Tecmnophyslcs, 117 (1985) 59-78

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

59

FAULT-PLANE SOLUTIONS AND SEISMICITY OF THE ITALIAN

PENINSULA

CALVIN0 GASPARINI ‘, GIOVANNI IANNACCONE 2 and ROBERTO SCARPA 3

’ Istltuto Nazionale di Geofisica, Via Ruggero Bonghi I1 /B, Rome (Italy)

’ Dipariimento di Geofisica e Vulcanologia, Uniuersira’ di Napoli, Large S. Marcellino IO, 80138 Naples

(Italy)

~’ Ossercatorio Vesuviano, 80056 Ercolano (NA) (Italy)

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

ABSTRACT

Gasparini, C., Iannaccone, G. and Scarpa, R., 1985. Fault-plane solutions and seismicity of the Italian

peninsula. In: C. Eva and N. Pavoni (Editors), Seismotectonics. Tectonophysics. 117: 59-78.

A new fault-plane solution map of the Italian peninsula is presented in this paper. The earthquakes

analyzed are included in the period 1905-1980, with magnitudes ranging 4-7, 75 earthquakes are located

in the crust, while 31 are related to the deep and intermediate zone of the Calabrian arc. The large seismic

events of the Italian peninsula are generally associated with normal faulting, while strike-slip motion is

mostly related to small earthquakes, located along lateral segments of the mountain chain.

The deep and intermediate earthquakes of the Tyrrhenian Sea indicate predominant down-dip

compression, and strike-slip motion at the boundaries of this Benioff zone. This last is interpreted as a

remnant of a subduction zone, active since Oligocene, extending to 500 km depth, with a very small lateral

size (about 300 km). The present tectonics of this Benioff zone is strongly conditioned by the lateral

bending, more so than the gravitational sinking process.

The coexistence of thrust and normal faulting motion associated to the earthquakes, within a few tens

of kilometers of each other, seems to be explained by the strong lateral inhomogeneities of the crustal

rocks present in this region, more so than to the depth of the seismogenetic zone and the nature of the

faulting process.

INTRODUCTION

The peninsular part of Italy, including Sicily, is a region of great geodynamical

interest since it is located at the center of the Mediterranean Sea between two

compressive zones belonging to the African and Eurasian plates. Geological and

geophysical evidences suggest that the tectonics is dominated by extensional features

along the seismic region of the Apennines (Ritsema, 1971; McKenzie, 1972; Tap-

ponnier, 1977; Scandone, 1979; Mantovani and Boschi, 1983; Gasparini et al., 1982)

while its southern part, the Calabrian arc, is a Benioff zone constituting a segment of

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

60

a broader subduction zone, active since Oligocene (30 Ma ago). In this last region earthquakes as deep as 500 km occur within a very small domain as compared to the lateral extent of the Pacific Benioff zones. The tectonics seems to be conditioned by the strong lateral deformation processes active along its boundaries, as indicated by the segmentation of the lithosphere evidenced by the reevaluation of the available fault-plane solutions and by the distribution of shallow and deep earthquakes (Gasparini et al., 1982).

The present paper discusses some relevant seismotectonic features of Italy, with the exclusion of the Alps, by reporting a new set of fault-plane solutions, obtained through a further revision of instrumental data collected from all seismic stations

operating around the world since the beginning of this century and by data from recent moderate and large Italian earthquakes.

DATA ANALYSIS

The data set used consists of polarity readings of P-waves from many sources which include mostly seismograms of the WWSSN network and other additional seismological observatories. In some cases, particularly for deep and intermediate earthquakes of the Tyrrhenian Sea, the BCIS, the ISS and ISC bulletins have been proved to be a reliable data source with respect to the double couple model, with a very low number of inconsistent readings. 106 earthquakes are analyzed, for a total number of about 6300 first motion polarities, with magnitudes ranging from 4 to about 7. Earthquakes with M lower than 5 are related to events recorded in recent years, at short period seismic stations and intermediate distances; consequently these data have to be treated with care due to the often unclear and ambiguous onsets. The selected fault-plane solutions are those characterized by the highest score (low number of inconsistent readings) and with the best azimuthal distribution of the observation points. The data have been plotted on a Wulff stereographic projection, lower hemisphere. The incidence angles at the focus have been calculated from Herrin et al. (1968) travel-time tables. The present analysis includes fault-plane solutions already available in the literature (Ritsema, 1971, 1979; Mckenzie, 1972; Cagnetti et al., 1978; Gasparini et al., 1980, 1982) which are in substantial agreement with the present results, notwithstanding the higher number of data considered here. Additional 22 solutions for crustal earthquakes are reported in the present paper. All 75 solutions related to crustal events are illustrated in Fig. 1. The 31 focal mechanisms of deep and intermediate earthquakes have been reviewed in detail by Gasparini et al. (1982). Tables 1 and 2 report the lists of the fault-plane parameters, the solutions related to the crustal earthquakes are shown in Fig. 2. Figures 3 and 4 show the stress pattern derived from the previous earthquake mechanisms.

