Tmmph.wic.~, 117 (1985) 7-38

Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands

CONSIDERATIONS ON THE SEISMOTECTONICS OF THE NORTHERN

APENNINES

M. BOCCALETTI ‘, M. COLI ‘, C. EVA 2, G. FERRARI 3, G. GIGLIA 4, A. LAZZAROTTO ‘,

F. MERLANTI 2, R. NICOLICH 6, G. PAPANI ’ and D. POSTPISCHL 3

’ Istituto Geologin, Universith di Firenre, Florence (Italy)

’ Jstituto Geofisrco e Geodetico, Universitb di Genova, Genoa (Ita[v)

’ fstituto di Topografia e Geofisica, Universitb de Bologna, Bologna (Italy)

4 Jstituto di Geologia, Unwersitb di Geneva, Genoa (Italy)

’ Departimento di Scienze della Terra, Universitir di Siena, Siena (Italy)

’ Jsirtuto di Miniere e Geofisica Applicata, Universltd di Trieste, Trieste (Italy)

7 Jstituto di Geologia, Universitb di Parma, Parma (Italy)

(Received June 20, 1983; revised version accepted June 4, 1984)

ABSTRACT

Boccaletti, M., Coli, M., Eva, C., Ferrari, G., Giglia, G., Lazzarotto, A., Merlanti, F., Nicolich, R., Papani,

G. and Postpischl, D., 1985. Considerations on the seismotectonics of the Northern Apennines. In: C.

Eva and N. Pavoni (Editors), Seismotectonics. Tectonophysics, 117: 7-38.

The Northern Apennines have been subdivided into homogeneous zones, on the basis of recent

structural evolution and crustal structure, in which the earthquake distribution can find a coherent

framework. These zones, whose physiography is in strict connection with their structure, are: the Internal

Peri-Tyrrhenian Belt: the External or Main Belt; the Buried Belt; and the Pede-Alpine Homocline.

Earthquake activity has a tendency to cluster along well-defined bands, particularly in the easternmost

border of the Peri-Tyrrhenian Belt, as well as along the zone between the External Belt and the Buried

Belt, i.e. along the Padanian margin of the Northern Apennines.

A minimum of seismic activity seems to be correlated with some zones of the External Belt, as well as

with the Late Tertiary and Quatemary magmatic province of Tyrrhenian Southern Tuscany.

The fault-plane solutions are coherent with the structural picture.

A tentative seismotectonic model of the Northern Apennines is discussed.

INTRODUCTION: THE GENERAL FRAMEWORK

The Northern Apennines are an example of a seismically active belt where

stresses, connected in a complex way to the Neogene or earlier deformation history,

are still active.

Over a consistent length, the Northern Apennines run parallel to the Alpine belt

the units which are present in northeastern Corsica and the Gorgona Island; as a

0040-1951/85/$03.30 0 1985 Elsevier Science Publishers B.V.

8

consequence, at least one part of the structural evolution of the whole belt must be

considered in the light of its relationships with the Western Alps.

The unraveling of the tectonic units of both belts invariably leads to a quite

simple paleogeographic organization at the end of the Jurassic or the beginning of

the Cretaceous: two passive continental margins-the European and the Insubro-

Apenninic-separated by a presumably deep-sea basin with an oceanic floor: the

Piedmontese-Ligurian paleo-ocean.

The structural history of these two paired belts can be subdivided into two main

stages (Boccaletti, 1977):

(1) A long late Cretaceous-Eocene stage, with the destruction of the Liguro-

Piedmontese oceanic trough, and the continental collision of the two crustal ele-

ments mentioned above. This episode led to imposing nappe formation, mainly with

European asymmetry; a quite linear eo-Alpine belt was formed. Flysch deposition

took place along the suture, on the residual non-consumed oceanic crust and

presumably even, to some extent, on the approached continental margins. These

thick flysch wedges had to be later involved in the successive structural events of the

internal zones of both the belts.

(2) With the closure of the Liguro-Piedmontese paleo-ocean, deformation was

displaced in the collided continental crusts. Ne deformational (and metamorphic)

phases overprinted the early Alpine building, and even more external zones of the

European crust. Deformation began in the Apenninic crust.

In the latter belt two particular groups of deformational events can be further

separated:

(a) In the Oligo-Miocene, when the emplacement of the main tectonic units, with

an Adriatic asymmetry, took place. Compressive deformation began to migrate

towards the Adriatic, presumably by ensialic shears, with the formation of successive

flysch troughs. Shortening of the basement was accompanied by decoupling of the

sedimentary cover, leading to the formation of folds and thrust nappes (e.g. the

Tuscan Nappe, in turn resting on the Tuscan metamorphics in the Apuane Alps;

Juratype folding in the Umbro-Marchean area, etc.). The more internal nappes (e.g.

the Ligurian nappes, in turn resting over the Tuscan Nappe) were translated towards

the external (Adriatic) side of the belt.

(b) Starting in the late Miocene, while translation and compressional movements

(at least on the cover) were confined to the external side of the belt, brittle

deformation began in its internal side, cross-cutting the whole nappe edifice. The

area underwent mainly tensile vertical movements, crustal attenuation and high heat

flow, possibly as a consequence of the opening of the Tyrrhenian Sea Basin. The

latter seems to have the same crustal and geological characteristics as the Tyrrhenian

slope of the emerged Apenninic belt (Giglia, 1974). Prevailingly elongated sedimen-

tary basins were formed in the areas depressed by vertical movements and block

faulting, filled by Messinian evaporites and Pliocene marine and Plio-Pleistocene

lacustrine deposits.

Fig

5.

1.

Srl

smot

ccto

nic

sk

etch

map

of

th

e N

orth

ern

A

pen

nin

es.

