-
ABSTRACT
The major seismogenic faults of Umbria, which most of
themoderate (M > 5) earthquakes of the region are related to,
consist ofa set of SW-dipping normal faults, aligned along a
definite NNW-SSE trend, from Sansepolcro to Norcia, referred to in
this paper asthe Umbria Fault System (UFS). The major recent
earthquakes, forwhich instrumental data have been recorded, as well
as a significantportion of the historical seismicity can be
associated with the activ-ity of the UFS.
Data on the active stress field, from both geological and
geo-physical sources (e.g. structural analysis, focal mechanisms,
break-out data) indicate that this region is presently affected by
crustalextension, with a minimum principal stress 3 oriented
SW-NE.Seismic profiles reveal that the UFS seismogenic faults are
antitheticto a major, ENE-dipping low angle normal fault, named
AltotiberinaFault (ATF).
The instrumental seismicity within the upper crust is
concen-trated in a relatively shallow layer (Shallow Seismogenic
Layer,SSL), which deepens from the internal to the external sectors
of theApenninic arc (that is, from WSW to ENE).
The geological sections show that both the ATF detachment
(astructural surface) and the envelope of the top of the basement
(alithological boundary) deepen towards the east, showing a
geomet-rical similarity with the SSL. Available data on the major
instrumen-tal earthquakes and on the aftershock sequences confirm
this simi-larity.
It is possible that the ATF detachment operates a structural
con-trol on the SSL: in fact the SW-dipping seismogenic faults
branchout from the ATF, and simply do not exist below it.
The evaporites/basement boundary may represent a
second,lithological, factor controlling the distribution of the
seismicity: thisboundary corresponds to abrupt variations both in
the mechanicaland in the permeability properties of the rocks.
Finally, we present two geological sections, suggesting that
bothGubbio and Colfiorito normal faults reactivate, in their deeper
por-tion, pre-existing thrusts. Considering the map view, however,
it isclear that it is not possible to hypothesise that the UFS,
consideredas a whole, reactivates a single, major thrust fault.
Because of thearcuate shape of the major thrusts, which is in
contrast with thestraight alignment of the normal fault systems,
the different seg-ments of the UFS reactivate different thrust
faults. Concluding, it ispossible that the basement steps related
to the Miocene-Pliocenethrusts mechanically control the location of
the single UFS seg-ments; but the individual thrusts are not
suitable to control the loca-tion of the UFS system, considered as
a whole.
The UFS segments are aligned along a straight, NNW-SSEtrend,
parallel to the ENE-dipping, ATF master fault, that is not
con-trolled by the west dipping thrusts.
KEY WORDS: Normal faults, Seismotectonics, InversionTectonics,
Seismic reflection profiles, Umbria-MarcheApennines.
RIASSUNTO
Fattori litologici e strutturali che controllano le faglie
sismogenetiche dellUmbria: osservazioni dai profili sismici
ariflessione.
Le maggiori faglie sismogenetiche dellUmbria, cui collegata
lamaggior parte degli eventi di magnitudo M >5, consistono in un
setdi faglie dirette, immergenti verso SW, allineate lungo la
direttriceS. Sepolcro-Norcia, che in questo lavoro viene denominato
UFS(Umbria Fault System).
I maggiori terremoti recenti della regione, per i quali esiste
unadocumentazione strumentale (Norcia, 1979; Gubbio, 1984;
Colfiori-to, 1997-98) e gran parte della sismicit storica sono
connesse allat-tivit di questo set di faglie.
I dati sul campo di sforzi attivo (istantaneo), ricavati da
mecca-nismi focali e dalle deformazioni dei pozzi (MONTONE et alii,
1999)coincidono quasi perfettamente con quelli del campo di sforzi
geo-logico (a medio termine), ricavati da misure di piani di faglia
stria-ti (LAVECCHIA et alii, 1994). Questa coincidenza induce a
ritenereche non esistano sostanziali variazioni del campo di stress
durantelevoluzione tettonica recente (i.e. quaternaria) di questa
regione.
Nel settore settentrionale (a nord di Gualdo Tadino), i profili
si-smici (BONCIO et alii, 1998; BARCHI et alii, 1999) mostrano che
le fa-glie del UFS sono antitetiche ad una faglia diretta a basso
angolo,immergente verso ENE (faglia altotiberina, ATF). Evidenze di
que-sta faglia sono ricavabili anche dalla geologia di superficie e
del sot-tosuolo dellarea dei Massicci Perugini (BROZZETTI, 1995).
Sulla basedi considerazioni di carattere regionale, si pu
ipotizzare che la ATFesista anche nel settore meridionale: tuttavia
in questo settore nonesistono buone immagini sismiche della faglia
stessa.
Analizzando i dati riguardanti la sismicit strumentale della
re-gione, emerge che la sismicit della crosta superiore si
disponeallinterno di uno strato relativamente superficiale (SSL,
ShallowSeismogenic Layer), che si approfondisce verso i settori
esternidella catena, da WSW verso ENE e da NNW verso SSE (BONCIO
&LAVECCHIA, 2000). La geometria dello strato sismogenetico SSL,
purricalcando quella ipotizzabile per il passaggio fragile-duttile
(FEDE-RICO & PAUSELLI, 1998), si trova allinterno dello strato
fragile, aprofondit comprese tra 6 e 12 km.
