Recent deep earthquake occurrence at Mt. Etna (Sicily, Italy)

PHYSICS O F T H E EARTH ANDPLANETARY INTERIORS

ELSEVIER

Physics of the Earth and Planetary Interiors 102 (1997) 277-289

Recent deep earthquake occurrence at Mt. Etna (Sicily, Italy) Mario Castellano a,*, Francesca Bianco a, Sebastiano Imposa b, Girolamo Milano a, Silvestro Menza b 1, Giuseppe Vilardo a a Osservatorio Vesuviano, Ercolano, Napoli, Italy b Ist. Geologia e Geofisica, Univ. di Catania, Catania, Italy

Received 10 January 1995; revised 10 July 1996; accepted 22 July 1996

Abstract

The seismic activity of Mt. Etna from April 1988 until the December 1991 eruption was monitored by means of permanent and temporary seismic networks. Volcanic activity that occurred during this period (tremor increase with lava fountains at the Summit Craters and eruptions) was preceded and accompanied by the occurrence of deep (Z>_ 15kin) seismicity. This deep seismic activity, occurring a few days up to some weeks before the volcanic phases, was characterized by typical mainshock-aftershocks sequences. Focal mechanisms of the more energetic events which preceded the eruptions show that the compression axis was nearly north-south, parallel to the direction of the compressive stress field acting in the area at regional scale. Both the observation of deep seismicity occurrence also before or during previous eruptions and the role played by tectonics as controller of the magma uprise suggest the hypothesis of a relation between the seismic energy released in the volcanic basement and the recharge mechanisms of the volcanic system. In this hypothesis the deep seismicity located in the Etnean area could be considered as a possible 'forerunner' of volcanic activity. © 1997 Elsevier Science B.V.

1. Introduction

Mt. Etna rises in the NE part of Sicily, Italy, and is bordered by two geostructural units: the Iblean Plateau southward and the mountain chains of Nebrodi and Peloritans northward (Cristofolini et al., 1979). Several tectonic trends have been recognized affecting the volcanic edifice (Lo Giudice et al., 1982). A structural sketch of Mt. Etna with the principal tectonic lineaments is shown in Fig. 1. The

* Corresponding author at: Osservatorio Vesuviano, Via Manzoni 249, 80123 Napoli. Italy. i Present address: Osservatorio Sismologico Protezione Civile, Acireale, Catania, Italy.

volcanic activity, mainly characterized by effusive basaltic eruptions, began around 0.7 my ago and has continued almost uninterruptedly until the present. Together with the very frequent effusive episodes, the seismic behaviour of this volcano also showed only rare interruptions. The seismic activity affecting the volcanic area can be subdivided into three categories defined on the basis of the nature of the generating source and the locations of the seismic events: 1. seismic activity strictly linked to the shallow volcanic dynamics; that is, shallow seismicity characterized by swarm sequences located in the first 5 - 1 0 k m of depth, which exhibits a time clustering of shallow earthquakes just before the erup-

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M. Castellano et aL / Physics of the Earth and Planetary Interiors" 102 (1997) 277 289

278 14"50'

14"55'

15"00'

15"05'

15"10'

15"I 5'

Fig. 1. Structural sketch of Mt. Etna: lines indicate the principal tectonic lineaments (after Lo Giudice et al. (1982); redrawn). SE, South-East Crater; VB, 'Valle del Bore'.

tive event (Cristofolini et al., 1987), and lowfrequency events and episodes of harmonic tremor associated with fluid movements in a shallow feeding system (Chouet et al., 1994) observed during a state of open conduit and during eruptive periods. 2. Shallow events located in seismogenetic areas, peripheral with respect to the volcanic edifice, at very shallow depth ( Z = 1 - 3 k m ) characterized by low energy release and, locally, by high macroseismic intensity (Lo Giudice and Rash, 1992; Alparone et al., 1994; Patan~, G. et al., 1994). This seismicity, mainly diffused on the eastern and northern flanks of Mt. Etna, is linked to shallow ground instabilities affecting the volcanic edifice and in some cases appears to be related to phenomena of strain accommodation after the end of eruptions (e.g. the 1984 eruption; Him et al., 1991). 3. More energetic earthquakes which occur beneath the volcanic edifice. This seismicity shows hypocentral depths up to 2 5 - 3 0 k m and magnitudes up to 4.5-5.0, and, owing to both the high energies and the deep hypocentres, is felt over wide areas.

