articolo pucci.pdf

Key words tectonic geomorphology – normal fault –
seismogenic sources – Umbria-Marche Apennines –
29 April 1984 Gubbio earthquake

1. Introduction

The Gubbio Basin is one of the most promi-
nent intramountain basins in the Central Apen-

nines. It is bounded to the east by an impressive
normal fault known as Gubbio Fault that is part
of the seismogenic Umbrian fault system (Bar-
chi, 2002). Even though the basin overlies infer-
red seismogenic sources including that responsi-
ble for the 1984 M 5.3 earthquake (Valensise and
Pantosti, 2001a,b), seismicity (both historical and
instrumental; CPTI, Working Group, 1999;
INGV, 1985-2000) is relatively infrequent and
not sufficient for a full understanding of the seis-
mogenic potential of the area.

To assess the seismic potential of the Gubbio
Basin with confidence we must first answer
these questions:

837

ANNALS OF GEOPHYSICS, VOL. 46, N. 5, October 2003

Mailing address: Dr. Stefano Pucci, Istituto Nazio-
nale di Geofisica e Vulcanologia, Sezione di Roma 1 -
Sismologia e Tettonofisica, Via di Vigna Murata 605, 00143
Roma, Italy; e-mail: pucci@ingv.it

Geomorphology of the Gubbio Basin
(Central Italy):

understanding the active tectonics
and earthquake potential

Stefano Pucci, Paolo Marco De Martini, Daniela Pantosti and Gianluca Valensise
Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Roma 1 - Sismologia e Tettonofisica, Roma, Italy

Abstract
The Gubbio Basin is a 22 km long, 4 km wide depression located within the North-Central Apennines fold-and-
thrust belt. The basin is bounded to the east by the Gubbio Fault, a W-dipping, normal fault dissecting a large
Jurassic-Oligocene anticline. Although located along one of the main seismogenic zones of the Peninsula, both
historical and instrumental is seismicity is limited with the only exception for the 29 April 1984, Ms 5.3 earth-
quake, which occurred about 10 km southwest of the basin. Most of the literature attributes this seismicity
to the Gubbio Fault. New geomorphic and geologic investigations based on field and aerial photo surveys and DEM
analyses provide new insights on the active faulting in the area and are used to infer potential seismogenic sources.
Limited evidence of ongoing deformation along the surface expression of the Gubbio Fault was found, possibly
because of low rates of deformation versus fast erosional processes. The western side of the basin appears to be con-
trolled by an east-dipping normal fault, antithetic to the Gubbio Fault. Standard dislocation modeling was used to
understand the role played by the Gubbio Fault and its antithetic. The Gubbio Fault was divided into a high-angle sec-
tion above 3.5 km and a low-angle section between 3.5 and 6 km depth. Based on different tests we conclude that
both sections of the Gubbio Fault as well as the antitethic fault contributed to the present setting of the basin. At pres-
ent the antithetic fault appears to be the most effective in producing a geomorphic signature and controlling the basin
width. The high-angle Gubbio Fault played a major role in the basin growth but now its activity rate appears minor.
Because of the characteristics and location of the 1984 earthquake, the low-angle Gubbio Fault is assumed to be
presently active and seismogenic. Based on the integration of geologic, geomorphic and seismological data we sug-
gest that the low-angle Gubbio Fault is formed by two individual sources capable of M 5.3-5.9 earthquakes. The
southern source ruptured in the 1984 earthquake while the northern source did not rupture recently nor historically.

838

Stefano Pucci, Paolo Marco De Martini, Daniela Pantosti and Gianluca Valensise

normal
faults

thrusts

C. di Castello

Gubbio

G. Tadino

Assisi
Perugia

Colfiorito

Norcia

Tiber basin

Altotiberina
fault

Umbrian
Faults

System

Plio-Quaternary
basins

study area

Sansepolcro

d
e

p
th

(
km

)

Tiber valley GubbioWSW ENE

1984 Gubbio Eq.
Ms 5.3

20 km Altotiberina fault

04/29/1984
Ms = 5.3

09/26/1997(11.40)
Mw = 6.0

09/26/1997(02.33)
Mw = 5.7

10/14/1997
Mw = 5.6

Fig. 1. Structural sketch of the Umbria-Marche domain (from Barchi et al., 2001, modified) showing the alignment of
the intramountain basins along the Umbrian Fault System (sensu Barchi, 2002), which coincides with the main seismo-
genic zone of Central Apennines defined by Valensise and Pantosti (2001a). Focal mechanisms and magnitudes are for
the 1997, Colfiorito sequence from Ekström et al. (1998) and for the 1984 Gubbio earthquake from Dziewonski et al.
(1985) and ISC (2001). The lower left inset includes a schematic regional geological section integrated with the interpre-
tation of seismic profiles and showing also the 1984 Gubbio earthquake mainshock location (after Boncio et al., 1998,
modified).

How many seismogenic sources lie beneath
the basin?

What is its (their) geometry?
What is the largest earthquake expected for

the area?
What is the expected seismic behavior of

this (these) source(s)?

Notwithstanding a rich literature describing
various aspects of the basin geology and struc-
ture, limited attention was devoted to active tec-
tonics and particularly to the study of landscape
as a direct response of repeated coseismic sur-
face deformation (Selvaggi and Sylos Labini,
1989; Menichetti, 1992; Boncio et al., 1996).

To contribute to the understanding of the
local earthquake potential we investigated the re-
lationships between seismicity and tectonics by
carrying out new geomorphic studies of the Gub-
bio Basin based on field and aerial photo surveys
and DEM analyses. Our approach is based on the
awareness that landscape evolution may contri-
bute to describing the seismic history of a region.
We also compared the deformation patterns of
different combinations of active faults based on
standard dislocation modeling. In the following
we summarize the present knowledge on the
area, present the new geomorphic data, and dis-
cuss different alternatives of seismogenic source
geometry and their possible implications for
defining the activity of the Gubbio Basin.

2. The Gubbio Basin

2.1. Geological and structural setting

The study area is located in the inner part of
the Umbria-Marche Apennines (Central Italy)
(Lavecchia and Pialli, 1980). A thick skinned
style of compressional deformation, characteri-
zed by a system of multiple superposed detach-
ments, generated a set of NW-SE trending, NE-
verging folds and thrusts during Langhian-
Serravallian (Barchi, 1991; Barchi et al., 1998).
The result of the eastward migration of the deep
detachment layers (base of the carbonatic mul-
tilayer) is the deformation of the more external,
partially grown structure such as the upper
detachment layers located within Miocene fly-
sch formations. The bedrock of the area is com-
posed by the upper part of the Umbria-Marche
sequence: a pelagic carbonatic multilayer (Juras-
sic-Oligocene) overlain by a Miocene flysch se-
quence (Bonarelli et al., 1952; Moretti and Perno,
1968; Damiani et al., 1983; Menichetti and Pialli,
1986). The Gubbio Basin is bounded to the east by
one major box-shaped, NE-verging carbonatic
anticline (referred to as Gubbio anticline), whereas
to the west it is bounded by folded flysch deposits
(Bonarelli et al., 1952; Moretti and Perno, 1968)
(fig. 2).

Starting in Lower Pleistocene, extension
took over the compressional tectonics produc-
ing structures with a main NW-SE direction.

West of the Gubbio Basin, high-angle normal
faults both SW and NE dipping are recogniz-
ed (Menichetti and Minelli, 1991; Menichetti,
1992). However, the most impressive exten-
sional structure bounds the basin to the east and
is likely the NW-striking, SW-dipping, Gubbio
Fault which dissects, parallel to its axis, the
Gubbio anticline down-throwing the SW flank
well below the basin sediments. It consists of a
120°- to 150°-striking, 50°- to 70°-dipping fault
planes, which, as a whole, show a gentle bend
(Collettini, 2001) (fig. 2).

The clear result of the long-term activity of
the Gubbio Fault is the topographic low, coin-
ciding with the Gubbio Basin, filled by con-
tinental deposits (GE.MI.NA, 1963) (fig. 2).
They consist of a sequence of three main sedi-
mentary units, resulting from the evolution of
the deposition from lacustrine toward fluvial
environment: 1) clayey-lignitic basal complex,
characterized by meter-thick lignite layers inter-
bedded with organic gray clay, locally with
sand and conglomerates; 2) clayey-sandy com-
plex comprising sand, sandy clay interbedded
with sandstone conglomerate lenses; 3) upper
alluvial complex, composed by organic sandy-
clayey matrix with calcareous conglomerate.
On the basis of palinological analysis, the unit
1 is referred to the interglacial period Günz-
Mindel (500-400 Ky) (GE.MI.NA, 1963) or to
the Lower Pleistocene (Lona and Bertoldi,
1972), this latter estimate is in agreement with
the fossil fauna (lacustrine facies) found in this
layer (Bonarelli, 1891).