Le sezioni geologiche costruite attraverso larea umbra,
inte-grando dati geologici di superficie con profili sismici a
riflessione,mostrano che la geometria del SSL potrebbe essere
controllata siada elementi strutturali che da cambiamenti
litologici, che influisco-no sul comportamento meccanico delle
rocce.
Lipotesi del controllo di tipo strutturale, gi avanzata da
BON-CIO & LAVECCHIA (2000), fa corrispondere la base di SSL
alla traiet-toria della ATF. Poich le faglie SW immergenti di UFS,
lungo cuisi sviluppano i terremoti, sono antitetiche alla ATF, che
ne costitui-sce lo scollamento basale, esse semplicemente non
esistono al di sot-to di essa.
Nellipotizzare un controllo litologico sulla geometria di SSL,
sipu notare che anche il basamento si approfondisce da WSW
versoENE. Ci dovuto principalmente ai sovrascorrimenti legati alla
tet-tonica compressiva, che hanno coinvolto almeno la parte pi
super-ficiale del basamento, e creano quindi una serie di gradini
del tettodel basamento, che si approfondisce verso ENE. Anche
lazione del-le faglie est-immergenti (in particolare ATF)
contribuisce allap-profondimento del top basamento verso i settori
pi esterni.
Linterfaccia evaporiti/basamento costituisce una
discontinuitimportante, in corrispondenza della quale avvengono
brusche va-riazioni delle propriet meccaniche delle rocce, messe in
evidenzadal forte contrasto nei valori della velocit di
propagazione delleonde sismiche, e della loro permeabilit. Questa
discontinuit favo-
Boll. Soc. Geol. It., Volume speciale n. 1 (2002), 855-864, 5
ff., 2 tabb.
Lithological and structural controls on the seismogenesisof the
Umbria region: observations from seismic reflection profiles
(*)
MASSIMILIANO R. BARCHI (**)
(*) Lavoro eseguito con il contributo finanziario del GNDT(PE98)
e della Regione Umbria. Resp. M.R. Barchi.
(**) Dipartimento di Scienze della Terra - Universit di Perugia
-Piazza Universit - 06100 Perugia.
-
risce la concentrazione degli sforzi ed influenza la
distribuzionedelle pressioni dei fluidi nel sottosuolo, favorendo
la nucleazione diterremoti.
Confrontando i dati disponibili per le profondit
ipocentrali(HAESSLER et alii, 1988; DESCHAMPS et alii, 1989; AMATO
et alii,1998; BONCIO, 1998), riferite sia alle scosse principali
che alle se-quenze di aftershock (terremoti di Norcia 1979, Gubbio
1984, Col-fiorito 1997-98) con le informazioni sulla profondit del
top del ba-samento e della faglia altotiberina nelle stesse zone,
si puconcludere che le ipotesi di controllo strutturale e
litologico sonocompatibili con i dati disponibili.
Infine, le sezioni geologiche integrate mostrano che le faglie
di-rette di Gubbio e di Colfiorito riattivano in profondit dei
pre-esi-stenti piani di sovrascorrimento (inversione tettonica).
Tuttavia, os-servando in pianta la distribuzione dei
sovrascorrimenti e dellefaglie dirette, risulta evidente la
differenza tra la traiettoria arcuatadei sovrascorrimenti e quella
rettilinea dei sistemi di faglie dirette.
Il fenomeno di riattivazione limitato ai singoli segmenti diUFS,
mentre il sistema nel suo complesso controllato dalla traiet-toria
rettilinea delle master fault est-immergenti.
TERMINI CHIAVE: Faglie dirette, Sismotettonica, Tettonicada
inversione, Profili sismici a riflessione,
Appenninoumbro-marchigiano.
INTRODUCTION
In the past few years the Structural Geology group ofthe Perugia
University began a multidisciplinary researchproject on the
seismogenesis of the Umbria region; thisactivity has been developed
after the 1997-98 Colfioritoseismic sequence. The project consists
of many branches:the active extensional faults have been studied
throughgeological mapping and structural analysis (MIRABELLA&
PUCCI, this Vol.), interpretation of seismic reflectionprofiles
(BONCIO et alii, 1998; BARCHI et alii, 1999; COL-LETTINI et alii,
2000; PAUSELLI et alii, this Vol.), numeri-cal modelling with
finite elements techniques (PAUSELLIet alii, 1998), analytical
models of fault mechanics (COL-LETTINI, this Vol.). This
interdisciplinary approach isessential for correctly modelling the
relationshipsbetween the geometry of the seismogenic faults and
theobserved seismicity: in particular, the interpretation ofthe
seismic profiles allows a 3-D reconstruction of thefault geometry
at depth (orientation, length, depth ofdetachment, ecc.),
independently from seismologicaldata, such as focal mechanisms and
shape of the after-shock sequence.