The time-space relation between seismicity (in its various manifestations) and volcanic activity is the geophysical method most widely used to identify precursors of eruptions (e.g. Malone et al., 1983; Karpin and Thurber, 1987; Nercessian et al., 1991; Nun~z-Cornh et al., 1994). In many studies concerning the characterization of seismic activity precursory of volcanic activity, encouraging results have been obtained correlating the occurrence of the seismic events located in the basement of a volcanic structure with the variation of volcanic activity (Wadge, 1977; Shaw, 1980; Ukawa and Ohtake, 1987). This type of seismicity seems to be linked to the modifications of the regional stress field acting on the volcano owing to the action of pressure transients at depth that trigger episodes of magma transport towards the surface (Shaw, 1980). Owing to the continuous seismic-volcanic dynamics which Mt. Etna exhibits it could appear to be an arduous task to find a correlation between eruptions and time distribution of the seismic activity. However, among the many manifestations of seismicity associated with the volcanic activity of Mt. Etna, a particular significance could given to the study of the occurrence of the deep earthquakes belonging to the third group defined above. In fact, the focal mechanisms of the deep Etnean earthquakes are correlated with the stress-field acting north-south on eastern Sicily at a regional scale (Lo Giudice et al., 1982; Scarpa et al., 1983). Structural studies of the Etnean region (Lo Giudice et al., 1982; Lo Giudice and Rash, 1986) compared with the regional framework suggest that the main fault system, oriented NNE-SSW, probably also affects the deep structures of the Etnean basement and seems to be a controller of the magma uprise (Frazzetta and Villari, 1981; Lo Giudice et al., 1982; Cristofolini et ai., 1985; Bousquet et al., 1988; McGuire et Pullen, 1989). Also De Natale et al. (1985), from the analysis of the seismic pattern preceding the 1981 eruption, pointed out the relevance of the role played by the regional tectonic stress-field on volcanic activity. To check a possible relation between deep seismic activity and volcanic processes, in this paper we consider the deep seismicity that occurred during the eruptions of the last two decades, and analyse in detail some deep earthquake sequences that occurred before and during the two last eruptions (1989 and

M. Castellano et al. / Physics of the Earth and Planetary Interiors 102 (1997) 277-289

1991). The depth threshold of 15 km for the analysed earthquakes has been chosen on the basis of geological and geophysical investigations (e.g. Cristofolini et al., 1979; Cardaci et al., 1993a) that have located at this depth a discontinuity marking the transition between the upper and lower crust. The observed occurrence of deep seismic activity before several Etnean eruptions and the framework of the feeding system seem to support the hypothesis that the deep seismicity affecting the Mt. Etna basement could be interpreted as the response of the brittle crust to pressure transients driving the recharge mechanisms of the volcanic system. Following this hypothesis, the occurrence of deep seismicity could indicate that an eruption is drawing near.

2. Occurrence of deep seismicity Several eruptions occurred at Mt. Etna in the last two decades and each of them was studied by many researchers by merging geophysical and volcanological information. Gasperini et al. (1990, 1992), performing statistical analysis on the seismic-volcanic behaviour of Mt. Etna, did not find evident correlations between eruptions and earthquakes sequences during 1978-1987. This result was obtained by analysing only the temporal occurrence of the earthquakes with magnitude M > 2.8, not taking into account the epicentral and hypocentral locations of the data set. Gresta and Patan~ (1983a,b) related the observed variations of the 'b-value' of the Gutenberg and Richter relationship before the 1981 and 1983 eruptions to possible changes of the stress field acting beside the volcano, whereas a correlation between low-frequency seismic events and flank eruptions was observed by Cardaci et al. (1993b) in the interval 1975-1987. In the following, a brief review is reported of the observed correlations between deep seismicity and volcanic activity available in literature. The western flank eruption of 1974 (30 January-16 February and 16-29 March) was preceded by shallow swarms from September 1973 and, just 10days before the eruption, deep (to 20 km depth) seismic activity occurred (Bottari et al., 1975; Tanguy and Kieffer, 1976). Cristofolini et al. (1985) related both the

279

occurrence and the spatial distribution of these deep earthquakes to the mechanism of magma uprise for the 1974 eruption. The lateral south flank eruption of 28 March-6 August 1983 was preceded by the occurrence of shallow events located around the effusive fracture (Patan~ et al., 1984), whereas a deeper swarm (to 40 km depth), located on the southern and western side with epicentre distribution along the NNW-SSE direction, occurred in June during the effusive phase (Glot et al., 1984). In 1984 (27 April16 October) a subterminal eruption from the SouthEast Crater occurred. No significant seismicity preceded this eruption whereas deep activity (12-23 km depth) occurred in September (Gresta et al., 1987). This deep seismicity was located on the western flank of the volcano and preceded a shallower seismic swarm located at the northern and eastern side of Mt. Etna (Gresta et al., 1987; Him et al., 1991; Patan~, G. et al., 1994). The October 1986-February 1987 eruption was preceded by deep seismic activity located on the western flank about 2-3 weeks before the eruption (Smithsonian Institution, 1986; Gresta and Patan~, 1987).