The results obtained from the analysis of the fault-plane solutions can be summarized as follows:

(1) the large crustal earthquakes along the peninsula are generally associated with normal faulting;

61

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+ \

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z + G I a.

\, +‘I I

‘\

+ : + t l + +t

z t ‘+

@

a’ i +I

\

’ +“++ G

62

63

64

65

66

67

68

69

(2) strike-slip motion characterizes small or moderate earthquakes (M lower than

6), in general it is related to transition zones or lateral segments of the chain;

(3) deep and intermediate earthquakes of the Tyrrhenian Sea indicate predomi-

nantly down-dip compression and minor strike-slip at the boundaries of the Benioff

zone; the distribution of the stress axes shows that the tectonics is strongly

conditioned by the lateral bending of this arc, one of the less active and most

deformed in the world.

In the northwestern Apennines the complex faulting and the epicenter distribu-

tion (Fig. 5) suggest that tectonic stress may be released by a continuous series of

small to moderate earthquakes (Eva et al., 1978). The geology is dominated by

tensional tectonics, with a horst-and-graben structure, while along the external

Fig. 2. Fault-plane solutions relative to 75 crustal earthquakes of the peninsular part of Italy. Data are projected into a Wulff net, lower hemisphere.

70

Fig. 3. Stress pattern derived from crustal earthquakes. Thick and thin lines represent the most horizontal

(dip d 30”) P and T axes. In the case of dip slip faulting (both P and T dipping > 30”) the length of

these lines has been reduced to a half.

Padanian margin compressive movements have been deduced from the buried folds

located beneath the sedimentary cover (Elter et al., 1975). The pattern derived from

the epicenter distribution and stress axes indicates a seismic zone elongated in the

NW-NNW direction, following the trend of the Apennines, while transversal

alignments are not recognizable from seismological data due to the size and poor

geometry of the seismic network. The fault-plane solutions available indicate both

normal and thrust faulting. This complex motion may be related to the flexure of the

lithosphere of this region, creating contemporaneous compressive and tensional

movements, at the same location but at different depths. In general no reliable depth

estimates exist for these earthquakes. A few recent earthquakes, which are char-

acterized by reliable depth estimates, based on several readings of later reflected

arrivals (pP or sP) have been located in the lower crust or in the mantle. Two

fault-plane solutions of earthquakes located in the northwest boundary of the

peninsular Italy (events 17 and 18 in Table l), indicating thrust faulting, may be

connected to the compressive stress pattern characteristic of the Alpine domain

tectonics.

71

ADRIATIC

TYRRHENIAN

SEA

Fig. 4. Stress pattern derived from deep and intermediate earthquakes of the Calabrian arc.

Along the central and northern Apennines the seismotectonic zoning maps

available (Gasparini and Praturlon, 1981; Lavecchia and Pialli, 1982) allow to

distinguish at least four areas characterized by different dynamic behaviour (Fig. 5):

(1) A zone elongated through the Tyrrhenian coasts, which is marked by a low

seismicity level and Quaternary volcanic activity.

(2) The Apenninic belt which includes the region with the higher topography.

This belt is associated with the largest seismic events and the highest frequency of

occurrence. The most reliable hypocentral depths are situated between 5 and 20 km.

The focal mechanisms indicate normal faulting motion, with T axes mostly oriented

orthogonal to the arc-like feature of the chain.

(3) The seismic activity decreases toward the Adriatic Sea: around the Ancona

region the available focal mechanisms indicate transcurrent left lateral motion,

perpendicular to the Apennines. The limit between zones 2 and 4 drawn in Fig. 5 is,

however, not clearly defined. Most of this region has been folded in the Quaternary,

and at present it is probably affected by compressive motions. Lavecchia and Pialli

(1982) interpreted the lack of significant seismicity along the Adriatic Sea as due to

the effects of anelasticity and to the different energy required to produce slip on a

thrust fault, with respect to that required to break a normal fault (Sibson, 1980).

The complexity of the stress field, as indicated by the scattering of the azimuths

of T and P axes in some regions, may be related to the relatively lower level of

seismicity and the limited extent of the rupture zones associated to the large

earthquakes, as deduced from the pattern of isoseismals.

Fig. 5. Epicenter map of crustal earthquakes located in the period 1975-1981. Data are from the bulletins

of Istituto Nazionale di Geofisica, Rome. A seismotectonic zoning of this region is also shown.

