Volcanomagnetic changes accompanying the onset of the 2002–2003 eruption of Mt. Etna (Italy

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Earth and Planetary Science

Volcanomagnetic changes accompanying the onset

of the 2002–2003 eruption of Mt. Etna (Italy)

Ciro Del Negroa,*, Gilda Currentia, Rosalba Napolia, Annamaria Vicaria,b

aIstituto Nazionale di Geofisica e Vulcanologia-Sezione di Catania, ItalybDipartimento di Ingegneria Elettrica, Elettronica e dei Sistemi-Universita di Catania, Italy

Received 18 March 2004; received in revised form 14 October 2004; accepted 14 October 2004

Editor: V. Courtillot

Abstract

Remarkable changes in the local magnetic field were associated with the onset of the 2002–2003 flank eruption at Mt. Etna.

After differential magnetic field measurements were filtered from the external noise by using adaptive filters, we recognized two

stages in the total intensity changes, which are closely related to different volcanic events: (a) rapid variations of about 4–5 nT

associated with October 26 seismic swarm recorded beneath the summit craters; (b) step-like variations of 9–10 nT coincident

with October 27 eruptive fissures opening up in the north flank. These observations are generally consistent with those

calculated from simple magnetic models of these volcanic processes, in which the magnetic changes are generated by stress

redistribution due to magmatic intrusions at different depth. The magnetic data not only allow the timing of the intrusive event

to be described in greater detail but also, together with other volcanological and geophysical evidences, permit some constraints

to be set on the characteristics of propagation of a shallow dike. Firstly, at around midnight on 26 October magma was rapidly

injected to a depth of 3–4 km just below the summit craters. Secondly, after 1:00 on 27 October, continued intrusion magma

occurred upward and culminated a few hundred meters below the free surface fractured along a N–E direction. Thirdly, at about

2:28, magma gave rise to an explosive fissural vent at the northern base of the NE crater near 3000 m a.s.l. Finally, at about

5:00, the first eruptive fissure became active along the eastern border of the NE rift at 2500 m a.s.l. The rate of growth of the

magnetic anomalies, moreover, leads to the interpretation that the magmatic intrusion travelled northward from base of the NE

crater to the NE rift at approximately 14 m/min.

D 2004 Elsevier B.V. All rights reserved.

Keywords: eruptions; monitoring; magnetic methods; volcanomagnetic modeling; Mt. Etna

0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.epsl.2004.10.033

* Corresponding author. Tel.: +39 95 716 5800; fax: +39 95 43

5801.

E-mail address: [email protected] (C. Del Negro).

1. Introduction

In the recent past, the magnetic method at Mt. Etna

has proved its ability to detect small changes in the

local magnetic field linked to volcanic activity. The

Letters 229 (2004) 1–14

C. Del Negro et al. / Earth and Planetary Science Letters 229 (2004) 1–142

first experiment goes back to the end of the seventies,

when two proton magnetometers operated for 4 years

on the volcano. The large anomaly identified in the

geomagnetic time series recorded in 1981 was asso-

ciated with the March 17–23 lateral eruption and was

considered to be the joint effect of piezomagnetism and

thermal demagnetization engendered by a intrusive

dike [1]. A convincing case of thermomagnetic effect

observation relates to the 1989 fissure eruption, when

repeated measurements at intervals of about 3 months

for 2 years revealed the slow buildup of a 130-nT

Fig. 1. Schematic map showing the area covered by the lava flows from 20

well. Inset shows the position of the L’Aquila geomagnetic observatory.

anomaly [2]. The anomaly vanished laterally within 0.2

km from the surface expression of the fissure system.

The nature and structure of the anomaly constrained the

location and time of cooling of a shallow dike. Later, a

close array of five protonmagnetometers ran for 2 years

on the upper southern flank of Mt. Etna. Geomagnetic

changes associated with the renewal of activity of the

NE crater were detected at the end of 1995. The center

of the magnetic anomaly source, which was thought to

be the region heated by high-temperature fluids and

gases originating from fresh magma, was estimated, by

02-2003 Etna eruption. Locations of magnetic stations are shown as

C. Del Negro et al. / Earth and Planetary Science Letters 229 (2004) 1–14 3

the spatial distribution of the variation rate, at a depth of

about 500 m near the 1991–1993 eruptive vents [3].