3. Data acquisition and location of events from 1988 to 1991 In April 1988 a temporary network of five seismographs was installed on Mt. Etna by Osservatorio Vesuviano. The array was composed of digital three-component seismic stations operating in trigger configuration with short-time/long-time signal average ratio and a sampling rate of 125Hz. The high dynamic range of the instruments (120dB) allowed the recording of unclipped events. During 1988 the number of the recording stations was gradually increased to 22 seismic stations contemporaneously operating on the volcano during the period May-July (Castellano et al., 1988). The seismic stations were equipped with Mark L4C-3D (1 Hz) and Mark L4A3D (2 Hz) seismometers. Seventeen of them recorded locally, and the other five used digital radio transmission on a digital multi-channel mixer unit (Station SGR; Fig. 2). In 1989 two seismic arrays, composed of four and three digital three-component seismic stations respectively, in addition to other three local recording

280

M. Castellano et al. / Physics of the Earth and Planetary Interiors 102 (1997) 277-289 14"50' I

14"55' I

15"00' [

15"05' I

15"10' I

Table l

15"15' [

Velocity model

LK .* -

Fig. 2. Osservatorio Vesuviano seismic network operating during 1988-1989. El, 1988 stations; A, 1989 stations. SGR and CRC are the acquisition centres operating during 1988 and 1989.

stations, were installed on the east and west side of the volcano and telemetered to two different acquisition centres in S. Gregorio (SGR) and Carcaci (CRC)

14'50' I

1~'

1~00'

I

I

15"05' I

1S10' I

1~1fi' I

Vp(km s - 1)

Z (km)

2.75 4.00 4.60 7.10 8.00

0.00 2.60 3.50 16.00 27.00

(Fig. 2) (Castellano et al., 1993; Ferrucci et al., 1993). After the end of the 1989 eruption, the information on the seismicity of Mt. Etna was collected by merging the data gathered by the analogue array run by the Istituto di Geologia e Geofisica (IGG, Universitb di Catania) and a temporary digital array run by Osservatorio Vesuviano (Fig. 3). Moreover, for the seismic activity that occurred during 1991, data gathered by the permanent seismic network of the Istituto Internazionale di Vulcanologia (IIV, Catania) are also available. In addition to the seismic information collected by the local networks, the seismic data concerning the seismicity analysed in this paper were supplemented by the data of the permanent analogue network of the Istituto Nazionale di Geofisica (ING, Roma) which operates in eastern Sicily. The seismicity was located using the HYPO71 routine (Lee and Lahr, 1975); the velocity model used is the horizontally layered velocity model obtained by Cardaci et al. (1993a) shown in Table 1 with a Vp/Vs ratio of 1.74 (Castellano et al., 1993). The use of a high number of reliable S-pickings in location routine (e.g. Gomberg et al., 1990), obtained Table 2 Quality characteristics of located deep seismic sequences during 1988-1991 Date

i

Maximum no. of

Gap (deg)

r.m.s. (s)

ERH (km)

ERZ (km)

55-91 107-113 97-135 98-102 105-130

0.08-0.30 0.04-0.20 0.10-0.58 0.41-0.50 0.27-0.59

0.5-1.5 0.1-1.7 0.8-2.0 0.8-1.0 0.9-2.0

0.3-1.5 0.3-1.8 0.7-2.6 0.3-0.4 0.4-2.6

P/S I

I

I

CATANIA ~,,~

~km I

Fig. 3. Seismic network recording the 1991 activity. Data have been provided by Istituto di Geologia e Geofisica (Catania University), Osservatorio Vesuviano and Istituto Internazionale di Vulcanologia (CNR, Catania).

