Geodynamic behaviour of eastern and western sides of Mount Etna

Tecfonophysics, 179 (1990) 81-92 Elsevier science Publishers B.V., Amsterdam - Printed in The Netherlands

81

Geodynamic behaviour of eastern and western sides of Mount Etna S. Gresta I, V. Long0 ’ and A. Viavattene i ’ Istituto di Scienze delta Term, ilniversitri di Catania, Corso iralio 55, 95129 CatDnia (IraY,, 2 C. N.R ktituto

Intemazionale

di Yulcanologia, mule Regina Margherita 6, 95123 Catania (Ita&)

(Received October 251988;

revised version accepted July 21,1989)

ABSTRACT

Gresta, S.. Longo, V. and Viavattene, A., 1990. Geodynamic behaviour of eastern and western sides of Mount Etna. In: E. Mantovani (Editor), Seismicity and Crnstal Deformation. ‘7’ectonophysics, 179: 81-92. The differing behaviour of the easternand western flanks of Mount Etna has been investigated by analyzing morphological, structural, volcanic and seismic data. Morphological and structural analyses have pointed out a higher “degree” of tectonization on the eastern side. From the volcanoiogical point of view, the analysis of some parameters of adventive eruptions during the last three centuries did not evidence great differences between the two sides of the volcano. only higher lava outputs on the eastern flank have been remarked. Historic and recent seismic activity has been analyzed for the number and magnitude of events, strain energy release and frequency-magnitude law. The shallowest and strongest earthquakes occur on the eastern side of the volcano, strictly linked to a regional structural system trending NNW-SSE. The western side is mostly characterized by deeper earthquakes that seem to control the volcanic activity. Lower seismic energy releases and higher values of the b coefficient in the Gutenberg-Richter’s (1956) law can be observed on the western side. The differences observed between the two sides of the volcano have been interpreted in terms of both regional and local tectonics, with the result that a higher extensional stress field is attributed to the eastern side of the volcano.

Introduction The central Mediterranean area is characterized by a very complex tectonic picture. In general this is a zone characterized by compression, due to the collision of the African Plate with the Eurasian one. However, no simple model can be proposed to explain the collision mechanisms clearly. This is because each of the many microplates that exist has a different pattern of movement which gives rise to local situations that are often in apparent contradiction with each other and with any model proposed (Mulargia et al., 1985a). A detailed discussion of these problems is outside the aim of the present paper, but, as an example, it is difficult to explain both the recent 0040-1951/90/$03.50

43 1990 - Elsevier Science publishers B.V.

talc-alkaline volcanic activity in the Eolian Islands and the tholeiitic volcanism of Etna (Barberi et al, 1973). The Etna volcano is located on the eastern side of Sicily, between two main tectonic units: the Northern Chain and the Iblean Foreland. The Northern Chain links the Apennines to the Atlas mountains and consists of a series of thrust sheets made up of sedimentary terranes deposited along the old cratonic edge of Africa (e.g. Lentini, 1982). The Iblean Foreland, in southeastern Sicily, consists of a carbonatic succession interlayered with many volcanic levels. It may be considered to be part of the African Plate margin. The progressive NE-SW downfaulting of the northwestern margin of the Iblean Plateau led to the formation

82

S. GRESTA

iYRRHENlAN

SEA

ET AL.

MESSINA

_ 380

Fig. 1. Tectonic setting of Sicily (see text).

of the Catania-Gela Foredeep, which is on the edge of the Caltanissetta Basin (see Fig. 1). Eastern Sicily is characterized by three main fault systems: the Messina-Corn&o line, trending NE-SW; its conjugate system, trending NW-SE; and the E-W Mt. Kumeta-Alcantara (e.g. Ghisetti and Vezzani, 1982). More detailed studies of the northeastern sector have allowed the identification of the TindariLetojanni line (Cristofolini et al., 1977), and of the Messina-Giardini line (Barbano et al., 1978). This has the NE-SW trend as the Messina-Comiso system, but is independent of it. Finally, a recent work (Lo Giudice et al., 1982) has shown that the Etnean region is intersected by four main structural trends (ENE-WSW, NNESSW, NNW-SSE, and WNW-ESE), and at their crossing the central crater system of the volcano can be located. However, distribution of these structural trends is not homogeneous ~ou~out the area, but concentrations of structures trending in the same direction are found on the flanks of the volcano. In order to insert the activity of Etna into the tectonic framework sketched above, morphological, structural, volcanic and seismic data have been used. In particular, the aim of this paper is to point out possible different features between eastern

