Contrasting triggering mechanisms of the 2001 and 2002–2003 eruptions of Mount Etna (Italy)

Journal of Volcanology and Geothermal Research 144 (2005) 235 – 255 www.elsevier.com/locate/jvolgeores

Contrasting triggering mechanisms of the 2001 and 2002–2003 eruptions of Mount Etna (Italy) Marco Neria,*, Valerio Acocellab, Boris Behnckea, Vincenza Maiolinoa, Andrea Ursinoa, Rosanna Velarditaa a

Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania; Piazza Roma, 2; 95123 Catania, Italy b Dipartimento Scienze Geologiche Roma TRE, Largo S.L. Murialdo 1, 00146 Roma, Italy Received 16 November 2004; accepted 16 November 2004

Abstract Mount Etna produced two significant eruptions in 2001 and 2002–2003, which we have analysed using geological, seismic and deformation data. These eruptions showed some similarities, such as the activating of two magmatic plumbing systems (central–lateral and eccentric), but they differed in their triggering mechanisms. While the 2001 eruption was largely the result of the emplacement of a N–S eccentric dike (independent from the central conduits) consistent with E–W regional extension, the 2002–2003 eruption occurred in response to a major flank slip on the eastern and southeastern sides of the volcano. This is demonstrated by the spatial and temporal distribution of seismicity and deformation preceding and accompanying the two eruptions. During the months prior to the 2001 eruption, most epicenters were concentrated on the southern flank, at depths of 5–15 km below sea level. During the 4 days before the eruption, earthquake hypocenters migrated to shallower levels (from 5 km bsl. upward) indicating the emplacement of the eccentric dike. This is confirmed by the patterns of ground fracturing observed in the field and deformation documented by electronic distance measurements (EDM). In contrast, the months before the 2002–2003 eruption were characterised by shallower seismicity, mainly concentrated along the active faults bordering the slipping flank sector. Flank slip accelerated in September 2002 and a second, more vigorous acceleration of flank slip occurred on 26–27 October 2002, accompanying the opening of eruptive vents. The very short (2 h) seismic crisis preceding the onset of eruptive activity stands in neat contrast with the 4 days of intense seismicity before the 2001 eruption. Subsequently, flank slip-deformation extended all over the eastern and southeastern flanks of the volcano, causing serious damage in this sector. The events of 2001–2003 can be seen as a continuous chain of intimately interacting processes including regional tectonics, magma accumulation and eruption, and flank instability. In this scenario the 2001 eruption led to increased flank instability that subsequently accelerated and

* Corresponding author. Tel.: +39 95 7165861; fax: +39 95 435801. E-mail addresses: [email protected] (M. Neri)8 [email protected] (V. Acocella)8 [email protected] (B. Behncke)8 [email protected] (V. Maiolino)8 [email protected] (A. Ursino)8 [email protected] (R. Velardita). 0377-0273/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2004.11.025

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culminated with the massive flank slip, which in turn facilitated the 2002–2003 eruption. This sequence of events points to a long-term feedback mechanism between magmatism and flank instability at Etna. D 2004 Elsevier B.V. All rights reserved. Keywords: eruption triggering; central–lateral vs. eccentric eruptions; flank instability and slip; volcano-tectonics; Mt. Etna; instrumental monitoring

extend from the summit to the east–northeast and southeast (Fig. 1; McGuire et al., 1997). Following a voluminous, long-lasting flank eruption in 1991–1993 (Calvari et al., 1994; Tanguy et al., 1996; Stevens et al., 1997), eruptive activity resumed at the summit craters in the summer of 1995. During the following 6 years, eruptive events were concentrated at the summit craters

1. Introduction Mount Etna in eastern Sicily (Italy) shows nearly continuous activity at its summit craters, and every few years eruptions occur on its flanks. Much of the flank activity is concentrated in certain areas, especially the Northeast and South Rifts, and two fissure swarms

0

5

10 km

50

0

Fig. 4

1,

E

ift R

Fig.2

PFS

N

ift WR

R

S Rift

TFS

TF

250

Ca

Tyrrhenian Sea

lab ria

500

ETNA

n nia Io ea S

Catania 15°00’

Sicily

Ion ian Se a

SV

1,0

00

37°40’

1

4

2

5 6

3

7

Fig. 1. Simplified tectonic map of Mount Etna. Key: 1=volcanics; 2=sedimentary basement, made up of units of the Apenninic-Maghrebian Chain (N and W sectors) and of early Quaternary clays (S and ESE sectors); 3=flank eruptive fissures; 4=pyroclastic flank cones; 5=Main faults (arrows indicate lateral component of movement); 6=direction of displacement of the unstable sector; 7=presumed segments of the boundaries of the unstable sector; PFS—Pernicana Fault System; TFS—Timpe Fault System; TF—Trecastagni Fault; R—Ragalna Faults; SV—Santa Venerina town. The areas shown in Figs. 2 and 4 are indicated.

M. Neri et al. / Journal of Volcanology and Geothermal Research 144 (2005) 235–255

(Neri and Tomarchio, 2000; Harris and Neri, 2002; Calvari et al., 2002; Aloisi et al., 2002; Alparone et al., 2003; Behncke and Neri, 2003a, Behncke et al., 2003) and then culminated in two flank eruptions in July– August 2001 (Behncke and Neri, 2003a; Lanzafame et

237

al., 2003) and October 2002–January 2003 (Neri et al., 2004; Andronico et al., in press). The 2001 and 2002–2003 eruptions had many similarities (Fig. 2). Both showed high degrees of explosivity, both affected two sides of the volcano

4 10 00

Central-lateral eruption

3

NEC

1? BN

NE

Ri ft

Pernicana fault system

VOR SEC

2

*

1

Piano Provenzana

2 3

Valle del Bove rim

4

5 eccentric eruption

b

6

7

Summit Craters

6 Central-lateral VOR eruption 1

NEC

3000

00 20

Valle del Bove

b

BN

SEC

7 2 3

La Montagnola

5 eccentric eruption

a 0

2.5

5 km

Valle del Bove rim

4

a

Fig. 2. Map showing the areas affected by the 2001 and 2002–2003 eruptions. Key: 1=pyroclastic cones formed during the 2002–2003 eruption; 2=2002–2003 lava flows; 3=pyroclastic cones formed during the 2001 eruption; 4=2001 lava flows; 5=2001 eruptive and dry fissures; 6=lava flows erupted between 1900 and 2001; 7=fault, arrow indicates horizontal direction of movement and ticks are on downfaulted side. Asterisk marks location of the Piano Provenzana tourist facilities. Insets (a) and (b) show the relationship between central–lateral and eccentric eruptive centers during the 2001 and 2002–2003 eruptions, respectively. The vents are numbered according to consecutive eruptive activity. Central– lateral activity occurred at vents fed from the central conduit system, which corresponds to the summit craters (NEC-Northeast Crater, VORVoragine, BN-Bocca Nuova, SEC-Southeast Crater), whereas the eccentric vents were not related to the central conduits.

