoothed in the depth converted section.

Section C (Fig. 3) crosses the Val Tiberina (south of the town of Umbertide), the Gubbio

anticline and reaches the westernmost part of the Apennine ridge. The Alto Tiberina fault

trajectory is underlined by some aligned oblique re¯ections and by the strong interruption of

the seismic markers in the hanging wall. The western part of the fault has a ¯at attitude, and

corresponds to the basal detachment of the `domino-like' structure of the Perugia Massifs area

(Brozzetti, 1995). The total displacement of the normal fault, as detectable by the position of

the Marne a Fucoidi Fm., can be evaluated as being about 5 km. The pro®le also shows the

geometry of the Gubbio anticline, located between a normal fault, which displaces its inner

limb, and a thrust fault, bordering its eastern limb. The normal fault, dipping westwards and

bordering the Gubbio intramountain basin, reaches its maximum throw (about 2 km) along

this section and is antithetic to the Alto Tiberina, which it is linked with, at 2 s. At a shallower

depth (about 1.5 s), the Gubbio normal fault joins the frontal thrust that generates the

anticline, representing a phenomenon of inversion tectonics.

The other seismic lines interpreted for this work show similar geometry to the Alto

Tiberina fault and to the antithetic, west-dipping faults bordering the intramountain basins.

Picking the depth value (in T.W.T.) of the Altotiberina fault along the available sections,

and using current interpolation and contouring techniques, an isochronous map for the Alto

Tiberina fault was constructed (Fig. 4). This map satisfactorily describes the regional

attitude of the fault, N1508 striking and ENE-dipping. Its staircase shape, shows a relatively

steep shallow part (until about 1.6 s), which is then followed by an almost ¯at intermediate

sector (1.6±2.0 s): the deeper part of the fault dips moderately (about 258) towards the east

(2.0±3.0 s).

This analysis demonstrates that the shape of the Alto Tiberina fault is not a local feature,

showing large variations depending on the selected section, but a regionally constant attitude.

The staircase shape of the fault can be seen as a function of the di�erent mechanical properties

of the litho-structural units which the Umbria-Marche sequence is made of, alternating

competent layers (carbonates), almost ductile sequences (turbidites), and internally complex

units such as the evaporites (alternating dolomites and anhydrites) and the phyllitic basement.

Moreover, we have to consider that the extensional tectonics have been superimposed on an

already structured area, a�ected by Miocene compression, and that pre-existing structures

might have had a great e�ect in in¯uencing the geometry of normal faulting in the area: the

inversion of the displacement along the Gubbio fault is a signi®cant example.

The seismic pro®les also show some signi®cant details of the internal geometry of the

continental deposits in®lling the Val Tiberina basin: an example is shown in section B (Fig. 5).

The thickness of the neo-autochthonous strata in®lling the basin reaches 1.4 s, corresponding

to a depth of about 1.5 km. The seismic re¯ections of the deposits show the asymmetrical

shape of the basin, with the progressive eastward onlap of the strata, and the thickening of the

sequences from east to west. The lateral variation of the basin stratigraphy signi®cantly ®ts the

sin-sedimentary activity of an east-dipping extensional fault.

Unfortunately, a precise dating of the bottom of the relatively thick (up to 1500 m)


continental sequence, depicted in the seismic pro®les, useful for the correct evaluation of the

long-term slip rate along the fault, is not available at present.

4. Characteristics of the seismicity

The Umbria-Marche Apennines are characterised by a well-documented historical seismicity,

which indicates a zone extending from Sansepolcro through Gubbio, Nocera Umbra,

Col®orito, as far as Norcia, a�ected by the strongest historical earthquakes in the area

(I > VIII) (Camassi and Stucchi, 1996). It is interesting to note that the epicentral location of

these earthquakes shows the same trend (NNW±SSE) as the normal faults bordering the

intramountain basins.

The recent major earthquakes which have occurred in this region, and which have been

instrumentally recorded, (Norcia 1979, Gubbio 1984 and Col®orito 1997±98) are located within

the intramountain basins, have a focal depth of between 6 and 10 km, and have extensional

focal mechanisms, with nodal planes matching the extensional structures of the area (Harvard

CMT database; Ekstrom et al., 1998; Amato et al., 1998). In particular, the distribution of the

hypocenters (main shock and aftershocks) of the Col®orito 1997±98 sequence (Amato et al.,

1998), is located at a depth shallower than 6±7 km.

All these data con®rm:

Fig. 5. Seismic pro®le `B', showing the internal details of the Val Tiberina basin. Dashed line is the base of the

continental Plio-Quaternary deposits; dashed-dot line is the trace of the Alto Tiberina fault. The trace of the pro®le

is reported in Fig. 1.


. the close connection between the Pliocene±Quaternary normal faults and the seismicity

observed in the Umbria region;

. the activity of the west-dipping normal faults bordering the intramountain basins, which can

be identi®ed as seismogenetic structures.

Recently, Boncio et al. (1997, 1998) have plotted the hypocenters of the May±June 1987

microseismic survey (Deschamps et al., 1989; Boncio et al., 1998) and the hypocenters of the

1991±1994 ReSiL-Umbria (the local seismic network) database along a regional geological

section, crossing the Val Tiberina and the Gubbio basins, drawn on the basis of surface

geology and seismic re¯ection data. This work shows that the distribution of instrumental

seismicity matches fairly well with the trajectory of the Alto Tiberina normal fault.