Detection of clear magnetic signals associated with

the renewal of the volcanic activity led to an increase

in magnetic monitoring of Mt. Etna. Since the end of

1998 a permanent magnetic network equipped with

Overhauser effect magnetometers has been set up [4].

The present network consists of five stations placed

on the volcanic edifice (CST, BVD, BCN, PDN, and

DGL), flanked by a sixth external reference station

(CSR) installed further west (about 50 km) on the

Nebrodi Mountains (Fig. 1). Stations on Etna are

located at elevations ranging between 2350 and 3000

m a.s.l. along a North–South profile crossing the

summit craters. All sites sample synchronously every

10 s and transmit data via mobile phone to the Catania

Section where these data are processed and analyzed.

The 2001 lateral eruption at Mt. Etna [5] provided

the first opportunity to verify the ability of our

network to detect volcanomagnetic effects. After

differential magnetic field measurements were filtered

from the seasonal thermic noise by using temperature

data, significant magnetic variations ranging between

3 and 7 nT were detected at several stations during

main volcanic events. These observations were mostly

generated by stress redistribution due to magmatic

intrusions at different depth and by the thermal

demagnetization at a rather shallow depth [6].

The latest eruption of Mt. Etna, occurring from late

2002 to early 2003, represented a further step towards

the definition of the real potential for continuous

magnetic studies as an effective volcano-monitoring

tool. During the night of 26–27 October 2002, two

fissure systems opened on the S and NE flanks of the

volcano, feeding explosive activity and two distinct

lava flows. The mainly explosive S flank eruption

lasted 3 months (26 October 2002–28 January 2003),

while the vents on the NE Rift erupted only for 8 days

and their activity was mainly effusive. The most

significant change in the magnetic field to occur here

is associated with the opening and propagation of the

vents along the NE Rift. In the following, we present

and discuss magnetic anomalies that were observed

during the night between 26 and 27 October 2002,

simultaneously with the onset of the 2002–03

eruption. The ash emissions and lava flows occurred

on the south flank entirely buried the BCN, BVD and

CST stations (they had been newly set up at the end of

2003), and the data were completely lost. Data

collected from the PDN and DGL stations located

on the north flank are analyzed below and the strategy

developed to eliminate disturbances and enhance

signal-to-noise ratios is presented. Simple models of

the main structures involved with the eruption, which

account for such magnetic changes, are proposed.

2. Magnetic observations during 2002

The geomagnetic field variations attributable to the

dynamics of volcano can be useful indicators of the

modifications of the stress field or of the thermody-

namic state within the volcanic edifice. However, the

volcanic geomagnetic field varies with amplitudes of

1~10 nT in most cases, while changes up to a few

hundreds nT are caused by ionospheric and magneto-

spheric currents, and secular variations. Therefore, it is

necessary to remove the unnecessary variations in

order to investigate an on-going physical process

inside a volcano. In general, the spatial distribution

of the secular variation and external field fluctuations

can be considered uniform (because of the large

distance of sources), and therefore, simple differences

in the total intensity with respect to the simultaneous

value at a remote reference (usually located at a

magnetically quiet site) are used to reduce these

effects. In Fig. 2a, the daily means of total intensity

variations from January to December 2002 observed at

PDN and DGL, relative to CSR are shown. Differ-

entiating these data more than 95% of magnetic

disturbances are reduced and the average of these

differentiated data on a daily basis reduces the standard

deviations by a factor of 10. Normally, measurement

accuracy for these data is better than 1 nT, depending

on site separation and local magnetic gradients.

