STRESS FIELD AND SUMMIT ERUPTION OF MT ETNA 251
GEOLOGICA CARPATHICA, 54, 4, BRATISLAVA, AUGUST 2003
CORRELATION BETWEEN LOCAL STRESS FIELD AND SUMMIT
ERUPTION OF MOUNT ETNA: THE 27 MARCH 1998 EVENT
SANTO LA DELFA
, VALERIA INNOCENTE
, GIUSEPPE PATANÈ
and JEAN CLAUDE TANGUY
Dipartimento di Scienze Geologiche, Università di Catania, Italy
Université de Paris 6 & IPGP, Observatoire de Saint-Maur, France
Corresponding Author: Giuseppe Patanè, Dipartimento di Scienze Geologiche, Università di Catania, Corso Italia 55,
95129 Catania, Italy; email@example.com
(Manuscript received February 28, 2002; accepted in revised form March 11, 2003)
Abstract: The eruption on 27 March 1998 at the NE Crater, from 21h22 to 23h59 GMT, with Strombolian explosions
and lava fountains, was the first of a long series of eruptive manifestations with relatively high energy, which affected
the volcano after the 19911993 lateral event. The seismic activity occurred late in 1997 and the first months of 1998
constitutes, in the authors opinion, a precursor of this eruption and the successive eruptive events at the SE Crater, since
it is believed to be associated with a conspicuous magma uprise. The continuous epicentre migration, in a brief time span,
in various sectors of the volcano owing to the dynamics of different tectonic trends, is considered related to a rapid
fluctuation in the local stress field, probably responsible for the sudden onset and rapid end of the eruptive event.
Key words: stress field, focal mechanisms, earthquakes, eruption, Mt Etna.
Etna is a volcano, circa 3300 m a.s.l., with a 40 km mean di-
ameter, located on the convergence between the African and
Eurasiatic plates. Its eruptive behaviour has evolved over the
last 500 ka, from a prevalently fissure type eruptive activity to
a central type one (Chester et al. 1985; Kieffer 1985; Tanguy
et al. 1997).
Monitoring the eruptive phenomena and the morphological
variations of the summit craters, was carried out carefully
from the second decade of the 20th century on; in fact, vulca-
nologists understood that the eruptive activity at the Central
Crater (where both the Voragine Centrale or Chasm and the
Bocca Nuova are located) and the NE Crater (Fig. 1), in many
cases, represented precursors of very high energy volcanic
phenomena, like the lateral eruptions.
Monitoring of seismic activity began much later, in 1967
(Bottari & Riuscetti 1967). The data quality was improved in
time and good reliability has been achieved since 1977. Most
of the data collected over the last 20 years, allowed the work-
ing out of various approaches, including statistical ones, to
studiyng the relationships between earthquakes and eruptions.
On the basis of statistics, Sharp et al. (1981) found an impor-
tant correlation between lateral eruptions and earthquakes. In-
stead, Gasperini et al. (1990) stated a contrasting opinion.
These researchers, by applying an algorithm developed by
Gasparini & Mulargia (1989) to the sequences of earthquakes
and eruptions in the period 19781987, concluded that there
is no correlation between lateral or summit events and seis-
According to Cardaci et al. (1993), the summit eruptions are
preceded by an increase in the number of low-frequency
events and seismic transients with spectral characteristics
equal to those of tremors; the source of low-frequency events
(spindle) is associated by Montalto et al. (1992) to slug break-
ing at the top of the magma column, which generates impul-
sive type pressure.
Cosentino et al. (1989) attempted to characterize the summit
and lateral eruptions from a seismological point of view.
These researchers conclude that there is no effective change in
seismicity close to an eruptive event at the summit craters, but
rather that the latter is the precursor of lateral eruptive activity,
an observation carried out by vulcanologists in the first half of
the 20th century only on the basis of the development of vol-
canic phenomena. Lastly, Vinciguerra et al. (1999), studying
the relationship between the main phases of seismic energy re-
lease and the principal eruptive episodes between 1977 and
1991, conclude that summit eruptions usually take place with-
out any preceding sudden change in seismic patterns. Howev-
er, the modest seismic activity some months before the onset,
suggests that the magma rises slowly, stopping temporarily in
a shallow magmatic batch, before the eruptive event.
This work considers the correlation between seismic and
eruptive activity occurring at the NE Crater on March 27th
1998, through a detailed analysis of the spatial and temporal
fluctuations in seismic activity and the local stress field.