-50

-10

-30

NEC

SEC

Eruptive activity

BN

1.00E+07

8.00E+06

2.00E+06

-5.00

0

panel 4

30

NE

10

S

SW

Seismicity

1.20E+07

Daily earthquake rate

panel 1

Ground deformation

19/02/03

fig. 4d

19/12/02

5

01-Feb-02

19/10/02

19/08/02

4

01-Oct-02

19/06/02

19/04/02

fig. 4c

01-Jun-02

19/02/02

19/12/01

3

01-Feb-02

19/10/01

panel 2

19/08/01

19/06/01

fig. 4b

01-Oct-01

5.00 19/04/01

2

01-Jun-01

19/02/01

19/12/00

fig. 4a

01-Feb-00

19/10/00

19/08/00

19/06/00

19/04/00

19/02/00

19/12/99

19/10/99

1

01-Oct-00

50

panel 3

01-Jun-00

0.00 19/08/99

Strain release (J 1/2) 1.80E+07

01-Feb-00

Depth (km)

FE

01-Oct-99

Cumulative areal dilatation (µstrain)

238 M. Neri et al. / Journal of Volcanology and Geothermal Research 144 (2005) 235–255

6

VOR fig.5a fig.5b

1.60E+07

250

1.40E+07

200

150

6.00E+06

100

4.00E+06

50

Time

10.00

15.00

20.00

25.00

30.00

M. Neri et al. / Journal of Volcanology and Geothermal Research 144 (2005) 235–255

(the southern and northeastern flanks), and both were actually btwo-eruptions-in-oneQ, representing both types of Etnean flank eruptions as defined by Rittmann (1964)—lateral and eccentric. In the earlier case it is assumed that the eruption originates from the lateral draining of the central conduit system—which is why in the following we apply the term bcentral– lateralQ. In contrast, eccentric eruptions (also named bperipheralQ by Acocella and Neri, 2003) are thought to result from magma ascent through new conduits independent of the central conduit system. In terms of pre-eruptive seismicity and deformation, however, the two eruptions were less alike, and in particular, the 2002–2003 eruption was accompanied by a major slip of Etna’s unstable eastern to southeastern flank, whereas no such phenomena had occurred in 2001 (Acocella et al., 2003; Neri et al., 2004). This paper presents the results of the integrated analysis of data obtained from geological and geophysical studies before, during and after the 2001 and 2002–2003 eruptions. Field observations of eruptive activity and slippage of Etna’s unstable sector (Fig. 1) are combined with ground deformation and seismic data. This analysis not only allows to depict that sector in three dimensions and infer on its internal behavior, but also reveals strikingly different dynamics of magma accumulation and ascent before and during the two eruptions, and the respective response of the volcanic edifice.

2. Eruptive activity Eruptive activity during the period under review was characterised by the latest stages of the intense summit eruptions initiated in July 1995, which lasted until mid-July 2001, and the two flank eruptions of July–August 2001 and October 2002–January 2003, with a brief period of intervening mild summit activity (June–September 2002). The distribution of the sites,

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fissure systems and products of the 2001 and 2002– 2003 eruptions is shown in Fig. 2, and panel 1 of Fig. 3 is a schematic representation of the timing, location and character of the individual eruptive events between August 1999 and March 2003. 2.1. Summit eruptions, 1999–2001 In the summer and autumn of 1999, all four summit craters (Figs. 2 and 3) showed intense eruptive activity, which varied from slow, practically non-explosive lava emission at the Southeast Crater (Neri and Tomarchio, 2000; Calvari et al., 2002) over strong Strombolian activity and voluminous lava overflows from the Bocca Nuova (Harris and Neri, 2002; Behncke et al., 2003) to extremely violent paroxysms with the generation of abundant tephra at the Voragine (Aloisi et al., 2002; Calvari and Pinkerton, 2002). After a temporary diminution of summit eruptive activity in mid-November 1999, a new phase of activity began on 26 January 2000 with the first in a series of 66 paroxysmal eruptive episodes at the Southeast Crater, which lasted until late August 2000 (Neri and Tomarchio, 2000; Alparone et al., 2003). The Southeast Crater reappeared on the stage in early 2001 with a 3-month period of slow lava effusion from a vent on its north–northeastern side, before its activity intensified in early May and culminated in another series of 16 paroxysmal eruptive episodes (Behncke and Neri, 2003a). The last of these events occurred on the early morning of 17 July 2001, just a few hours before the first eruptive fissures opened on the upper south flank of Etna, and the first flank eruption since 1991–1993 began. 2.2. 2001 Flank eruption This eruption lasted from 17 July until 9 August 2001 (Behncke and Neri, 2003a) and occurred from a total of seven systems of eruptive fissures (Billi et al., 2003; Lanzafame et al., 2003). Most of these were

Fig. 3. Summary diagram of eruptive activity, seismicity, and deformation at Mount Etna, during August 1999–March 2003. Panel 1 distinguishes the styles and locations of eruptive activity: 1=more or less continuous mild Strombolian activity; 2=sporadic Strombolian activity and ash emissions; 3=more or less continuous emissions of (mostly lithic) ash; 4=episodes of violent fire-fountaining, tephra emission, and often with fast-moving lava flows; 5=lava overflows onto the external flanks of Etna; 6=flank eruption. VOR, BN, SEC, NEC are indicated as in Fig. 2; FE stands for flank eruption. Panel 2 shows the daily number of earthquakes (black vertical bars) and the cumulative strain release (shaded area). Panel 3 plots the depth of earthquake hypocenters in time. Panel 4 shows cumulative areal dilatation measured on the three EDM networks on Etna’s southwestern, southern and northeastern flank. The changes measured by the EDM networks gain more significance if considered in the long term (see Fig. 5).

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located between 2100 and 2900 m elevation on the southern flank, while two small fissures were active on the southeastern and north–northeastern sides of the Southeast Crater, and an eruptive fissure formed at about 2600 m elevation, on the northeastern side of the volcano. As shown in inset (a) of Fig. 2, all of the fissures above 2600 m (1–3, 6 and 7) were sites where magma laterally drained from the central conduit system, while two fissures at 2100 and 2570 m on the southern flank (4 and 5) were eccentric, receiving their magma from a presumably newly-tapped magma reservoir below the southern flank (Acocella and Neri, 2003). Fissure 5 produced nearly all of the tephra of this eruption (estimated at 5–10106 m3 as freshly fallen; Behncke and Neri, 2003a), the bulk of which was generated by phreatomagmatic activity during the first and last phases of the activity, and 5 days of magmatic activity led to the rapid growth of a large, 100 m-high pyroclastic cone. About 80% out of a total of ~25106 m3 of lava was emitted from fissure 4, which was also the last to shut down at the end of the eruption (Behncke and Neri, 2003a). One of the most remarkable features of this eruption was the simultaneous emission of two compositionally distinct magmas (INGV-CT Scientific Staff, 2001; Andronico et al., in press) from the eccentric and central–lateral portions of the eruptive fissure system. The central–lateral activity produced a magma that is identical to all post-1669 eruptive products at Etna. In contrast, the lavas and pyroclastics from the eccentric vents are slightly more mafic and contain minor though significant amounts of amphibole (Pompilio and Rutherford, 2002; Andronico et al., in press), a mineral that is quite uncommon at Etna. Besides that, the eccentric products are very rich in inclusions of sedimentary basement rocks, which appear in negligible quantities in the laterally emitted lavas. 2.3. 2002–2003 Flank eruption The 2002–2003 eruption was preceded by 3 months of renewed summit eruptive activity, concentrated at the Bocca Nuova and the Northeast Crater. Between late June and late September 2002, Strombolian activity was nearly continuous at the Northeast Crater. This activity diminished drastically after an earthquake on the Pernicana Fault System (PFS; see Fig. 1) on 22 September, which was accompanied by