Volcanomagnetic signals, unfortunately, are very

small and, even if data reduction processes are properly

employed, we often see geomagnetic variations regard-

less of the state of the volcanic activity. Very much

larger time variations, with periods from minutes to

years, are present in these differences. For example,

also in the absence of any volcanic activity, some of the

time changes can occur because of secondary fields

induced in the crustal rocks by external current

systems. Where the electrical conductivity of the rocks

of a volcano changes over short distances, time changes

Fig. 2. Daily mean differences of total magnetic intensity with respect to CSR station from January to December 2002 (a) and after removal of

external effects (b). The local K index values (c) obtained from L’Aquila geomagnetic observatory are shown as well.


can vary correspondingly. Differences in underground

conductivity between two close (order of 10 km apart)

sites will make the time variable fields different, even

if the primary field from the magnetosphere is the same

[7]. In addition, variable-induced magnetization, due

to large susceptibility contrasts, could locally modify a

magnetic disturbance field by an amount up to 5 nT

[8]. These effects are highlighted at PDN station,

where in correspondence of strong external activity

(geomagnetic K index values more than 5; Fig. 2c)

high geomagnetic components clearly appear in the

differences.

At different positions, this disturbance appears

with different properties and characteristics because of

its interaction with the local environment [9], but it

should be predictable, because the variations at the

different stations are correlated by their electromag-

netic impedances [10], which should remain constant

in time [11]. Therefore, a number of methods of

predictive filtering were proposed to further reduce

the changes in the difference fields due to contrasting

responses at magnetometer sites, with the filters

giving the relative responses between sites. A first

type of approach, relative to predictive methods based


on classical Wiener filters theory, was in particular

developed by Davis et al. [11]. This method provides

an estimation y(t) of the sequence y(t) by means of a

weighted linear combination of past samples from

other correlated sequences u1(t),. . ., up(t) and from

the same sequence to be estimated:

yy tð Þ ¼Xn

i¼1

a0iy t � ið Þ þXp

i¼1

Xmt

j¼0

aijui t � jð Þ ð1Þ

where aij are the filter parameters, p stations, n and mi

predictors.

The predictive filtering estimates the variations that

are common to different sites, and the filter output is a

prediction of the signal based on observations in other

sites. The residual field (the difference between the

observed values and the predicted values) contains

only the effects that are spatially local. This one

represents the signal to be analyzed [12].

It is worth noting that total field differences are

dependent on the direction of the disturbating field

[13]. The directional effect has meant that vector

information as well as that from total fields must be

used as input to the filter to take this into account [11].

In our case, the vector magnetic field data from

L’Aquila Geomagnetic Observatory, which is the

closest vector magnetometer, are utilized (Fig. 1).

Even if L’Aquila Geomagnetic Observatory is many

kilometers away from Mt. Etna (about 700 km),

signals correlate well. The correlation coefficients,

calculated between total magnetic field data at Etna

stations and L’Aquila observatory for the whole year

2002, are more than 0.90. In this case, the three

component fields of the vector magnetometer at

L’Aquila and the total field at CSR reference station

(of Etna array) represent inputs to the filter, and one of

the two stations (DGL or PDN) is the output, as

reported below:

yy tð Þ ¼Xn

i¼1

a0iy t � ið Þ þXm1

i¼0

a1iX t � ið Þ

þXm2

i¼0

a2iY t � ið Þ þXm3

i¼0

a3iZ t � ið Þ

þXm4

i¼0

a4iCSR t � ið Þ ð2Þ

where y is the station to be filtered, X, Y, Z the vector

components of LTAquila, CSR the total field recorded

at reference station, a the filter parameters, n and

m1,. . ., m4 predictors.

However, predictive filters designed during quiet

solar times fail during periods of intense geo-

magnetic activity, characterized by rapid transient

variations. On the contrary, filters working well

during severe geomagnetic storm are less efficient.