The aim of this paper is to offer a synthetic view ofthe results
achieved untill now, and in particular to dis-cuss the possible
control of structural (faults) and/or lith-ological
(stratigraphical boundaries) factors, on the spa-tial distribution
of the seismicity of the region.
Although alternative views have been presented byBOCCALETTI et
alii (1995) and CELLO et alii (1997), mostauthors converge in
describing both the recent tectonicevolution and the active faults
in the Umbria-Marcheregion as related to an extensional stress
field. In partic-ular, the major seismogenic faults of Umbria,
which mostof the moderate (M > 5) earthquakes of the region
arerelated to, consist of a set of SW-dipping normal faults,aligned
along a definite NNW-SSE trend, from Sansepol-cro to Norcia (fig.
1), referred to in this paper as theUmbria Fault System (UFS). Both
the major recentevents, for which instrumental data have been
recorded(Norcia 1979, Gubbio 1984, Colfiorito 1997-98, GualdoTadino
1998), and a significant portion of the historicalseismicity
(BOSCHI et alii, 2000), can be associated, at
least from the geometrical point of view, to the activity ofthe
UFS (fig. 1). Available knowledge on the UFS faults,along with a
comprehensive list of references, have beenrecently summarised by
BARCHI et alii (2000).
It is worth noting, however, that some significant his-torical
earthquakes (e.g. Fabriano 1741, Cagli, 1781) arelocated too far
east to be referred to the UFS activity.Moreover, at least in the
southern sector, paleoseismicitydata document the activity of a
second, more easterlylocated, alignment of seismogenic faults (M.
Bove-M. Vet-tore alignment, see BARCHI et alii, 2000, and
referencestherein).
The data on the active stress field, from both geolog-ical and
geophysical sources (e.g. structural analysis,focal mechanisms,
breakout data) indicate that thisregion is presently affected by
crustal extension, with aninferred minimum principal stress 3
oriented SW-NE(LAVECCHIA et alii, 1994; MONTONE et alii, 1999). In
thistectonic environment, the presence of minor, oblique
orstrike-slip earthquakes (e.g. the 16th October 1997, 4.3
Mwearthquake of the Colfiorito sequence, EKSTROM et alii,1998) can
be related to the activity of near-vertical, trans-fer faults,
reactivating pre-existing faults (e.g. PUCCI &MIRABELLA, this
Vol.): the main set trends about N-S,shows left lateral movement,
and possibly reactivatestranspressive lateral ramps of the regional
thrusts.
At least in the northern sector of the UFS (north ofGualdo
Tadino), the seismic profiles reveal that the seis-mogenic faults
are antithetic to a major, ENE-dippinglow angle normal fault, named
Altotiberina Fault (ATF,BONCIO et alii, 1998; 2000; BARCHI et alii,
1999), whosepresence was previously inferred by BROZZETTI
(1995)from surface mapping of the extensional faults of theregion.
Regional geology suggests that the ATF (or avicariant, ENE-dipping
fault) could be present in thesouthern sector as well, but no good
seismic image ofsuch fault is available.
Finally, it is important to consider the spatial rela-tionships
between the UFS (as well as the other exten-sional systems) and the
compressional structures of thispart of the Northern Apennines. The
Northern Apenninesare a typical arc-shaped belt: from the
hinterland (west)to the foreland (east), all the main structural
features ofthe region (e.g. the Tuscan Nappe front, the internal
andthe external border of the Umbria-Marche Ridge, respec-tively
indicated as WUT and ORT in fig. 1) are similarlyarc-shaped, with
an eastward convexity (BALLY et alii,1986; DEIANA & PIALLI,
1994). On the contrary, the laterextensional structures, which
bound Pliocene-Pleistoceneneo-autochthonous basins, are not
arc-shaped and areessentially aligned along NNW-SSE trends: the
Tiber Val-ley-Umbria Valley system and the UFS (from Gubbio
toNorcia) are good examples of this straight shape. As aresult the
compressive arcs of the Northern Apenninesare obliquely dissected
by these later, extensional faultsystems (fig. 1).
THE SHALLOW SEISMOGENIC LAYER (SSL)AND THE MECHANICAL
STRATIGRAPHY
OF THE UMBRIA UPPER CRUST
Instrumental data show that most of the crustal seis-micity of
the Umbria-Marche region is confined in theupper crust (depth <
15 km), even if, in the same region,
856 M.R. BARCHI
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LITHOLOGICAL AND STRUCTURAL CONTROLS ON THE SEISMOGENESIS
857
Fig. 1 - Shaded relief and schematic structural map of Umbria,
showing the major contractional and extensional faults of the
region. Thetraces of the ATF and of the UFS segments are reported.
The map is based on the compilation of published data (BARCHI et
alii, 2000, andreference therein), and on unpublished field work.