Data to attempt a reconstruction of the
subsurface setting of the basin are available
too. Deep boreholes by GE.MI.NA (1963) were
used to infer the geometry of the basin and of
the filling deposits. By correlating their stratig-
raphy GE.MI.NA (1963) imaged: 1) NE-dip-
ping deposits, with dips generally increasing
toward the Gubbio Fault; 2) a deepening toward
NE of the bedrock/basin filling deposit contact;
3) a Pleistocene depocentre, up to 500 m below
the present basin surface, located close to the
Gubbio Fault. However, because of the uneven
distribution of the boreholes in the basin (main-
ly located along the western side and in the
southern part) this reconstruction contains large
uncertainty. Menichetti (1992) re-interpreted

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Geomorphology of the Gubbio Basin (Central Italy): understanding the active tectonics and earthquake potential

these data integrating them with more recent
boreholes stratigraphy and new electrical and
seismic surveying parallel and perpendicular
to the basin (Giaquinto et al., 1991), reconstruct-
ing the geometry of the pre-Pleistocene bedrock.
He describes the basin as the typical result of the
evolution of an half-graben, with a wedge geom-
etry, and progressive thickening of the lacustrine
deposits controlled by a SW-dipping normal fault:
the Gubbio Fault. However, this reconstruction
shows a peculiar bathtub shape, about 400 m-
deep, with flat bottom and long steep facing
flanks. The pre-Pleistocene depocentre, deeper in
the northern part of the basin, appears to be cen-
tered within the valley, with a geometry that is not
expected for a typical half-graben (fig. 3a).

Recently, commercial reflection profiles al-
lowed Barchi et al. (2000) to perform a detailed
reconstruction of the Gubbio fault isobaths. The
resulting picture of the fault plane at depth
shows an high-angle dip of the upper fault por-
tion and a low-angle dip below ~ 3.5 km, due to
the reactivation of a pre-existing thrust plane.
The interpretation of these seismic profiles
images the Gubbio Fault as an antithetic of the
E-dipping, low-angle, Altotiberina Fault (fig. 1;
Barchi et al., 1998; Boncio et al., 1998). The
Altotiberina Fault is a major extensional dis-
continuity in the Umbria-Marche Apennines. It
affects the upper crust, at least up to 15 km
depth, beneath the thrust and fold belt. The
Altotiberina Fault follows a staircase trajectory

840

Stefano Pucci, Paolo Marco De Martini, Daniela Pantosti and Gianluca Valensise

Ponte D'Assi

Gubbio Gubbio Basin
watershedwatershed

Mocaiana

Gubbio

Padule

Ponte d'Assi

Branca

regional divide

Mt. Urbino

study area

Chiascio River4 km

carbonates
(Jurassic-Oligocene)

alluvial deposits
(Holocene)

fluvio-lacustrine
deposits

(Pleistocene)

fluvio-lacustrine
deposits

(Pleistocene)

fluvio-lacustrine
deposits

(Pleistocene)

turbidites (Miocene)turbidites (Miocene)turbidites (Miocene)

thrust faultsthrust faultsthrust faults

normal faultsnormal faultsnormal faults

alluvial fan (Holocene)
and slope debris

(Late Pleistocene-Holocene)

alluvial fan (Holocene)
and slope debris

(Late Pleistocene-Holocene)

alluvial fan (Holocene)
and slope debris

(Late Pleistocene-Holocene)

Fig. 2. Simplified geological map of the Gubbio Basin. Thick normal faults are discussed in this work (after
Bonarelli et al., 1952; Moretti and Perno, 1968; Menichetti and Pialli, 1986; Collettini, 2001, modified).

and shows an offset as much as 5 km. The iso-
baths of the Gubbio Fault confirm that the left
bending of the structure, shown by its surface
trace, is well recognizable even at depth (Barchi
et al., 2000). The isobaths of the Gubbio Fault
also display a rise of the branch line with the
Altotiberina Fault from 6 km in the southern
part to 4.5 km in the central part to get deeper
again toward the north. The analysis of denser
seismic profiles array allowed Mirabella (2003)

to derive additional characteristics of the Gub-
bio Fault. The fault boundaries are quite consis-
tent with the end of the basin to the north, where-
as, to the south the fault appears to continue
even outside the basin for a total length of about
30 km although with minor evidence (fig. 4).
The total throw of the Gubbio Fault along strike
reaches a maximum of about 2500 m near the town
of Gubbio and shows a sharp decrease toward the
northern and southern tips (Mirabella, 2003).

841

Geomorphology of the Gubbio Basin (Central Italy): understanding the active tectonics and earthquake potential

Fig. 3a,b. a) 3D reconstruction of the pre-Pleistocene basement (after Menichetti, 1992, redrawn); b) 3D view
of the northeastern side of the Gubbio Basin derived by 5-m resolution DEM. Hypothesized Gubbio Fault splays
are reported along with to the location of the northern part of the pre-Pleistocene depocentre suggested by
Menichetti (1992).

Slope debris

carbonates

Gubbio Fault trace

alluvial complex

Gubbio

Mocaiana

alluvial fan

left step

artificial cut (fig. 14)

pre-Pleistocene depocentre

paleosoil sample location
fanhead entrenched

paleosurface

500

ele
vat

ion
(m

) a.
s.l.

Mocaiana

Gubbio

left step

splay

pre-Pleistocene depocentre

2.2. Seismicity

The Gubbio Basin is located within the main
NW-SE Apennines seismogenic belt, which
extends from Tuscany to Calabria (Valensise and
Pantosti, 2001a). In the Umbria-Marche region,
most of the historical and instrumental seismici-
ty is related to the activity of the NNW-SSE
trending SW dipping normal faults alignment
(Umbria fault system, sensu Barchi et al., 2000).
This system extends through the intra-mountain
basins of Norcia, Colfiorito, Gualdo Tadino,
Gubbio and Sansepolcro (fig. 1).

Historical earthquakes of I > VII MCS (Mer-
calli-Cancani-Sieberg scale) are located both to
the north (Citerna-Monterchi, April 1917 Imax =
= IX; Cagli, June 1781 Imax = IX-X; Monterchi,
December 1352 Imax = IX; Città di Castello, April
1458 Imax = IX; Valtiberina, September 1789 Imax =
VIII-IX; Bocca Serriola, October 1389 Imax = IX;
CPTI, Working Group, 1999) and to the south
(Gubbio, April 1593 Imax = VII; Gualdo Tadino,

July 1751 Imax = X; Fiuminata, April 1747 Imax =
= IX; Camerino, April 1279 Imax = X; CPTI,
Working Group, 1999) of the Gubbio Basin.

Instrumental seismicity (Md > 1.5), recorded
in the region by the Istituto Nazionale di Geofi-
sica e Vulcanologia in the period 1985-2000
(INGV, 1985-2000), is mainly located south of
the Gubbio Basin and it is related to the 1997-
1998 Colfiorito earthquake sequence character-
ized by normal faulting events (fig. 5).

On April 29, 1984 the study area was
struck by a Ms 5.3 (NEIC) earthquake (MO
3.4 1024 dyne. .cm; Dziewonski et al., 1985)
located at Mt. Urbino, about 10 km south of
the town of Gubbio at 7 km depth (ISC, 2001).
A local network, set in the epicentral area two
days after the mainshock, recorded 300 after-
shocks, 4 of which had Ms > 4.0. The after-
shocks occurred in an area of about 20 × 6 km
elongated in a N140° direction. Their distribu-
tion appears to delineate two linear parallel
clusters, the southernmost of which is denser
and includes also the mainshock (see inset in
fig. 4). These clusters describe two high-angle,
west-dipping planes, the northernmost being
characterized by superficial events with hypo-
central determinations between 3 and 5 km
(Haessler et al., 1988). These planes are locat-
ed 7-12 km west of the Gubbio Fault and seem
to be not directly related to this major structure
(Collettini et al., 2003). The CMT (Dziewon-
ski et al., 1985) and first-motion (Westaway et
al., 1989) focal mechanism solutions of the
mainshock agree substantially on fault strike
and kinematics but not on fault dip (see inset
in fig. 5). The 1984 Gubbio earthquake instru-
mental data has been used in the definition of
three seismogenic sources recently published
in Valensise and Pantosti (2001b). They solved
the discrepancy between aftershock distribu-
tion and CMT focal mechanism, which better
fits the Gubbio fault at depth, as possibly due
to the fact that aftershocks were recorded a few
days after the mainshock and thus may not
directly be related to the main fault but to stress
readjustment within the fault hangingwall.

In May and June 1987 a local network was
installed again in the area (Deschamps et al.,
1989) and four hundreds events with magnitude
between 0.8 and 3.1 were recorded. These were

842

Stefano Pucci, Paolo Marco De Martini, Daniela Pantosti and Gianluca Valensise

Jurassic-Oligocene Fm.