This eruptive phenomenon, as well as the other eruptions at
the NE Crater, was not immediately preceded by considerable
seismicity. Nevertheless, numerous earthquake swarms of
modest magnitude (MD £ 3.5) took place between October
1997 and March 1998 and affected the whole volcanic area.
A seismic activity maximum was recorded during January
1998 and, in the authors opinion, was associated with magma
uprise, which caused the eruptive event at the NE Crater and
the volcanic manifestations at the other summit craters during
252 LA DELFA et al.
History of the NE Crater
The creation of the NE Crater (NEC) took place on 27 May
1911 through the non-eruptive collapse of the lower part of
the central cone, at about 3100 m elevation, 3.5 months be-
fore a lateral eruption on the NNE flank of the mountain (Ric-
cò 1911, see details in Tanguy 1981). During the following
two years, the NEC merely appeared as a steaming chasm.
Weak explosive activity was noticed for the first time on 15
July 1913 (Riccò et al. 1917). However, Strombolian explo-
sions occurred only from March 1915 onwards, either jointly
with, or independently of, similar activity at the Central Cra-
ter. On 24 June 1917 an 800 m high lava fountain gushed
forth for twenty minutes from the NEC, but the first sluggish,
«subterminal» lava flows occurred in MarchJuly 1918 (Tan-
guy & Patanè 1996). Strombolian explosive activity was re-
newed in June 1922 and a typical lava effusion in May-June
1923 preceded another lateral eruption on the NNE flank. On
the other hand, the 2 November 1928 eruption (ENE flank)
was heralded by only one hour of explosive activity from the
NEC, and both NEC and Central Crater showed intermittent
lava fountains in the weeks before the 1947 eruption on the
Conversely, and although intermittent weak explosions oc-
curred there between 1931 and 1941, no activity from the
NEC happened during the 1942 eruption on the Southern
flank, nor during the 1949 one (S, N and NW flanks) or dur-
ing the large Eastern flank 195051 eruption (125´10
From 1955 to 1971, the NEC was in almost continuous, or
«persistent» explosive and effusive activity, producing more
of flows that covered the Northern and
Eastern summit area, sometimes in connection with strong ex-
plosions at the Central Crater (e.g., 1960, 1964). However, no
lateral eruption occurred before 1971 (Southern and NE
flanks), when all activity at the NEC had already ceased (Tan-
guy & Patanè 1996).
Explosions resumed at the NEC on 28 September 1974, fol-
lowed by lava outflows and, in February 1975, by opening of a
new fissure on the Northern flank (Kieffer et al. 1975). Alter-
nation of lateral and subterminal (NEC) activity lasted in this
region until January 1977. On 16 July of the same year, a se-
ries of strong lava fountains began and very fast outflows oc-
curred at irregular intervals (a few days to several weeks) be-
tween July 1977 and March 1978, no connection being evident
with the 1978 (AprilJune, August, November) and 1979 later-
al eruptions (SE and NE flanks). Paroxysms similar to those of
the 197778 period were renewed at the NEC in September
1980 and February 1981, perhaps heralding the violent March
1981 eruption on the NNW flank (Tanguy & Patanè 1984).
Then the NEC was quiet for several years, although other later-
al eruptions occurred in 1983 and 1985 (S and E flanks).
In September 1986, a renewal of subterminal explosive and
effusive activity at the NEC eventually culminated in a violent
phreatomagmatic paroxysm (on the 24th), perhaps related to
the opening of new fissures leading to the November 1986
February 1987 Eastern flank eruption. Then the NEC was dor-
mant for a long period, although large lateral eruptions oc-
curred in 1989 (NE flank) and 199193 (SE flank, this latter
, Calvari et al. 1994).
In recent years, the activity of the NEC was rather sporadic,
alternating between quiet long-lasting degassing and violent
paroxysms, usually of a few hours, namely in 1995 and 1996.
A peculiar effusive event was a lava outflow cascading within
the neighbouring Voragine of the Central Crater (21/7 to 19/8
1996). Since then, only occasional explosions have occurred
Fig. 1. 3D schematic picture of the summit craters.
STRESS FIELD AND SUMMIT ERUPTION OF MT ETNA 253
from time to time, as in October 1999. Finally, continuous
abundant degassing took place at the NEC before, during and
after the July 2001 lateral eruption (Tanguy et al. 2001).