On the right side the focal mechanisms of the major recent
earthquakes of the regionare reported (after DESHAMPS et alii,
1984; HAESSLER et alii, 1988; AMATO et alii, 1998). Thrusts: WUT =
Western Umbria Thrust; IRT = InnerRidge Thrust; ORT = Outer Ridge
Thrust. Boreholes: SnD = S. Donato1; PG2 = Perugia2. Topografia e
carta strutturale schematica dellUmbria. Oltre alle principali
strutture compressive ed estensionali della regione, sono riporate
inevidenza le tracce della faglia Altotiberina (ATF) ed i segmenti
del sistema sismogenetico dellUmbria (UFS). La mappa deriva dalla
compilazio-ne di dati gi pubblicati (BARCHI et alii, 2000, and
reference therein), integrati con i risultati di rilevamenti di
terreno, ancora inediti. Sulladestra sono rappresentati i
meccanismi focali dei principali terremoti recenti che hanno
colpito lUmbria, ripresi dai lavori di DESHAMPS et alii,1984;
HAESSLER et alii, 1988 e AMATO et alii, 1998. Sovrascorrimenti: WUT
= Western Umbria Thrust; IRT = Inner Ridge Thrust; ORT = OuterRidge
Thrust. Perforazioni profonde: SnD = S. Donato1; PG2 =
Perugia2.
-
deeper, subcrustal earthquakes (>30km) have been recor-ded,
related to the deep structural setting of the region(SELVAGGI &
AMATO, 1992). In this paper, we will essen-tially deal with the
shallow seismicity, which can be rela-ted to the activity of the
UFS.
BONCIO et alii (1998) and BONCIO & LAVECCHIA(2000) observe
that the instrumental seismicity, if con-sidered in a cross-view
with respect the UFS (i.e. alonga SW-NE trending section), defines
a wedge-shapedseismogenic volume, whose base deepens from WSW
toENE, showing a fairly good fit with the ATF trajectory.The main
shocks are located close to the base of thisvolume, that we will
name shallow seismogenic layer(hereinafter SSL). The same authors
note that in a tieview, i.e. moving along the UFS, the SSL
progressivelydeepens from NNW (Citt di Castello) to SSE
(Norcia).This observation is consistent with the data reported
intab. 1, showing the hypocentral depth of the mainshocks and of
the aftershock sequences for the recentearthquakes of Gubbio 1984
(HAESSLER et alii, 1988),Colfiorito 1997-98 (AMATO et alii, 1998;
BASILI &BARBA, 2000) and Norcia 1979 (DESCHAMPS et alii,1989;
BONCIO, 1998). This data set is not homogeneous,since the accuracy
of the depth determination is grea-ter for the more recent events
(Colfiorito) and less pre-cise for the oldest (Norcia). Major
uncertainties regardthe main shock locations, due to the lack of a
tempo-rary local network. Nevertheless, available data
consis-tently indicate a deepening of the SSL from SSW toNNE.
Considering the data in a 3-D view, the SSL deep-ens from the
internal to the external sectors of theApenninic arc, which is
obliquely dissected by the UFS(fig. 1).
The geometry of the SSL can be compared with thedepth of the
brittle/ductile transition, which is controlledby the temperature.
Thermal modelling of the brit-tle/ductile transition through the
Northern Apennineslithosphere has been performed by FEDERICO &
PAU-SELLI (1998) and PAUSELLI & FEDERICO (This Vol.),using heat
flow data. The modelled boundary can becompared with the bottom of
the SSL below the studiedregion: both surfaces deepen towards the
east, but thedepth of the brittle/ductile transition is at a depth
of 15-20
km, whilst the base of the SSL is found at of 8-12 kmdepth (fig.
2).
Therefore the mechanical stratigraphy of the brittleupper crust
is to be considered, searching for the litholog-ical and/or
structural features, capable of controlling thefrictional
parameters along the fault surfaces (SCHOLZ,1990), which influence
the observed distribution of theseismicity with depth and the
geometry of the SSL.
From a lithological point of view, the Umbria-Marcheupper crust
can be divided into five main litho-structuralunits, clearly
recognisable in all the seismic profilesthroughout the region
(BARCHI et alii, 1998). They are,from top to bottom: Miocene
Turbidites, Oligocene-Juras-sic Carbonates, Triassic Evaporites,
Phyllitic Basement,Crystalline Basement. Each of these units can be
consid-ered internally homogeneous from a lithological
andmechanical point of view.
PAUSELLI et alii (1998) evaluated the Young modulusE of these
major litho-structural Units, using the equation(BARTON, 1986):
E = Vp2 (1+) (12)/(1-).The values of P-wave velocity Vp were
measured in
deep wells of the region (BALLY et alii, 1986; BARCHI etalii,
1998), or derived from the results of DSS seismicrefraction
experiments (PONZIANI et alii, 1995; DEFRANCO et alii, 1998). The
results are reported in tab. 2.
These data show that the brittle upper crust ismarkedly layered,
consisting of alternated strong and
858 M.R. BARCHI
TABLE 1
Recent instrumental earthquakes in the Umbria region.Data
sources: DESHAMPS et alii, 1984; HAESSLER et alii,
1988; AMATO et alii, 1998; EKSTROM et alii, 1998. Principali
terremoti recenti dellUmbria. Dati ricavati da:DESHAMPS et alii,
1984; HAESSLER et alii, 1988; AMATO et
alii, 1998; EKSTROM et alii, 1998.