Miocenic Fm.
Pleistocenic Fm.Pre-Pleistocene depocentre

6
7

3000

2500

2000

1500

1000

500

0
0 5 10 15 20 25

fault lenght (km)

d
is

p
la

ce
m

e
n
t
a
lo

n
g
t
h
e
f
a
u
lt

p
la

n
e
(

m
)

t
o
p
o
g
ra

p
h
ic

t
h
ro

)

2500

2000

1500

1000

500

250

measure points along topographic
profiles and seismic lines

NW SE

geologic displacement

topographic throw

footwall
hangingwall

basin lenght (km)

Fig. 4. Comparison between geologic displacement
and topographic throw along the Gubbio Fault strike.
Geologic throw has been calculated by using seismic
lines on the base of the Marne a Fucoidi beds (115-95
My) offset (from Mirabella, 2003). Topographic throw
is measured as difference between highest elevation
and surface of the basin. The position of the pre-
Pleistocene depocentre, as reconstructed by Meni-
chetti (1992), the basin length and the lithologies
cropping out in the fault footwall are reported too.

organized in two clusters NW-SE trending located
near Gubbio and Valfabbrica towns. 58% of the
hypocentral determinations concentrated between
3 and 7 km depth, with a clear deepening toward

SE. A selected group of 33 well-constrained focal
mechanisms indicates a minimum horizontal prin-
cipal stress tensor ( .3) oriented 219 (Boncio et al.,
1996).

843

Geomorphology of the Gubbio Basin (Central Italy): understanding the active tectonics and earthquake potential

Fig. 5. Instrumental and historical seismicity of the Gubbio Basin area. Historical seismicity with intensity larger
than VII MCS is reported for the period 217 B.C.-1991 A.C. (CPTI, Working Group, 1999). The inset shows the
4/29/1984 Gubbio earthquake sequence; gray star shows the mainshock, white stars show aftershocks with M > 4 and
rectangles show aftershocks with M < 4 (modified after Haessler et al., 1988). Focal mechanism parameters: strike
143 dip 21 rake – 72, strike 304 dip 70 rake – 97 (Dziewonski et al., 1985); strike 135 dip 55 rake – 90, strike 315
dip 35 rake – 90 (Westaway et al., 1989).

Boncio et al. (1998), interpreting all the avail-
able instrumental seismic data, propose that the
Altotiberina Fault controls the location of the local
seismicity, all concentrated in its hanging-wall,
close and east of the branch-line with the Gubbio
Fault. Consequently, the historical damaging
earthquakes are located east of Gubbio Basin,
where the Altotiberina Fault deepens (fig. 1). On
the basis of the substantial agreement between the
kinematics of the major active faults, the Qua-
ternary tectonics, paleostress computations and
the seismologic extensional tensor Boncio et al.
(1996) suggest that the stress field in the area is
constant at least since Upper Pleistocene.

These observations are not definitive in the
understanding of the present activity of the
Gubbio structure but, on the contrary, raise
important questions about the true role of the
Gubbio Fault in the seismogenesis of the region.

3. Geomorphic evidence of active tectonics

To analyze the geomorphic, recent tectonic
and geologic evolution of the Gubbio Basin
we performed a field survey focused on geo-
logic mapping at 1:25.000 scale, supported by
1:33.000 scale aerial photos analyses (IGM,
years 1955/1956, 1991 and 1994). We mapped
drainage, erosional and terrace surfaces, scarps,
peaks, saddles, sharp ridges, deep linear ero-
sions, structural lineaments, faults, alluvial
fans, etc. (fig. 6). At the same time we used dig-
ital cartographic data at a 1:5000 scale, provid-
ed by the Umbria region, to build a 5 m resolu-
tion DEM (Digital Elevation Model). By using
a GIS program we geo-referenced all the data
collected and performed morphometric analy-
ses of the topography and of the drainage pat-
tern. We used the GE.MI.NA. (1963) stratigraph-
ic scheme as a reference for the classification of
the continental units outcropping in the study
area (fig. 2).

3.1. Physiography

The alluvial complex crops out over most of
the Gubbio Basin (fig. 2). Its sedimentary facies
has been interpreted as the final stage of the

Pleistocene lacustrine infill (GE.MI.NA., 1963).
It is composed by low gradient alluvial fans that
have been fed by transversal streams flowing
through the carbonatic anticline from NE to SW
(fig. 6). Four principal fans can be distinguished
along the eastern side of the basin: one isolated
is near Mocaiana village, while the other three
coalesced north and south of Gubbio town. The
alluvial complex, which hides the lower lacus-
trine sequence (GE.MI.NA., 1963; Giaquinto et
al., 1991), appear to control the SW dip of the
basin floor (fig. 7, cross sections 3 to 9). In gen-
eral, the alluvial complex does not experience
strong erosion, with the exception for the allu-
vial fan near Mocaiana, the northern part of
which has been eroded by the Assino River (fig.
6). Moreover, the alluvial fan, just north of
Gubbio (fig. 7, cross section 13), acts as local
divide for the drainage basin (see Section 3.2.).

The Gubbio Basin is a 22 × 4 km, NW-
trending elongated depression. The slopes
bounding the basin exhibit a different morpho-
logy depending both on their lithology and
structural setting. The basin floor lies at about
400 m a.s.l. whereas the eastern and western
ranges reach elevations of about 950 and 650 m
a.s.l., respectively.

The eastern flank of the basin has the steep-
est slope (fig. 7) and presents an important
debris deposition at its foot. The declivity of its
northern section shows a typical regularized
slope replacement profile (Bartolini, 1992) with
a homogeneous inclination of about 30° (fig. 8,
cross sections 5 and 6). Differently, the south-
ern part exhibits a higher inclination (35°) and
profile irregularities, such as scarps, can be
observed on the slope debris, but not in corre-
spondence of the bedrock fault plane (fig. 8,
cross sections 1, 2, 3 and 4). The eastern slope,
carved by wide triangular facets also detectable
from DEM (see inset of fig. 8), is modeled on
back-dipping Jurassic-Oligocene bedding of the
footwall formations. It represents the source of
high-angle young fans developed over slope
debris and older fans of the alluvial complex
(fig. 6). This flank appears strongly controlled
by the activity of the Gubbio fault and, because
of the significant slip accommodated, only the
eastern limb of the Gubbio anticline crops out.
The resulting occurrence of strong erosional

844

Stefano Pucci, Paolo Marco De Martini, Daniela Pantosti and Gianluca Valensise

processes is testified by the larger amount of
continental sediments, which buried most of the
down-thrown anticline back-limb. We dated a
paleosoil, found inside the slope debris deposits
north of Gubbio by means of radiocarbon analy-
sis (fig. 3b for location), this yielded an age of
39 ky BP. Between Gubbio and Mocaiana (figs.

2 and 3b), where two sections of the fault over-
lap, this slope debris has the largest surface
extension and reaches the highest elevations. To
the northwest, the surface slope of these deposits
is higher than their structural dip (about 15°)
indicating that its dynamic development is pre-
dominantly governed by a strong erosional activ-

845

Geomorphology of the Gubbio Basin (Central Italy): understanding the active tectonics and earthquake potential

Fig. 6. Map of Quaternary geology and geomorphology of the northern and central portion of the Gubbio Basin
from aerial photo and field survey. The lower left inset shows the location of the study area (see also dashed area
in fig. 2) on the 5-m resolution DEM.

alluvial deposits (Holocene)

eluvial-colluvial deposits (Holocene)

alluvial fan deposits (Holocene)

slope debris deposits (Holocene-Late Pleistocene)

alluvial complex (Late Pleistocene)

clayey-sandy complex (Middle Pleistocene)

clayey-lignitic complex (Early Pleistocene)

bedding

saddle

peak

400-420 m a.s.l.

420-430 m a.s.l.

430-440 m a.s.l.

440-450 m a.s.l.

450-480 m a.s.l.

480-540 m a.s.l.

540-640 m a.s.l.

fan

strong linear erosion

scarp

tectonic lineament

sharp ridge

paleosurfaceslitology

ity interpretable as due to the regional tectonic
uplift or as caused by local lowering of the basal
level. Anti-dip streams participate in the model-
ing of this slope by removing debris deposits and
by forming minor entrenched Holocene alluvial
fans (figs. 3b and 6). Sinuosity of the mountain-
piedmont contact is quite high, typical of slopes
controlled by low activity faults (Bull and Mc-
Fadden, 1977).

An along strike comparison between the
geologic displacement on the Gubbio Fault and
the topographic throw (difference between the
maximum elevations of the Gubbio anticline
and the elevations of the basin floor, measured
along transects perpendicular to the Gubbio
Fault) is shown on fig. 4 (Collettini et al., 2003).
The location of the two maximums does not
coincide, suggesting that the differential ero-
sion has a strong influence in the landscape
evolution. It is also to be noted that the pre-
Pleistocene depocentre (Menichetti, 1992) is
located exactly where the geologic displace-
ment reaches its maximum. In conclusion,
along this flank of the basin the action of the
surface processes appears to be in strong com-
petition and mostly prevailing over tectonics.

The western flank of the basin has a com-
pletely different morphologic expression with
respect to the eastern one. It does not show any
important folding structure and has low relief
energy. In fact, at the contact mountain-pied-
mont only colluvial deposits and low gradient
alluvial fans are found, as a consequence of
the range consumption (fig. 6). The bedrock
shows basinward, gently dipping beds, generally
with oblique strike with respect to the contact
bedrock/basin infill. Gravitational processes
have a strong control on the landscape evolution,
the bedrock being constituted by flysch deposits.
In the field, relevant fault mirrors do not crop
out, probably they are not preserved because of
their rheology. In addition, the long-term
deformation appears to be accommodated by
minor tectonic structures. The competition of the
landscape modeling with tectonics is minor and
thus the potential for the preservation of sur-
face evidence of recent faulting is higher. The
mountain-piedmont junction is noticeably dis-
sected, but the contact between the 15 and the
30 percent grades of the slope shows a clear
straight NW-SE trend, typical of fault-con-
trolled slopes.