ground rupturing near Piano Provenzana (Neri et al., 2004). This was the first significant seismo-deformative event at the PFS for 14 years, and it probably played a significant role in facilitating the eruption that began 5 weeks later. Following a very brief but violent episode of lava fountaining from the Northeast Crater and/or nearby vents shortly before midnight on 26 October 2002, two eruptive fissure systems opened almost simultaneously on the upper southern flank (site 2 in inset b of Fig. 2) and along the Northeast Rift (sites 3 and 4). The opening of a ~4 km-long discontinuous system of eruptive fissures on the Northeast Rift coincided with the reactivation of flank slippage along the PFS, which resulted in vigorous seismic activity and spectacular ground fracturing (Neri et al., 2004). These led to the nearly total destruction of most of the tourist facilities of Piano Provenzana (Fig. 2) before the area was overrun by lava flows later on the first day of the eruption. New vents continued to open along this fissure during the early morning of 28 October, and the activity remained vigorous through 31 October, after which there was a rapid diminution in the levels of eruptive activity. By 5 November, the Northeast Rift eruption had ended, although slow movement of the main lava flow (extending about 6 km to the east) continued for another few days. On the southern flank, eruptive activity occurring from an initially 1000 m-long fissure was highly explosive and generated a dense ash column which darkened the skies over the southern flank of the volcano. Minor lava emission occurred during the first days of the eruption from this site, but then the activity concentrated at a single vent in the upper portion of the fissure (2750 m) and became purely explosive, leading to the growth of a large pyroclastic cone. Starting on 13 November, lava emission resumed and continued almost uninterruptedly until the end of the eruption, on 28 January 2003. Explosive activity on the southern flank alternated between Strombolian explosions and periods of ash emission. Between 25 November and 10 December, this activity shifted to a vent at 2800 m elevation, building a second pyroclastic cone to the north of the earlier one, which again became the main site of explosive activity after 10 December. Lava flows on the southern flank did not exceed 4 km in length, forming a fan-shaped lava field to the SW and S of the eruptive fissure (Fig. 2).

M. Neri et al. / Journal of Volcanology and Geothermal Research 144 (2005) 235–255

At the end of the eruption, 33–46106 m3 of lava and about 40–50106 m3 of tephra had been emitted (Andronico et al., in press); two-thirds of this lava and more than 95% of the pyroclastics had come from the vents on the southern flank, where the activity lasted ten times as long as that on the Northeast Rift. Like during the 2001 eruption, there had been simultaneous central–lateral (fissures 3 and 4 in inset b of Fig. 2) and eccentric activity (fissure 2), with the emission of two magma types (Andronico et al., in press). One of the most conspicuous aspects of the 2002– 2003 eruption is the major flank slip that affected the

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eastern to southeastern side of Mount Etna (Neri et al., 2004). Slippage started, in the upper portion of the unstable sector, as early as 5 weeks before the beginning of the eruption, and accelerated on the first day of the eruption, with an impressive meter-scale displacement along the section of the PFS closest to the Northeast Rift. The movement then extended eastwards, affecting the lower eastern slope of the volcano down to the sea and the Timpe fault system lying in that area (TFS in Fig. 1), where strong seismicity on 29 October 2002 caused heavy damage in the village of Santa Venerina and surrounding towns (Fig. 1). On 26

Table 1 Main parameters of earthquakes (with Mdz3.0) for which the focal solutions shown in Fig. 4 have been calculated Code

Date

Time (hh,mm,ss)

Md

Lat

Long

H (km)

RMS

Nl

Gap (8)

ERZ

ERH

DD

Dip

1 2 3 4 5 6 7 9 11 15 16 18 21 22 23 24 25 31 36 38 43 45 53 55 56 57 58 61 62 65 67 68 75

30/09/1999 26/12/1999 21/09/2000 09/01/2001 25/04/2001 27/04/2001 13/07/2001 13/07/2001 13/07/2001 15/07/2001 16/07/2001 16/07/2001 17/07/2001 17/07/2001 17/07/2001 28/10/2001 24/03/2002 27/10/2002 27/10/2002 27/10/2002 27/10/2002 27/10/2002 29/10/2002 29/10/2002 29/10/2002 29/10/2002 29/10/2002 30/10/2002 31/10/2002 01/11/2002 03/11/2002 04/11/2002 02/12/2002

14:52:05.18 14:19:50.17 17:51:25.21 04:31:39.45 19:33:07.62 21:09:14.74 03:13:33.18 05:11:47.99 21:59:04.01 07:45:30.57 02:44:09.09 19:31:40.76 10:48:31.39 13:40:19.25 15:18:53.16 15:05:11.63 23:04:06.25 00:35:03.80 01:23:48.66 01:28:17.59 02:50:25.01 05:46:45.86 10:02:20.23 10:56:09.16 11:02:35.23 15:49:50.52 16:39:46.78 15:25:43.31 10:41:04.22 18:01:42.11 10:21:59.01 10:52:35.59 12:28:14.01

3.1 3.1 3.1 3.1 3.1 3.1 3.5 3.3 3.0 3.5 3.1 3.1 3.3 3.3 3.2 3.2 3.0 3.0 3.0 3.5 4.2 3.4 4.4 3.6 4.0 3.8 4.0 3.2 3.2 3.4 3.5 3.0 3.6

37.777 37.709 37.744 37.707 37.741 37.733 37.708 37.731 37.712 37.734 37.716 37.710 37.707 37.709 37.716 37.657 37.725 37.726 37.758 37.797 37.763 37.780 37.686 37.821 37.807 37.804 37.657 37.811 37.745 37.798 37.811 37.752 37.708

15.111 15.082 15.121 15.068 15.039 15.026 15.040 15.000 15.037 15.003 15.014 14.994 15.008 15.007 15.007 15.131 15.060 14.995 14.993 15.039 15.025 15.021 15.100 15.074 15.085 15.045 15.129 15.117 15.108 15.061 15.079 15.064 15.128

6.89 6.43 8.91 5.02 5.42 5.43 2.42 1.14 2.35 0.38 0.23 1.04 0.97 0.29 0.47 4.04 5.46 0.01 0.05 0.09 4.38 4.56 1.32 2.69 1.35 1.36 3.41 2.82 4.53 4.13 3.80 2.82 0.52

0.25 0.23 0.23 0.14 0.19 0.14 0.14 0.16 0.12 0.13 0.15 0.17 0.15 0.18 0.15 0.23 0.12 0.22 0.16 0.22 0.21 0.17 0.17 0.15 0.38 0.20 0.21 0.19 0.17 0.19 0.16 0.17 0.13

19 25 27 32 38 42 31 33 28 39 24 32 29 27 29 24 25 21 17 17 22 21 15 20 25 21 28 22 21 21 23 32 21

122 55 97 62 45 58 40 47 51 44 71 47 40 55 51 128 83 51 53 85 112 123 109 87 101 72 163 67 146 98 82 78 154

0.7 0.4 0.6 0.3 0.2 0.2 0.3 0.3 0.4 0.2 0.4 0.3 0.3 0.2 0.3 0.4 0.3 0.5 0.6 1.7 0.7 0.7 0.3 0.3 0.3 0.4 0.2 0.2 0.4 0.7 0.3 0.2 0.5