I =

Ext

ern

al

Bel

t.

wrt

h

rece

nt

com

prea

sion

al

mov

rmrn

tb

(bu

ried

u

nder

th

e Q

uat

ern

ary

sedim

ents

):

2 =

Mai

n

Bel

t.

prev

ailin

g re

cen

t u

plift

m

ovem

ents

: 3

= In

tern

al

Bel

t,

wit

h

exte

nsi

onal

m

ovem

ents

: 4

= La

te

Mio

cen

e an

d Pl

io-Q

uat

ern

ary

inte

rmon

tan

e bas

ins

of

the

Infe

rnal

bel

t:

3 =

Ter

tiar

y an

d Q

uat

ern

ary

plu

ton

ic

and

volc

anic

: ro

cks:

6

= n

orm

al

fau

lts

(squ

ares

to

war

ds

the

dow

n-t

hro

wn

bl

ock)

of

maj

or

impo

rtan

ce

in

rccc

nt

stru

ctu

ral

evol

uti

on;

7 =

cwer

th

rust

s (t

rian

gles

to

war

ds

the

over

ndin

g bl

ock)

: X

=

fau

lts

of

nor

mal

an

d tr

anse

urr

ent

type.

of

th

e

inte

rnal

bel

t;

V =

de

form

atio

n

or

dlsc

ontl

nu

ity

ban

ds;

10

=

drpt

h

of

the

Moh

o (fu

ll lin

rs).

Das

hed

lin

es

repre

sen

t th

e lim

its

of

the

low

er

cru

st

char

acte

nre

d by

a ve

loci

ty

of

7.4

km/x

c;

II

= fa

ult

-pla

ne

solu

tion

an

d fo

cal

dep

th;

I 2

= tr

aces

of

g

eolo

gic

al

cros

s-se

ctio

ns

of

Fig

. 2.

pp

. 11

-12

Fig.

2.

Sc

hem

atic

ge

olog

ical

se

ctio

ns

acro

ss

the

Nor

ther

n A

penn

ines

. Se

ctio

n tr

aces

ar

e in

dica

ted

in

Fig.

1.

I

= Q

uate

rnar

y se

dim

ents

of

th

e PO

Pl

ain:

2 =

Mio

-Plio

cene

an

d Q

uate

rnar

y se

dim

ents

of

th

e in

tern

al

basi

ns;

3 =

Plio

cene

se

dim

ents

of

th

e PO

Pla

in;

4 =

Tert

iary

ex

tern

al

tlysc

h (C

erva

rola

, U

mbr

ian.

flys

ch,

etc.

); 5

= Te

rtia

ry

flys

chs

of

the

Tus

can

unit

s;

6 =

Mes

ozoi

c co

vers

of

th

e T

usca

n an

d U

mbr

ian

unit

s;

7=

Ligu

rian

al

loch

thon

ous

unit

s;

8 =

subs

trat

um

(Pal

eozo

ic

to

llppe

r T

rias

sic)

: 9

= ea

rthq

uake

hy

poce

ntre

s (d

iam

eter

s pr

opor

tion

al

to

foca

l vo

lum

e).

13

The Tyrrhenian-Adriatic main divide itself prograded eastwards, annexing successive and more external parts of the belt to the Tyrrhenian slope (Mazzanti and

Trevisan, 1978).

GEOLOGICAL ORGANIZATION AND STRUCTURAL EVOLUTION OF THE NORTHERN

AI’ENNINES

The general structural organization of the belt is shown in the map of Fig. 1 and the cross-sections of Fig. 2, where the depth of the Moho has also been recorded. Considerations on the crustal structure will be made later; a short summary on the different te~tono-sedimental units appearing in the cross-sections, as well as on their deformation phases, is given herein.

The different Ligurian units (once deposited in the Ligurian-Piedmontese paleo- ocean) occupy the top of the nappe pile and in turn rest over different tectono-sedi- mentary units deposited on continental crust. From the internal (Tyrrhenian) towards the external (Adriatic) side of the belt, these are:

(1) The Tuscan Domain, subdivided in two Tectonic units: the Tuscan Nappe and the Tuscan metamorphic core of the Apuane Alps. The Tuscan Nappe (mainly represented by the Triassic to Oligocene sedimentary cover) was emplaced on the Apuane metamo~~cs in the upper Oligocene or perhaps lower Miocene, in corre- spondence with an ensialic shear belt with polyphase deformation (Carmignani et al., 1978, 1981). In the lower unit even the Paleozoic basement is involved. since the earlier deformation phases are present, at the core of isoclinal, recumbent syn-meta- morphic shear folds where formations up to Oligocene are represented. The early- phase axial-plane schistosities have been later refolded in a complex mega-antiform, presumably connected with crustal diapirism (Carmignani and Giglia, 1983).

(2) The ModinooCervarola Domain, outcropping with a thick flysch sequence and Upper Oligocene to Middle Miocene in age. This unit was deposited on top of the Tuscan Nappe flysch (presumably already emplaced on the Apuane Metamor- phics), from which it is separated by more or less thick advanced tongues of the Ligurian Allochthon.

(3) The Umbro-Marchid Domain, outcropping with a thick sedimentary sequence of late Triassic to Miocene age. The overall concentric geometry of the

Umbro-Marchid compressive structures renders probable the existence of a decolle- ment tectonics at depth, under the Burano evaporites, at the contact with the siliciclastic Paleozoic and Triassic basement. The basement is possibly underthrusted to the westernmost alignment of structural highs (A. Apuane, M. Pisani, etc.). The age of the top of the sequence involved in fold deformation becomes younger from west to east: Middle Miocene to Late Miocene in the outcropping belt; still younger for that part of the Domain buried under the PO Plain, as will be described in detail in the following paragraphs.

14

Homogeneous zone in the recent structural evolution of the Northern Apennines

From the point of view of morphology and recent structural evolution, the

Northern Apennines and immediately surrounding areas can be subdivided into

homogeneous longitudinal bands (Bartolini et al., 1983). These are (Figs. 1 and 2):

the Internal Belt, corresponding to the Tyrrhenian slope of the Northern Apennines;

the External Belt or Main Belt, corresponding to the Adriatic slope; the folded and

thrusted area under the PO Plain alluvial deposits (Buried Belt), and the Pede-Alpine

Homocline (Pieri and Groppi, 1981).

Transverse1 lineaments, evidenced by both physiographic and structural features,

are also present in the Northern Apennines. Their importance has been stressed

since long (Signorini, 1935; Boni, 1962; Zanzucchi, 1963; Ghelardoni, 1965; Borto-

lotti, 1966). Their structural expression is marked by both brittle and ductile

deformation at the surface (see in Boccaletti et al., 1982) mainly corresponding to

transcurrent movements at depth, active since the Miocene (Ricci Lucchi, 1975;

Boccaletti et al., 1980; Chiocchini et al., 1982). Some of these lineaments may

correspond to old rejuvenated features, since they have influenced the Mesozoic and

early Tertiary sedimentation (prior to compressive deformation), but they also mark

an interruption and fragmentation of the Neogenic and Quaternary intermontane

basins of the internal belt, or the deflection of recent fold axes in the Buried Belt.