This practical result shows that the assumption of

considering the signal stationary is restrictive [14]. If

we assume the process to be time-variant, a non-

stationary approach is required to describe the

natural geomagnetic fluctuations. For this reason,

we implemented an adaptive type approach, and the

Eq. (2) becomes:

yy tð Þ ¼Xn

i¼1

a0i tð Þy t � ið Þ þXm1

i¼0

a1i tð ÞX t � ið Þ

þXm2

i¼0

a2i tð ÞY t � ið Þ þXm3

i¼0

a3i tð ÞZ t � ið Þ

þXm4

i¼0

a4i tð ÞCSR t � ið Þ ð3Þ

where the aij are now time-varying parameters. The

objective of the adaptive filter is to estimate and

update the filter parameters continuously by means

of the new observations, so that predictions closely

match the observed data. The aij are automatically

determined by using the recursive least squares

algorithm [15], with the aim of minimizing the error

(i.e. the residual field). The choice of number

parameters mki relies on the estimation of a quality

factor by using the Akaike Information Criterion

[16]. An efficient filter was accomplished when the

number of filter coefficients reaches to five.

Inclusion of more coefficients led to no further

improvements.

Finally, we used the adaptive algorithm for filtering

the signal from DGL and PDN stations by means of

the vector magnetic components from L’Aquila and

the reference station CSR. The residual components

are now further reduced (Fig. 2b), and the application

of adaptive filtering is clearly gainful during disturbed

days. The adaptive filter is effective enough and the

changes in October 2002 are the only significant ones

during the observed period.


3. Magnetic modeling

In Fig. 3a, the 10-min means of total intensity

variations at PDN and DGL stations, located on the

northeast flank of volcano, are shown after removal

of external components between 20 October and 3

November 2002. Comparison of the experimental

data with the different volcanic events occurring here

showed the close temporal correspondence between:

(a) sudden drop in geomagnetic total intensity and

October 26 seismic swarm recorded beneath the

summit craters; and (b) step-like magnetic variations

and October 27 eruptive fissures opening up in the

north flank. Therefore, the geophysical sources,

which could produce these magnetic field variations,

were searched separately. We will briefly summarize

results obtained for these two stages in chronological

order.

Fig. 3. Plot of 10-min means of total intensity variations (a), cumulative str

20 October and 3 November 2002.

3.1. Before the eruption onset

The seismic swarm began at about 21:25 local

solar time on 26 October, and continued without any

fall in energy release till the early hours of 28 October

(Fig. 3b). Until 1st November, about 500 shocks were

recorded [17]. The great majority of seismic events

(about 110) were recorded in the first 5–6 h and, in

this time interval, the seismicity occurred in a NS

elongated area crossing Mt. Etna summit craters.

Epicenters time pattern evidenced, in fact, an almost

stationary distribution in the central part of the

volcano until 2:28 local solar time on 27 October

[18,19]. In the early morning of 27 October, a set of

extensional fractures trending N–S (horizontal dis-

placements between 10 and 50 cm) formed at the base

of the NE crater [20]. In concomitance with the

seismic crisis, during a 48-h period encompassing the

ain release and earthquake hourly rate (b; after Patane [17]) between


start of the 2002 NE Rift eruption, marked tilt

variations were recorded at the closest stations to the

fractures field. The deformation recorded by the

tiltmeters (ranging between 10–15 Arad on the south

flank and 50–60 Arad on the northwestern flank) was

mainly cumulated through local midnight on 26

October [21]. Also the continuous microgravity

sequence, acquired at the station on the N slope

(2800 m a.s.l.), showed a marked decrease (about 400

mGal in less than one hour) during the night between

26 and 27 October [20].

Until October 26, no significant changes were

observed at all magnetic stations (Fig. 3a). On

October 27, after the beginning of the seismic swarm

but before the northeast lava flows emission, large

changes in local magnetic field occurred only at PDN.

The total intensity decreased by about 5 nT at PDN

within a few hours coinciding with the quick

propagation of the fractures at the base of the NE

crater. Before eruption, ascending magma has to push

rocks apart and this perturbs stress distribution and

pore fluid pressures, commonly resulting in fracturing

and numerous earthquakes. The minimum peak in the

local magnetic field at PDN was reached at the same

time as the temporary fall in seismic energy release

(Fig. 3b), when the total intensity at DGL slightly

decreased about 1 nT.