846

Stefano Pucci, Paolo Marco De Martini, Daniela Pantosti and Gianluca Valensise

20 1 3

500

600

700

800

900

4
400 4

20 1 3 4 5 6

500

600

400 1

700

500

600

400
20 1 3 4 5

500

600

700

400

800

900

20 1 3 4 5 6
3

500

600

20 1 3 4 5 6
5

500

400

distance (km)
0 2.5 5 7.5 12.5 15 17.5 20 22.510

500

600

400
20 1 3 4 5 6 7

500

600

700

400
20 1 3 4 5 6 7

500

600

700

400
20 1 3 4 5 6 7 8

500

600

700

900

800
SW

0 1 2 3 4 5 6 7 8 9 10
7

500

600

700

800

900

20 1 3 4 5 6
6

500

600

400
20 1 3 4 5 6 7

500

600

400

21 3 4 5 6
12

e
le

v
a

ti
o

n
(

m
)

fault trace
1

3 4

5 6

7
8 9

10 11

Gubbio

location map of the
topographic profiles

13
3 km

Fig. 7. Topographic profiles across and parallel to the Gubbio Basin performed by means of the 5 m resolution
DEM. The position of the Gubbio Fault trace is shown in the profiles.

3.2. Drainage network

The complex drainage network of the
Gubbio Basin is shown in fig. 9. It is formed by
two secondary hydrographic networks, connec-
ted with the Assino and Chiascio rivers. These
streams, located at the northern and southern
end of the basin, respectively, flow perpendicu-
larly to the NW-SE trending compressional
structures. The whole Gubbio drainage basin
has a watershed that does not follow the main
elevations of the area, which coincides with the
carbonatic structure but, anomalously, it runs
always on the highest crests of the flysch for-
mations (fig. 9). As a consequence the north-
eastern part of the Gubbio Basin watershed is
peculiarly shifted to the east with respect of the
carbonatic anticline and coincides with the
Regional Divide. To the west the watershed is
more irregular and enlarges north of Mt. Urbino.

In the northeastern part of the Gubbio
drainage basin, a trellis water-network developed
with a NE-SW and a NW-SE trending patterns
(fig. 9). While the latter is organized in along-
strike valleys, emphasized by selective erosion,
the NE-SW creeks fed the major fans of the allu-
vial complex, cutting perpendicularly the carbon-
atic structure. Some of these streams evolved in
deep watergaps, which incised the slope, shap-
ing smoothed triangular facets (see inset of fig.
8). On the contrary, in the western flank of the
basin, composed by flysch formations, the
drainage is organized as a dendritic water-net-
work (fig. 9).

The Gubbio drainage network shows a
peculiar diverging trend, where the main fan
(fig. 6), close to the Gubbio town, acts as a
water divide between the North Saonda Creek
and the South Saonda Creek flowing NW-SE
with opposite directions (fig. 9, anomaly d).

847

Geomorphology of the Gubbio Basin (Central Italy): understanding the active tectonics and earthquake potential

1 trace of the
Gubbio fault

trace of the
topographic

profiles

paleosurfaces

2 km

Gubbio

0 400 800 1200 1600

400

800

1200

distance (m)

e
le

va
tio

n
a

.s
.l.

(
m

)

carbonates
fluvio-lacustrine complexes

alluvial complex

slope debris

Gubbio Fault

slope rupture

SW NE

400

800

0 400 800 1200 1600
distance (m)

e
le

va
tio

n
a

.s
.l.

(
m

)

alluvial complex

slope debris

Gubbio Fault

slope rupture

SW NE

fluvio-lacustrine complexes

carbonates

0 400 800

400

800

distance (m)

e
le

va
tio

n
a

.s
.l.

(
m

)

0 800

400

800

400
distance (m)

e
le

va
tio

n
a

.s
.l.

(
m

)

0 400 800 1200

400

800

distance (m)

e
le

va
tio

n
a

.s
.l.

(
m

)

0 400 800 1200

400

800

distance (m)

e
le

va
tio

n
a

.s
.l.

(
m

)

Gubbio Fault

slope rupture

SW NE

SW NE SW NE

SW NE

4 6

3Mocaiana

Fig. 8. Detailed topographic profiles, across the Gubbio Fault. All the profiles show the location of slope rup-
tures, the bedrock fault trace and a schematic geological setting.

These main creeks show a clear linearity, flow
parallel to the maximum basin elongation, and
are located close to its western flank. Thus they
are in asymmetry with respect to the axis of the
basin and also in disagreement with the pre-
Pleistocene depocentre (fig. 3a,b). This asym-
metry may be explained with a progressive mi-
gration of the rivers westward as the fans pro-
graded. However, the rivers linearity and low fan
dip, coupled with the observation that the South
Saonda Creek flows close to the western edge of
the basin, even where large alluvial fans are not
present, are suggestive of a tectonic control.

Within the Gubbio Basin, the South Saonda
Creek incises the lacustrine sedimentary se-
quence up to the lowest units, both along the
NE-SW gradient direction and perpendicular to

it. On the contrary, the North Saonda Creek is
less developed, in fact Middle Pleistocene depo-
sits are incised only near the confluence with
the Assino River and the alluvial complex is still
well preserved. South of Mocaiana, the gentle
geomorphology between two coalescent allu-
vial fans, deposited at the end of lacustrine
cycle, presents a local convergence of drainage
(fig. 9, anomaly c).

Most stream anomalies have been recog-
nized in the southern part of the basin, where
the South Saonda Creek appears to be left step-
ped and composed by two main sections, NW-
SE trending (fig. 9, anomaly e). South of Ponte
D’Assi, where this creek reduces the ampli-
tude of its Holocene deposition and assumes
a degradational behavior, its sinuosity index

848

Stefano Pucci, Paolo Marco De Martini, Daniela Pantosti and Gianluca Valensise

4 km

Gubbio Gubbio Basin
north watershednorth watershed

Gubbio Gubbio Basin
south watershedsouth watershed

Mocaiana

Gubbio

Padule

Ponte D'Assi

Branca

regional divide

local divides

Mt. Urbino

North Saonda Creek

South Saonda Creek

Chiascio River

Assino River
c

e
a

carbonates
(Jurassic-Oligocene)

alluvial deposits
(Holocene)

fluvio-lacustrine
deposits

(Pleistocene)

fluvio-lacustrine
deposits

(Pleistocene)

fluvio-lacustrine
deposits

(Pleistocene)

turbidites (Miocene)turbidites (Miocene)turbidites (Miocene)

drainage anomaliesdrainage anomaliesdrainage anomalies

normal faultsnormal faultsnormal faults

alluvial fan (Holocene)
and slope debris

(Late Pleistocene-Holocene)

alluvial fan (Holocene)
and slope debris

(Late Pleistocene-Holocene)

alluvial fan (Holocene)
and slope debris

(Late Pleistocene-Holocene)

Fig. 9. Drainage map of the Gubbio area. Thick dashed line divides the Gubbio area in two smaller drainage
basins: the North and the South Saonda creek basins. Simplified geology is reported.

849

Geomorphology of the Gubbio Basin (Central Italy): understanding the active tectonics and earthquake potential

0 0.4 0.8
400

440

480

520

560

2
0 0.4 0.8

380

420

460

500

540

1
0 0.4 0.8

420

460

500

540

0 1 2
400

440

480

88885
0 1 2

400

440

480

520

0 0.4 0.8 1.2 1.6
380

420

460

500

540

107

0 1 2
420

460

500

540

lenght (km)
0 4 8 12 16 20

320

360

400

440

South Saonda Creek

A
B

0 4 8 12
360

400

440

480

520

North Saonda Creek

Assino River
C

Holocene
deposits

location
of knickpoints

1 streams

normal faults

area of increased sinuosity

area of increased degradation

3 km

1 km

Mocaiana

Gubbio

Assino River

Ponte D'Assi

Padule

North Saonda Creek

South Saonda Creek

Ponte D'Assi

5
6

Fig. 10a,b. a) Longitudinal profiles of the western basin streams performed from 5 m resolution DEM with
knick-points location; b) detail of the South Saonda Creek showing changing behavior running from the footwall
to the hangingwall of a fault section.

850

Stefano Pucci, Paolo Marco De Martini, Daniela Pantosti and Gianluca Valensise

decreases from 1.6 to 1.1 (fig. 10b). Two knick
points have been detected along the South
Saonda Creek (fig. 10a), one is located near
Ponte d’Assi (knick point B) and seems to be
related to regressive erosion, while the south-
ernmost (knick point A), being in line with a
tectonic lineament (mapped through aerial pho-
tos interpretation), has been tentatively inter-
preted as controlled by the activity of the struc-
ture. In the same area, west of Ponte D’Assi, the
hierarchic organization of several NE-SW ori-
ented creeks may suggest, again, a recent tec-
tonic control on the development of the drain-
age network.