Quaternary activity, even in the last 700.000 years, has been demonstrated in the

structural evolution of some transverse1 lineaments.

In the seismotectonic map (Fig. 1) we have marked the transverse1 lineaments that

have seismotectonic relevance, since they define compartments with a somewhat

different evolution in geologic time and presumably reflect deep tectonic dis-

turbances in the crust.

The Peri-Tyrrhenian Internal Belt In the complex structural setting of this area, the most striking character is the

presence of ridges and basins, mainly elongated parallel to the belt (NNW-SSE). In

the basins, unfolded (or only locally warped) marine and continental formations

outcrop.

The age of the base of the sedimentary infilling varies between late Tortonian and

Quaternary; the more recent ages are clearly confined to the eastern limits of the

area, close to the main divide of the belt. The substratum, represented by strongly

folded and thrusted Paleozoic to Tertiary formations, outcrops in the ridges separat-

ing the basins. These are limited by sets of normal faults superimposed on all the

pre-existing folds, thrusts and nappes. Vertical movements connected to normal

faulting have controlled the deposition inside the basins; local compressive episodes,

whose importance in the general framework is still a matter of debate, have been

described within these basins (Cerrina Feroni and Plesi, 1980). Subsidence velocities

of up to 50 cm/1000 yr. have been ascertained in the lower Pliocene sediments,

15

whose thickness can reach 1000 m (Volterra basin, see in Costantini et al., 1980).

The elevation and, more generally, the physiography of the basins can be strongly

disturbed by the emplacement of intrusive stocks, in connection with crustal

magmatism of late Miocene to Quaternary age, as in Radicofani and Monte Amiata,

where marine Pliocene is elevated to 800-1000 m a.s.1.. Detailed geological and

sedimentation-rate studies in some of these areas (Costantini et al., 1980, 1982) have

shown that these structures are strongly asymmetric; they are in fact, frequently

characterized by a master fault dipping WSW (with a throw of some thousand

metres) on their eastern margin, and with antithetic faults with minor throw, dipping

ENE, on their western margin. Tilting of the basins towards the east, with a

migration of the axis of maximum depth from west to east has been ascertained in a

time interval between the lower and middle Pliocene in the Siena basin (Costantini

et al., 1979) as well as in the Val di Chiana (Ambrosetti et al., 1980) and in the mid

and upper Tiber Valley (Baldi et al., 1974).

The same situation can be invoked in the northernmost sector of the belt (see

cross-section A in Fig. 2) where much evidence points to the Apuane Alps being a

NNW tilted block (Eva et al., 1978).

As a conclusion, the whole peri-Tyrrhenian area appears, rather than being a

series of horsts and grabens, as a succession of eastwards-tilted step blocks grading

down towards the Tyrrhenian depression, limited to the east by master faults of

presumably crustal importance, with antithetic faults dipping eastwards, on the

western side.

The External or Main Belt

In a longitudinal sense, the External Belt can be subdivided into three segments,

separated by major transverse1 lineaments having relevance to the recent structural

evolution:

(1) North of the transverse1 Passo della Cisa-Val D’Enza lineament, the Ligurids

outcrop extensively, with the more external units appearing only in minor tectonic

windows (ex: Umbro-Marchid Domain in the Salsomaggiore window).

(2) The Tuscan Nappe and the Modino-Cervarola Unit outcrop in the inter-

mediate longitudinal segment of the external belt. South of the transverse1 Passo

della Cisa-Val D’Enza lineament, they have been uplifted by many hundreds of

metres since the middle Pleistocene. This compartment is limited to the south by the

transverse1 Prato-Sillaro lineament.

(3) The Umbro-Marchid Domain outcrops extensively in the southernmost

segment of the external belt of the Northern Apenmnes, South of the Prato-Sillaro

line, where it has been uplifted by many thousands of metres compared to the

north-western sector.

Since at least the Upper Pleistocene, the whole easternmost part of the Main Belt

has been uplifted by many hundreds of metres compared to the Padanian margin.

The structures separating the External or Main Belt from the PO Plain are flexures

16

and strongly dipping reverse faults, representing the surface expression of the

Pede-Apenninic Thrust Front (PTF). The bundle of reverse faults representing the

Pede-Apenninic Thrust Front have been active at least since the Late Miocene

(Iaccarino and Papani, 1981). In correspondence with the PTF, the internal part of

the Pliocene foredeep of the Northern Apennines has been uplifted, compressively

deformed and annexed to the belt in the Quaternary. The throw at the base of the

Pliocene deposits can reach many thousands of metres.

Compressive stresses in correspondence with the PTF have been active during the

whole Quaternary time interval, up to the present. The direction of maximum

compression, deduced from meso-structures in the middle Pleistocene formations

near Parma is in accordane with the fault-plane solution of the 1971 earthquake

(Bernini and Clerici, 1983).

The Buried Belt

This longitudinal band of the Northern Apennines represented in the Pliocene the

foredeep of the belt. Its depocentre has migrated towards the northeast, contempora-

neously with compressive deformation, in Plio-Quaternary times.

The structure at depth has been retraced by AGIP work (Pieri and Groppi, 1981)

and can be schematically defined as a blind thrust system (with northeastern

asymmetry), represented by many leading imbricate fans (in the sense of Boyer and

Elliot, 1982).

Different arched groups of folded structures can be distinguished in the Buried

Belt: the Emilia folds in the northwest; the Ferrara, Adriatic and Romagna folds in

the southeast (see Fig. 1).

The Emilia folded arch has a width of 25-35 km and is represented by a

succession of blind imbricate thrusts, presumably detached from the basement at the

level of the Mesozoic successions. Folding dates back to the Pliocene and has been

active in the Quaternary.

The southeastern arch (Ferrara, Adriatic and Romagna folds) is formed by two

distinct blind imbricate leading fans: the internal Romagna and the external Ferrara

and Adriatic folds, separated by a major thrust with northeastern asymmetry. In

correspondence with this thrust, the Pliocene base dips down to about - 8000 m

b.s.1.. The age of the folds in Plio-Quaternary.