Consequently, they suggest that the active, high angle, west-dipping normal fault that borders

the Gubbio basin is antithetic to an east-dipping, low angle detachment, and that seismicity is

concentrated along the fault planes, with the major events located at their intersection.

5. Stress analysis

The action of the Alto Tiberina fault during the extensional tectonic phase was simulated

using the ®nite element method. With this technique the equilibrium equation of a two

dimensional (2D) elastic body was resolved: in this way, with the appropriate boundary

condition, it was possible to determine the shifts and, therefore, stresses and deformations

within the studied body. The program used is a commercial code Algor 3.18 that in this case

was applied to a 2D geometry with the plane strain approximation.

The 2D geometry was reconstructed from the geological section of Barchi et al. (1995): this

section has a length of around 80 km and a depth of 14 km and stretches from the Val

Tiberina up to easternmost thrust of the Umbria-Marche Apennines (M. San Vicino area).

Considering the aim of the work, the geological section has been simpli®ed by considering a

model with plane and parallel layers.

The presence of earthquakes and the results of rheological pro®les inferred in this zone from

study of the thermal state (Federico and Pauselli, 1998) make it possible to assume an elastic

behaviour for the material of this region with reasonable con®dence.

The formations that characterise the Mesozoic±Tertiary multilayer were grouped together in

litho-structural units (from top to bottom: turbidites, carbonates, evaporites, basement) with

di�erent values of density, Young's modulus and Poisson's ratio to simulate materials with

di�erent mechanical characteristics. To simulate the relative weakness of the fault zone, the

material in this region was assigned a value for Young's modulus lower than that used for the

surrounding units.

Fig. 6 shows the distribution of the Von Mises obtained: this value expresses the di�erence

between the principal components of the stress and thus gives an indication of the

concentrations of shear stress. The maximum value of the Von Mises (about 50±60 MPa), and

so the expected region of concentration of seismicity, is achieved in the easternmost and

deepest section of the fault, on its hanging wall. Another local maximum (about 20±30 MPa) is

located at a shallower depth, in the evaporites (Anidriti di Burano Fm.), which is assigned a


value of Young's modulus higher than the surrounding formations. A third local maximum

(about 10±15 MPa) is found at the topographic emergence of the fault.

Modelling was repeated for a series of di�erent boundary conditions to appraise the

in¯uence of these on the ®nal result: in particular, the depth of the model was increased to

check whether the concentration of the stresses at the foot of the fault was independent of the

fact that the fault terminates at the bottom of the model.

The distribution of the Von Mises obtained shows a close correspondence with the

distribution of the earthquakes in the zone under study: in fact, we can see how the fault

separates an active hanging wall from an almost aseismic footwall. The model, moreover,

shows how stresses originating in the deepest part of the fault correspond with the

concentration of stresses within the more brittle rocks of the shallower multilayer.

6. Conclusions

1. The Umbria region is a place of active extensional tectonics, as univocally testi®ed by

surface geology, seismological and geophysical evidence. In the area under investigation the

extensional tectonics are driven by a major, east-dipping normal fault (Alto Tiberina fault),

which represents the easternmost fault of a system, disrupting the compressional architecture

of the northern Apennines orogene. The Alto Tiberina fault is characterised by a staircase

trajectory, a longitudinal continuity of more than 60 km and reaches a depth of about 13

km.

2. The other extensional features of the area, including continental intramountain basins and

west-dipping normal faults (e.g. the Gubbio fault) are related to this major detachment. In

particular, the west-dipping normal faults can be regarded as being antithetic to the Alto

Tiberina fault. The network of seismic pro®les does not show any evidence of major strike-

slip faults a�ecting the basement: the faults with strike-slip displacement, mapped in the

area, are to be interpreted as transfer structures with shallow detachment.

3. The focal mechanisms of the main earthquakes which have occurred in the region show an

extensional character, with nodal planes ®tting the geological extensional structures. The

pattern of the epicentres of the historical earthquakes documented in the area show the

Fig. 6. Finite element modelling. Distribution of the Von Mises along the Alto Tiberina fault.


same trend as the major extensional structures. The geometry of the faults revealed by the

seismic pro®les ®ts satisfactorily with the microseismicity survey and with instrumental

seismicity recorded by local seismic networks in the area.

4. The distribution of the Von Mises obtained by a 2D ®nite element modelling of the Alto

Tiberina fault shows a close correspondence with the distribution of the earthquakes in the

zone: in fact, we can see how the fault separates an active hanging wall from an almost

aseismic footwall.

5. Finally, from a methodological point of view, we would like to underline the utility of the

interpretation of seismic re¯ection pro®les as an important tool in constraining the geometry

of seismogenetic faults. In general, seismic pro®les are not easily replaced in showing the

geometry of potentially seismogenetic faults at the hypocentral depth. In our particular case,

even if in the Perugia Massifs area there is ®eld evidence for east-dipping normal faults, the

importance and the structural continuity of the Alto Tiberina faults could not have been

detected without seismic re¯ection data.

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Seismic expression of active extensional faults in northern Umbria (Central Italy)

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