In the northern part of the Gubbio Basin, the
longitudinal profile traced along the North
Saonda Creek shows the presence of a knick
point at the northern threshold of the Gubbio
plain (fig. 10a, knick point C) and consequently
testifies the capture of the North Saonda Creek
by the Assino River. The location of this cap-
ture through a threshold located in an area of
local topographic high, induced us to consider a
possible tectonic control by a NW-SE trending
lineament bounding the northwest side of the
basin (fig. 6). Similar analyses have been per-
formed along the tributaries of the North and
South Saonda creeks. Several of these draina-
ges, incised on the flysch deposits of the west-
ern side of the basin, show non-equilibrium pro-
files and knick points, which align in a NW-SE
direction, this is also suggestive of tectonic control
(fig. 10a).

3.3. Paleosurfaces

Within the Gubbio Basin we have mapped
several relict surfaces (fig. 6), which are evi-
dence of paleolandscapes; these include both
erosional surfaces, carved on the local bedrock,
and lacustrine depositional terraces. The major-
ity of these surfaces are found in the northwest-
ern part of the basin at different elevations.
Because of the lack of direct dating, we attempt
their analysis by sorting them by height in
seven groups, which are reported in the fig. 6
with different colors. A correlation among all
this surfaces being very difficult, we looked
for their possible relations with other geomor-

phic elements such as scarps, lineaments and
streams. The majority of these surfaces cannot
be unequivocally attributed to variations of the
Pleistocene lake level or to recent tectonic dis-
locations.

Along the western slope of the basin it is
possible to observe a large number of surfaces,
widely distributed, and, in particular, the best-
preserved sequence is located near Ponte d’Assi
(fig. 6). Differently, the eastern slope of the
basin does not show any comparable sequence
of surfaces nor indented on the slope debris or
on the alluvial complex. A series of topograph-
ic profiles, both parallel and perpendicular to
the western basin flank, are shown in figs. 11
and 12a-d to evidence the relative height of the
surfaces. At the northern termination of the
basin, the principal surfaces, both erosional and
depositional, progressively decrease in eleva-
tion toward south. In fact, the highest deposi-
tional surfaces, belonging to the sandy-clay com-
plex, lie at a maximum height of 540-560 m
a.s.l. in the northern area whereas, appear, at an
elevation of 450-480 m a.s.l. near Ponte d’Assi
(fig. 11, cross-section 5 and 6; fig. 12a-d, cross-
section 12).

In order to analyze the distribution and rela-
tion of the different surfaces with respect to the
present topography, we simulated a draining se-
quence of the Gubbio Basin by subsequently set-
ting the lake level at 535, 480, 440 and 420 m
a.s.l. (fig. 15a-f). When we fix the lake level at
the highest elevation (535 m a.s.l.) (fig. 13) only
few surfaces appears. At the northern termina-
tion of the basin, depositional surfaces topping
the lacustrine sequence can be seen where the
geologic throw of the Gubbio Fault tapers to
zero (figs. 4 and 7, between cross sections 1 and
2). Along the central part of the eastern flank of
the basin, some NW-SE elongated small sur-
faces, indented on the debris deposit, show up.
Taking into account their position, shape and
substratum, these surfaces have been interpret-
ed as slope ruptures, probably related to sec-
ondary splays of the Gubbio Fault (fig. 3b). On
the western side of the basin, surfaces carved in
bedrock up to 640 m a.s.l. determine a staircase
topographic profile about 3 km NW of Ponte
D’Assi and present their outer edges bound-
ed by tectonic lineaments. The western flank

851

Geomorphology of the Gubbio Basin (Central Italy): understanding the active tectonics and earthquake potential

Fig. 11. Detailed topographic profiles, across and along the western slope of the basin, performed from 5-m
resolution DEM. The location of the main paleosurfaces (dark gray) together with the traces of the observed tec-
tonic lineament is shown. Profile 1, 2 and 3 are shown in overlap to highlight the elevation decrease of the paleo-
surfaces between north and south.

852

Stefano Pucci, Paolo Marco De Martini, Daniela Pantosti and Gianluca Valensise

8
a)

0 200 400 600 800

380

400

420

440

460

480

12
0 200 400 600

380

400

420

440

460

480

distance (m) distance (m)

e
le

va
tio

n
(

m
)

e
le

va
tio

n
(

m
)

Fig. 12a-d. 3D view of some paleosurfaces and scarps, from 5 m resolution DEM, along with their aerial pho-
tograph view (a and b) and their topographic profiles (12 and 8). c) Landscape photograph detail of a paleo-
surface of fig. 12a. d) Landscape photograph of a scarp crossing a recent alluvial fan (see fig. 11 for the loca-
tion).

assumes an almost rectilinear, NW-SE trending
attitude only when we decrease the water level
at 480 m a.s.l.; conversely, the eastern side
appears strongly controlled by the Gubbio Fault
since the earlier stages (fig. 13). Going on with
the draining sequence simulation up to 420 m
a.s.l., we observed: 1) the presence of several sur-

faces at the northern end of the basin, main-
ly on lacustrine deposits; 2) a discontinuous
distribution of surfaces, both on bedrock and
continental deposits, often organized as stair-
case flats included within parallel tectonic lin-
eaments along the western flank; 3) the absen-
ce of analogous surface sequence along the

853

Geomorphology of the Gubbio Basin (Central Italy): understanding the active tectonics and earthquake potential

Gubbio

Padule

Mocaiana

Ponte d'Assi

Gubbio

Padule

Mocaiana

Ponte d'Assi

Gubbio

Padule

Mocaiana

Ponte d'Assi

Gubbio

Padule

Mocaiana

Ponte d'Assi

3 km 3 km

lake level
535 m a.s.l.

3 km
3 km

lake level
480 m a.s.l.

lake level
440 m a.s.l.

lake level
420 m a.s.l.

N N

Fig. 13. Draining sequence simulation of the Gubbio Basin obtained by subsequently setting the lake level at
535, 480, 440 and 420 m a.s.l. See legend of fig. 6 for the reported paleosurfaces. Topographic base acquired
from 5-m resolution DEM.

eastern side, where alluvial deposition dom-
inates. We also noted that the lower surfac-
es along the western slope of the basin are
formed only on Pleistocene continental de-
posits, with the outer edge scarps general-
ly parallel to the North and South Saonda
creek.

3.4. Morphotectonic observations

Morphostructures such as tectonic linea-
ments, escarpments and bedrock fault planes
have been mapped in the Gubbio Basin (fig. 6).

Different authors suggested the existence
of active tectonic in the Gubbio Basin. Boncio
et al. (1996) describe deformed slope debris
directly in contact with the fault plane, at a
location about one kilometer SE of the town of
Gubbio. However, we did not notice any clear
deformation possibly linked to recent activation
of the Gubbio Fault at the site. Selvaggi and
Sylos Labini (1989) observed a 2 km long
scarplet displacing the deposits of the Gubbio
fans. Archeological literature (Branconi and
Manconi, 1982-1983) describes in this site a
buried ancient Roman wall. Although the pub-
lished evidence appears to be highly question-
able, the signature of active tectonic processes in
the area can be found through detailed analysis.

Along the eastern flank of the basin, we
noticed that the Gubbio Fault Zone exhibits a
clear geometric segmentation with a left step of
the fault plane exposed between Gubbio and
Mocaiana. The same zone of stepover is also
recognized at depth, from the reconstruction of
the bedrock/basin infill contact (Menichetti,
1992). As a whole, the Gubbio Fault Zone has a
concave shape with dip directions from N240°
to N220° moving northward. Topographic pro-
files show several irregularities on the slope de-
bris identified as morphological scarps (fig. 8,
cross sections 1, 2, 3 and 4). Even though part
of them may be related to human activities, it is
interesting to note that they are often rectilinear
and aligned for a maximum length of two kilo-
meters. Calcareous bedrock has been exhumed
from the debris cover in the hangingwall of the
outcropping Gubbio Fault, due to erosion and
to extensive quarrying. Such bedrock appears to

be located between slope ruptures and the
bedrock fault planes, suggesting the existence
of at least two main fault splays below the con-
tinental deposits (fig. 3b). At the northeast-
ern edge of the basin, several shear structures,
affecting the debris polycyclic depositional
horizons, can be recognized on the walls of
large artificial cuts. One of these cuts was stud-
ied in detail (1/20 scale) (fig. 3b for location).
In fig. 14a,b, vertically re-oriented clasts and
offset, of the order of few centimeters, of dis-
tinct layers indicate the presence of small NW-
trending and SW dipping normal faults. Con-
sidering the steep slope related to this side of
the basin, the discrimination between tecton-
ic and gravitational origin of these structures re-
mains unclear.