19.06.1988 03.08.1989 23.09.1989 10.10.1991 23.10.1991

43 33 35 43 36

M. Castellano et al. / Physics of the Earth and Planetary Interiors 102 (1997) 277-289

by both digital and analogue three-component seismic stations, allowed us to obtain extremely well constrained hypocentral locations. Moreover, the use of both local and regional stations provided good hypocentral control, especially for the earthquakes having greater hypocentral depths, according to theoretical considerations on seismic network optimization (Lee and Stewart, 1981). D u r i n g the analysed period (1988-1991) five deep sequences of events were recorded (Z > 15 km). In Table 2 the values of the location quality parameters are summarized for the events belonging to each deep sequence.

Seismic and volcanic activity during 1988

20 ,~ ~15-

Increase of tremor and Iowa fountoining

June,19 sequence (Z=14-19 kin)

o~10"

" Z 5-

o

n ,,,tub, it

APR

MAT

~ffN

JUL

AUG

SE'P

~)

Seismic a n d volcanic activity during 1989

50 m *~40"

4. Seismic-volcanic activity during 1988-1991

September,23-24 swarm (Z=11-24 km)

IL

August,3 sequence (z=16-20 km)

~,30 o

During the years 1988-1991 the volcanic activity of Mt. Etna was characterized, in the period JulyAugust 1988, by strombolian activity with high lava fountaining but without lava flow, and by two important eruptions, which occurred in September-October 1989 and December 1991-March 1993. The earthquake occurrence during the whole of 1988 and early 1989 was very low (Castellano et al., 1993), and an increase in the number of events was observed just before the September-October 1989 eruption (Ferrucci, 1990; Ferrucci et al., 1993). A similar feature was observed for other volcanic events of Mt. Etna (Patanb et al., 1984; Cristofolini et al., 1985; Gresta and Patanb, 1987), suggesting that this kind of seismicity was linked to the changes in the stress field owing to magma movements. From the beginning of 1988 until the start of the 1989 eruption, more than 400 earthquakes located between 5 and 25 km depth were recorded. Of these events about 70 belong to three deep seismic sequences located at depths of 15 km or greater. This deep activity occurred on 19-20 June 1988, 3 August 1989 and 23-24 September 1989 (Fig. 4(a), Fig. 4(b)). After the earthquakes of January and February 1990, which marked the end of the eruptive phase linked to the September-October 1989 eruption, the seismic activity was characterized by a low number of events and low energy release, with the occurrence of swarms with hypocentres in the first 10km of the volcanic structure. These events showed epi-

281

{

Eruptive period

2010-

ul,d i.~ I' MAy JUN JIIL

AUG

SEP

OCT

NOV

DEC

b)

Seismic a n d volcanic activity during 1991 90-

~

Start of the 1991-1993 eruption October,23 sequence ~ (Z=l 5-24 km)

80" 70-

g60'

>

~5o. October.lO sequence (Z=15 kin) ~

~20 10 0

JUN

JUL

AUG

SEIP

OCT

NOV

DEC

o)

Fig. 4. Dally seismic frequency during 1988 (a), 1989 (b) and 1991 (c). Arrows indicate the seismic activity discussed in the text. In (b), u'iangles indicate (1) the beginning of strombolian activity preceding the eruption, (2) the change of eruptive pattern that occurred on 27 September and (3) the shallow seismic swarm linked to the SE fracture. In (c), arrows indicate the deep seismic sequence preceding the eruption and the beginning of the 19911993 eruption marked by the seismic activity that occurred on 14 December.

centres corresponding to the Summit Craters, Valle del Bove and the southwestern flank of Mt. Etna (Patanb et al., 1991). During 1991 the seismic activity was more intense, with several hundred events,

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M. Castellano et al. / Physics of the Earth and Planetary Interiors 102 (1997) 277 289

The source locations of these earthquakes are very reliable as they occurred during the maximum deployment of the array; the most energetic events were recorded at 26 stations and located by means of 4 3 P / S phases. About 10days after the 19-20 June sequence, intense tremor activity began (Fig. 4(a)), with the uprise of magma as far as the summit craters, and lava fountain activity. This activity con-

s

N

]0000019.06.88

I

W

E

,t0000019.o6,8~ I

a) l

M~B~NA

7.500

37,950

?j

I

Fig. 5. Epicenttal location of the deep seismic activity recorded during 1988-1989 and 1991. Lozenge, June 1988 sequence; square, August 1989 sequence; triangle, September 1989 sequence; star, 10 October 1991 sequence; circle, 23 October 1991 sequence.

I °': b,) 07.500

37.950

and two deep sequences occurred on 10 and 23 October 1991, about 2months before the eruption (Fig. 4(c)). In the following, we will describe the space-time characteristics of the deep sequences detected in the period 1988-1991.