and western sides of the volcano, and to explain them in terms of geodynamic behaviour. On the basis of previous works (Cristofolini et al., 1978, 1981; Lo Giudice et al., 1982) the areal distribution of tectonic and volcanic structures is discussed, as well as the morphology of the volcano. An analysis of some parameters relative to the adventive eruptions which have occurred at Etna during the last three centuries has been performed, while seismicity is studied from both historical and recent instrumental data. Analysis of data (a) Morphological and structural &a

Mount Etna is a typical strato-volcano with a basal diameter of about 40 km and a height of about 3350 m. The present volcano is the result of a succession of quiet lava eruptions and intense explosions associated with caldera collapses (for a detailed descfiption of the eruptive history of Mount Etna, see Romano, 1982). One of the most noticeable morphological features of Mount Etna is the eccentric location of the surmnit craters. This is due partly to the steeper slopes on western and northern sides, where the

GEODYNAMIC

BEHAVIOUR

OF EASTERN

TYPICAL

AND

WESTERN

SIDES

OF MOUNT

83

ET’NA

SECTION OF THE WESTERN FLANKS

NF

CENTRALCRATER

FREOURNCYOF

Km

TYPICAL

CENTRALCRITER

Km 1

5

la

2%

1

Km

SECTION OF THE EASTERN FLANKS

15

l0

20

25

Fig. 2. Slope profile and distribution of pyroclastic cones (C) and volcanic linear elements (V) for eastern and western sides of Mt. Etna (from Lo Giudice et al., 1982).

flanks abut the surrounding mountains (Guest, 1982) and partly to the migration (from SE to NW) of the eruptive centers which has characterized voleanic activity of the last 100,000 years (Romano, 1982). Generally speaking, slopes are gentle with a concave profile over almost all the volcano; but a sharp slope variation can be observed (Fig. 2) on both eastern and western flanks at about 1750 m a.s.1. More in detail, according to Lo Giudice et al. (1982) a variation can be observed at about 750 m a.s.1. (see Fig. 2) on the eastern side of the volcano only. The eastern flank is characterized by the great calderic depression of the Valle de1 Bove (zone no. 4 in Fig. 3), to which zone no. 7 (see Fig_ 3) corresponds on the western side. This zone is characterized by a high density of parasitic cones and by an apparent “mass excess”, which is due to the very high rate of effusion during the last

0

5

IO rm

Fig. 3. Sketch map of Mount Etna showing the eight main zones with different morphostructural features (redrawn after Cristofolini et al., 1978 and 1981).

84

2000 years (R. Cristofolini and R. Romano, pers. commun.). Previous morphostructural studies of the Etna volcano (Cristofolini et al., 1978, 1981) identified four main types of linear morphological elements, on the basis of their recent activity: fault scarps, alignments of “facets”, fault line scarps and morphologic flexures. Eight main zones have been recognized based on the azimuthal distribution of the morphotypes described above (Fig. 3). A recent paper (Rasa’ et al., 1982) introduced volcanic elements (eruptive fissures, parasitic cones, caldera rims) to obtain a more detailed structural survey of the volcano. The results of a thorough st~~tur~ analysis based on this approach were reported in Lo G&dice et al. (1982) and we have mainly used this paper to develop the following ideas. Study of the areal distribution of tectonic linear elements based on number and length shows two main trends (N60 o W and N60° E) on the NE flank, while the SE flank is characterized by a N20 * W trend. The western side shows a scattering of trends in the range N60° W-N60* E. A subdivision between NW and SW flanks has not been made due to the scarcity of elements for a good statistical significance (E. Lo Giudice, pers. commun.). The volcanic linear elements have a complex, but generally radial distribution, over most of the volcano. On the NE flank, the trends N60 o E and N30”E are dominant; on the SE flank most elements trend N-S and N30° W, on the NW flank N40°WandN800W~do~theSW~~N300E and N80 o E. A comparison between structural features of eastern and western sides of the volcano shows that the area1 density of faults is higher (0.32 km/km*) on the eastern flank than the western one (0.13 km/kmz); while the density of the eruptive systems (pyroclastic cones and volcanic linear elements) is only slightly higher on the eastern side (see fig. 13 in Lo Giudice et al., 1982). Finally, it does not seem accidental that the maxima observed in the distribution of eruptive systems as a function of altitude on both eastern and western sides correspond to the points of