0.3 0.3 0.3 0.2 0.2 0.1 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.2 0.3 0.2 0.2 0.2 0.3 0.3 0.3 0.3 0.2 0.2 0.2 0.3 0.3 0.4 0.3 0.3 0.2 0.2

195 100 285 60 240 345 270 295 195 165 100 160 280 105 250 255 230 250 250 170 155 155 155 155 205 95 240 110 140 165 190 235 205

55 90 85 90 30 85 60 75 85 35 85 90 40 75 70 70 75 60 70 90 90 80 20 50 55 65 85 50 40 90 80 20 40

Rake 10 80 150 40 10 100 40 100 170 60 50 40 60 150 20 140 10 160 140 20 20 40 70 140 20 50 150 60 50 30 130 40 110

Err DD

Err dip

Err rake

5 5 3 8 5 3 13 8 3 3 5 10 15 8 3 5 3 3 0 13 3 3 8 3 3 10 5 3 5 3 10 5 8

10 0 3 3 3 3 8 8 10 3 8 18 10 10 10 25 20 8 0 20 20 15 5 3 10 8 10 3 8 8 15 5 5

15 0 10 15 10 10 10 0 35 0 5 10 20 150 5 20 5 5 5 20 10 20 5 10 15 10 5 5 5 10 15 0 5

H-focal depth referred to sea level; RMS-root main square expressed in s); NI-number of readings including P-phase and S-phase; Gap-max azimuthal angle between used stations in degree; ERH and ERZ-errors (in km) for epicentral coordinates and focal depth, respectively; DD-dip direction of one of two nodal planes; D-dip of the nodal plane.

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M. Neri et al. / Journal of Volcanology and Geothermal Research 144 (2005) 235–255 37.90

19 Aug 1999 - 30 Sep 2000

01 Oct 2000 - 11 Jul 2001

Magnitude 1

M>=4 3<=M<4 2<=M<3 1<=M<2

5

37°40’ 6

15°00’

3

a

2

0

3.5

4

b

7

Kilometers

14.75

0

0

3.5

7

Kilometers

37.50 14.75 15.25

15.25

0 5

10

10

depth (km)

5

15 20

15 20 25

25

30

30 Longitude (°)

35 14.75

14.85

14.95

15.05

35 15.25 14.75

15.15

Longitude (°)

14.85

14.95

15.05

15.15

15.25

37.90

01 Sep 2001 - 25 Oct 2002

12 Jul 2001 - 31 Aug 2001 I 15 9

11 25

16

7

18

21

23

c

22

0

3.5

Kilometers

14.75

d

7

0

7

3.5

Kilometers

15.25

0

0

5

10

10

depth (km)

5

15

15

20

20

25

25

30

30 Longitude (°)

35 14.75

24

37.50 15.25 14.75

14.85

14.95

15.05

15.15

35 15.25 14.75

Longitude (°)

14.85

14.95

15.05

15.15

15.25

Fig. 4. (a–d) Seismicity (Mdz1) at Etna in different periods between 19 August 1999 and 31 January 2003. The numbers of the focal mechanisms refer to the numbers in Table 1. The displayed intervals have been chosen in function of the changes in time and space of the distribution of seismicity related to volcano-deformative and eruptive events. See text for details. (e–f; continued).

M. Neri et al. / Journal of Volcanology and Geothermal Research 144 (2005) 235–255 37.90

26 Oct 2002 - 31 Jan 2003 55

243

01 Feb 2003 - 31 Mar 2003

61

67

57

56 38

45 65

36

43

62

Magnitude

59

M>=4 75

31

e

0

68

53

58

37.50

0

7

3.5

Kilometers

15.25

0

5

5 depth (km)

10 15

10 15 20

20

25

25 30 14.75

0

14.75

15.25

14.75

f

7

3.5

Kilometers

3<=M<4 2<=M<3 1<=M<2

Longitude (°)

14.85

14.95

15.05

15.15

30 15.25 14.75

Longitude (°)

14.85

14.95

15.05

15.15

15.25

Fig. 4 (continued).

November 2002 ground fracturing at the Trecastagni Fault (TF in Fig. 1) marked the propagation of flank movement to a more southerly portion of the unstable sector. Movement of the unstable flank of the volcano continued unabated after the end of the 2002–2003 eruption, although at a much slower rate, through at least April 2004.

3. Instrumental data In the following two sections, we present both seismic and ground deformation (EDM) data acquired before, during and after the 2001 and 2002–2003 eruptions. The periods covered by these data are different, starting in August 1999 for the seismic data, but going back to 1980 in the case of the EDM data. As shown in panels 2 and 3 of Fig. 3, August 1999 marks the beginning of a reasonably stable period, from the seismic point of view, with a fairly regular seismic energy release and the distribution of focal depths showing few variations with time. This is followed,

starting in September 2000, by a gradual increase in the seismic rate, which precedes the 2001 eruption. The period covered by the EDM data presented here is longer because the observations related to the 2001 and 2002–2003 eruptions can be better evaluated in a long-term context. The fourth panel of Fig. 3 shows the cumulative areal dilatation measured between August 1999 and March 2003, and although major deformation occurred, its significance is not well represented in this short-term view. 3.1. Seismicity The permanent seismic network of Mt. Etna is run by Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania (INGV-CT). It is composed of 52 seismic stations: 44 are equipped with vertical geophones and 8 with three-component geophones (two of these with broad-band high-dynamic sensors). We have analysed the seismicity of Mt. Etna volcano from August 1999 to March 2003. The quality of the locations of earthquakes is satisfactory

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because the high number of stations and since the epicenters lie inside the seismic network. Magnitudes have been estimated by the signal duration from recorded signals, and hypocentral parameters were computed with the HYPOELLIPSE code (Lahr, 1989) and using the 1-D velocity model of Hirn et al. (1991). In order to better understand the kinematic processes of the eastern flank of the volcano, we focus on a data set including earthquakes with Mdz3.0. Fault-plane solutions (FPS) are computed, assuming a double couple source type, according to the Reasenberg and Oppenheimer algorithm (1985) using earthquakes with Nz8 polarities of the first arrival. A total of N=129 with Mdz3.0 events have been processed, of which we have selected 76 focal mechanisms with an error of V358 and a discrepancy of V20% in the polarity numbers (Table 1 and Fig. 4). Fig. 3 (panel 2) shows the strain release trend and daily earthquake rate for the period analysed in which it is possible to observe periodic rapid increases of the strain due to occurrence of seismic swarms and/or isolated earthquakes. Two major increases in strain release are connected with the onsets of the eruptions in 2001 and 2002. On basis of the strain release trend, we have divided the time span since August 1999 in the following 6 periods, that include the pre- and posteruptive intervals and the eruptive periods themselves. 3.1.1. 19 August 1999–30 September 2000 The seismicity during this period is distributed more or less evenly over the whole volcanic edifice (Fig. 4a), and the temporal distribution of shocks and strain release (see Fig. 3, panel 2) underlines a seismicity connected with both several relevant seismic swarms and isolated earthquakes (Privitera et al., 2001, 2003). This time span covers the major eruptive events at the Voragine and the Bocca Nuova in September–November 1999, and the 66 episodes of vigorous lava fountaining on Southeast Crater in 2000. In the E–W cross-section (Fig. 4a, lower panel) it can be seen that hypocenters are mainly located within the first 15 km below the sea level (bsl). There is an almost complete lack of linear clustering of earthquakes in this period. This corresponds to inactivity during 1999 and 2000 of the seismogenic faults that cut a large sector of Etna from the PFS to the north over the numerous faults on the