Fig. 2 - Relationships between the geometry of the
Brittle-Ductiletransition (modelled along the CROP03 profile, see
location in fig. 1)and the distribution of the instrumental
seismicity (modified afterPAUSELLI & FEDERICO, This Vol.).
Microseismicity is confined in aShallow Seismogenic Layer (SSL, see
text for explanations), locatedwithin the upper crust, shallower
than the estimated brittle-ductiletransition. Note also the
relationship between the distribution of thehypocenters and the
trace of the Altotiberina Fault (ATF). Relazioni tra la geometria
della transizione fragile-duttile (modellocostruito lungo il
profilo crostale CROP03, la cui localizzazione ri-portata in fig.
1) e la distribuzione della sismicit strumentale (modi-ficato da
PAUSELLI & FEDERICO, This Vol.). Si nota che la microsi-smicit
della crosta superiore risulta confinata in uno stratosuperficiale
(Shallow Seismogenic Layer SSL) a profondit inferioria quelle
stimate per la transizione fragile-duttile. anche evidente
larelazione tra la distribuzione della sismicit strumentale e la
traietto-ria in profondit della Faglia Altotiberina (ATF).
-
weak layers. The terms weak and strong are hereused in a
relative sense: the phyllitic basement is a rela-tively weak layer,
lying between stronger units.
The boundary between the Evaporites (Vp > 6 km/s)and the
phyllitic basement (Vp about 5 km/s) is a zone ofinversion of the P
wave velocity (i.e. across this boundarythe velocity decrease with
depth). It is also worth to notethat the Evaporites, consisting of
alternated dolomitesand anhydrites (GHELARDONI, 1962; MARTINIS
& PIERI,1963) and traditionally considered as the main
dcol-lemnt level of the Apennines (e.g. BALLY et alii, 1986),would
represent at present a relatively strong layer withinthe
sedimentary cover.
GEOLOGICAL SECTIONS THROUGHTHE UMBRIA FAULT SYSTEM
Two geological sections through the Umbria seismo-genic region
are shown in fig. 3 (see location in fig. 1):from NW to SE, they
cross respectively the seismogenicdistricts of Gubbio-Valfabbrica
(section A) and Colfiorito(section B).
Section A crosses the southern termination of theGubbio
anticline and the Gubbio fault, representing asegment of the UFS
(fig. 1). It is based on surface geol-ogy data (BROZZETTI, 1995),
on the interpretation of anetwork of closely spaced seismic
reflection profiles(BARCHI et alii, 1999; PAUSELLI et alii, this
Vol.), cali-brated on deep wells, and on the results of DSS
seismicrefraction experiments (BIELLA et alii, 1993).
In the westernmost part of the section, the S. Donato1well
(ANELLI et alii, 1994) drilled at about 180 m a.s.l. theATF
surface, tectonically juxtaposing the Miocene Turbi-dites
(Marnoso-Arenacea Fm.) on the Triassic Evaporites(Burano Fm.). At
greater depth (2390 m and 3977 m b.s.l.,respectively) the same well
shows the presence of tectonicimbrications, involving the Triassic
Evaporites and thePhyllitic basement: these structures demonstrate
theinvolvement of at least the shallower part of the
basement(Phyllitic basement) in the major thrust sheets, as
con-firmed also, in a regional framework, by the seismicreflection
profiles (BARCHI et alii, 1998). The thrust sur-
faces drilled by the S. Donato1 well crop out about 30 kmfurther
to the east, in correspondence with the front ofthe Inner Ridge of
the Apennines (IRT in fig. 1).
In the eastern part of the section, the seismic reflec-tion
profiles effectively constrain the geometry of theGubbio fault,
which is shown to be antithetic to the ATF:the intersection between
the Gubbio fault and the ATFoccurs at a depth of about 5 km. In its
shallower part, theGubbio fault displaces the back-limb of the
Gubbio anti-cline, whilst at depth it inverts the displacement of a
pre-existing thrust fault.
Section B crosses the northern part of the M.Subasioanticline,
the Valtopina synclinorium and finally theAnnifo and Colfiorito
intermountain basins. It is based onrecently acquired surface
geology data and on the inter-pretation of a seismic reflection
profile (MIRABELLA &PUCCI, this Vol.). The seismic profile
shows that the base-ment is imbricated at depth by two major
thrusts, pro-ducing the stepping and deepening of the
basementtowards east.
The seismic profile does not reveal the trajectory ofthe ATF,
whose presence is hypothesised only on thebase of regional geology
speculations. Along this section,the UFS is represented at surface
by the M. le ScaletteFault, which in this zone dips 55-70 towards
SW. Somemovements were observed along the M. Le Scalette
faultsurface during the Colfiorito (1997-98)
earthquakes,interpreted as genuine surface faulting by CELLO et
alii(1998) and as secondary phenomena by BASILI et alii(1998) and
CINTI et alii (1999). The continuation at depthof the fault
surface, dipping towards SW, has been tra-ced by the interpretation
of the seismic profile: the aver-age dip (about 40) corresponds to
the slip plane as infer-red from the focal mechanism of the main
shock(EKSTROM et alii, 1998) and from the alignment in crosssection
of the aftershock sequence (AMATO et alii, 1998).At a depth of
about 8 km, the trace of the active normalfault corresponds to the
position of the innermost base-ment step: this observation suggests
that the position ofthe basement steps, generated by
Miocene-Pliocenethrust tectonics, may have controlled the location
of thelater normal faults. The intersection with the ATF of
theactive, SW-dipping fault has to be hypothesised at adepth of at
least 10 km, beyond the resolution of theavailable seismic
data.