The situation is somewhat different in the southern Apennines, where the meisoseismal zones of the larger historical earthquakes have a pronounced elliptical shape, extended up to f-2 thousands squared kilometers (Martini and Scarpa, 1983). Moreover the linear extent of the aftershock zone of the November 23, 1980 earthquake is in a very good agreement with the strike of the normal faulting indicated by the focal mechanism (N118-145E). Here the scattering in the fault-plane solutions of the events in certain portions of the chain are in close agreement with the anti-Apenninic sinistral strike-slip motion and the segmentation of the chain. This fact also allows to hypothesize on the boundaries of the rupture zones

13

associated to the large earthquakes, characterized by a maximum extent reaching up

to 70-80 km and located in parallel with volcanoes along the Tyrrhenian margin and

with a more diffused epicenter distribution.

In southern Italy a high degree of correlation exists between the stress field

deduced from the analysis of brittle deformations affecting the middle-late Pleisto-

cene sediments and the pattern deduced from the analysis of the fault-plane

solutions (Cello et al., 1982). In particular in the southern Apennines and Calabria,

the geostructural analysis has shown a close correspondence between the orientation

of the T axes, which are the predominant horizontal components of the stress field,

associated to larger earthquakes. The release of seismic energy in the Calabrian arc

has been explained to be connected with brittle deformations in the upper crust,

induced by the activity of the shear zone at depth (Ghisetti and Vezzani, 1982). The

coexistence here of fault-plane solutions of normal and thrust type may be interpre-

ted in terms of the different depth of the seismogenetic zones.

Along the Calabrian arc and western Sicily the neotectonic evolution of the

regional structures, as well as the orientation of the regional stress field and its

connection with the deep structures, are indicated on a detailed seismotectonic map

(Ghisetti and Vezzani, 1982; Gasparini et al., 1982). Among these active structures,

the southern part of the Calabrian arc is characterized by the higher seismicity levels,

with magnitudes (MS) reaching up to 7.0-7.5, such as the events occurring at the

beginning of this century (1905 and 1908). The fault plane solution of the 1908 event

(Ms = 7.0) indicates normal faulting, consistent with the pattern of vertical displace-

ments (Loperfido, 1909), showing a relative subsidence of the concave block,

plunging along a quasi vertical plane in the Tyrrhenian Sea (Schick, 1977). Another

interpretation is a couple of contiguous synthetic and antithetic normal faults,

dipping in the Messina strait and southern Calabria (Mulargia and Boschi, 1983).

The direction stress T axis is connected to the strong lateral stretching of the

Calabrian arc (Gasparini et al., 1982)

DISCUSSION

The seismotectonic implications of these fault-plane solutions have been revised

in detail. One of the most interesting problems deriving from the present stress field

analysis is the coexistence, within a narrow belt, of parallel normal and thrust faults,

as evidenced from the geology and neotectonics. The tensional motion is associated

to the large and moderate seismic events distributed along the Apenninic mountain

belt, while the existence of compressive stresses is inferred from the presence of the

“Calabrian ridge” in the Ionian Sea, and thrust fault systems active since the middle

Miocene-lower Pliocene. According to Dalmaryac and Molnar (1981) these con-

trasting tectonic styles are a common feature of many seismogenetic regions. They

can be explained by buoyancy forces due to the presence of mountains and their

roots, and thus limiting the upper value of this type of force at about 500 bar. The

model has been applied, however, to the region of Peru, where the seismogenetic

74

zone is mainly determined by thrust faults, exactly the reverse of that which is the

case in the Apennines. It is likely that in the present region the rule played by the

high level of the crustal and lithospheric inhomogeneities is fundamental, more so

than the action of the tectonic forces which have been related to the rifting process

migrating from the Tyrrhenian Sea and presently being active beneath the Apen-

nines (Scandone, 1979; Calcagnile et al., 1981).

From the present results and the analysis of the neotectonic and structural models

derived for the region Italy (Gasparini and Praturlon, 1981; Ghisetti and Vezzani,

1982) it is possible to distinguish the following sectors marked by different seismo-

tectonic behaviour (Fig. 5):

Zone 1. The perityrrhenian zones with very low and shallow seismicity. This

sector is characterized by distinctive features as a thinned crust, positive Bouguer

anomalies and volcanic activity, related to the rifting process in the Tyrrhenian Sea.

Zone 2. The central and northern Apennines, with higher seismic activity than the

previous region but lower than in the southern Apennines and the Calabrian arc.

Fault-plane solutions show a predominance of normal faulting. This region is also

characterized by the most elevated mountains and includes the most external

Plio-Quaternary graben and other lowered zones. The transition between the positive

and negative values of the Bouguer anomalies limits the western border of this

region.