Although the available magnetic data are not dense

enough to uniquely identify the physical process that

generated the observed anomalies, a preliminary

interpretation is possible by taking into account

available geophysical data. The time–space evolution

of seismic events indicated that firstly a rapid dike

intrusion occurred vertically to a depth ranging

between 0 and 5 km bsl in the central part of the

volcano, along the direction NS of the maximum

compressive stress [17]. Thenceforth, the deformation

pattern unequivocally showed a response to a tensile

Table 1

Summary of fault parameters used in two piezomagnetic models

Event Fault X center Y center D

(km) (km) (m

Seismic swarm Tensile 5 3.4 1

Eruptive fissures in the North flank Tensile 6.5 3.9

Tensile 9 5.2

Strike 10.5 7.5

Magnetization 2 A/m, inclination 53.38, declination 1.88, and sensitivity 0

mechanism coherent with a magmatic intrusion

occurring upward and culminating a few hundred

meters below the free surface fractured along a N–E

direction [22]. The location of the tensile source lies

within the zone of the seismic swarm that occurred

during the magma ascent. Branca et al. [20] inter-

preted the strong gravity decrease, recorded in the

hours before the eruption onset, as the gravity effect

produced by a two-dimensional vertical sheet, with

width ranging between 3 and 4 m, cutting through the

volcano pile slightly northeast of the crater area. The

magnetic change could have resulted from stress

redistribution due to dike emplacement at a rather

shallow depth, which took place in a few hours. In

these cases, the piezomagnetic effect (stress-induced

changes in rock magnetization) is the principal

mechanism, since it could justify both the amplitude

and the time-scale of the magnetic changes. It is worth

noting that the magnetic field changes might also be

expected to accompany magmatic intrusions as a

result of electrokinetic effects generated by fluid flow

[23]. However, to explain the observed rapid and

reversible changes in terms of electrokinetics would

require rapid and implausibly intense fluid flow

[24,25]. Thus, even if the electrokinetic mechanism

cannot be ignored, we favor a more straightforward

explanation in terms of the piezomagnetic effect.

A piezomagnetic model, based on the techniques

described in Utsugi et al. [26], who revised solutions

given by Sasai [27], was employed to calculate the

expected magnetic field change at the earthTs surfacefor a dike. Model parameters were based on estimated

dike geometry using gravity and seismic data [19,20],

and induced rock magnetization was calculated from

surface samples near the various magnetometer sites

(Table 1). Fig. 4 shows the calculated anomaly from

this model. Predicted values at the magnetometer sites

provide a reasonable fit to the observed data.

epth Length Width Strike about North Dip Dislocation

) (km) (km) (8) (8) (m)

000 2.5 3 15 90 3.0

800 2.1 1 15 90 2.5

300 3.6 1 45 90 2.5

250 2.9 1 78 90 0.7

.0001 bars�1.

Fig. 4. Computed piezomagnetic field change related to the 26 October 2002 seismic swarm, i.e. an intrusive dike. The geometry and physical

properties used to model are reported in Table 1.


3.2. After the eruption onset

In the early morning of 27 October, a short N–S

trending fissure at the northern base of the NE crater

(3000 m a.s.l.) produced a short-lived explosive event,

marking the onset of the 2002 NE-Rift eruption (Fig.

1). At this time, epicenters time pattern evidenced a

clear migration of seismic events in the NE direction,

involving the volcano-tectonic structure named NE

Rift [18,19]. The downslope propagation of the dike

was subsequently disclosed by the opening of an

eruptive fissure system at 2500 m a.s.l. It extended

along the eastern border of the NE rift, covering a

distance of 3.5 km up to 1900 m a.s.l. The eruptive

fissures were mainly characterized by effusive activ-

ity. During their propagation downslope, they gradu-

ally became inactive. From 29 October, the effusion

rate from the 1920–1900-m vent gradually decreased,

until it stopped on 4 November. On the whole, about

10�106 m3 of lava was erupted forming two narrow

lava flow fields a few kilometers long [28].