Considering the geological sections, we can see howboth the ATF
detachment (a structural surface) and theenvelope of the top of the
basement (a lithological boun-dary) deepen towards the east,
showing a geometricalsimilarity with the SSL.
Moving from NW to SE, the horizontal distancebetween the ATF and
the SW-dipping faults of UFSincreases, as well as the depth of
their intersection. Wecan expect that the intersection of the Citt
di Castellofault with ATF would be shallower than the
Gubbio/ATF(i.e. less than 5 km): the CROP03 profile (BARCHI et
alii,1998, location in fig. 1) shows the presence of an anti-thetic
fault at the eastern border of the Tiber basin, alsoconfirmed by
geomorphologic data (CATTUTO et alii,1995); another, antithetic
fault has been placed more tothe east on the basis of seismic
reflection interpretationand on re-interpretation of the existing
geological maps.The intersection of the ATF with these antithetic
faults is
LITHOLOGICAL AND STRUCTURAL CONTROLS ON THE SEISMOGENESIS
859
TABLE 2
Mechanical stratigraphy of the Umbria-Marche upper crust.Data
sources: BARCHI et alii, 1998; PAUSELLI et alii, 1998. Stratigrafia
meccanica della crosta superiore in Umbria-Marche. Dati ricavati da
BARCHI et alii, 1998; PAUSELLI et
alii, 1998.
-
at relatively shallow depth (about 4 km), and is locatedjust
below the Tiber Valley.
Symmetrically, moving towards SSE, the depth of theintersection
between the different UFS segments and theATF is expected to deepen
progressively, becoming grea-ter than 10 km in the Norcia area.
COMPARING SEISMICITY,LITHOLOGY AND STRUCTURES
Fig. 4 presents a plot of the available data on thedepth of the
instrumental seismicity, of the top of thebasement and of ATF,
moving from northwest (Citt di
860 M.R. BARCHI
Fig. 3 - Two geological cross-sections through the Gubbio fault
(Section 1, modified after COLLETTINI & BARCHI, submitted) and
theColfiorito fault (Section 2, modified after MIRABELLA &
PUCCI, This Vol.). Sezioni geologiche attraverso la faglia di
Gubbio (Sezione 1, modificata da COLLETTINI & BARCHI,
submitted) e le faglie di Colfiorito (Sezione 2,modificata da
MIRABELLA & PUCCI, This Vol.).
-
Castello) to southeast (Norcia) along the UFS align-ment.
a) Earthquakes: the hypocentres of the main shocks,as well as
the associated aftershock sequences, deepenfrom Gubbio (6-7 km) to
Norcia (10-11 km). A further,slight deepening of the instrumental
seismicity movingtowards south-east (LAquila) has been shown by
BONCIO& LAVECCHIA (2000). No instrumental data are availablefor
the Citt di Castello-Sansepolcro area, that is thelocus of
historical seismicity (e.g. 1458, 1789, 1917, BO-SCHI et alii,
2000). If the trend illustrated in fig. 4 isregionally significant,
in this zone the seismicity is expec-ted to be shallower (4-5 km?)
than in the Gubbio zone.
b) Basement: using seismic reflection data, the depthof the top
of the basement (D) can be established, withreasonable
approximation, in the northern part of thestudied region, from
Sansepolcro (D = 4-5 km) to Colfio-rito (D = 8-9 km). No seismic
profile is available in theNorcia area. However, it can be noted
that in the Umbria-Marche region the top of the basement deepens
fromwest to east, due to the steps produced by the
east-vergingthrusts (fig. 3): these features have been clearly
evidencedby the CROP03 profile (BARCHI et alii, 1998 fig.
4).According to most authors, the basement depth is alsoexpected to
increase from the northern to the southernpart of the thrust belt,
as a consequence of the increaseof the shortening (e.g. BALLY et
alii, 1986). This regionalframework suggests a relatively high
value (11-12 km?)for the basement depth in the Norcia area.