eastern flank to the Ragalna Fault (R in Fig. 1) in the southwest. 3.1.2. 1 October 2000–11 July 2001 During the 9 months preceding the 2001 eruption (but excluding the last 4 days before the beginning of the eruption), seismicity begins to be clustered in a more restricted area centred below the upper southern flank of the volcano (Fig. 4b). At the same time, the strain release increases, especially in connection with several seismic swarms, most notably in late 2000 and in early 2001. In particular, the late-April 2001 seismicity in the time-depth diagram (Fig. 3, panel 3) is characterised by a brief burst of relatively deep (N10 km bsl) seismicity. A notable drop in the overall seismicity occurred between May and mid-July 2001. The E–W crosssection (Fig. 4b, lower panel) shows that the hypocenters in the western part of the volcano are concentrated between 5 and 10 km bsl, while under the eastern part of the edifice they are much shallower (3–5 km). As during the preceding time frame, no significant clustering of epicenters along the seismically active faults in Etna’s unstable sector can be recognized. 3.1.3. 12 July 2001–31 August 2001 This period includes the 4 days of intense seismicity preceding the 2001 eruption and the eruption itself (Fig. 4c). The shallow intrusion and the opening of the eruptive fracture system, which mostly occurred during 12–18 July, were accompanied by an intense seismic swarm affecting the southern to southeastern flank (Patane` et al., 2002, 2003b) with a maximum magnitude estimated at 3.9. A total of 2694 earthquakes (Mdz1) were recorded from the beginning of the swarm (12 July) until the end of the eruption (9 August), most of which occurred between 12 and 17 July (note the rapid decrease in seismic rate in Fig. 3, panels 2 and 3). Focal depth (see Fig. 4c, lower panel) was almost exclusively located within the uppermost 5 km bsl. Interestingly, seismicity begins to show some clustering along at least two of the fault systems on the southeastern flank (the NW–SE trending TFS in Fig. 1), indicating that the unstable eastern flank of the volcano was responding to the magmatic (intrusive and eruptive) processes.

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3.1.4. 1 September 2001–25 October 2002 The period following the 2001 eruption (and preceding the 2002–2003 eruption), compared with the pre-2001-eruption seismicity, shows a highly different behavior. While the seismicity is again distributed over a much broader sector of the volcanic edifice (Fig. 4d) with some seismic swarms located mainly in the southern flank of volcano, there is a marked clustering of the seismicity along the active faults in the eastern to southeastern sector of the volcano. Comparing the pre-2001 and the pre-2002– 2003 eruption seismicity (Fig. 4b and d) it is seen that in the latter interval there is no marked clustering of earthquakes in the western sector of the volcano, and most of the focal depths are confined within 5 km bsl. The strain release trend in the second period (Fig. 3, panel 2) is less evident than pre-2001 eruption, due to a minor number of earthquakes and/or seismic swarms. However, as in 2001, a relatively strong and short-lived burst of deep (~8–18 km bsl) seismicity in late August– September 2002 precedes the ensuing flank eruption by several weeks. The epicentral area of this swarm is developed along a well-defined NNW–SSE plane underneath the summit craters, with an almost vertical dip (Gambino et al., 2003). 3.1.5. 26 October 2002–31 January 2003 The time window considered here includes the brief (~2 h) but very intense premonitory seismicity that preceded the new eruption, and the full 3-month duration of the eruption itself. A striking concentration of the seismicity along the faults activated during the massive flank slip predominates in the distribution of epicenters shown in Fig. 4e, and significant seismicity is also seen along the full length of the eruptive fissures on the Northeast Rift. Most importantly, however, there is nothing similar to the dense clustering of epicenters in the southern-central portion of the volcano seen in connection with the 2001 eruption (Fig. 4c). Much of the seismicity along the Northeast Rift and the PFS occurred during the first day of the eruption, while the remarkable clusters of earthquakes on the southeastern flank are largely related to the 29 October seismic crisis around S. Venerina (Fig. 1). As during the 2001 Etna eruption, focal depths are concentrated within the uppermost 5 km bsl (Fig. 4e, lower panel; Barberi et al., 2003a,b).

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3.1.6. Post-2002–2003 eruption seismicity During the 2 months following the end of the 2002–2003 eruption, seismic activity picked up again, as is clearly visible in panel 3 of Fig. 3. This included a significant seismic swarm at the PFS, which was accompanied by ground rupturing, in mid-February 2003, and several notable events on the southeastern flank in early March (Fig. 4f). The levels of seismicity, although with a low daily earthquake rate, follow the same seismogenetic areas as during the 2002–2003 eruption period. All the hypocenters of these earthquakes are mainly located within 5 km bsl. 3.2. Ground deformation Ground deformation studies on Mt. Etna have been systematically carried out since 1978, using periodical distance measurements (EDM) and continuous recording tiltmeters. Here we describe EDM data obtained from the networks located on the northeastern, southwestern and southern flanks of the volcano (Figs. 5 and 6), respectively, between 500 and 3000 m asl, with particular emphasis on the data from the northeastern flank. For reasons explained above, the whole 23-year period between June 1980 and June 2003 is considered here (Fig. 5). The three EDM networks, established during the early 1980s, have been strategically placed in areas with a high eruption probability, closely corresponding to the northeast, south and western rifts (Figs. 1 and 6). Each network consists of about 14 to 16 geodetic benchmarks and a total of 36 to 47 lines, and has commonly been surveyed once per year, by means of infrared or laser geodimeters (accuracy of F5 mm +1 mm for each km of distance between benchmarks). In case of eruptive activity the measurements were repeated more frequently to investigate in detail the deformation patterns linked to the event. The EDM data show significant correlations between deformational events and eruptive activity since 1978 (Falzone et al., 1988; Bonaccorso et al., 1990; Bonaccorso et al., 1995), the deformation being more pronounced in relation with flank eruptions. They furthermore show a clearly differential dynamic behavior of the three sectors of the volcano corresponding to the three networks (Fig. 5). These are mainly controlled by the locations of the respective

1991-93

1983

1981

Cumulative areal dilatation (microstrain)

40

1989

2002-03

1985

60

2001

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1986-87

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NE

20

0

S -20

-40

SW 28/6/03

5/11/01

15/3/00

24/7/98

1/12/96

11/4/95

19/8/93

28/12/91

7/5/90

14/9/88

23/1/87

2/6/85

11/10/83

18/2/82

28/6/80

-60

Fig. 5. Twenty-three years of cumulative areal dilatation measured by the three EDM networks on Etna’s southwestern, southern and northeastern flanks (see Fig. 6 for locations), and correlation with flank eruptions (vertical white arrows). This representation gives little qualitative information about the type and orientation of deformation in the distinct areas, but shows whether there was an overall dilatation (interpretable as inflation) or contraction (deflation) of the volcanic edifice. The period from 1980 until the major 1991–1993 flank eruption is characterised by a net contraction of the volcanic edifice, which can be correlated with the frequent draining of magma from the shallow plumbing system, reaching a climax with the 1991–1993 eruption. The trend inverted for the eight years following that eruption, showing net inflation until 1999, when there was a notable increase in the output rate of the summit eruptions initiated four years earlier, and a temporary slowing of inflation. Rapid inflation resumed several months before the 2001 eruption and once more after that event. Detail of the deformation related to the 2001 and 2002–2003 eruption is shown in Figs. 3 and 6.