The opening of the eruptive fissures was clearly

seen at the close PDN and DGL stations, where

progressive changes in the total intensity were

recorded in coincidence with the outpouring lava

flows. According to Fig. 3a, the minimum peak in the

total intensity at DGL was reached, while the local

magnetic field at PDN turned to increase of about 7

nT. The total intensity sharply decreased by about 4

nT at DGL at the same time as the secondary rise in

seismic energy release (Fig. 3b), which we may

reasonably associate with the outbreak at 2500 m a.s.l.

The magnetic changes, started along with the onset of

the eruption, continuously increased by about 2 nT at

PDN and near 10 nT at DGL within a few hours,

while the eruptive fissures extended northward up to


1900 m a.s.l. (Fig. 1), and remained stable at a new

level. The fact that a jump in the magnetic field

remained constant implies that the field is maintained

by the stress buildup accompanying the opening of the

eruptive fissures.

Based on the opening and the propagation patterns

of this eruptive fissures field, we developed a

mechanical model that successfully explained the

on-going crustal movement associated with this event.

The analysis of seismic events points out that, during

the early hours on 27 October, dike intrusion

modified the preexistent stress field generating a re-

orientation of the main axes able to induce magma

injection along the NE Rift and subsequent activation

of northeastern sector [18]. We presumed that with

progressing extension and upward propagation of

dike, failure reached the ground surface, and the

eruption started. The opening of eruptive fractures

Fig. 5. Expected piezomagnetic field change produced by the northward ex

parameters are reported in Table 1.

was also seen at tilt station, located in the Volcano-

logical Observatory of Pizzi Deneri, where the radial

tilt variation decelerated in coincidence with the

outpouring lava flow [22]. The extensional fractures

(horizontal displacements between 2 and 3 m) were

interpreted as an expression of the lateral propagation

of a shallow radial dike fed by the central conduit.

The quick propagation of the vents along the NE Rift

sharply slowed down after the crossing with the E–W

trending Pernicana fault system. Moreover, contem-

poraneously with the opening of eruptive fissures, the

westernmost Pernicana fault system (Fig. 1) became

active again, with an almost purely left-lateral

displacement of about 70 cm [29]. We considered

the entire northeastern block (confined at north by the

Pernicana fault system and at west by the NE rift)

subjected to a sliding in NW–SE direction. Ground

fracturing along the Pernicana fault system migrated

tension of a dyke up to 2500 m a.s.l. on October 27, 2002. The dyke


eastward from the NE Rift, accommodating the

extension caused by the intrusion.

The variations occurred at the two closest sites

(PDN and DGL) and the simultaneity and proximity

in space and time point to the piezomagnetic effect as

the primary physical mechanism driving these anoma-

lies. Two models at different time instants based on

stress redistribution due to intruding magma could

explain the relationships between the observed

anomalies and the eruptive events. Firstly, after the

magmatic intrusion occurred below the summit craters

(Fig. 4), the increase in the total intensity at PDN can

be recognized as due to northward extension of a new

dyke up to 2500 m a.s.l. As the dyke approached

DGL, the total intensity sharply decreased there. The

expected piezomagnetic anomaly caused by a tensile

fault was calculated (Fig. 5). The geometry and the

Fig. 6. Expected piezomagnetic field change produced by the 27 October 2

a left-lateral strike-slip fault. The geometry and physical properties are re

slip of the fault have the same form as that determined

by earthquake distributions and surface fracturing

[19]. Secondly, growth of another dyke up to cross the

Pernicana fault system gave rise to rapid increase in

the total intensity at DGL, while it also contributed to

gradual increase at PDN. The piezomagnetic field

change due to two typical faults was then computed

(Fig. 6): the former is a tensile fault due to intrusive

magma, whose volume was estimated to be in the

order of lava volume emitted, and the latter is a left-

lateral strike-slip fault corresponding to the displace-

ment of the westernmost Pernicana fault system. Data

and model prove that the strike-slip fault (Pernicana

fault) does not affect the local magnetic field at both

stations, because it was located rather far away and its

dislocation was relatively small. The fault parameters

together with estimates of rock magnetization are

002 eruptive fissure system in the North flank, i.e. a tensile fault and

ported in Table 1.


listed in Table 1. The calculated values from the two

models are in good agreement with the observed local

magnetic field data (Figs. 5 and 6).