c) ATF: seismic reflection profiles effectively describethe
geometry of the ATF from Sansepolcro to GualdoTadino: in this
northern sector the intersection betweenATF and the antithetic,
west-dipping faults of UFS deep-ens from 4-5 km to 7-8 km. South of
Gualdo Tadino, thegeometry of ATF (or other, east-dipping
detachments) isnot traceable in the available seismic reflection
profiles. Itwould be possible that the ATF is only a local feature,
bor-dering the western side of the Tiber Basin from Citt diCastello
to Perugia, and that no east-dipping normal faultis present in the
southern sector. However, no significantdifference can be observed
in the geometric characters ofthe seismicity and of the active
normal faults north andsouth of the Gualdo Tadino area. The
continental basinrelated to the ATF (Tiber Basin) continues towards
SSEfrom Perugia to Spoleto (Umbria Valley, fig. 1), withtrend,
sedimentary features and age which are grosslysimilar to those of
the northern part. At a regional scale,studies by many different
authors, based on surface geo-logy data and/or interpretation of
seismic reflection pro-files, demonstrate the role of gently
east-dipping, exten-sional master faults in the formation and
evolution of theMiocene-Pliocene basins of the Northern
Thyrrenian-Tuscany domain (BARTOLE et alii, 1991; KELLER et
alii,1994; LIOTTA & SALVATORINI, 1994; BARTOLE, 1995; JOLI-VET
et alii, 1998). The geometry at depth of these faultshas been
revealed by the CROP03 profile (BARCHI et alii,1998b), showing that
the activity of east-dipping, relativelyflat, normal faults is a
common feature in the Miocene-Quaternary tectonic evolution of the
Northern Apennines.In conclusion, the presence of an east-dipping
detach-ment, geometrically and cinematically equivalent to theATF,
has to be hypothesised also in the sector south ofGualdo Tadino:
probably such a detachment is deeper,beyond the resolution of the
seismic profiles.
The presented data show that both the ATF (a struc-tural
surface) and the top of the phyllitic basement (astratigraphic
boundary) deepen towards the east, reach-ing a depth that is
comparable with the thickness of theSSL. From a geometrical point
of view, both surfaces arelikely to control the distribution of the
seismicity and thedepth of the SSL.
The structural control operated by the ATF trajectoryon the
geometry of the SSL, is quite evident (BONCIO &
SEISMOGENESIS OF THE UMBRIA REGION 861
Fig. 4 - Comparison between seismicity, lithology and
structuresalong the UFS system. Seismological data are derived from
DESHAMPSet alii, 1984; HAESSLER et alii, 1988; AMATO et alii, 1998;
EKSTROMet alii, 1998 (see also tab. 1). Basement and ATF depths are
derivedby seismic reflection profiles (BARCHI et alii, 1998; BARCHI
et alii,1999; MIRABELLA & PUCCI, This Vol.). Confronto tra dati
sismologici, profondit del basamento e strutturelungo il sistema
delle faglie sismogenetiche umbre (UFS). I dati sismo-logici sono
derivati da DESHAMPS et alii, 1984; HAESSLER et alii, 1988;AMATO et
alii, 1998; EKSTROM et alii, 1998, mentre le profondit delbasamento
e della faglia Altotibeina derivano dalla interpretazione diprofili
sismici a riflessione (BARCHI et alii, 1998; BARCHI et alii,
1999;MIRABELLA & PUCCI, This Vol.).
-
LAVECCHIA, 2000; BONCIO et alii, 2000): assuming thatATF is the
master fault, from which the SW-dippingfaults (UFS) splay out, the
UFS seismogenic faults simplydo not exist below the ATF,
corresponding to the base ofthe SSL.
The evaporites/basement boundary may representanother,
lithological, factor controlling the distribution ofthe seismicity
and the nucleation of the main ruptures.The evaporites/basement
boundary is a lithological andmechanical discontinuity,
corresponding to abrupt varia-tions in the mechanical properties
(as related to the seis-mic velocity) and possibly in the
permeability of the rocks.
Seismic reflection and down-hole data show that theevaporites,
as well as the overlying carbonates, are hori-zons of relatively
high seismic velocity (5.5-6.1 km/s, tab.2), constituting a highly
competent level. High Vp valueshave been measured in rocks
consisting of alternatedanhydrites and dolomites. The present-day,
brittle behavi-our of the sedimentary cover (carbonates and
evaporites)is suggested by the distribution of the instrumental
seis-micity: the main shocks are generally located in the lowerpart
of the sedimentary cover (within the evaporites?),while the
aftershock volume is located within the carbo-nates/evaporites
imbrications.
On the contrary, the phyllitic basement is to be con-sidered a
relatively weak layer, as suggested by sensiblylower values of
seismic velocity (about 5 km/s, tab. 2).Experimental studies of the
seismic properties the phyl-lites of the Elba Island (Tuscan
basement) confirmed lowvalues of Vp (BURLINI & TANCREDI, 1998).
It is reasonableto suppose that the changes in the mechanical
propertiesof the rocks can induce corresponding variations in
thefrictional properties of the fault zone and of the sur-rounding
rocks (COLLETTINI & BARCHI, submitted).
Considering this mechanical stratigraphy, character-ised by the
presence of a weak basement below astrong sedimentary cover
(evaporites and carbonates),the lower part of the sedimentary
cover, located some kmabove of the brittle/ductile transition, has
superiorstrength properties. This layer can therefore act as a
stress guide (LISTER & DAVIS, 1989), whose failure pro-duces
the largest earthquakes.