,2t . ZS[ 57.500

4.1. The 1988 activity The sequence of 19-20 June 1988 ( M m a x = 3.7; 01:44h UTC) occurred during a period of very low seismic and volcanic activity following the 19861987 eruption and was located at 14-19km depth. The observed T~-Tp values are in the range 3.54.0 s (e.g. Station POM). Epicentral locations show a clustering around the Summit Craters (Fig. 5 and Fig. 6(a), Fig. 6(a')). This sequence shows a time distribution of magnitudes with a mainshock followed by less energetic events (Fig. 7(a)). Such a time distribution of seismic activity characterizes the energy release in high rigidity seismogenetic volumes such as the basement of Mt. Etna.

1 4 . ~ 0 0 ¢[ANV~23.09.89 C')

!it 3_7.500

1 4 ~ 2 0 0 a]~10"1091

~-~...~.50

~1o.lo.gl

"

d)

{21

"

~,E,I~00 2"3"10"91

37.9,50

e)

"

I

~ ~ . ~ o o

e')

Fig. 6. Cross-sections of hypocentre distribution of the four deep seismic activities. The hypoeentres are projected along the southnorth section ((a)-(e)), and along the west-east one ((a')-(e')).

283

M. CasteUano et al. / Physics of the Earth and Planetary Interiors 102 (1997) 277-289

tinued for about 3 months, without lava overflow (Luongo et al., 1988; Castellano et al., 1991). 4.2. The 1989 activity

The 3 August and 2 3 - 2 4 September 1989 earthquakes were recorded at fewer stations, but the use of three-component digital stations allows a reliable estimate of S phases and the constraint of the hypocentral locations. The average T ~ - T p values during the two sequences at stations such SLN and T U R are about 3.8-4.0 s and 3.8-5.4 s, respectively. The 3 August 1989 sequence ( M m a x = 3.4; 08:30 h UTC), recorded by 20 stations and with up to 33 P / S phases, was located at 1 6 - 2 0 k m depth, and was preceded and followed by shallower swarms (2 km < Z < 12kin). Epicentres are located on the southwestern flank of the volcano (Fig. 5 and Fig. 6(b), Fig. 6(b')). Also, this seismicity was characterized by a mainshock at the beginning of the sequence

(Fig. 7(b)). At the end of August a strong explosive phase started at the Summit Craters with strombolian activity and very high ( 5 0 0 - 6 0 0 m ) lava fountains (Bertagnini et al., 1990). After a strong strombolian activity on 11 September the eruption began with a lava flow from the South-East Crater. The deep seismic activity occurred on 2 3 - 2 4 September 1989 during the eruption and was located on the western flank at 1 1 - 2 4 km depth (Fig. 5 and Fig. 6(c), Fig. 6(c')). This sequence (Mma ~ = 3.3; 13:17h UTC) was recorded by up to 19 stations with a maximum number of 35 P / S phases, and does not show a decrease of the magnitudes with time as observed in the previous seismic sequences. Also, the focal volume of this swarm does not reflect the clustering of the hypocentres displayed by the previous sequences, and the spread of the hypocentral depths is marked by the larger range of T ~ - T p observed at the stations of the array (e.g. TUR: T ~ - T p = 3.8-5.4 s). This sequence occurred during

4.0

4.O"

Md

Md 3.5

3.5

3.0

3.0

2.5

2.5

03.08.1989 Secl~e~ce

2.0

2.0

a)

b) 1.5

1.5

Progressive events

12

Progressive events 4,0 ,

4.0

Md

15 fi

23.09.1989 Swarm.

Md

23.10.1991 Sequence

3.5 ~

3.5

3.0 1

2.5

2.5-

2.0

2.0-

d)

o) 1.5

1.5

Progressive events

~ ,~ g ~ 1'o 1'2 1'4 1'e 1~ 2b ~ if4 2'6 2'~ 3b 32 Progressive events

Fig. 7. Magnitude-time distribution for deep seismic activity recorded during 1988-1991. During the seismicity of June 1988 (a), August 1989 (b) and 23 October 1991 (d) a mainshock at the beginning of each sequence is observed, whereas during the September 1989 sequence (c) a more chaotic distribution of energy is evident.