The lava fountaining of 27 March 1998 was one of the last
significant events of the NEC. After moderate and transient
explosive activity in the early afternoon on this day, Strombo-
lian jets started again at about 21h30 GMT (22h30 Local
Time) and evolved into a lava fountain between 22h50 and
23h50 GMT, accompanied by two phases of unusually high
amplitude tremor. Strong detonations were heard all around
the volcano as large lava blocks and scoriae were thrown 300
to 400 m above the crater rim, but relatively little material was
found outside the large chasm. During this paroxysmal event,
the other summit craters (especially Bocca Nuova and SE Cra-
ter) continued their moderate magmatic activity without sig-
nificant change (Tanguy et al. 1999).
As a conclusion to this brief summary, it seems that the
NEC, although clearly related to the lateral eruptions of 1911
and 1923 and, to a lower degree, than the 1928 and 1947 one,
subsequently evolved as an independent summit vent. This is
particularly true since the appearance of the Bocca Nuova (or
West Chasm of the Central Crater) in 1968, and that of the
new subterminal South-East Crater in 197178.
Seismic activity before the 27th March 1998 eruptive event
at NE Crater prevalently affected the top of the volcano and,
only moderately, the other sectors.
The local magnitudes of the events are in the range 2 to 3.
Earthquakes were recorded by University of Catania seismic
stations and by the Seismological Observatory of Acireale.
Seismic signals are acquired by 1 Hz 3D digital seismome-
ters located in Adrano (ADR) and Acireale (ACR) and by
1 Hz single-component seismometers in S. Venerina (SVN),
Pennisi (PNS), S. Leonardello (SND), S. Alfio (CSA), and
Mount Pomiciaro (PMC), in the eastern sector of Mount Etna,
and Serra La Nave (SLN), Pedara (PDR) and Torre del
Filosofo (TDF), in the southern sector (Fig. 2).
A selection of the data has been performed by considering
the earthquakes recorded at a minimum number of 7 stations
and the standard errors of the coordinates ERH, ERZ £ 3 km
and RMS < 0.3 sec. This selection meant a dramatic reduction
in the number of the events, from more than a thousand to a
few hundred. The hypocentre location was performed by the
calculation program Hypo71 (Lee & Lahar 1975) and the ve-
locity model used is that of Hirn et al. (1991).
The seismic activity, in the considered period (October
1997March 1998), is located in the summit area, in the east-
ern, western and southern sectors. In October and in Novem-
ber, the epicentre distribution is associated with the NE-SW,
NNE-SSW and NW-SE structural trends and also affected the
middle-low altitudes (Fig. 3a,c). In December the seismic ac-
tivity decreased, and was located at middle-high altitudes with
two evident trends, NE-SW and NNW-SSE, (Fig. 3e). In Jan-
uary 1998 about 500 events were recorded, only 138, with
MD > 2.0, were located and affected almost the whole volca-
Fig. 2. Mt Etna map: location and names of the seismic stations.
The inset on the upper-left corner offers a schematic picture of the
nic area (Fig. 4a). Two main structural trends were associated
with the seismic swarms: NNE-SSW and WNW-ESE. In the
following months, February and March, the seismicity de-
creased and the structural trends involved were NE-SW, NW-
SE and NNE-SSW, NNW-SSE respectively (Fig. 4c,e). The
vertical sections (Figs. 34: b,d,f) show that the hypocentres
were distributed in a depth range between 0 and 24 km; a re-
duction of the seismic foci density was observed below
12 km. The highest focal depths are to be found in the western
sector and in relation to the main axis of the magma uprise; the
highest hypocentre density was located at a depth lower than
Similar eruptions occurred between July 1977 and March
1978 at the NE Crater (Vinciguerra et al. 1999), and seismic
activity preceded the 27th March 1998 eruption by a few
months. In fact, in January about 500 earthquakes with
MD < 3.5 (Fig. 5) occurred over the whole volcanic area
along the NNE-SSW regional tectonic trend and the WNW-
ESE volcano-tectonic one (Romano 1982).
Similar considerations were made on occasion of other sum-
mit phenomenologies (Tanguy & Patanè 1984; Cosentino et
al. 1989); this may mean that the source mechanisms of erup-
tive phenomena are substantially unchanged over time.