Along the western flank of the basin two
new morphologic lineaments have been recog-
nized (fig. 6). These two NW-SE striking linea-
ments are parallel. The western one is defined
by several linear sections, arranged in a right
stepping system, and is entirely located on bed-
rock. Several geomorphological features con-
tribute to the identification of this structure: sad-
dles, tops, escarpments and staircase surfaces
included within parallel sections (fig. 6). The
eastern lineament is located at the bottom of the
slope and is identified by aligned scarps, both in
colluvium and bedrock, and by paleosurface
outer edges (fig. 11, topographic profiles 7, 9
and 11). Although taking in consideration the
possibility for a fluvial-lacustrine origin for the
alignment of these outer edges, we propose a
tectonic origin, mainly because of a clear eleva-
tion change in their along strike topographic
profile (fig.11, topographic profile 13). In gen-
eral, the two lineaments have been recognized
and mapped in detail for more than 15 km, from
the northern termination of the basin to the
south. These lineaments are interpreted as the
expression of an east-dipping normal fault,
which describe, as a whole, a concave geometry
mirroring that of the Gubbio Fault (fig. 2).

3.5. Defining active faults in the Gubbio Basin

The Gubbio Basin is clearly bounded to the
east by the prominent NW-striking, SW-dip-

854

Stefano Pucci, Paolo Marco De Martini, Daniela Pantosti and Gianluca Valensise

ping, about 22 km long Gubbio normal fault.
This structure is clearly responsible for the
basin inception and for a maximum total throw
of the carbonatic multilayer (Jurassic-Oligo-
cene) of about 2500 m (Mirabella, 2003). The
detailed reconstruction of the Gubbio Fault iso-
baths by Barchi et al. (2000) shows an abrupt
change in the plane dip from 50° to 20° below
3.5 km. All the geomorphic elements used to
discuss the Gubbio Fault activity should refer to
the steeper and more superficial part of the
fault: no obvious geomorphic evidence for the
characterization of the low-angle part of the
fault exists. Therefore, in the following we will
discuss the Gubbio Fault as composed of a high
and a low-angle portion.

Simple topographic profiling across the
fault trace shows a gently dipping regularized
profile with the section on the calcareous bed-
rock having a grade similar to that built on de-

bris deposits (fig. 8). Even if the Gubbio Fault
strongly controls the eastern flank of the basin,
separating two different lithological sectors, no
evidence for a recent important activation of the
bedrock fault plane has been observed. The pre-
dominance of the erosional and depositional
processes with respect to tectonic activity along
the main Gubbio Fault Plane is shown by the
presence of young entrenched alluvial fans
(figs. 3b and 6) and by the misfit between the
curves of topographic and geologic throw along
the fault strike (fig. 4). Here differential erosion
dominates due to the presence of the pre-exist-
ing carbonatic anticline. However, field and air-
photo investigations of the debris deposits
along the fault hangingwall are suggestive of
minor recent displacements located west of the
main bedrock plane. The presence of slope rup-
tures and elongated small suspended benches
(fig. 8, cross section 2), associated with small

855

Geomorphology of the Gubbio Basin (Central Italy): understanding the active tectonics and earthquake potential

20 cm

Fig. 14a,b. a) Photo of the NW wall of the artificial cut affecting the slope debris near the Gubbio Fault. In fig-
ure are highlighted normal faults (continuous line) and main depositional horizon (dot line); b) detail showing
the re-oriented clasts and the offset amount.

a b

normal faults affecting slope deposits (fig.
14a,b), and the unconformity of the debris lay-
ers dip with respect to the present erosional sur-
face slope, suggest the existence of two parallel
fault splays located about 500 m west of the
Gubbio Fault between Gubbio and Mocaiana
(fig. 3b). Some of these variations of the topo-
graphic gradient represent landslide crowns,
although their linearity and continuity (up to
1.5 km) would support a tectonic origin. The
clear predominance of the erosional processes
over tectonics, partially controlled by the
Apennines regional uplift (Doglioni et al.,
1998), indicates low or negligible rates of tec-
tonic activity of the high-angle Gubbio Fault.

The activity of the low-angle portion of the
Gubbio Fault can be inferred on the basis of the
location of the April 29, 1984 earthquake. The
main shock location (fig. 5) is close to the
branch line of the Gubbio Fault with the
Altotiberina Fault (see inset in fig. 1). The CMT
solution for the mainshock (Dziewonski et al.,
1985) indicates the low-angle Gubbio Fault as
the probable seismogenic source.

The Gubbio Basin is the long-term evidence
of the activity of a ca. 22 km long fault. It is
well known that geometry and size of a fault
together with its slip amount and distribution,
are the major factors controlling the evolution,
length, and depth of a fault related basin, cou-
pled with the regional uplift context. With a
fault of a such length, which produced 2000 m
of displacement, and assuming that both sec-
tions of the Gubbio Fault (high and low-angle)
were active simultaneously, we would expect a
fault related basin 8 to 10 km wide. This is in
clear contrast with the present setting of the
basin which is peculiarly narrow, not exceeding
a width of 4 km. Our investigations highlighted
that the western flank, although without an asso-
ciated impressive topographic relief, exhibits a
peculiar geomorphology which suggest a tecton-
ic control on the basin. Looking in detail, two
mains, NW- trending, parallel lineaments appear
to dominate all along this flank (fig. 6). The
westernmost is clearly arranged in a right step-
ping system with several creeks, flowing across
it, showing aligned knick-points suggesting
normal faulting with east side down (fig.
10a,b). The second lineament is located close to

the contact with the basin infill (figs. 6 and 11),
where it affects both Miocene and Quaternary
deposits. This lineament is composed by a
series of aligned scarps, bordering several flu-
vial-lacustrine surfaces at different elevation
and intersecting recent alluvial fans (fig. 12d).
The interpretation of these lineaments as E-dip-
ping normal faults is supported also by the loca-
tion and linearity of the North and South
Saonda creeks, flowing close and parallel to the
basin western flank.

Thus, we propose that a normal fault anti-
thetic to the Gubbio Fault exists along the west-
ern side of this basin, close to the contact bed-
rock/infill. This antithetic fault has an average
strike parallel to the Gubbio Fault and may
explain the reduced 4 km width of the basin.

4. Modeling of faults in relation with
landscape evolution

In the previous sections we suggested that
the present setting of the Gubbio Basin is the
result of long-term activity of two W-dipping
normal fault (a low and high-angle sections con-
fined between depth 6 and 3.5 km and above 3.5
km, respectively) and of a secondary antithetic
high-angle normal fault located above 2.8 km
(fig. 15b). To test this hypothesis with an inde-
pendent approach, we tried to verify the influ-
ence of these fault sections on the landscape
evolution of the Gubbio Basin. The idea is to
compare the deformation expected to be pro-
duced at the surface by each fault, or by a com-
bination of faults, with the observed basin set-
ting. To achieve this goal we modeled the
expected deformations by using a code, based
on standard dislocation theory developed by
Ward and Valensise (1989). Consequently our
calculations assume uniform slip on planar, rec-
tangular faults embedded in elastic half-space.

To establish the fault parameters of our mod-
el, we adopted for the low and high-angle por-
tions of the Gubbio Fault a simplified fault
geometry (fig.15b, faults 1 and 2), derived from
the detailed reconstruction of the fault isobaths
(Barchi et al., 2000). To determine the fault
length of the antithetic structure, we used the
data derived from field survey and air photo

856

Stefano Pucci, Paolo Marco De Martini, Daniela Pantosti and Gianluca Valensise

857

Geomorphology of the Gubbio Basin (Central Italy): understanding the active tectonics and earthquake potential

20°

50°

60°

Gubbio Fault

antitethic fault
3

6.0

5.0

4.0

3.0

2.0

1.0

0 2 4 6 8 10 12 14

distance (km)

A A'

d
e
p
th

(
km

)

basin

e
le

va
tio

n
(

m
)

500

600

700

800

900
0 2 4 6 8 10 12 14

A A'topographic profile

distance (km)

basin

unitary slip

unitary extension

3 km

N
Antithetic fault

Gubbio Fault

A A'

distance (km)

e
le

va
tio

n
c

h
a
n
g
e

2
3

-1.4

-1.2

-0.8

-0.6

-0.4

-0.2

0.2

0.4

0 2 4 6 8 10 12 14

0.0

-1.0

basin

A A'

A A'
distance (km)

e
le

va
tio

n
c

h
a
n
g
e

distance (km)

e
le

va
tio

n
c

h
a
n
g
e

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0 2 4 6 8 10 12 14

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0 2 4 6 8 10 12 14

basin

3 2

1+2+3
combined contributions

distance (km)

A A'

e
le

va
tio

n
c

h
a
n
g
e

A A'

distance (km)

e
le

va
tio

n
c

h
a
n
g
e

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0 2 4 6 8 10 12 14

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0 2 4 6 8 10 12 14

basin

2
3

1+3
1+2

individual contributions

combined
contributions

individual contributions
individual contributions

Fig. 15a-f. Modeled elevation changes due the activity of the two sections of the Gubbio Fault and its antithetic
structure. a) Location map of the modeled transect. b) Sketch of the faults geometry adopted. c) Individual con-
tributions of the faults, assuming a unitary slip for each of them. d) Topographic profile along the modeled tran-
sect. e) Individual and combined contributions of the fault sections. Applying a unitary horizontal extension, NE-
SW oriented, and assuming faults 2 and 3 accommodate alternatively the extension. Combination of faults 1 and
2 or faults 1 and 3 is shown in the lower part. f ) Individual and combined contributions of the fault sections
applying a unitary horizontal extension, NE-SW oriented, assuming the three faults work simultaneously. Each
fault 2 and 3 has to accommodate half of the unitary extension. The combination of the three sections seems to
fit well the topographic setting of the basin.

interpretation assuming a constant 60° dip
down to the intersection with fault 2 to obtain
its width (fig.15b, fault 3). For faults 2 and 3,
we assume they are both surface-breaking and
with a pure normal dip slip movement. Thus,
we adopted a length of 22 km and a total width
of 11.5 km for the two portions of the Gubbio
Fault, a length of 18 km and a width of 2.8 km
for the antithetic fault (fig. 15b, fault 3) and a
general strike of 130°. All the elevation changes
obtained by the dislocation modeling are cal-
culated along the same transect (fig. 15a) for
which a topographic profile is also plotted in
fig. 16d. In the following, on the basis of our
previous discussion, we consider that the low-
angle portion of the Gubbio Fault (fig. 15b fault
1) is definitely active.