284

M. Castellano et al. / Physics of the Earth and Planetary Interiors 102 (1997) 277-289

the effusive phase. It preceded by about 3 days both a change in the eruptive pattern and the rapid propagation of a SSE fracture without lava emission (Frazzetta and Lanzafame, 1990; Ferrucci et al., 1993). The end of the propagation on the ground surface of this fracture was accompanied by an intense shallow seismic swarm (Fig. 4(b)) (Ferrucci et al., 1993). The major effusive emission of the September-October 1989 eruption began on the same day from a series of eruptive vents that opened in the upper part of the 'Valle del Bove' (Bertagnini et al., 1990).

4.3. The 1991 activity In October 1991 two deep seismic sequences occurred (10 and 23 October). The time distribution of magnitude for the second sequence shows the same trend as observed for the other sequences preceding volcanic activity (Fig. 7(d)), whereas for the first one, owing to the small number of events, it is not possible to make a reliable time distribution of energy. The seismic activity occurred on 10 October 1991 ( M m a x = 3.2; 09:28h UTC), during a period of general low seismicity; it was located on the north-western side of the volcano and the hypocentres were closely clustered around 15 km depth (Fig. 5 and Fig. 6(d), Fig. 6(d')) with a Ts - T p at Station VEN of about 4.2 s. The earthquakes were located using up to 23 stations and 43 P / S phases. The seismic sequence of 23 October 1991 (Mma x = 3.5; 21:36h UTC) was located on the northwestem flank of Mt. Etna at 1 5 - 2 4 k m depth (Fig. 5 and Fig. 6(e), Fig. 6(e')), with T~ - Tp ranging between 3.9 and 4.6s at Station VEN. This activity was located by means of 19 stations and up to 3 6 P / S phases. After less than 2 months (14 December 1991), one of the longer eruption of this century started (Fig. 4(c)) (Barberi et al., 1993). This eruption, which occurred through the same SSE fracture system that opened during the September-October 1989 eruption, was preceded by a short seismic sequence that occurred just before the magma overflow and was located in the same area of the effusive fractures (between the South-East Crater and the western edge of the 'Valle del Bove') at very shallow depth (Ferrucci and Patanb, 1993). Throughout 1992 the seismicity was characterized by a weak energy re-

lease with a very small number of earthquakes (Patan~, D. et al., 1994).

5. F o c a l m e c h a n i s m s

Fault plane solutions of Etnean earthquakes were discussed by Scarpa et al. (1983) and Gresta et al. (1985), and with respect to some sequences by Ferrucci et al. (1993), Patan~, D. et al. (1994) and Patan~, G. et al. (1994). Scarpa et al. (1983) and

@1 @7 ~ 13

3

9

15

4

10

16

11

17

5

Fig. 8. Selected fault plane solutions (equal-area, lower hemisphere projection) for the analysed five deep sequences. C), Dilatation; 0 , compression; P and T are maximum compressional and tensional axes, respectively.

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M. Castellano et al. / Physics of the Earth and Planetary Interiors 102 (1997) 277-289

Table 3 Date, depth, magnitude, plane parameters and relative errors (expressed in degrees) of the computed fault plane solutions of Fig. 8 No. Date h:min Z (km) M Strike Dip Rake AStrike ADip ARake 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

880619 880619 880620 890803 890803 890803 890923 890923 890923 890923 890923 911010 911010 911010 911010 911023 911023 911023

01:44 02:27 05:02 08:30 13:07 17:57 08:20 08:30 08:31 11:19 13:17 09:25 09:28 09:30 13:01 21:36 21:41 21:44

19.0 17.5 16.0 20.0 18.0 18.5 19.5 19.0 20.5 17.5 20.5 16.0 15.0 15.5 15.5 16.0 17.0 16.5

3.7 3.1 2.4 3.4 3.2 2.9 3.0 2.5 2.6 2.5 3.3 2.8 3.2 2.5 2.7 3.5 2.2 3.1

130 190 110 230 5 55 120 260 240 205 270 225 195 5 185 210 200 195

Gresta et al. (1985), in particular, found that shallow earthquakes (depth less than 7 kin) are characterized by normal faulting, whereas earthquakes with focal depths between 7 and 16km show both normal and thrust fault motions with a strike-slip component. For shallower earthquakes, a good correlation between fault plane solutions and surface tectonic structures can be observed, whereas a more complex pattern characterized the source mechanisms o f the deeper ones. In the period 1988-1991 the analysed events are confined to depths greater than 15 k m and we are not able to confLrm the differences in fault plane solutions between shallow and deep events observed by other workers. Focal mechanisms have been computed by means the F P F I T program (Reasenberg and Oppenheimer, 1985), selecting the more energetic earthquakes with at least seven P-polarities (Fig. 8). In Table 3 the fault plane parameters are summarized for the events shown in Fig. 8. The sequences analysed in this study that occurred during quiescence periods preceding volcanic activity (June 1988, August 1989 and October 1991) show a n o r t h - s o u t h compressional axis (P-axis) direction for the first event (the more energetic one) o f each sequence (fault plane solutions (FPS) Nos. 1, 4, 12 and 16; Fig. 8), according to m a x i m u m regional