S. GRFSTA

ET AL.

slope change (Fig. 2). According to Lo Giudice et al. (1982), this fact can have a structural meaning; it should confirm the higber “degree” of tectonization of the eastern flank of Etna. (b) Volcanic data

Volcanic activity at Mt. Etna can be divided into two main types: persistent activity at the summit craters and flank eruptions. Persistent activity can be of different types: degassing, strombolian and hydromagmatic explosions, lava filling or collapses and low flow rate (< 1 m3/s) lava emissions. Flank eruptions can be classified as: subterminal, lateral and eccentric based on the distance from the summit of the eruption site (~ttm~n, 1973). In a more recent paper (Roman0 and Sturiale, 1982), a classification based on the kind of the eruptive fracture system (radial or tectonic regional) has been proposed. This paper also gave a catalog of historical Etnean eruptions, which has been used in the present study. In order to work on accurately recorded data, only lateral and eccentric eruptions since 1600 have been used (see Mulargia et al., 1985b), which gave a total of 33 eruptions. The three eruptions, of 1607-1610, 1651-1653 and 1879, were excluded because there are uncertitudes about or lack of some parameters. In addition, seven of them occurred in a thin N-S trending rift zone on the southern side, and in order to find evidence of possible differences between eastern and western sides of the volcano, they were not analysed, even if they are important in the whole eruptive history of Etna. For each of the remaining 23 eruptions (16 of them on the eastern flank and 7 on the western one), the following parameters have been considered: location of the fractures that erupted, duration of emission, volume of both lava and pyroelastic material, explosive index and effusion rate. In order to point out possible different volcanological features between eastern and western sides, the range of variation of each eruptive parameter was considered (see Table 1). Eruptions on the eastern side seem to be characterized by longer durations and, consequently, higher lava outputs.

GEODVNAMIC

BEHAVIOUR

OF EASTERN

AND

WESTERN

SIDES

OF MOUNT

85

ETNA

TABLE 1 List of the eruptions considered in the present study (with relative parameters) for eastern (a) and western (b) sides of the volcano Start

Duration

Lava volume

Pyrocl. volume

Rate

(days)

(X106 ms)

(X106 d)

(m3/s)

58 84 194 380 14 180 70 280 150 1 13 32 18 16 372 69

180 100 75 80 36 50 48 120 96 1 63 78 40 10 157 75

E.I.

(a) 1646.11.20 1763X36.18 1766.06.27 1792.05.26 1809.03.27 1811.10.27 1819.05.27 1852.08.20 1865.01.30 1908.04.29 1911.09.09 1923.06.16 1928.11.02 1947.02.24 1950.11.25 1971.04.05

10.5 50.0 1.8 0.1 2.1 3.0 0.8 8.0 6.5 0.1 1.6 0.5 2.5 0.02 0.06 3.0

95.0 20.7 4.6 2.4 31.5 3.4 8.0 5.3 7.9 20.8 57.5 28.4 27.3 7.6 5.0 13.0

0.055 0.333 0.023 0.001 0.055 0.056 0.020 0.060 0.063 0.110 0.025 0.006 0.060 0.002 0.004 0.040