eruptive sites (and dike intrusions) relative to the networks. The main strain parameters calculated before and after the 2001 and 2002–2003 eruptions again clearly show the different behavior of the investigated areas (Tables 2 and 3 and Figs. 5 and 6). 3.2.1. Southwestern network This network, consisting of 16 benchmarks connected by 36 baselines, lies approximately between 1100 and 2000 m elevation and was first measured in September 1980. Generally, all major flank eruptions since 1980 have been accompanied by an overall areal contraction on the southwestern flank of Etna (Fig. 5), and strong contraction was observed also during both the 2001 and the 2002–2003 eruptions. While in 2001 the maximum contraction occurred in a NW–SE direction (Fig. 6a), it was oriented ENE–WSW in 2002–2003 (Fig. 6b). Furthermore, the 2002–2003

deformation in this area was less strongly expressed than in 2001. 3.2.2. Southern network Installed in November 1983, the southern EDM network consists of 16 benchmarks connected by a total of 47 baselines, and lies between about 650 and 2100 m elevation (Fig. 6). Strong deformation has affected this network during several flank eruptions during the 1980s, most notably in 1989 (Fig. 5). Strong deformation related to the 2001 eruption was recorded by the network, showing a contraction that is strongest in a NNE–SSW sense (Fig. 6a). In contrast, a very small amount of deformation (essentially NW–SE contraction) was recorded during the 2002–2003 eruption (Fig. 6b). Cumulative areal dilatation calculated over the entire network shows only negligible areal contraction related to the 2001 eruption, while the following period was characterised by moderate positive values, includ-

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247

37.90

2002-2003

2001 a

b

NE

NE Jun 02-Jul 03

May 02-Jun 03

SW May-Jul 01 May-Aug 01

SW S

S Aug-Dec 02

Jun-Jul 01 -10 µstrain

-10 µstrain

+10 µstrain

0

Kilometers

15.25

14.75

37.50

0

7

3.5

Kilometers

14.75

15.25

NE

NE

Oct 02-Jul 03

Jun 02-Oct 02

May-Jul 01

-30 µstrain

-30 µstrain +30 µstrain

+10 µstrain

7

3.5

0

3.5

7 km

+30 µstrain

0

3.5

7 km

Fig. 6. Location of the three EDM networks (SW, S and NE) and deformation of the volcanic edifice in correspondence with the 2001 and 2002– 2003 eruptions, shown as the principal strain axes (expansion and contraction). In the case of the NE network, details of the southeastern portion are shown in two enlargements (lower panels). See text for details.

ing the 2002–2203 eruption (Fig. 5). In the case of the 2001 eruption, this is basically due to the fact that the principal strain axes, in spite of their very high values (Fig. 6a), have nearly equal modules but are of opposite polarity and thus virtually eliminate each other. At the same time they indicate an extremely high strain. The measuring campaigns following the 2001 eruption, including also the 2002–2003 eruption, show that the

axis of maximum expansion E1 (Table 2) always maintains a module that is higher than the axis of contraction E2, indicating a modest though continuous expansion of the area covered by the southern network. 3.2.3. Northeastern network The northeastern EDM network was established in 1982, consisting of 14 benchmarks and 38 baselines

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Table 2 Main strain parameters measured on the Etna trilateration networks (see Fig. 6 for location) Period (month/year) bSW Q network 06/00–10/00 10/00– 05/01 05/01– 08/01 08/01– 05/02 05/02– 06/03

E1 (ppm)

TH1 (8)

E2 (ppm)

0.5F0.9 5.2F1.2 3.5F3.3 4.8F1.2 4.4F3.3

134.9F42.3 173.2F20.4 35F42.9 5F7.1 158F4.6

1.5 F 0.9 3.2 F 1.2 30.2F3.3 1F1.2 23.5F3.3

bSQ network 09/00–06/01 06/01–07/01 07/01–02/02 02/02–08/02 08/02–12/02

2.7F0.6 23.8F1 1.7F1.1 3F0.6 3F1.3

68.7F14.2 97.9F6.7 166.8F14.7 59.8F8.3 144F16.2

bNEQ network 07/00–05/01 05/01–07/01 07/01–06/02 06/02–07/03

4.7F1.1 7.6F3.2 4.3F0.9 66.5F65.8

151.1F17.4 87.7F6 102.9F11.4 136.8F29.7

MS (ppm)

D (ppm)

1F1.2 2F1.7 33.7F4.2 5.8F1.8 27.9F4.5

2.1F1.5 8.4F1.7 26.7F5 3.8F1.8 19.1F4.8

1F0.6 24.1F1 0.9F1.1 0.01F0.6 0.7F1.3

1.7F0.8 48F14 2.6F1.5 3F0.8 3.7F1.6

3.7F0.9 0.3F14 0.8F1.6 3F0.9 2.4F2.1

2F1.1 10.8F3.2 1.5F0.9 29.5F65.8

2.7F1.5 18.5F4.5 2.8F1.2 96F86.9

6.6F1.7 3.2F4.5 5.8F1.2 37F98.9

E1: maximum extension axis; TH1: azimuth of the maximum extension axis, referred to north and measured in clockwise direction; E2: minimum extension axis; MS: maximum shear; D: areal dilatation.

and occupying an area between 1250 and 2750 m elevation. In order to better document the deformation related to the 2001 eruption, the network that had just been measured in May 2001, was re-measured at the end of July 2001. Two further surveys were carried out in June 2002 (4 months before the 2002–2003 eruption) and in July 2003, thus including the 2002–2003 eruption. Following the 1991–1993 eruption, the overall areal dilatation shows a general expanding trend, interrupted only by two modest phases of contraction in June 1998–June 2000 and in May–July 2001, the latter in correspondence with the 2001 eruption (Fig. 5). From the orientations and values of the principal

strain axes a strong deformation appears between the May and July 2001 measurements, the axis of maximum compression being slightly longer to that of maximum expansion, resulting in a modest areal contraction but an elevated shear stress. The subsequent measurement revealed a significant dilatation, with both principal strain axes showing positive values, and the main expansion axis being oriented more or less E–W (Table 2). The whole network was surveyed again in July 2003. The obtained strain parameters indicate a very high value of the cumulative areal dilatation (Fig. 5). The main strain axes are very impressive in modulus, indicating a strong dilatation of the area in a NW–SE

Table 3 Main strain parameters measured on a quadrilateral (eastern portion) of the northeast network (see Fig. 6 for location), comparing data from the two eruptions Period (month, year)

E1 (ppm)

TH1 (8)

07.00–05.01 05.01–07.01 07.01–06.02 06.02–10.02 10.02–07.03

7.5F3.9 114.6F28.6 4.9F2.3 27.6F15.9 15.3F5

146.6F15.5 126.8F8.2 117F13.9 18.5F4.1 76.1F9

E2 (ppm) 3F3.9 38.4F28.6 1.8F2.3 114.8F15.9 4.4F5

MS (ppm) 10.5 F 5.1 153F36.4 6.7F3 142.4F21.7 19.7F7

D (ppm) 4.5F6 76.3F44.1 3F3.4 87.2F23.2 11F7.3

E1: maximum extension axis; TH1: azimuth of the maximum extension axis, referred to north and measured in clockwise direction; E2: minimum extension axis; MS: maximum shear; D: areal dilatation.