70 60 65 55 90 10 30 65 75 85 80 85 80 90 75 85 50 30

0 120 20 - 110 170 -50 160 120 110 -10 120 160 170 180 - 170 160 - 150 -120

0 10 5 10 8 20 5 10 8 13 20 3 5 10 5 10 8 8

0 5 8 5 5 23 10 3 8 3 8 3 5 5 8 3 10 0

5 0 0 50 5 15 0 10 10 20 10 0 5 0 5 10 30 0

compressive stress (Lo Giudice et al., 1982; Cristofolini et al., 1987; Bousquet et al., 1988). This fact is a further indication that the stress-release in the Etnean basement is linked to a stress field acting at regional scale. In contrast, focal mechanisms of deep earthquakes that occurred during volcanic activity (September 1989 in our case) are characterized by a P-axis direction rotated with respect to the regional stressfield showing an E N E - W S W alignment. This trend was noted also during the December 1 9 9 1 - M a r c h 1993 eruption (Caccamo et al., 1994; Patan~, D. et al., 1994). This could be due to the modification o f the stress field acting at a regional scale caused by the superposition of the driving mechanism of the m a g m a uprise. The 23 September 1989 sequence, moreover, shows prevalent thrust movements, according to the stress field acting on the volcano during the magma uprise (FPS No. 7, 8, 9 and 1 l; Fig. 8).

6. Geochemical constraints on the m a g m a storage and uprise The current eruptive cycle o f Mt. Etna that started in 1971 is characterized b y a considerable increase

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of the effusion rate with respect to the average of the last 200 years (Wadge and Guest, 1981). The erupted lava products, since 1971, show an almost constant geochemical composition astride the boundary between the hawaiite and alkalic basalt fields, with a trend toward hawaiite (Armienti et al., 1989). Both the compositional uniformity and the almost constant effusive activity suggest that the feeding system of Mt. Etna is continuously filled up with new magma injections (Armienti et al., 1984, Armienti et al., 1987). The Sr and Nd isotopic composition variations observed on the erupted products during the period 1971-1987 seem to suggest the lack of a wide magmatic chamber in the feeding system, indicating that magma reaches the surface through a network of dykes fed by subsequent magma injections (Armienti et al., 1989). The strombolian and effusive phases of the volcanic activity during the period 1988-1991 have been characterized by distinct magma injections whose geochemical compositions had been already observed in detailed studies of the Etnean eruptions (e.g. Clocchiatti et al., 1986, Clocchiatti et al., 1988). This fact puts in evidence the observability of significant differentiation even after rest periods of only a few months or years (Tonarini et al., 1995). This was observed also during the September-October 1989 eruption, which occurred 1 year after the strong strombolian activity of July-August 1988. In fact, the 1989 lavas show an alkalic basalt composition slightly different from both the mean compositional trend recognized for the Mt. Etna and the composition of 1988 explosive products. In particular, during the 1989 eruption, the lava flow in the Valle del Bove that started on 27 September, during the second effusive phase, was characterized by a lower degree of evolution than the SE crater eruption's products of the beginning of the eruption (11 September), suggesting that a new supply of less differentiated magma occurred during the eruption (Armienti et al., 1990). Also, the December 1991 eruption has been fed by a new Hawaiian magma injection partly mixing with the magma that was geochemically distinct and already standing in the upper part of the feeding system (Armienti et al., 1994). Geochemically, the identification of new magma pulses in a continuous activity model seems to con-

firm an upwelling mechanism that is activated when a variation of stress field occurs acting on the feeding dykes. In this context, the occurrence of seismic sequences at depths greater than 15kin, preceding volcanic phases or their rapid variations, could be interpreted as evidence of an energetic release in the bedrock strictly related to recharge mechanisms of the volcanic system.