Between January and April 1998, the macroseismic activity
with intensity Io
IVV (M.S.K.), mainly affected the south-
west and southern sectors of the volcano (Fig. 6). The mesos-
eismic areas of some events are prevalently aligned in a NE-
SW direction (events No. 1 and 2), in agreement with the
orientations of the eruptive fractures opened up in the NE Cra-
ter and along the NW-SE structural trend (events 4 and 5).
Fig. 6 shows all the macroseisms occurring in 1998; all these
events show the particular dynamism of the southern sector of
Etna, where all the summit eruptions following the NE one
254 LA DELFA et al.
and the eruption of July 2001 took place (La Delfa et al.
By using the earthquake data collected during March, 19 re-
liable Fault-Plane Solutions (F.P.S.) have been evaluated (Ta-
ble 1 and Fig. 7a). As can be seen from this figure, 9 events
show a focal mechanism linked mainly to thrust faulting and
only 6 of them show mechanisms connected with normal
faulting (Fig. 7b); 2 events are linked to strike-slip faults and
lastly, only 2 of them show a sub-vertical focal plane.
Generally, it can be observed that the Fault-Plane Solutions
(F.P.S.) are in good agreement with the outcropping structures
(Fig. 7a,b; Fig. 8); in some cases this correspondence seems
ambiguous, in particular in the sites where faults and fractures
outcrop with variable orientations; lastly, there is a complete
lack of correspondence for the F.P.S. No. 3, 7 and 15.
From the comparison between the focal mechanisms and
the tectonic structures outcropping in the area it may be seen
that the most active structural trend is the NNW-SSE one
(events No. 2, 4, 11, 12, 13, 14, 15, 18, 19). The WNW-ESE
(events No. 3, 5, 7, 8, 17), NNE-SSW (events No. 1, 6, 10)
and ENE-WSW (events No. 9 and 16) trends are less active,
at least in the studied period.
The eruption at the NE Crater is associated with the open-
ing of an eruptive fracture, oriented circa NE-SW (Fig. 9a),
the distribution of the azimuthal direction of the P-axes fluctu-
ates prevalently around an average value of roughly 45°50°
Fig. 3. Maps indicating the seismic activity at Mt Etna in October, November and December 1997: (a, c, e) epicentres and principal
trends (inside delimitate areas); (b, d, f) hypocentres.
STRESS FIELD AND SUMMIT ERUPTION OF MT ETNA 255
Fig. 4. Maps indicating the seismic activity at Mt Etna in January, February and March 1998: (a, c, e) epicentres and principals trends
(inside delimitate areas); (b, d, f) hypocentres.
(Fig. 9b), in agreement with the direction of the same fracture.
In particular, during the third week of March, the orientation
of the P-axis is very close to the above mentioned discontinui-
ty (Fig. 7a: F.P.S. Nos. 17, 18, 19; Fig. 9b).
Discussion and conclusions
It is known that seismic energy release before an eruptive
event, during the final phase and after its end, can give differ-
ent information about the origin and evolution of the eruptive
event (Yokoyama 1988; Patanè et al. 1996; La Delfa et al.
Fig. 5. Cumulative curve concerning the earthquakes recorded in
the period between October 1997 and April 1998; 1 seismic
swarm in January 1998; 2 NE Crater eruption (March 27, 1998).
Table 1: Focal parameters of main earthquakes during March 1998. A.P. pressure axis; A.T. tension axis; P.A. and P.B. are the nodal planes from fault-plane solutions.
ELFA et al.
STRESS FIELD AND SUMMIT ERUPTION OF MT ETNA 257
Similarly to the eruptions at the NE Crater in 19771978,
no seismic event of significant energy occurred shortly before
the 27 March 1998 eruption.
This eruption, like those in the second half of 1998 at the
SE Crater (La Delfa et al. 2001), justifies the hypothesis that
the magma migration towards higher crustal levels, began be-
tween the second half of 1997 and the first half of 1998, a
time interval within which numerous seismic swarms took
The hypocentres affected both the surface crustal as well as
relatively deeper layers, constituted by the carbonate units
cropping out to the south in the Iblean area (Cristofolini et al.
1979). In such a way one may hypothesize that the volcanic
phenomenon at the NE Crater, albeit to a modest degree, rep-
resents the manifestation of a dynamism, associated with a
very unstable stress field, which affects a large crustal thick-
ness. Cocina et al. (1997) have arrived at the same consider-
ations; according to these researchers, the trend of the stress is
rather heterogeneous in the Etnean crust eastwards from the
fifteenth meridian which is the eastern sector of the volcano
where the eruptive phenomenon of the NE Crater is located.