3.7 2.6 2.9 0.1 0.02 2.0 1.1

14.0 22.4 57.8 36.3 7.6 3.0 2.0

0.093 0.060 0.053 0.060 0.002 0.460 0.340

(b) 1763.02.06 1832.11.01 1843.11.17 1942.06.30 1949.12.02 1974.01.30 1974.03.11

32 22 11 1 3 17 19

36 40 52 1.6 10.5 2.4 2.1

No other significant differences result from the comparison of the two sets of parameters, confirming that probably too many “factors” are needed to describe the start and the evolution of an eruption properly (Cristofolini et al., 1987a). From the present analysis, the only remark that can be made is that, in the last three centuries, the eastern flank has been more active than the westem, with a distinctly higher number of eruptions and lava output (see Table 1). (c) Seismic data The analysis of the historic seismicity at Etna took into consideration earthquakes occurring in the region ranging from 37.48” to 37.85 o N and from 14.82O to 15.23OE, found in the Progetto Finalizzato Geodinamica-ENEL (1983) Italian seismic catalog. When local magnitudes (ML) were not available, we estimated the macroseismic magnitude (MM) from the empirical relationship between

magnitude MM and epicentral intensity I,,. The value of this for the Etnean area (Gresta, unpublished data) is: Mrvr= 0.41, + 0.8

(I)

In order to take into account the fact that the catalog is not complete for seismic events of lower magnitude, we only considered seismic events with a magnitude M 2 2.4 (I,, 2 IV), from 1865 to 1980. A set of 1065 earthquakes was selected for the whole Etnean area. The first interesting consideration is that most of the events (892) occurred on the eastern side of the volcano, while only 173 occurred on the westem flank. Moreover, the highest magnitudes were recorded for earthquakes occurring on the eastern flank. This is also confirmed by other authors (Mulargia et al., 1987) who carried out an analysis over a longer period. The value of the b coefficient for the Gutenberg-Richter’s (1956) relationship has been calculated (for both eastern and western sides of

86

S. GRESTA

the volcano) method

following

the maximum

likelihood

by Utsu (1965):

ET AL.

TABLE 2 Comparison of seismologicat parameters for the two sides of Mount Etna

b = -0-4343

(2)

M-M,

Seismological data

Eastern flank

Western flank

892

173

(1865-1980)

where G and MO are the average and the smallest magnitude

of each data set respectively.

The error at 95% of the confidence

(M>2.4)

limit (Aki,

1965) is: Ab=p

1.96. b

Number of events M

4.6

4.0

b yzuue

1.1kO.l

1.5_+0.2

(3)

J;;

where n is the number

of earthquakes

in the data

Several during

set. The b value

related

to earthquakes

occurring

on the western side of Etna gives higher results (1.5 + 0.2) than that of the eastern flank (1.1 + 0.1). The above mentioned results are all summarized in Table 2. Finally the seismic energy released by each event was calculated using the formula: log E=

11.8 + 1.5M,

(4)

Strain release curves have been obtained for eastern and western flanks, showing (Fig. 4) a distinctly higher release of seismic energy on the eastern flank of the volcano.

*

studies

of Etna

the last decade

Cosentino

have been

using instrument

performed data (e.g.

et al., 1982; Scarpa et al., 1983; Glot et

al., 1984; Patane’ et al., 1984; Gresta and Patane’, 1987; Gresta et al., 1987). All these works pointed out that more than 80% of the earthquakes occurring at Etna are very shallow (h -c 5 km), and mainly located on the eastern side of the volcano. This seems to confirm the higher activity of the eastern flank suggested by the analysis of the historic seismicity. However, if only deeper events (h > 10 km) are analyzed a very interesting observation can be made. In this case the greater part of the earthquakes occur on the western side of the volcano,

E’ldo.rp)

20_

1865

1900

19so

1980

t(yoars)

Fig. 4. Strain release curves for earthquakes on eastern (a) and western (b) sides of Etna, during 1865-1980. occurrence of eruptions on the respective sides.