M. Neri et al. / Journal of Volcanology and Geothermal Research 144 (2005) 235–255

direction. The high error values are due to the elongation of a few lines crossing the eruptive fracture which reach from 1 to about 2 m. During October 2002, it was possible only to remeasure the lower portion of the network (see enlargement in Fig. 6b and Table 3). The results of this survey, carried out soon after the beginning of the 2002–2003 eruption, at the end of October 2002, show quite strong variations (up to 30 cm) in most of the lines of the network. The overall picture is that of an accentuated WNW–ESE contraction and much minor NNE–SSW expansion. If these values are compared to those obtained from the same limited sets of lines during the 2001 eruption (see enlargement in Fig. 6a), the two deformational events appear to be of nearly opposite character, with strong NW–SE expansion and minor NE–SW contraction in 2001. In the period October 2002–July 2003, the same lines show E–W expansion once again.

4. Discussion The geological, seismic and EDM data presented in the previous sections indicate how complex and changeable the dynamics of Mount Etna are. From the analysis and comparison of these data, it is possible to obtain crucial information on (1) how the two eruptions in 2001 and 2002–2003 were triggered differently, though once initiated they were strikingly similar; (2) what is the relationship between flank slip and eruptive activity. 4.1. Different triggering of the 2001 and 2002–2003 eruptions The time–space distribution of seismicity preceding the 2001 eruption (Section 3.1.1.) differs from the common shallow (V5 km bsl) seismicity associated with movement along the faults that are believed to confine the unstable slide blocks on the eastern– southeastern flanks (Neri et al., 1991; Lo Giudice and Rasa`, 1992; Borgia et al., 1992; Rust and Neri, 1996). It is plausible that this seismicity is related to the fracturing of the sedimentary substratum of Etna in response to vigorous magma accumulation in one or more reservoirs located about 5–10 km bsl (Patane` et al., 2003a). In fact, although some authors (Borgia et

249

al., 2000; Froger et al., 2001; Lundgren et al., 2003) observe that deformation of the eastern and southern flanks of the volcano had begun shortly after the end of the 1991–1993 flank eruption, no accelerated, coseismic flank slip occurred until September 2002. Starting on 12 July 2001, earthquake hypocenters began to migrate to shallower levels, between 5 km bsl and 2 km asl, with their epicentral areas clustering on the southern and southeastern flanks of the volcano (Fig. 4c). This seismicity clearly marks the forceful uprise of magma from the eccentric reservoir that opened the flank of the volcano like a wedge. Tilt and GPS data presented by Bonaccorso et al. (2002) confirm this scenario. Also the strong deformation measured on all three EDM networks during the 2001 eruption is clearly a result of the intrusion of the eccentric dike below the southern flank, but also shows, in the case of the northeast network, a strong NW–SE expansion, possibly representing accelerated aseismic flank slip. The expansion is clearly visible in the enlargement of the lower portion of the northeast network (Fig. 6a). On the other two networks, compression is much stronger than in the northeast network, but a strong E– W expansion is also evident in the south network, which is probably due to the wedging open of the southern flank by the eccentric dike. The 2001 eruption can thus be inferred to be the result of the emplacement of this N–S trending eccentric dike, associated with an E–W extension direction. Such a direction is consistent with the regional stress field, with the E–W extension possibly associated to N–S compression (Lanzafame et al., 2003). This dike reached the surface at 2100–2570 m elevation on the southern flank (inset a of Fig. 2, vents 4–5). The opening of the eruptive fissures located higher upslope on the southern and northeastern flanks (inset a of Fig. 2, vents 1–3 and 6–7) occurred in response to the extensive fracturing of the upper portions of the edifice and they can be supposed to result from an initial, limited stage of flank slip of the eastern sector of the volcano. Moreover, these fissures served to drain the magma that had been present for 6 years in the central conduit system, representing the central–lateral portion of the 2001 eruption. Fig. 4c, which largely covers the 2001 eruption period, shows some clustering of seismicity along linear trends, corresponding to at least two of the

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faults on the southeastern flank of the volcano, and points to an increased structural instability of the eastern flank—possibly incipient accelerated flank slip. This becomes much more evident in Fig. 4d, which represents the period between September 2001 and October 2002. Seismicity was now largely concentrated along faults confining the unstable slide blocks on the eastern–southeastern flanks. During this period, swelling of the southern flank of Etna indicated resumed magma accumulation, probably in the reservoir that had fed the eccentric part of the 2001 eruption (Calvari and INGV-CT, 2002). The distribution of epicenters related to the 2002– 2003 eruption implies a quite different scenario as compared to the 2001 eruption (Fig. 7). Also, there is almost complete lack of earthquakes in that part on the southern flank affected by strong seismicity before and during the 2001 eruption. This stands in apparent contrast with the vigorous eruptive activity that began in this area at the same time as the Northeast Rift eruption and continued for 3 months, producing about twice as much lava and pyroclastics as in 2001. On a more general level, in October 2002 vigorous seismicity started only 2 h before the first magma reached the surface. In this 2 h magma rose from a depth of about 3–5 km bsl (as shown in Fig. 4e), much more rapidly than the dike ascent in 2001. Much of the October 2002 seismicity was concentrated along the Northeast Rift, the western portion of the PFS (Neri et al., 2004), and, following the beginning of the eruption, extended to the Timpe fault system (Fig. 7b). While the earthquakes along the rift are in part related to the uprise of magma to the surface, many of them are clearly linked to the vigorous displacement of the unstable flank of the volcano, which also caused the PFS seismicity. According to the scheme proposed by Acocella and Neri (2003), we interpret the emplacement of the dike along the Northeast Rift as a passive response to the unlocking of the rift induced by the flank slip (Fig. 8). This may have developed fractures, soon filled by magma from the main conduits, at depths less than few hundreds of meters, consistently with theoretical calculations (Gudmundsson, 1992, 1998). Similarly, the lack of seismicity on the southern flank indicates that the uprise of the residual eccentric magma, already emplaced at shallow levels during the 2001 eruption, occurred passively, somewhat enhanced by the flank slippage.