In order to test the role played by the geo-
metric parameters of the modeled faults, we
assigned to each fault the same unitary slip (fig.
15c), being aware of the fact that this is not a
realistic model for their long-term activity.
From this model it is possible to note that: fault
1 generates a gentle, almost symmetric wide
signal at the surface with its maximum subsi-
dence in correspondence with the western edge
of the basin; fault 2 produces an asymmetric
sharp depression, about 6 km wide deepest in
the eastern part of the basin; fault 3 produces an
asymmetric sharp depression, deepest in the
western part of the basin, the only completely
confined within the basin. The location of the
antithetic fault in the area of maximum subsi-
dence produced by fault 1, could be interpreted
as the consequence of the deformation pattern
resulting from the activity of fault 1.

In order to make the expected elevation
changes more realistic and because of the lack
of direct measurements of extension across the
area, we apply a unitary horizontal NE-SW ex-
tension on the system (fig. 15e,f) with fault 1
always active. In particular, in the model shown
in fig. 15e, we assume that the whole crustal
extension above 3.5 km is accommodated only
by one of the high-angle faults: thus only by
fault 2 or by fault 3. Whereas, in fig. 15f faults
2 and 3 are active simultaneously accommodat-
ing half of the extension each. We are aware of
the fact that probably this is a reasonable model
only for the short-term activity of these faults.

An important implication of this test is that,
summing the contribution of fault 1 alternative-
ly with that of faults 2 or 3 in fig. 15e, no satis-
factory match with the basin setting is found. In
fact, looking in fig. 15e at the combined contribu-
tion of faults 1 and 2 (forming the whole Gub-
bio Fault), we remark that the width of the mod-
eled depression is quite larger with respect to
the actual one (fig. 15d), and that the resulting
asymmetry toward NE is in disagreement with
the present drainage network within the basin,
being clearly asymmetric toward the SW (fig.
9). Only the gentle dip toward NE of the conti-
nental deposits filling the basin (GE.MI.NA.,
1963; Menichetti, 1992) seems to fit this model.
On the other hand, the deformation distribution
produced by modeling the combination of faults 1
and 3 (even if this setting is mechanically unlike)
in fig. 15e, shows a better fit with the basin
width, linearity, and position of the North and
South Saonda creeks, but is in strong contrast
with the basin Quaternary sedimentary infill
geometry (GE.MI.NA., 1963; Menichetti, 1992).

The lack of fit of the model shown in fig. 15e,
which assumes only one of the faults 2 or 3 to be
active along with fault 1, and the geomorphic
results discussed above, highlights the need for a
combined contribution of the three faults in a
graben-like setting (fig. 15f ). An immediate con-
sequence of this test is that the relative impor-
tance of the signal produced by fault 1 at the sur-
face increases. We can also clearly note how the
actual basin width is in good agreement with the
modeled one (fig. 15d,f). Moreover, the overall
shape of the computed elevation changes for the
three faults combination (fig. 15f ) is well compa-
rable with the topography as well as with the pre-
Pleistocene basement shape according to Meni-
chetti (1992) (fig. 3a). Finally, the fig. 15f model
would fit better also the gentle dip of the basin
infill deposits (fig. 3a) and admits some complex-
ity in the near fault (e.g., high dip angle of the
clayey-lignitic basal complex, GE.MI.NA., 1963).

4.1. The Gubbio Fault System

We have concluded that three main fault sec-
tions can be imaged in the Gubbio Basin, how-
ever, no unequivocal data exist to place some

858

Stefano Pucci, Paolo Marco De Martini, Daniela Pantosti and Gianluca Valensise

constraints on the age of activity of the individ-
ual fault sections, nor on their slip rates even on
a relative basis. For the Gubbio Fault the authors
(Selvaggi et al., 1989; Menichetti, 1992) set an
inception of the activity between Late Pliocene
and Early Pleistocene. During this period a maxi-
mum throw of 2500 m (Mirabella, 2003) has
been accumulated, which would translate to a
maximum slip rate higher than 1.4 mm/yr.

No constraints for the antithetic fault (fault
3) age of inception exist. The long-term evi-
dence of this fault does not suggest an activity
lasting since Early Pleistocene and producing
kilometric displacement like the Gubbio Fault.
The fact that the antithetic fault should have a
smaller role than the Gubbio Fault in the basin
development can be also inferred by comparing
figs. 15f with 15d and 3a. We can notice a dis-
crepancy between the modeling, where faults 2
and 3 are given the same weight (i.e. each ac-
commodating the same amount of extension)
and the present basin setting. To obtain a better
fit we should reduce in the model the role of the
antithetic fault with respect to the Gubbio Fault.
It is hard to define whether this minor role of
the antithetic fault is due to its lower slip rate,
its more recent activation, or by a combination
of these two. Even though the antithetic fault
has had a minor role in the basin development,
at present this appears to be the fault section
most effective in producing a landscape signa-
ture and thus, to be considered active.

On the basis of the dislocation modeling we
have argued that the combined activity of the
two sections of the Gubbio Fault and its anti-
thetic, is needed to explain the present setting of
the Gubbio Basin (fig. 15f ). However, we have
also to consider erosion, driven by the regional
uplift, as an important contributor to the present
basin shape. The rates of uplift of the Apen-
nines are expected to be as much as 1 mm/yr
(Doglioni et al., 1998); if these exceed the rate
of subsidence in the basin, sedimentation in the
hangingwall would not probably be enough to
compensate the tectonic offset. At present, both
footwall and hangingwall undergo erosion and
is possible that sedimentation was never able to
entirely fill the basin. However, this does not
seem to be enough to assume the basin narrow-
ness could be derived only by the activity of the

Gubbio Fault (fig. 15e, faults 1 and 2). In fact,
the western edge of the basin is linear and not
shaped according the distribution of displace-
ment on the Gubbio Fault (wider in correspon-
dence of the maximum throws).

Summarizing, on the assumption that the
low-angle Gubbio Fault is the main active fault
in the area below a depth of 3.5 km, it is still
unclear if today the antithetic fault takes over
most of the extension across the upper few kilo-
meters of crust, with an essentially inactive high-
angle Gubbio Fault, or if this latter is active too.
The short-term geomorphic signal seems to
suggest the Gubbio Fault to slip at a very low
rate or to be inactive, whereas the antithetic
fault to lead the present landscape evolution of
the area. On the other hand, the maximum slip
rates inferred for the Gubbio Fault (>1.4
mm/yr) on its long-term activity appear to be
too high in relation to its poor present geomor-
phic signature even in consideration of the
expected erosional rates for the area. This clear
discrepancy points out that the Gubbio Fault
activity changed through time, allowing at an
unknown time the inception of the activity of
the antithetic fault.

5. Seismogenic sources of the Gubbio Basin

Based on the considerations discussed in the
previous sections we conclude that the Gubbio
Basin contains active and potentially seismo-
genic faults. However, the extent of the Gubbio
Fault is quite large with respect to the presum-
able rupture length of the 1984 earthquake (Ms
5.3). At the same time, geomorphic data from
the basin and the long-term evidence for Plei-
stocene fault activity do not suggests the exis-
tence of permanent fault boundaries.

The Database of Potential Seismogenic
Sources for Earthquakes Larger than M 5.5 in
Italy (Valensise and Pantosti, 2001b) proposed
a preliminary scheme for the seismogenic
sources of the Gubbio area based on available
literature. The new data and considerations pre-
sented in this work allow us to propose an
updated version of this scheme.