4. Magnetic timetable

Geomagnetic time sequences encompassing the

onset of the 2002 NE-Rift eruption (Mt. Etna) were

Fig. 7. Plot of 1-min means of total field (a) and seismic tremor (b

acquired at two stations very close to the eruptive

fissures, and above all with very high sampling

frequency (0.1 Hz), unusual for volcanomagnetic

monitoring. Fig. 7 shows the 1-min means of total

intensity variations at all stations between 26 and 28

October 2002, after removal of the external noise by

using adaptive filters. The magnetic data not only

allow the timing of the intrusive event to be described

in greater detail but also, together with other volcano-

; after Barberi et al. [18]) between 26 and 28 October 2002.


logical and geophysical evidences, permit some

constraints to be set on the characteristics of prop-

agation of a shallow dike. In the following, we use the

timetable of magnetic events to describe step-by-step

the actual temporal sequence of dike intrusion along

the NE rift in the early hours on 27 October 2002.

Until at 1:00 local time, no significant changes were

observed at all magnetic stations. At this time, magma

was rapidly injected just below the summit craters to

large depth, more than 3–4 km bsl. No variations were

predicted at the two sites, DGL and PDN. In fact, the

dike intrusion is too deep and far from them to record

appreciable changes at these stations.

After 1:00 local time, the first variation of

magnetic field is seen at PDN, the station nearest to

central craters. The decrease of the total intensity at

PDN began when the seismic tremor reached the

maximum amplitude (Fig. 7b). While continued

intrusion magma occurred upward, vertical fractures

and perhaps faults developed around the growing dike

top in the subsurface at the northern base of the NE

crater. At DGL station, located about 2.5 km north-

ward, the first magnetic change was observed about

70 min later.

After 2:28 local time, the magnetic activity at PDN

reversed soon at high rate toward the level observed

before the seismic swarm, while the seismic tremor

showed an abrupt decrease (Fig. 7b) and the seismic

events migrated northeastward. We mark the opening

of the short eruptive fissure at the northern base of the

NE crater at about 3000 m a.s.l. at this time. The

magnetic offset at the close PDN station is ascribed to

the piezomagnetic effect due to a rapid mechanical

event.

At 3:50 local time, a rapid transient variation of

more than 2 nTwas recorded at the DGL station, while

the transient at PDN has an amplitude of �0.6 nT (by

readings at 10 seconds). The magnetic transients are

coincident with one of the largest earthquakes,

magnitude Md=4.2, recorded during the seismic

swarm preceding and accompanying the opening of

fractures on the volcano. This earthquake was located

roughly midway between DGL and PDN, with focal

depth 3.4 km. The magnetic changes related to seismic

activity were reversible; in fact they cancel at the same

time as the fall in seismic energy release.

Right after 5:00 local time, a very strong and rapid

magnetic increase took place at DGL, and the seismic

tremor and the earthquake hourly rate started rising

again. More rapid volcanomagnetic variations are

well associated with rapid modifications of the stress

field induced by magma that reaches the ground

surface through volcanic vents and fissures. We

located the onset of the eruption along the NE rift

at this time, when the first eruptive fissure opened at

2500 m a.s.l. producing a lava flow that spread in the

direction of Piano Provenzana (Fig. 1). The offsets in

magnetic time series occurred in different time points

spaced approximately 2.5 h apart, near 2:28 at PDN

located at 2783 m a.s.l. and around 5:00 at DGL

situated at 2372 m a.s.l.