7. Discussion and conclusions On the basis of epicentral distribution of the events recorded during 1988-1991 it is not possible to make structural or tectonic correlations. However, it can be stressed that the distribution of the P-axis directions as derived by the fault plane solutions of the stronger deep earthquakes that occurred during non-eruptive periods is in accordance with maximum regional compressive stress, confirming that the basement of Mt. Etna is prevalently subjected to a regional stress-field acting along the north-south direction (Lo Giudice et al., 1982). On the other hand, the presence, during the eruptive activities, of the stress-field linked to magma movements seems to produce a change in the pattern of the orientations of the P-axis; these display a rotation toward the east-west direction, as evidenced by the analysis of the data collected during the last two eruptions of Mt. Etna (September-October 1989 and December 1991-March 1993). The dynamics of the main fault systems, such as the NNE-SSW system and its conjugate NNW-SSE faults, subjected to the regional stress-field, seems to play a very important role in the control of the magma uprise (Lo Giudice et al., 1982; Cristofolini et al., 1985, Cristofolini et al., 1987; Gresta and Patanb, 1987). Moreover, as already suggested by Ferrucci et al. (1993), the 1989 and 1991-1993 eruptions could be ascribed to a single intrusive episode along the NNW-SSE structure, with the magma uprise ruled by the regional tectonics. Under such conditions, the deep earthquakes which occurred before the eruptions may be the evidence of a sudden energy release governed by the stress-field acting on the Etnean region. The influence of the regional stress-field was pointed out also by statistical analyses of the seismic patterns during previous

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eruptions (e.g. 1981 and 1983 eruptions; Gresta and Patan~, 1983a, Gresta and Patan~, 1983b; De Natale et al., 1985). In this picture, the crossing of the deep structures, on which the regional stress-field acts, with the feeding system provides favourable conditions for the magma movements towards the surface. These movements have been supposed passive and aseismic by some workers (Gresta and Patanb, 1987;Cristofolini et al., 1987). The occurrence of earthquakes at depths greater than 15 km, moreover, argues against the presence of a definite wide magmatic reservoir at such depth as suggested by Sharp et al. (1980) and Cristofolini et al. (1987). The sequences of mainshock-aftershocks are also indicative of regions that can sustain high stress and do not agree with partial melting. These arguments could confirm the hypothesis, inferred from petrologic and geophysical data (Armienti et al., 1989; Loddo et al., 1989; Him et al., 1991; Cardaci et al., 1993a), on the absence of a definite magma chamber beneath Mt. Etna. To evaluate if deep earthquake occurrence is strictly linked to volcanic eruptions, one should compare the elapsed time between the two phenomena and the magma uprising velocity. Data on the magma velocity inside the feeding system of Mt. Etna are not available. However, an attempt to evaluate how long the magma takes to rise from depth up to the surface can be made using information inferred from other volcanic districts (e.g. Hawaii volcanic area) which display physical and chemical magma properties very similar to those observed in the erupted lava of Mt. Etna (Armienti et al., 1989). Klein et al. (1987) determined the velocity of the pulsed flow of magma during several eruptions of Kilauea volcano (Hawaii); with the physical and chemical characteristics of Hawaiian magma, a velocity of 0.010.05 kmh-~ was estimated. Taking into account this velocity value and for a starting depth at about 20km, a time of 16-80days is required for the magma pulse to reach the surface. Such estimated time, for the examined period at Mt. Etna, results in agreement with the time elapsed between the detection of the deep earthquakes (June 1988, August 1989 and October 1991) and the corresponding following occurrence of volcanic activity. A shorter time-lag between the deep seismicity and the sudden changes, observed both in composition and effusion

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rate, during the September 1989 activity could be explained considering that, in this case, the feeding system of Mt. Etna was already filled. In this framework, the time needed for the magma uprise would be highly reduced. In conclusion, although the analysis of the seismic-volcanic activity observed at Mt. Etna during 1988-1991 is not sufficient to determine a reliable statistical correlation between deep earthquakes and volcanic activity, the data presented in this paper seem to support this hypothesis. Further comparisons between the seismic behaviour and observations of volcanic activity are needed to consider earthquakes located in the basement, at more than 10-15km depth, as a 'forerunner' of eruptions.

Acknowledgements The authors are grateful to L. Villad and E. Privitera, and to the data coordinator of Istituto Nazionale di Geofisica for the availability of the IIV (Catania) and ING (Roma) data. A. Him (Paris) and J. Dorel (Clermont Ferrand) are also acknowledged for their co-operation during 1988 and 1989. G. Patanb supplied digital data of IGG (Catania). Special thanks are due to F. Ferrucci for helpful discussions. This work was financially supported by the Gruppo Nazionale per la Vulcanologia (National Council of Research, Italy).

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