The external margin of the Buried Belt is again marked by a group of blind thrust

faults (External Thrust Front: ETF). On the Adriatic side of the structure, the base

of Pliocene Formations is again strongly depressed, reaching - 7000 m b.s.1.

Northeast of the ETF a discontinuous band, a few kilometres wide, of gentle

compressional deformation separates the buried belt from the Pede-Alpine Homo-

cline.

The Pede-Alpine Homocline

This buried area can be considered as the present-day foredeep of the Apenninic

17

Orogeny. It can be subdivided into two main sectors, on the basis of its deforma-

tional history.

In the eastern sector (East Lombardy and Venetian Plain) the Mesozoic and

Tertiary buried successions dip more or less regularly SSW, with dipping values of

less than 10”. Pliocene and Quaternary deposits are locally reduced or discontinuous

in some active areas, but the sector as a whole is generally characterized by moderate

subsidence and a more regular and thicker stratigraphic succession.

In the northwestern sector (central and western Lombardy Plain) the area is, on

the contrary, gently deformed, showing a blind thrust system with SSW asymmetry,

formed in the late Miocene (Pieri and Groppi, 1981). Plio-Quaternary compressional

stresses, in correspondence with the External Thrust Front, are mainly concentrated

around the V-shaped sector of the Pede-Alpine Homocline, defined by the conver-

gence of the two main arches of the Buried Belt.

CRUSTAL ORGANIZATION

Gravity data

The Bouguer gravity pattern (Ballarin et al., 1972; Morelli, 1973) gives a rough

indication of the crustal structures. First of all, a gravity high ( + 200 mGa1) occurs

in the Ligurian Sea, whereas values of - 150 mGa1 are found in correspondence with

the buried Pede-Apennine. The zero-line approximately follows the coastline be-

tween Genoa and La Spezia, but extends far inland in Tuscany (at the limit between

the internal and external belts), being accompanied by the maximum gradient

towards the negative values in the External Belt. This feature indicates an abrupt

change in the crustal thicknesses.

Free-air anomalies are positive in the Tyrrhenian Sea as well as in the Peri-Tyr-

rhenian Internal Belt, and negative in correspondence with the western margin of the

Buried Belt (Morelli, 1973). This distribution of free-air anomalies is indicative of

the existence of isostatic desequilibria. In the Tyrrhenian slope, these may be

correlated with distensive tectonics, with consequent magmatic intrusions and the

presence of high-density materials towards the base of the crust. In the Padanian

margin, the apparent contradiction of subsidence coupled to negative free-air

anomalies can be attributed to the upthrusting of the Apennine emerged belt over

the PO Plain (Elter et al., 1975) as is indicated by the presence of very recent

compressive structures in the surface geology (Bartolini et al., 1983).

Crustal structures from deep seismic soundings (DSS)

The isobaths of the Moho discontinuity derived from DSS data collected in the

area (Fahlquist and Hersey, 1969; Morelli et al., 1977; Giese et al., 1981; Ferrucci et

al., 1982) have been contoured on the seismotectonic map (Fig. 1).

18

They refer to the computed depths in correspondence with velocity values close to

8.0 km/s, commonly assumed for the identification of the crust-mantle boundary.

For most of the profiles the interpretation is based on the indication of a precise

velocity gradient at the base of the crust. In places the Moho remains only

tentatively defined, but critical analysis of peculiar seismic features of the lower

crust and of crust-mantle transition zone allow us to delimit different sectors and

make correlations with the geological evidence

An example is given by the Tuscany-Latium geothermal region, where a thinned

crust of 22-25 km has been computed. The upper crust is characterised by velocities

of 6.0-6.3 km/s and thicknesses of 14-18 km. Velocities of 6.8-7.5 km/s identify

the top of the lower crust or of a peculiar interval that could represent a thick and

upward-extended crust-mantle transition zone. No hypocentral depth has been

computed inside this interval. Moreover, the downwards extension of the heat-flow

data, according to different models, accepts temperatures higher than 700°C in this

interval. Thus the interaction between acid and basic rocks results in a pronounced

vertical and lateral heterogeneity with materials of variable viscosities, perhaps in

places partially melted, that give origin to multiple reflections caused by alternating

high and low velocity layers.

At the bottom of this 5-10 km thick interval the velocity reaches values of nearly

7.9 km/s. However, the Moho is not clearly defined and also the upper mantle does

appear homogeneous. Thus, it is possible to imagine that the lower crust is, affected

by “absorption phenomena” from the upper mantle, and (or) that a soft mantle may

be present at depth.

These features characterise the Tuscany-Latium geothermal area and the north-

ern Tyrrhenian Sea as well. Eastwards, in the region of the “Ancona-Anzio” line,

crustal structures are of normal continental type with well-marked Moho and

thicknesses of nearly 30 km. Along the Peri-Tyrrhenian Internal Belt, high velocity

values (nearly 6.5 km/s) have been observed at shallow depths (5-6 km) which can

support the hypothesis of intra-crustal thrusting connected to post-collisional

orogenetic processes. In correspondence with the limit of the External Belt, a

lowering of the Moho from about 25 to at least 35 km is observed. On the lowered

side, the Moho reflections are poor or uncertain. However towards the buried

Pede-Apennine an increased crustal thickness of 35 km or less is found on the

northern side of the PO Plain.

Again in the Northern Apennines some of the anti-Apenninic or Apenninic

structural elements also affect the Moho. Nearby La Spezia one of the anti-Apen-

ninic lineaments bounds the Tyrrhenian structures towards the North, and its

seaward extension separates the Corsica Block from the mantle uplift in the Ligurian

Sea, where a velocity of 7.9 km/s has been found at depths greater than 15 km.

Between La Spezia and Livorno the base of the crust in Fig. 1 corresponds to

velocities of only 7.4 km/s; velocities of 8.0 km/s have been observed here at depths

greater than 50 km.

19

The continental structures of Corsica are well defined as far as the N-S trending

boundary with the Tyrrhenian where the features of the lower crust and upper

mantle appear more complicated. Only the upper structures of the crust exhibit very

similar velocities and thicknesses (20 km) on both sides of the two sectors.

Aeromagnetic data

The aeromagnetic surveys carried out by AGIP in the period 1970-1980 (Bolis et

al., 1982) have made it possible to obtain a picture of the structural features of the

basement and to insert them into a regional context.