The prolonged activity on the southern flank during the 2002–2003 eruption was probably facilitated by the extension that resulted from the eastward displacement of the unstable flank (Figs. 7 and 8). This picture is confirmed by the EDM measurements in 2002. The strong ENE–WSW compression, well visible at the Southwest network (Fig. 6b), reflects the intrusion of magma in the Northeast Rift. In contrast, deformation on the southern flank is close to negligible. This is consistent with the fairly insignificant seismicity below the southern flank. In contrast, the deformation on the northeastern flank, notwithstanding the limited extent of the measurable network, shows a strong WNW– ESE compression that stands in apparent contradiction with the assumption that major slippage affected this sector at the same time the measurements were made. In reality, this compression is possibly the result of both the intrusion of magma in the Northeast Rift and the differential movement of the unstable flank, which was much more rapid in the uppermost portion of the slide block than in its lower part (Neri et al., 2004). The measured portion of the northeast network lies in the central portion of the slide block and thus recorded the discrepancy between fast movement in its upper part and slower movement in its lower portion. This interpretation is confirmed by the results of the July 2003 survey on the fully re-established northeast network, which show that after the initial strong movement of the upper portion of the slide block, the movement there slowed and instead accelerated in its lower, eastern portion. The notable scale of movement (up toN2 m) revealed by these data is consistent with the displacements measured in the field. 4.2. Two eruptions, one story The compositional heterogeneities and differences in eruptive styles observed during the latest two flank eruptions show that there are currently at least two magma storage areas. One of these corresponds to the central conduit system, while the other seems to have appeared only shortly before the 2001 eruption (Behncke and Neri, 2003a; Acocella and Neri, 2003). From seismic (Patane` et al., 2003a), gas emission (Allard, 1997; Le Cloarec and Pennisi, 2001) and

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Fig. 7. Summary and interpretation of seismicity, deformation, eruptive activity, and flank slip in 2001 (a) and 2002–2003 (b). Key: 1=unstable, mobile sector on the eastern to southern flanks of Etna, showing slow aseismic spreading prior to the 2001 eruption (frame a; Froger et al., 2001; Lundgren et al., 2003), and slide blocks (Blocks 1, 2 and 3) becoming distinguishable during the 2002–2003 eruption (panel b); 2=fronts of the spreading eastern, southeastern and southern flanks of Etna, well evident from deformation of sediments on the southern margin of Etna (Borgia et al., 1992, 2000) and inferred from bathymetric data in the Ionian offshore (Borgia et al., 1992); 3=Etnean volcanics; 4=preEtnean sedimentary basement; 5=direction of slippage of slide blocks 1 and 2; 6=assumed boundaries of the unstable sector. Thick dark gray lines indicate location of eruptive fissures of the 2001 and 2002–2003 eruptions.

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W

MaghrebianApenninic Chain

5

E depth (km)

0

Etnean volcanics Dike complex

5 kms-1

Hyblean Plateau

? -1

6 kms

10 15 14.75

W

14.85

2002-2003

14.95 15.05 Longitude (°)

15.15

Etnean volcanics MaghrebianApenninic Chain

5

15.25

E Dike complex

-1

5 kms

Hyblean Plateau

? -1

6 kms

10 15 14.75

?

?

depth (km)

0

?

?

?

2001

?

14.85

14.95

15.05

15.15

15.25

Longitude (°)

Fig. 8. Interpretive E–W cross-sections of Mount Etna at the time of the 2001 (bottom) and 2002–2003 (top) eruptions, showing the distribution of earthquake hypocenters during the two eruptions (modified after Rust and Neri, 1996). Note how in 2002–2003 earthquakes occupy a sector extending much further to the east than in 2001, indicative of the mobilization of the unstable eastern flank of the volcano. The slow initiation of the flank slip preceding the 2001 eruption (Froger et al., 2001; Lundgren et al., 2003) may have been caused by the expansion of a magma reservoir (dike complex) above ~5 km below sea level. The arrows within the eastern flank in the lower panel stand for: black—incipient movement, gray to white—latent movement. Accelerated flank slip in 2002–2003 (represented by black arrows in upper panel, which indicate movement affecting the entire eastern flank) was a result of both the 2001 dike emplacement and continued expansion of the subvolcanic magma reservoir.

InSAR data (Lundgren et al., 2003) it is known that subvolcanic magma accumulation is voluminous and causes vigorous inflation of the volcanic edifice even in times of intense eruptive activity, as during the 1995–2001 summit eruptions. We agree with Patane` et al. (2003a) and Lundgren et al. (2003) that this magma accumulation and the resulting inflation of the volcano are the prime cause of instability of its eastern flank, and in the long term many lateral eruptions as well as a marked cyclicity in the eruptive behavior of Etna (Behncke and Neri, 2003b) might in fact be the consequence of these processes. However, flank instability was not the prime cause of the 2001 eruption, but rather seems to have been accelerated by this event. The ensemble of magmatic (magma accumulation and eruption) and volcano-tectonic (seismicity, deformation and flank slip) processes can thus be envisaged as a chain of closely related events, of which the eruptions are merely the surficial and comparatively

short-lived end products. Between 1993 and 2001 all the Etna eruptions were fed by the central conduit of the summit craters and the volcano did not shows any major flank slip. The uprise of the eccentric dike in 2001 increased the instability of its eastern flank. This finally culminated with the two-step acceleration of flank slip (September and October 2002) and the 2002–2003 eruption. The gradual resumption of summit activity in the spring of 2004 suggests that the central conduit system is refilling once more, and the same is probably true for the eccentric magma reservoir below the south flank. Renewed accelerated flank slip is thus likely to occur again and trigger more flank eruptions in the forthcoming years. This interplay between magma accumulation and flank slip and the triggering of flank eruptions will continue until the volcano returns to a state of temporary stability. This will probably be after a very voluminous flank eruption, as in 1993, and, further back in time, in 1951 or 1892 (Behncke and Neri,

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2003b). At the current point of understanding of the dynamics of Etna, this seems to be one of the most likely mechanisms capable of interrupting, for a certain period that may last a few years to a few decades, this highly hazardous type of volcanotectonic dynamics.

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between magmatism and flank instability, which results in series of flank eruptions (the culminating stages of eruptive cycles according to the model of Behncke and Neri, 2003b) and repeated episodes of accelerated flank slip in relatively short (15–20 years) periods.

5. Conclusions

Acknowledgements

Based on the evidence presented and discussed in the preceding sections, we draw the following conclusions.

The authors thank the pilots and technicians of the Civil Defence helicopters who made the overflights during the 2001 and 2002–2003 eruptions possible. Thanks go to the colleagues of the Istituto Nazionale di Geofisica e Vulcanologia for fruitful scientific discussions. G. Tomarchio is acknowledged for sharing his observations with us. Helpful criticism was provided in the reviews by S.L. Brenner and S.J. Day and in additional comments by A. Gudmundsson, who is also acknowledged for the editorial handling of this paper.

1. The integration of geological, seismic and deformation data has been proved to be a potentially powerful tool for the identification of the triggering mechanisms of flank eruptions at Etna. 2. The 2001 and 2002–2003 eruptions of Mount Etna were similar in a number of respects, such as simultaneous central–lateral and eccentric activity, emission of two compositionally distinct magma types, and strong explosive activity. In contrast, the geodynamic processes that led to the two eruptions were fundamentally different. 3. Based on the seismic and deformation data, the 2001 eruption occurred when a dike rose from an eccentric magma reservoir below the south flank. The clearest evidence for this lies in the vigorous seismic crisis and ground deformation which heralded the 2001 eruption by 4 days. The fracturing of the volcano flanks caused by the ascent of the dike also led to the lateral draining of magma from the central conduit system. The data furthermore indicate that flank instability was enhanced by the forceful dike intrusion and continued to increase thereafter, probably accelerated by the recharging of the central and eccentric plumbing systems. 4. The 2002–2003 eruption, which was preceded by only 2 h of seismic crisis, occurred in correspondence with an acceleration of flank slip. Extension at the head of the sliding sector allowed both the lateral draining of magma from the central conduit system and the reactivation of the eccentric system that had generated the 2001 eruption. 5. The sequence of events described in this paper indicates that there is a feedback mechanism

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