THE ROLE OF INVERSION TECTONICS
The geological sections of fig. 3 suggest that the loca-tion of
the main SW-dipping normal faults (UFS) is con-trolled by
pre-existing thrusts, which may have been par-tially reactivated,
following the modes of negativeinversion tectonics. The influence
of pre-existing faults onthe location and geometry of the active
normal faults, andin particular the possible inversion of the
thrust faults inthe present-day extensional stress regime has been
largelydebated in the literature. The hypothesis of a
reactivationhas been considered by different authors at
differentobservation scales and different depths (e.g. BALLY et
alii,1986; CALAMITA et alii, 1994; LAVECCHIA et alii, 1994).
At the surface, the normal fault planes cropping outalong the
UFS (e.g. Gubbio, Colfiorito and Norcia faults)are characterised by
relatively high dip (50-70). Theavailable focal mechanisms (fig. 1)
indicate that the seis-mogenic rupture planes, possibly related to
the samefaults at hypocentral depth, are characterised by
relativelylow dip (30- 40). In cross-section view, the resulting,
lis-tric geometry of the normal faults is likely to join at
depththe west-dipping thrust faults (fig. 5), representing a caseof
partial, extensional reactivation of an inherited, reversefault
(negative inversion). The process of fault reactiva-tion
mechanically favours the movement along a non-Andersonian
(relatively low-angle) normal fault (SIBSON,1990). Many authors
(e.g. CELLO, 2000; GHISETTI & VEZ-ZANI, 2000) also underlined
the role of high fluid pres-sures, which can facilitate the
reactivation of partiallymisoriented (e.g. relatively low angle)
normal faults, e.g.in the Colfiorito area (COLLETTINI, this
Vol.).
As previously discussed, integrated geological andgeophysical
investigations demonstrate that at least theshallower levels of the
basement were involved in themajor thrust sheets, probably during
the last stages of thecompressional deformation (BARCHI et alii,
1998). From amechanical point of view, the basement steps
resultingfrom the thrust sheets imbrications, correspond to
placesof lateral heterogeneity between weak basement (phyl-lites)
and strong sedimentary cover (evaporites and car-bonates). These
lateral discontinuities might constituteplaces of stress
concentration and consequent preferen-tial nucleation of the later
normal faults.
In fact, the sections of fig. 3 suggest that both theGubbio and
Colfiorito faults reactivate, in their deeperportion, pre-existing
thrust faults.
Considering the map view (fig. 1), however, it is clearthat,
even if the thrust reactivation is suitable for the sin-gle normal
fault segments (e.g. Gubbio and Colfioritofaults), it is not
possible to hypothesise that the UFS, con-sidered as a whole,
reactivates a single, major thrust fault.
Because of the contrast between the arcuate shape ofthe major
thrusts and the straight alignment of the normalfault systems, the
different segments of UFS reactivate dif-ferent thrust faults. In
particular, moving from NW to SE,the normal faults reactivate
progressively more easternand younger thrusts: the Gubbio fault
reactivates the Gub-bio thrust (a portion of WUT in fig. 1), the
Colfiorito faultreactivates the Inner Ridge thrust (IRT in fig. 1),
the Nor-cia fault reactivate the Outer Ridge thrust (ORT in fig.
1).
862 M.R. BARCHI
Fig. 5 - Scheme of negative tectonic inversion for the Umbria
FaultSystem. The scheme is based on dip data measured along the
Gub-bio fault and on seismic profiles through the same structure.
Schema di inversione tettonica negativa, applicata alle faglie del
si-stema UFS. Lo schema basato sulla struttura di Gubbio, e prende
inconsiderazione le immersioni rilevate in affioramento, lungo il
pianodi faglia principale e le interpretazioni dei profili sismici
disponibili.
-
In conclusion, some important structural features ofthe UFS
normal faults can be summarised:
the basement steps related to the Miocene-Pliocenethrust systems
mechanically control the location of thesingle UFS segments;
the arc-shaped thrusts possibly control the segmen-tation of the
UFS system: the N-S trending portions of thethrusts are reactivated
as transfer faults in the extensionalstress field;
the single thrusts do not control the location of theUFS system,
considered as a whole: in fact UFS segmentsare aligned along a
straight, NNW-SSE trend, obliquelydissecting the Umbria-Marche fold
and thrust belt andprogressively disrupting more eastern and
younger arcsmoving from North to South.
The latter point can be explained keeping in mindthat the UFS
consists of fault segments, which are anti-thetic to the
East-dipping, ATF master fault, whose tra-jectory is not likely to
be controlled by the west dippingthrust faults.
ACKNOWLEDGEMENTS
The researches here summarised were supported by GNDT andRegione
dellUmbria grants. The paper benefits by the constant dis-cussion
with the other members of the research group, in
particularCristiano Collettini, Costanzo Federico, Francesco
Mirabella, Cri-stina Pauselli and Stefano Pucci. I am very grateful
to Luigi Burliniand Gianluca Valensise for their comments, that
greatly improvedthe manuscript.
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864 M.R. BARCHI
Manoscritto pervenuto il 20 Dicembre 2000; testo approvato per
la stampa il 21 Settembre 2001; ultime bozze restituite il 27 Marzo
2002.