Also in aeromagnetic data, a clear subdivision appears in NNW-trending bands

roughly coinciding with the partitions adopted on the base of surface geology,

fragmented by transverse1 lineaments.

(1) The Peri-Tyrrhenian Internal Belt corresponds to the Northern Tuscany and

Tuscany-Latium-Campania magnetic Provinces. In the region between the Apuane

Alps and Pisa, the magnetic substratum is characterized by medium-low intensities,

referred to the basement of the Apuane Alps. The susceptibility contrasts (100-300

x 10e6 C.G.S.U.) are compatible with crystalline metamorphic rocks (mainly acidic).

AGIP interpretation places this magnetic substratum at a depth of 3-4 km, with a

progressive deepening towards the north and south of the Apuane Alps.

In Southern Tuscany, medium-high frequency anomalies are found, referable to

recent magmatic activity.

(2) The low-to medium-high magnetic province of the Internal Belt strongly

contrasts with the adjacent Apenninic magnetic province. The peculiar characteristic

of this area, corresponding to the previously described External or Main Belt, is a

general lack of magnetic response. The data seem to be consistent with the presence

of a very deep, westwards-dipping basement.

(3) In the PO Valley magnetic province (corresponding to our Buried Belt and

Pede-Alpine Homocline together) magnetic anomalies present a low intensity (20-30)

and wide wavelength, witnessing a deep acidic crystalline basement, deepening

irregularly towards the southwest. Calculated depths are in fact, less in the PO Plain

(8-10 km at Alessandria: 8 km between Milan and Brescia, about 6 km in the

Venetian offshore with a local high along 4752 m in a narrow strip facing the lagoon

of Venice) than along the edge of the Emilia Apennines, where they reach depths

greater than 12 km.

Mafic intrusions and volcanites, both Permian or Triassic and Middle Miocene,

recorded at depth in oil drillings, can further complicate the local magnetic anomaly

pattern.

SEISMICITY

Historical seismicity

The Italian finalized project “Geodinamics” has started since 1979 a national

20

program of revision and completion of the Italian Seismic Catalogue from the years

1000 up to 1975, drawn up by ENEL (ENEL 1977).

In particular, the program started with the revision of the strongest historical

earthquakes (about 200 shocks with I,,, = IX MCS) carried out on the basis of

published and unpublished contemporaneous documents concerning historical events,

which in some cases also allowed of completing the catalogue with the smallest

shocks.

From the historical and statistical point of view, the maximum magnitudes that

occurred in the last 1000 years have been observed in the regions of

Garfagnana-Lunigiana (inner belt), Mugello (inner belt), Forll and some other

zones of the External Belt.

Analyzing the historical main events (C.N.R./P.F.G., 1983) in each area, it is

possible to summarize these principal behaviours.

The GarfagnanaaLunigiana was affected in different periods (1481-174661837-

1820) by events with intensities greater than VIII MCS. The orientation of the main

axis of the isoseismal lines is mainly in agreement with the general trend of the

Apenninic structures. The reconstruction of the macroseismic field of Sept. 7th, 1920

earthquake, the strongest recorded in the area, shows most efficient propagation

towards the PO (Ferrari et al., 1983).

The Mugello zone was affected by severe earthquakes in 1542, 1611, 1762. and

1919, with epicentres ranging between Firenzuola, Barberino Mugello, Scarperia,

and Vicchio. Also is this area, the main trend of the isoseismal lines is of Apennines

strike. In the Forli area the most part of the heaviest shocks (1279,1661,1768, 1918).

was localized in an area included between S. Sofia, Galeata, and Civitella di

Romagna. Another seismicity cluster is connected with the Brisighella-Faenza

territory (1509, 1781, 1882).

The isoseismal lines of maximum degree are normally aligned with the Apenninic

trend (Ferrara et al., 1983).

The PO Valley includes the following areas affected by destructive earthquakes:

Rimini, Lugo di Romagna and Ferrara. Rimini and other coastal sites felt the

seismic activity generated in the Adriatic Sea (1308, 1672, 1786, 1875, 1916). This is

to show that all macroseismic maps of these events are very similar and they show a

good propagation towards Ferrara and Venice.

In the region of Lugo some earthquakes were felt with high values of intensity

and characterized by very low values of focal depth.

In the Ferrara area, where buried folded structures of the PO Plain are very

shallow, the occurrence of shocks with intensity of VIII MCS was observed (1467,

1570, 1624, 1787).

The earthquake that occurred in the 1570 showed the main isoseismal axis

oriented with the direction of the Ferrara faulted belt at the limit between the buried

Apenninic belt and the Pede-Apenninic Homocline.

All earthquakes that occurred in the PO Valley are characterized by their

pp. 21-32

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33

shallowness of foci and by the raising of long seismic periods in connection with low

values of realized energy. The Ferrara earthquake of 1570 was followed by a long

sequence (4 years) with, on the basis of historical information, more than 2000

shocks with intensities ranging between III and VIII MCS.

Instrumental seismicity

The catalogue has been completed up to 1980, with data derived from the

national seismic network (Cassinis, 1981; CNR/PFG, 1982).

As far as the northwestern part of the Apennines is concerned, the seismic activity

occurring in the last 10 years has been revised with a relocation of foci (Eva et al.,

1978).

The epicentral distribution of shocks occurring in the area from the years 1000 up

to 1980 is shown in Fig. 3, where the epicentres are represented by circles whose

radius is proportional to the radius of the equivalent sphere of focal volume

determined according the Bath relationship (1964) (Gasparini and Postpischl, 1980).

From the figure, the Northern Apennines appear to be affected by a diffuse

seismicity, even if in terms of occurrence frequency and released energy, a tendency

to concentration in bands of activity is shown (Cattaneo et al., 1981). This happens

particularly along the eastern border of the Tyrrhenian margin, corresponding with

the maximum horizontal gradient of the Moho and the zero-line of the Bouguer

anomaly, and along the PO-Plain and Adriatic borders in connection with the

minima of this anomaly.

On the contrary, the seismic activity is moderate or nearly absent in the eastern

Ligurian Sea and Southern Tuscany, in connection with the Tuscany-Latium

geothermal region, characterized by a rise of the Moho and by positive values of

gravimetric anomaly.