After 8:00 local time, geomagnetic total intensity at

both stations turned almost flat at the higher level

(Fig. 7a), while new lava flows outpoured from the

lower fractures (at 2150 and 1900 m) and expanded

on the north flank of the volcano. These events were

associated with a new local maximum of the seismic

tremor (Fig. 7b) and the earthquake hourly rate. Since

the eruption started and the stress field became stable,

the induced magnetic changes were permanent at both

sites. The modifications of the local magnetic field

lasted longer than duration of the seismic swarm and

of the NE rift eruption (Fig. 3).

In short, the rate of growth of the magnetic

anomalies allows describing how the eruption

evolved on NE flank of Etna. The basic features

were inferred by examining resemblances and differ-

ences in the characteristics of magnetic anomalies at

two sites. The similar characteristics (the shape and

the amplitude) point to a common geophysical

mechanism as source of the anomalies. On the other

hand, the differences in the anomalies (the phase shift

and the duration) are useful to determine the changes

of source positions. The anomaly at DGL has a delay

of 70 min, but it lasts about 2.5 h more than PDN.

This behaviour is a clear indication that the anoma-

lies are generated by a progressive intrusion of

dykes. The time interval between the reverses at

PDN and DGL amounting to 2.5 h (Fig. 7a) could

represent the time taken by magma to reach the

ground surface through the first eruptive fissure

opened at the base of the NE Crater and the

subsequent eruptive fissures field activated along

the NE rift spaced about 2.2 km apart. The 2.5-h time

delay simply indicates the time interval until the

intrusion of a new dyke starts.


5. Concluding remarks

The early stages of 2002–2003 lateral eruption at

Mt. Etna were accompanied by local magnetic field

changes recorded at the continuously operating Over-

hauser effect magnetometers set up on the volcanic

edifice. When the natural magnetic fluctuations are

removed using the adaptive filtering, no noticeable

precursory long-term signal is detected before the

eruption, while conspicuous short-time changes

anticipated and accompanied the eruptive fissure

openings. The comparison between magnetic data,

seismicity and surface phenomena imply that the

piezomagnetic effect is the primary physical mecha-

nism driving these anomalies. The position and the

geometry of sources, which could explain the total

intensity changes at each stage, were estimated. In

general, our data indicate that magnetic networking is

an efficient tool for detecting, locating, and modeling

even a narrow dike. This exceptional set of observa-

tions provided valuable information on what hap-

pened at a rather shallow depth beneath the volcano.

The various sources within the volcano of magnetic

changes at each stage suggest the most likely scenario

occurring during the eruption. Firstly, at around

midnight on 26 October, magma was rapidly injected

to a depth of 3–4 km just below the summit craters.

Secondly, after 1:00 on 27 October, continued

intrusion magma occurred upward and culminated a

few hundred meters below the free surface fractured

along a N–E direction. Thirdly, at about 2:28 magma

gave rise to an explosive fissural vent at the northern

base of the NE crater near 3000 m a.s.l. Finally, at

about 5:00, the first eruptive fissure became active

along the eastern border of the NE rift at 2500 m a.s.l.

The rate of growth of the magnetic anomalies,

moreover, leads to the interpretation that the magmatic

intrusion travelled northward from base of the NE

crater to the NE rift at approximately 14 m/min.

Acknowledgments

We are indebted to all personal of Geomagnetism

Laboratory of National Institute of Geophysics and

Volcanology (Catania Section) who ensure the regular

working of the permanent magnetic network. Thanks

are due to Paolo Palangio for making the data of

L’Aquila magnetic observatory available. We are

grateful to Yochi Sasai for constructive and helpful

comments that improved greatly the manuscript. An

anonymous reviewer also gave valuable comments.

This research was carried out in the frame of the

project EPOT of the Gruppo Nazionale per la

Vulcanologia of the INGV.

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Volcanomagnetic changes accompanying the onset of the 2002–2003 eruption of Mt. Etna (Italy

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