Dots indicate the

GFiODYNAMIC

BEHAVIOUR

OF EASTERN

AND

WESTERN

SIDES

OF MOUNT

often clustered in swarms that seem to be related to a NNW-SSE structural trend (see for example Fig. 5). As has often been observed (Clot et al., 1984; Gresta et al., 1987), this “activation” of deep structures on the western flank of Etna seems to be linked to important changes in both seismic and volcanic activity. Further instrumental studies of the seismic activity have allowed focal solutions of some events to be obtained (e.g. !karpa et al., 1983; Gresta et al., 1985). Unfortunately very few focal mechanisms have been determined for events on the western side, which hinders possible comparison between the behaviour of the two sides of the volcano. However, interesting results can be obtained by the integration of ~st~ent~ studies and macroseismic ones (Benina et al., 1984) to give a better understanding of features of some seismogenic structures. Only a few shallow faults, mainly trending E-W, and probably linked to the Mt. KumetaAlcantara regional system, are active on the northem flank of the volcano. The greatest amount of shallow seismicity (which allows detailed macroseismic maps of events to be drawn) occurs on the southern side of Etna. Figure 6 shows both macroseismic fields and focal solutions for two earthquakes that are typical of the shallow (h < 2 km) seismicity of the southeastern and southwestern sides of the volcano. Elongations of mesoseismic areas indicate the directions of the seismogenic structures. These are in general agreement with the fault plane solutions of focal mechanisms. As shown in Fig. 6, and also confirmed by the two focal solutions, shallow earthquakes occurring on the eastern side of Etna are mainly caused by structures trending NNWSSE, while on the western side the NE-SW trend seems dominant. Discussion and conclusions

From the analysis of mo~holo~~, structural, volcanic and seismic data several differences between the eastern and western sides of the Etna volcano have been pointed out.

ETNA

87

Analysis of the morphology has shown that the slopes of Mt. Etna show a concave profile with a sharp change at about 1750 m a.s.1.; in addition, the eastern side is characterized by a second slope variation at about 750 m a.s.1. It is interesting to note that slope changes have been observed which coincide with maxima in the distribution of volcanic elements (see Fig. 2). According to Lo Giudice et al. (1982), this relationship is not fortuitous and may have a structural meaning. The above data confirm the higher “degree” of tectonization of the eastern flank with respect to the western one, that was suggested by the analysis of the areal density of faults (0.32 km/km2 and 0.13 km/km2 for the eastern and western sides respectively). Recent studies (Mulargia et al., 1985b, 1987) showed that eruptions at Etna occur following a Poisson process and the volcano becomes active at random intervals. Analysis of some parameters (duration, volume and rate of lava output, explosivity index) for the adventive eruptions which have occurred at Etna during the last three centuries almost completely confirm this random behaviour. It has also been shown that lava outputs are of greater volume on the eastern flank due to effusions being of longer duration (Roman0 and Sturiale, 1982). From the volcanological point of view, the only appreciable differences between eastern and westem flanks of the volcano are the higher number of eruptions and total lava output for the eastern side. We suggest that ma8ma intrusions into the shallowest layers of the crust (and the subsequent effusions) occur more easily on the eastern flank. This would be favoured both by the above mentioned higher “degree” of t~to~~on and by possible higher extensional stresses on the eastern side. Analysis of both historic and instrumental seismic data shows that more than 80% of earthquakes occur on the eastern flank of the volcano. In addition, higher magnitudes (see Table 2) and consequently higher energy releases are observed (Fig. 4) on the eastern side, giving a significantly lower value for the b coefficient in the frequencyma8nitude law for the seismicity of this flank,

88

S. GRESTA

This is in agreement with the hypothesis that there are higher extensional stresses acting on the eastem flank.

-r 14’45

ET AL.

Analysis of focal mechanisms has shown that, on the eastern flank of Etna, extensional features can be attributed to the shallow (h < 7 km) crust

T

15’00 15’15 Fig. 5. Epicentral (a) and hypocentral (NE-SW cross section) (b) distribution of a typical swarm of deep (h > 10 km) earthquakes (black dots) on the western side of Etna (from Gresta et al., 1987).