At the present state of knowledge, more detailed interpretations do not seem

significant because the accuracy in the definition of epicentres, as reported in the

catalogue, is on average 25 km.

As regards the focal depths, it is necessary to outline that this parameter has been

determined only for 27% of shocks occurring in the studied area and in many cases it

has been derived from the macro-seismic field.

A different behaviour is shown by foci, instrumentally determined, with a

systematic deepening of the hypocentres. This systematicity may depend on the fact

that the catalogue is prevailingly a macroseismic one and the macroseismic depth

evaluation, when it is restricted to the highest isoseismal lines (as happens for the

shocks occurring in the past centuries), gives informations on the shallower part of

the fracture plane (Shebalin, 1972).

Nevertheless, considering the frequency distribution of depths, the data tend to

concentrate at different levels.

The first level of seismic activity, affecting the whole Apenninic structure, is

34

connected with very shallow sources lying in the first 10 km of the crust.

The second level points to a deeper crustal seismic activity and is mainly

connected with the central part of the belt in coincidence with the major deepening

of the Moho.

More evidence of very deep crustal seismic activity, with foci deeper than 50 km,

exists in two narrow areas: Lunigiana and the Northern sector of Marche, respec-

tively in the inner and external Apennines.

These data are related to only 14 shocks that occurred during the last 20 years,

and one of these events (earthquake occurring on 25th Oct. 1972 in Lunigiana),

studied by Bossolasco et al. (1973) shows a focal depth of about 50 km.

The distribution of foci is shown in cross-sections of the Northern Apennines

drawn in Fig. 2.

The focal mechanisms, available for the studied area, are shown in Fig. 1. Some

of them have been collected from the literature (Bossolasco et al., 1974; Eva et al.,

1978; Cagnetti et al., 1976; Scarpa et al., 1982); others have been revised and some

evaluated for this study.

The distribution of focal solutions in so wide and area of complex geological

evolution does not allow the drawing of unambiguous conclusions.

Indeed, the focal mechanisms outline the presence of both transcurrent and

vertical (either normal or overthrust) faults. Particularly the inner belt is affected by

mechanisms prevailingly of distensive type passing to transcurrent type in the

northwestern sector. The directions of fault planes are consistent with either the

Apenninic trend or the transverse1 systems of structural discontinuity.

An anomalous behaviour is shown by the earthquake with a focal depth of 50 km

(25th Oct. 1972 shock) which presents a clear overthrust solution (Bossolasco et al.,

1974; Scarpa et al., 1982).

The External Belt, along the Apenninic margin with the PO Valley and Adriatic

Sea, is mainly characterized by transcurrent mechanisms, which assuming a fault

plane parallel to the Apenninic trend, are of sinistral type. In this area, overthrust

solutions have been determined in the regions near Parma and Ferrara in connection

with the buried folding systems.

A POSSIBLE SEISMOTECTONIC MODEL

A seismotectonic model of the Northern Apennines arises from previously

described data on its recent geological evolution and deep structural setting, com-

bined with the location of seismic events and the study of fault-plane solutions.

Leaving apart the early Alpine phases, a good starting point can be represented

by the continental collision which took place after the complete consumption of the

oceanic crust once separating (in the late Mesozoic) the European and Insubro-

Apenninic crustal elements. Since oceanic crust is consumed more easily, the

formation of ensialic shear belts in both the above-mentioned crustal elements can

35

be considered as indicative of the beginning of continental collision. Post-Middle

Eocene syncinematic metamorphism in the Alpine Brianconnais, Upper Oligocene

and Miocene metamorphism and deformation in the Apuane Massif for the Apen-

nines seem sufficient evidence that, at least in the upper Oligocene, the two

above-mentioned crustal elements were already appressed and collided.

The ensialic shear model appears as one of the possible mechanisms in the

compressive post-Eocene deformational history of the Northern Apennines. The

existence of an Upper Oligocene ensialic shear belt in the Apuane Alps is demon-

strated by rotation of fold axes into parallelism with extension lineation in the

course of tangential ductile deformation (Carmignani et al., 1981).

A present-day ensialic shear belt could be represented by the external margin of

the Northern Apennines, where crustal thickening and compressive events at the

surface, indicative of shortening, have been evidenced in the Quaternary evolution.

These geological data are not in conflict with the few fault-plane solutions available

for the area, showing compressive and or transcurrent character.

Following some of us (Boccaletti, 1977; Boccaletti et al., 1980) as well as other

authors (Panza et al., 1980), the post-collisional intercontinental deformation, re-

sponsible of the formation of the belt and still active, would have implied detach-

ment of the upper crust in relation to the underlying lower crust and lid. These

episodes would be controlled by important systems (Boccaletti et al., 1982).

The ensialic shear model for collisional belts has been extensively applied to the

Himalayan orogeny by Mattauer (1975). In Mattauer’s reconstruction the migration

of compressive deformation, from the internal Main Central Thrust towards the

more external (and presently seismically active) Main Boundary Thrust, is strictly

connected to ensialic shears. The buoyancy of continental crust makes in fact the

ensialic shear a rapidly self-sealing process; successive shortening is consequently

displaced towards the external side of the belt, where further thickning of the crust is

already possible.

In the Northern Apennines, such a model would be in perfect accordance with the

migration of flysch throughs, described by Italian authors since the fifties (see in

Merla, 1951) for which it would provide an adequate mechanism.

Compressive stresses at the external margin of the Northern Apennines make the

present-day structural evolution of this side of the belt the continuation of its

post-collisional history.

Mainly tensile deformation in the internal Peri-Tyrrhenian Belt, overprinted on

compressive deformation appears, on the contrary, as having an origin outside the

geodynamic framework which led (and is presently leading on the external side) to

the formation of the folded and thrusted belt.

From the surface structure (see, for references, in Giglia, 1974), crustal and

lithospheric thinning at depth (Calcagnile and Panza, 1981) high heat flow (Loddo

and Mongelli, 1979) and emplacement of magmatic events, the internal belt appears

as an emerged equivalent of the Northern Tyrrhenian Sea. The whole Western

36

Mediterranean appears as a superimposed structural (and physiographic) feature,

since its emplacement is without connection with the previously established Alpine

lineaments. The Western Provencal-Ligurian Sea basin cuts obliquely across all the

Western Alps tectonic units; and the Tyrrhenian Sea occupies the internal root zones

of both the Apenninic units and Alpine units of Corsica which in earlier times

certainly had represented convergent plate margins. Rifting and “oceanization”,

acting in the Western Mediterranean Sea and in the internal zone of the Northern

Apennines, coexist with crustal thickening, separated in space and time.