The analysis of the causes about the stress field and its vari-
ability in the Etnean area, is still a matter of debate among re-
searchers, since it is not well defined if the convergence be-
tween the African and European plates is still active and what
is the role of volcanism about the mentioned stress field.
Fig. 6. Map indicating the mesoseismic areas of some earthquakes
during 1998. The macroseismic scale used is MSK. 1 01.10.1998,
08h46m (GMT), I
IVV; 2 03.02.1998, 06h10m, 06h27m
III; 3 03.02.1998, 21h10m (GMT), I
03.03.1998, 21h21m (GMT), I
IVV; 5 04.01.1998, 07h29m
IV; 05.22.1998, 11h54m (GMT), I
21h24m (GMT), I
IV; 07.22.1998, 14h19m (GMT), I
6 08.27.1998, 06h59m (GMT), I
IVV; 7 10.26.1998,
11h54m (GMT), I
IV; 8 11.03.1998, 15h26m (GMT), I
9 12.05.1998, 04h38m (GMT), I
V (La Delfa et al. 2001).
Fig. 7. (a) Fault-plane solution plots of the main earthquakes
which occurred during March 1998 and (b) relative enumerated
epicentres. The numbers inside the squares relate to the normal
fault-plane solutions (see Table 1 for the list of focal parameters
associated with each event).
However the researches carried out in the last thirty years
(Cristofolini et al. 1979; Sharp et al. 1980; Tanguy et al. 1997,
to quote some of them) have underlined the mantle uprising at
the deep crust correspondence, which may be a component re-
sponsible for the stress field and therefore for the magma up-
rising towards the surface. Furthermore the south-vergences
nappe chain outcropping in the North of the volcano, with
clear gravitate balance loss, could represent another compo-
nent of the tangential type stress field, with North-South ori-
entation, acting on Etna. The intensity and direction of time
variation about these two strengths combination could be re-
sponsible for the local stress field fluctuation and therefore the
volcano geodynamics having different time scale (La Delfa et
In particular, the main results obtained in this research, may
be listed as follows:
258 LA DELFA et al.
a The eruptive style of the NE Crater on 27 March 1998
does not differ from that observed in this century many times.
a The epicentral pattern preceding the eruptive phase of this
volcano suggests that the terminal activity is related in a more
complex way to the seismic activity. Analysis of seismicity
between October 1997 and March 1998 in fact shows that be-
fore the eruption at the NE Crater, numerous earthquake
swarms affected all sectors of Etna along different tectonic
and volcano-tectonic trends; however, most seismic events are
located in the summit area and the eastern sector (Figs. 34:
a,c,e), where the crust shows the greatest degree of fracturing
Fig. 8. Structural map of Mt Etna (from Romano et al. 1979, modified). This map emphasized faults (1), fractures (2) and P-axis (3). The
numbers represent the progressive order of the associated F.P.S.
In addition, in the studied period there were two seismic
peaks, in October 1997, before a moderate eruptive activity at
the SE Crater begun in November, and in January.
All these considerations allow us to conclude that the mag-
ma uprise is associated with a marked crustal deformation in
the volcanic area; however, the release of elastic energy is
higher in the more brittle zones.
a The migration of the magma to the surface is slowed
down by a notable hysteresis of the crust, probably deter-
mined by a non-favourable stress field; in fact the time inter-
val separating the summit eruptive event from seismic parox-
ysms is generally in the order of a reduced number of months.
STRESS FIELD AND SUMMIT ERUPTION OF MT ETNA 259
and following the eruption at the SE Crater in 1984, have fur-
ther confirmed these hypotheses.
The results in this work demonstrate a notable analogy with
those obtained by these last authors. In fact, the opening up of
eruptive fractures oriented circa NE-SW, accords well with
the orientation of the P-axis, which shows the same direction
at least during the last ten days of March (F.P.S. Nos. 17, 18,
19). The instability of the local stress field was most likely re-
sponsible for the brief duration of the eruptive phenomenon.
Acknowledgments: The authors wish to thank the referees
and particularly Dr. M. Ripepe for the improvement of the
early version of the paper. This work was carried out by
means of 60 % MURST grants.
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260 LA DELFA et al.
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