In the previous sections, we defined the
Gubbio Fault System as formed by three 130°-

859

Geomorphology of the Gubbio Basin (Central Italy): understanding the active tectonics and earthquake potential

striking normal fault sections: a W-dipping,
low-angle fault located below 3.5 km depth and
two high-angle faults arranged in a graben-like
setting located above 3.5 km depth (fig. 15b).
This structure is limited by sharp northern and
southern boundaries which limit its length to
about 22 km. The northern boundary is set on

the basis of: a) the position of the northern tip
of the Gubbio Fault as shown by geophysical
data (fig. 4); b) the northern end of the basin
as defined by the presence of several deposi-
tional paleosurfaces located at the highest eleva-
tion within the basin (fig. 6), indicating an area
of lower subsidence typical of a fault tip, and

860

Stefano Pucci, Paolo Marco De Martini, Daniela Pantosti and Gianluca Valensise

Mocaiana

Gubbio

Padule

Branca

Mt. Urbino

Chiascio River

Assino River

Ponte D'Assi

Gubbio seismogenic
sources

Fossato di Vico-Valle dell'Esino
tectonic lineament

projected sources cut-off

transversal
alignment

intersection
with the

Altotiberina Fault

Gubbio seismogenic
sources parameters

width
strike

carbonates
(Jurassic-Oligocene)
carbonates
(Jurassic-Oligocene)
carbonates
(Jurassic-Oligocene)

continental deposits
(Pleistocene-Holocene)
continental deposits
(Pleistocene-Holocene)
continental deposits
(Pleistocene-Holocene)

turbidites (Miocene)turbidites (Miocene)turbidites (Miocene)

anomaliesanomalies

normal faultsnormal faultsnormal faults

N4 km

length
dip

thrust faultsthrust faultsthrust faults

divides

8 km

1 2

bottom depth
top depth

7 km
130°

10 km
20°

4 km
6 km4.5 km

130°
7 km
10 km

20°

2.5 km

Fig. 16. Hypothesized seismic sources in the Gubbio area. Simplified geology and basin divides are reported.
The thick normal faults are the high-angle sections of the Gubbio Fault System. The sources are represented as
boxes, which are the projection of the low-angle plain of the Gubbio Fault to the surfaces. Dashed gray line is
the hypothetical transversal lineament highlighted by structural and geomorphological anomalies. The inset
shows the position of the sources with respect to the reconstruction of the Gubbio Fault isobaths, its intersection
with the Altotiberina Fault and the 1984 seismicity (after Barchi et al., 2000, modified).

c) some Assino River captures (Cattuto, 1973).
Seismic reflection profiles suggest that the Gub-
bio Fault probably continues south-eastward
outside the basin (Mirabella, 2003). Because of
the total lack of geomorphic evidence we assume
this southern part of the fault is the witness of an
early, longer structure, not active today. Thus we
infer the length of the Gubbio Fault as directly
derived from the basin size. The southern bound-
ary is less constrained and coincides with the
southern end of the basin marked by the Fossato
di Vico-Valle dell’Esino transverse tectonic lin-
eament (Cencetti, 1988; Dramis et al., 1991 and
references therein) and with some Chiascio River
captures (Cattuto, 1973).

On the basis of the 1984 inferred seismogenic
source and of similarities with the geometry of
the 1997, Colfiorito earthquake sources, we may
approximate the main seismogenic source of the
Gubbio area with the low-angle section of the Gub-
bio Fault. This corresponds to a 22 km long, 7 km
wide, 20° dipping fault plane which, according to
Wells and Coppersmith’s (1994) empirical rela-
tions could produce earthquakes in the magnitude
range 5.9 to 6.4.

The geomorphic evidence of the Gubbio
Fault suggests that this structure has been active
for a long time over its entire 22 km length. How-
ever, no strong basis exists for deciding whether
the fault will rupture as a whole segment or in
shorter independent sections. In both cases the
Gubbio Basin would be evidence for long term
activity of the fault but in the second case it
would be the result of the coalescence of small-
er basins. Understanding which of these two
scenarios is more realistic has a critical impact
on seismic hazard issues.

In the middle part of the basin there is evi-
dence for a NE-SW alignment highlighted by
the following anomalies (fig. 16): a) abrupt
southern termination of the carbonatic anticline;
b) lateral termination of Miocene thrust fronts
(Menichetti et al., 1986); c) low hierarchic grade
of some NE-SW creeks; d,e) divide and wat-
ershed flexures (Cencetti, 1988). This align-
ment is also consistent with discontinuities with-
in the Altotiberina Fault and the bending of the
Gubbio Fault Plane at depth (see inset fig. 16;
Barchi et al., 1999, 2000) and may suggest
internal complexities of the Gubbio Fault

System that could act as geometric barriers in
the propagation of earthquake ruptures. This
potential segmentation of the Gubbio Fault
System is also in agreement with observations
on the 1984 earthquake. As discussed in the
previous part of the paper, this Ms 5.3 earth-
quake is interpreted to have ruptured the south-
ern part of the low-angle Gubbio Fault. Mo-
reover, the 1984 aftershock distribution shows a
peculiar step over zone (Collettini et al., 2003)
(inset in figs. 5 and 16) occurring along the NE-
SW alignment.

Based on these considerations we suggest
that the Gubbio area may comprise two smaller
seismogenic sources whose boundaries are rep-
resented by the basin fault system boundaries
and by the hypothesized NE-SW transverse a-
lignment (fig. 16). The maximum magnitude
expected for these sources can be estimated
both by assuming the 1984 earthquake as the
typical size earthquake for the area, and also on
the basis of the fault length and width using em-
pirical relations (Wells and Coppersmith, 1994).
On this basis, these sources may generate M
5.3-5.9 earthquakes. However, the activation of
the 22 km long fault for its entire length cannot
be completely ruled out.

No information on the long-term seismic be-
havior of these sources exists. Taking into ac-
count that the historical record of seismicity can
be considered complete for 5.0 < M < 5.6 in the
past 400 years and for M > 5.6 in the past 800
years (Zonno et al., 2002) and that there is no
evidence for 1984-type earthquakes during this
period, we expect recurrence intervals longer
than four centuries.

6. Conclusions

We used geomorphic analysis coupled with
DEM analysis and dislocation modeling to un-
derstand the present tectonic activity of the Gub-
bio Basin and infer the potential seismogenic
sources threatening the area.

The present setting of the Gubbio Basin ap-
pears to be controlled by three main fault sec-
tions: two of them form a W-dipping fault
known as the Gubbio Fault (fig. 15b, faults 1
and 2), while the third one is a E-dipping high-

861

Geomorphology of the Gubbio Basin (Central Italy): understanding the active tectonics and earthquake potential

angle fault (fig. 15b, fault 3) antithetic to the
Gubbio Fault. The antithetic fault has a clear
surface expression and several lines of evidence
suggest its recent activity: aligned scarps in
young deposits and bedrock, drainage anom-
alies, basin asymmetry, staircase-like uplifted
erosional and depositional surfaces. The exis-
tence of this antithetic fault contributes to ex-
plain the narrowness of the Gubbio Basin. The
Gubbio Fault, which certainly played an impor-
tant role in the basin construction, does not
show sizable ongoing activity. No stratigraphic
data are available to decide conclusively if the
high-angle section of the Gubbio Fault is: 1)
totally inactive; 2) slipping at very low rates
(easily compensated by erosional processes), or
3) slips at a substantial slip rate (> 0.5 mm/yr).
Neverthless, we believe that the 1984 earth-
quake provides living evidence that the low-
angle portion of the Gubbio Fault (fault 1) is
active.

After testing different combinations of the
fault sections of fig. 15b, we conclude that
only the joint action of all three faults can fully
explain the present setting of the basin (fig.
15f ). The antithetic fault and the low-angle
section of the Gubbio Fault appear to be the
main active features although their long-term
contribution is smaller than that of the high-
angle Gubbio Fault. No firm constraints on the
relative age and rates of activity of the differ-
ent fault sections exist at present.

Rupturing of the whole Gubbio Fault could
produce a M 5.9-6.4 earthquake. However,
based on the existence of a transverse linea-
ment in the middle part of the basin that may
act as a fault boundary, we hypotesize the exis-
tence of two smaller, independent seismogenic
sources, each one capable of a M 5.3-5.9 earth-
quake (fig. 16). These sources are represented
by low-angle, W-dipping normal faults that
may propagate to the surface through the
adjoining high-angle sections. The southern
source is thought to have ruptured entirely in
the 1984 earthquake, but no evidence for
earthquakes rupturing the northern source
exists in the historical record. Under these cir-
cumstances this latter source may be the cause
of a future damaging earthquake. The peculiar
geometry of these sources and their relatively

small size, which sets an upper bound to the
size of the expected earthquakes, appears to be
a typical feature for this part of the Central
Apennines seismogenic belt. This observation
may explain the frequent but moderate magni-
tude earthquakes reported for this region (CPTI,
Working Group, 1999) and has important
implications on seismic hazard evaluations.

Acknowledgements

We thanks C. Collettini, F. Mirabella, R.
Basili and M. Moro for the constructive discus-
sions and suggestions, T. Tamashvili, M. Mar-
chetti and his group for their field support, and
the countess Torlonia-Borghese for permission
for working in her land. We are grateful to M.
Barchi and M. Meghraoui for their valuable
comments and suggestions, which helped sub-
stantially in preparing the final version of the
paper. This research was funded by the GNDT
project «Terremoti probabili in Italia tra l’anno
2000 ed il 2030: elementi per la definizione di
priorità degli interventi di riduzione del rischio
sismico» and INGV «Seismology and Tecto-
nophysics» Department.

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Stefano Pucci, Paolo Marco De Martini, Daniela Pantosti and Gianluca Valensise