The two stress fields at the origin of the present-day crustal organization of the

belt can be connected in many ways, but further expansion of the model, as part of

the general picture of the whole Mediterranean Sea basin, merely on the basis of the

Northern Apennines data, seems hardly practical to the authors.

REFERENCES

Ambrosetti, P., Carboni, M.G., Conti, M.A., Costantini, A., Esu, D., Gandin, A., Girotti, O., Lazzarotto,

A., Mazzanti, R., Nicosia, U., Parisi, G. and Sandrelli, F., 1979. Evoluzione paleogeografica e tettonica

dei bacini tosco-umbro-laziali nel Pliocene e nel Pleistocene inferiore. Mem. Sot. Geol. Ital.. 19:

573-580.

Baldi, P., Decandia, F.A., Lazzarotto, A. and Calamai, A., 1974. Studio Geologico de1 substrato della

copertura vulcanica laziale nella zona dei laghi di Bolsena, Vito e Bracciano. Mem. Sot. Geol. Ital.,

13: 575-606.

Ballarin, S., Palla, B. and Trombetti, C., 1972. La gravita in Italia. Carta delle anomalie topografiche e

topografico-isostatiche. Boll. Geod. Sci. Aff., 31: 4: 461-489.

Bartohni, C., Bemini, M., Carloni, G.C., Castaldini, P., Costantini, A., Federici, P.R., Francavilla. F.,

Gasperi, G., Lazzarotto, G., Mazzanti, A., Papani, G., Pranzini, G., Rau, A., Sandrelli, F. and Vercesi.

P.L., 1983. Carta neotettonica dell’Appennino Settentrionale. Note illustrative, Boll. Sot. Geol. Ital..

in press.

Bath, M. and Duda, S.J., 1963. Strain release in relation to focal depth. Geofis. Pura Appl., 56: 93-100.

Bernini, M., Boccaletti. M. and Clerici, A., 1977. The role of the foreland in the late geotectonic evolution

of the Mediterranean arcs at the continental stage. Rend. Accad. Naz. Lincei, Sci. Fis. Mat. Nat., 6

Ser., 63: 561-570.

Boccaletti, M., Coli, M. and Napoleone, G., 1980. Landsat lineation pattern in the Apennines and its

geodynamic significance. Modem Geol., 7: 95-103.

Boccaletti, M., Coli, M., Decandia, F.A., Giannini, E. and Lazzarotto, A., 1981. Evoluzione dell’Appen-

nino Settentrionale second0 un nuovo modello strutturale. Mem. Sot. Ital., 21: 359-373.

Boccaletti, M., Conedera, C., Dainelli, P. and Gocev, P., 1982. The recent (Miocene-Quaternary)

regmatic system of the Western Mediterranean Region. J. Petrol. Geol., 5(l): 31-49.

Bolis, G., Cappelli, V., Marinelli, M., 1981. Aeromagnetic data of the Italian area: instrumental to a better

comprehension of the basement main characteristics in Italy. 43rd E.A.E.G. Meet.. Venice, presented

paper.

Boni, A., 1962. Lignes et problemes tectoniques du secteur Nord-Quest de 1’Apennin Septentrional. Bull.

Sot. Geol. Fr., Ser. 7, 4: 644656.

Bortolotti, V., 1966. La tettonica trasversale dell’Appennino. I-La linea Livomo-Sillaro. Boll. Sot.

Geol. Ital., 85: 529-540.

Bossolasco, M., Eva, C. and Pasquale, V., 1973. 11 terremoto de1 25 Ottobre in Lunigiana. Riv. Ital.

Geofis.. 22: 314-322.

31

Bossolasco, M., Eva, C. and Pasquale, V., 1974. On seismotectonics of the Alps and Northern Apennines.

Riv. Ital. Geofis., 23(1/2): 57-63.

Boyer, SE. and Elliott, D., 1982. Thrust Systems. Am. Assoc. Pot. Geol. Bull., 66(9): 1196-1230.

Cagnetti, V., Pasquale, V. and Pollinari, S., 1976. Focal mechanisms of earthquakes in Italy and adjacent

regions. Comitato Naz. Energia Nucleare, RT/AMB, 76, 4.

Calcagnile, G. and Panza, G.F., 1980. The main characteristics of the lithosphere-astenosphere system in

Italy and surrounding regions. Pure Appl. Geophys. 119: 865-879.

Carmignani, L. and Giglia, G., 1983. “Autoctono Apuano” e Falda Toscana: sintesi dei dati e

interpretazioni pih recenti. Mem. Sot. Geol. Ital., in press.

Carmignani, L., Giglia, G. and Khgfield, R., 1978. Structural evolution of the Apuane Alps: an example

of continental margin deformation. J. Geol., 86: 487-504.

Carmignani, L., Giglia, G., and Kligfield, R., 1981. Nuovi dati sulla zona di taglio ensialica delle Alpi

Apuane. Mem. Sot. Geol. Ital., 21: 93-100.

Cassinis, R., 1981. Prime correlazioni tra struttura crostale e sismicita della regione italiana. In: CNR,

Gruppo Naz. Geofis. Terra Solida. Atti de1 lo Convegno, 2: 507-532.

Cattaneo. M., Eva, C. and Merlanti, F., 1981. Seismicity of Northern Italy: a statistical approach. Boll.

Geofis. Teor. Appl., 89: 31-42.

Chiocchini, V., Cipriani, N. and Giardini, G., 1981. Pretrologia delle arenarie dei bacini minori torbiditici

di Camerino e di S. Donato Cantia (Marche). Boll. Geol. Ital., 100 (2): 213-231.

Costantini. A., Gandin, A., Guasparri, G., Mazzanti, R.. Lazzarotto, A. and Sandrelli, S.. 1980.

Neotettonica dei Fogli: 111 Livorno-112 Volterra-113 Castelfiorentino-119 Mass marittina-120

Siena-121 Monte pulciano-126 Isola d’Elba-127 Piombino-128 Grossetto 129 S. Fiora. In: Contrib-

uti Preliminari alla Realizzazione della Carta Neotettonica d’Italia. CNR P.F. Geodin., Pubbl. 356:

1075-1186.