Zone 3. This region was already folded during the Pliocene with a probable

extension into the Quaternary. The region has not yet been uplifted, it is char-

acterized by negative Bouguer anomalies and by a moderate level of seismicity as

compared to the region 2.

Zone 4. The seismic activity becomes higher in this region, comparable to the level

of zone 2. Focal mechanisms reveal transcurrent motion in its southern part (Ancona

area), while complex motion is found in the northern part with the presence of both

normal and thrust faulting. The different depth of the seismogenetic zones may be

related to a possible bend of the lithosphere, which can reach 40-50 km, and may

justify this complex behaviour. This region corresponds to the most advanced part of

the Apenninic chain.

Zone 5. This region is not seismically active, corresponding in its southern part to

the Adriatic foreland and in the northern sector to the area of contact between the

Adriatic microplate and Apenninic chain.

Zones 6a and 6b. The region 6a (Gargano) in the past has been characterized by

maximum values of seismicity up to 10 MCS intensity scale. Presently, and in

average, the seismic activity is somewhat lower than the seismicity of the contiguous

Apennines. The zone 6b corresponds to the Ofanto fault zone, marked by very low

intensity earthquakes. Some amplification of the intensity, however, has been

observed in correspondence of the largest earthquakes in the southern Apennines

(De Vivo et al., 1979).

Zone 7. This region, extending for about 100 km in the Apenninic direction, has

75

been characterized in the past by earthquakes with size and frequency comparable to

the events in the region 8. The geology and tectonics are similar for these two

regions. The last large earthquake occurred here in 1688 and no seismic activity has

been located at magnitudes higher than 3 since 1975 and higher than 4 since 1960

(Martini and Scarpa, 1983).

Zone 8. This region is characterized by the most intense vertical movements in the

whole italian pensinsula, with the present uplifting rates amounting to 1-2 mm/year.

Earthquakes have reached magnitudes up to 7-7.5, and were characterized by

normal faulting and depth up to 20-25 km. The depth of the November 23, 1980

southern Italy earthquake (Ms = 6.9) was reliably determined at 16 km, with

aftershocks confined to 6-16 km depth range (Del Pezzo et al., 1983). The crustal

thickness varies from 20-25 km to 35-40 km in the central part.

Zone 9. These two regions are characterized by the same rate of uplifting as in the

zone 8; but the historical seismicity is much lower, reaching up to magnitude 6 only

(or intensities 8-9 MCS) as reported in the available 2000 years catalogues. The

seismotectonic explanation proposed by Ghisetti and Vezzani (1982) is that the

strike of the main shear zone at these places is nearly parallel to the regional slip

direction, with a prevailing of aseismic creep processes.

Zone 10. This region is presently characterized by a low seismicity, which is in

agreement with the neotectonic data. Large seismic events, however, have occurred

here in 1169, 1542 and 1693 A.D.. Purcaru and Berckhemer (1982) defined a seismic

gap in the area, but seismological data are very poor and no reliable isoseismal maps

exist for the mentioned earthquakes. Geological data show important deformations,

but with lower intensity than those in the chain itself.

Most of the boundaries of the zones indicated are not well defined, particularly in

relation to the seismic activity. This uncertainty is due to the present inaccuracy of

earthquake locations, to the relatively low level of seismic activity, to the short time

interval and to the high magnitude threshold of the seismic stations.

The discussion in this section is mainly based on historical data and the epicenter

maps obtained through the NEIS bulletins and the bulletins from the Istituto

Nazionale di Geofisica, Rome, in the period 1975-1981.

In conclusion it can be stated that the present stress field and the seismicity

pattern reflect the state of strong disequilibrium of the lithosphere. One of the

possible causes of this disequilibrium is the strong lateral variation of the crustal

thickness and its properties (Artyushkov, 1973) particularly for the Apenninic

mountain range. In fact (Fig. 6) the crustal thickness varies from lo-15 km in the

center of the Tyrrhenian Sea to 35-40 km below the most elevated part of the

Apennines. The velocity properties too are strongly inhomogeneous along the Italian

region (Cassinis et al., 1979; Scat-pa, 1982). A sharp transition of the stress field and

of the seismicity pattern exists in the central part of the Apennines, around the

Ancona-Anzio line, which corresponds to a lateral bending of the lithosphere and a

variation of the velocity properties in the lid (Panza et al., 1981).

: . . a

.

Fig. 6. Epicenter map of earthquakes in the period lOOf-1975. Data are from the catalogue of Carrozzo et al. (1973) and NEIS bulletins. Contour lines represent the crustal thickness from DSS data (after Cassinis et al., 1979; Guerra et al., 1981; Nicolich, 1981 and Scarascia, 1982).

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