GEODYNAMIC

BEHAVIOUR

OF E.ASTJZRN AND

WESTERN

SIDES

OF MOUNT

ETNA

N

89

0 I

0

D 0

0

S. GRESTA

(h > 10 km) earthquakes western side (see Fig. 5).

that

are typical

We do not agree with Lo Giudice who attribute

to the NNW-SSE

of a deep way for the magma Etnean

main magma

the

uprise

6. Macroseismic

typical

shallow

fields

earthquakes

and

focal

mechanisms

on southeastern

for

two

and southwestern

flanks of Etna.

the lack of (Gresta et al., 1985). Unfortunately focal solutions for many events on the western side did not allow comparison between the two flanks of the volcano. It is interesting to note that shallow (h < 5 km) seismicity is mainly originated by seismogenic structures trending NNW-SSE on the eastern side, and NE-SW on the western one (Fig. 6). Conversely, deeper (h > 10 km) earthquakes mainly occur in swarms on the western flank, with foci distribution trending about NNW-SSE (Glot et al., 1984; Gresta et al., 1987). Sharp variations in seismic and/or volcanic activity have been observed (Gresta and Patane’, 1987) which coincide with these swarms; this could confirm the important role of the NNW-SSE structural trend in controlling the activity of the Etna volcano (Lo Giudice and Rasa’, 1986). It is also worth noting that shallow seismogenic structures of the eastern flank are mainly oriented in the NNW-SSE direction (see, for example, Fig. 6), as well as the clustered swarms of deeper

bringing

the role

it up to the

(Cristofolini

et al.,

to the elongation

of the

(Sharp trend

et al., 1980).

probably

and consequently activity,

and Patane’,

Whatever

Fig.

chamber

NNW-SSE

volcanic

(Gresta

et al. (1982)

structure

system of fractures

1987b) which are parallel

magma

of the

region. This is more likely to occur along a

NNE-SSW

The

ET AL.

as well

controls

the evolution as of

the of

seismicity

1987).

else is occurring,

it is evident

that

there are several differences between the eastern and western sides of Mount Etna; even if it is not easy to include them in the geodynamic picture of eastern Sicily. A simple model of the collision of the African Plate with the Eurasian one is not enough to explain the existence and the behaviour of the Etna volcano. In fact, if the Eolian Islands and the Northern Chain are taken to be the internal and the external arc of a typical subduction zone (Barberi et al., 1973), Mount Etna would be located in a compressional area, excluding volcanism. Furthermore, its volcanic products (tholeiites and alkaline basalts (Cristofolini and Romano, 1982)) suggest an extensional behaviour for the stress field acting on the Etnean region. In fact the collision between the African and Eurasian blocks takes place through a number of microplates, each moving in its own way. This complex picture suggests that the extensional crustal behaviour of eastern Sicily could be explained by a sequence of angular grabens (Fig. 7) the largest one being located in the Strait of Messina (Ghisetti and Vezzani, 1979). It is possible that the opening of these grabens induces extensional stresses on Mount Etna, which are higher on the eastern than on the western side. Furthermore, the eastern side could be characterized by tectonic sliding, due to movement downwards towards the Ionian Abyssal Plain, whereas a similar mechanism toward North and West is hindered by the surrounding mountains. Such a sliding mechanism could partially explain the location of a “rift zone” (Kieffer, 197%

GEODYNAMIC

BEHAVIOUR

OF EASTERN

AND

WESTERN

SiDES

OF MOUNT

ETh’A

91

This work was supported by CNR grant no. 88.00168.62.

References Aki,IL, I965. Maximum likelihood estimate of

Fig. 7. Sketch map showing the angular grabens in the Ionian Abyssal Plain.

and of the great caldera depression of the Valle de1 Bove on the eastern flank (in the zones 5 and 4 of Fig, 3, respectively). We suggest that both the progressive downshift of “mega-blocks” of the volcano and its basement towards the Ionian Sea and the higher extensional stresses induced by the opening of angular grabens (see Fig. 7), have allowed a higher “degree” of tectonization on the eastern flank. Nevertheless, the most important factor deter~~g the different beha~our of the eastern and western flanks of the Etna volcano is the regional tectonic structure. In particular the NNW-SSE trend seems to be the most active* according to Lo Giudice and Rasa’ (1986). The present study has shown evidence of some differences in behaviour between eastern and western sides of Mount Etna and an attempt to insert them in the geodynamic picture of eastern Sicily has been made. Further interdisciplinary approaches (e.g. more focal solutions for western flank earthquakes, in situ stress measurements, deformational data) are now needed to obtain a better understanding of the geodynamic behaviour of the Ema volcano. Acknowledgements The authors are grateful to Prof. R. Cristofolini and to Dr. G, Frazzetta for useful discussions.

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