Costantini. A., Lazzarotto, A. and Sandrelli, S.. 1982. II graben di Siena. Conoscenze geologico-strutturali.

CNR, P.F. Geodin.. R.S. 9: 11-33.

C.N.R., P.F. Geodinamica, 1982. Italian Seismic Catalogue from the year 1000 up to 1980. In press.

Elter, P., Giglia, G., Tongiorgi. M. and Trevisan, L.. 1975. Tensional and compressional areas in the

recent (Tortonian to present) evolution of the Northern Apennines. Boll. Geofis. Teor. Appl.. 77:

3318.

ENEL. 1977. File of Italian earthquakes from the year 1000 up to 1975. Unpubl.

Eva. C.. Giglia, G., Graziano, F. and Merlanti. F.. 1978. Seismicity and its relation with surface structures

in the North-Western Apennines. Boll. Geofis. Teor. Appl., 20: 2633277.

Fahlquist. D.A. and Hersey, J.B.. 1969. Seismic refraction measurements in the Western Mediterranean

Sea. Bull. Inst. Ocean. Monaco, 67 (1386).

Ferreari, G., Guidoboni. E. and Manenti, E., 1981. Criteri per la ricerca dei dati sismici storici. Atti

Convegno Sismicita dell’ltalia, Udine, 12-14 Maggio 1981. In press.

Ferrucci, F., Makris, J., Nicolich, R., Snoek, M., Steinmetz, L. and Weigel, W.. 1982. Deep seismic

soundings by observation in the Tyrrhenian Sea. Proc.-Verb. Congr.-Assem. C.I.E.S.M.. 28e, Cannes.

2-11 Dec. 1982.

Gasparini, P. and Postpischl, D., 1980. Criteri per la rappresentazione automatica dei dati ipocentrali

CNR/PFG, N. 322: 1-31.

Gasparini, C.. Iannaccone, G. and Scarpa, R., 1983. Fault-plane solutions and seismicity of the Italian

peninsula. Tectonophysics. 117: 59-78.

Ghelardoni, R., 1965. Osservazioni sulla tettonica trasversale dell’Appennino settentrionale. Boll. Sot.

Geoi. Ital., 84: 277-290.

Giese, P., Wigger. P., Morelli, C. and Nicolich, R., 1981. Seismische Studien zur Bestimmung der

Krustenstruktur-Anomalien der Toskana, Comm. Europ. Communities, EUR, 75-78.

Giglia, G., 1974. L’insieme Corsica-Sardegna e i suoi rapporti con 1’Appennino Settentrionale: rassegna

di dati cronologici e strutturali. In: Paleogeografia de1 Terziario Sardo nell’ambito de1 Mediterraneo

Occidentale. Rend. Sem. Fat. Sci. Univ. Cagliari, pp. 245-275.

38

Iaccarino, S. and Papani, G,, 1980. II Messiniano dell’Appennino Settentrionale della Val d’Arda alla Val

See&a: stratigrafia e rapporti con ii substrato ed il Pliocene. In: Volume deddicato a S. Venzo. Parma,

pp. 15-46.

Loddo, M. and Mongelli, F., 1979. Heat flow in Italy. In: V. Cermak and L. Rybach (Editors), Terrestrial

Heat Flow in Europe. Springer, Berlin, pp. 221-231.

Mattauer, M., 1975. Sur Ie mecanisme de formation de la schistosite dans 1’Himalaya. Earth Planet. Sci.

Lett., 28: 144-154.

Mazzanti, R. and Trevisan, L., 1978. Evoluzione d&a rete idrografica nel~Appennino Centro-Settentrio-

nale. Geogr. Fis. Din. Quat., 1: 55-62.

Merla, G., 1951. Geologia dell’Appennino Settentrionale. Boll. Sot. Geol., 70: 95-382.

Morelli, C., 1973. La gravimetria dell’area italiana. In: Modeme vedute sulla Geologia dell’Appennino.

Accad. Naz. Lincei, quad. N. 183.

More% C.. Giese, P.. Carrozzo, M.T., Colombi, B., Guerra, I., Hirn, A., Letz, H., Nicolich, R., Prodehl,

C., Reichert, C., Rower, P., Sapin, M., Scarascia, S. and Wigger, P., 1977. Crustal and upper mantle

structure of the Northern Apennines, the Ligurian Sea and Corsica, derived from seismic and

gravimetric data. Boll. Geofis. Teor. Appl., 19 (75-76): 1999260.

Panza, G.F.. Calcagnile, G., Scandone, P. and Mueller, S.T.. 1980. La struttura profonda dell’area

Mediterranea. Le Scienze, 24: 60-69.

Pieri. M. and Groppi, G., 1981. Subsurface geological structure of the PO Plain, Italy. C.N.R., P.F.

Geodin., Sottoprogetto Modello Strutturafe, PubI. 914: 23.

Plesi. G. and Cerrina Feroni, A., 1980. Contributo alla conoscenza delle deformazioni de1 neosutoctono

della Toscana: segnalazione di due fasi di raccorciamento attraverso lo studio degli stilotiti impressi su

ciottoli. Boll. Sot. Geol. Ital., 98: 15-25.

Postpischl, D., 1979. The process of catdoging historical seismic data. IUGG 17th Gen. Assem.,

Canberra.

Postpischl, D. (Editor), 1983. Atlas of Isoseismal Maps of Italian EarthquakesCNR. P.F.G., in press.

Ricci Lucchi, F., 1975. Miocene Paleogeography and Basin Analysis in the periadriatic Apennines. In: C.

Squyres (Editor), Geology of Italy, Pet. Explor. Sot. Lybia, Tripoli, pp. 129-236.

Shebalin, N.V., 1972. Macroseismic data as information on source parameters of large earthquakes. Phys.

Earth. Planet. Inter., 6(4): 315-323.

Signorini, R., 1935. Linee tettoniche trasversali neB’Appennini Settentrionale. Rend. Accad. Lincei, Cl.

Sci. Fis. Mat. Nat., Ser. 6, 21: 42-45.

Zanzucchi, G.. 1963. Sulla vergenza di alcune strutture trasversali nel flysch dell’Appennino Emiliano.

Ateneo Parmense, 34 (suppl. 2): 23.