Intrusion of eccentric dikes: The case of the 2001 eruption and its role in the dynamics of Mt. Etna volcano

The 2001 eruption represents one of the most studied events both from volcanological and geophysical point
of view on Mt. Etna. This eruption was a crucial event in the recent dynamics of the volcano, marking the
passage from a period (March 1993–June 2001) of moderate stability with slow, continuous flank sliding and
contemporaneous summit eruptions, to a period (July 2001 to present) of dramatically increased flank
deformations and flank eruptions. We show new GPS data and high precision relocation of seismicity in
order to demonstrate the role of the 2001 intrusive phase in this change of the dynamic regime of the
volcano. GPS data consist of two kinematic surveys carried out on 12 July, a few hours before the beginning of
the seismic swarm, and on 17 July, just after the onset of eruptive activity. A picture of the spatial distribution
of the sin-eruptive seismicity has been obtained using the HypoDD relocation algorithm based on the
double-difference (DD) technique. Modeling of GPS measurements reveals a southward motion of the upper
southern part of the volcano, driven by a NNW–SSE structure showing mainly left-lateral kinematics. Precise
hypocenter location evidences an aseismic zone at about sea level, where the magma upraise was
characterized by a much higher velocity and an abrupt westward shift, revealing the existence of a weakened
or ductile zone.
These results reveal how an intrusion of a dike can severely modify the shallow stress field, triggering
significant flank failure. In 2001, the intrusion was driven by a weakened surface, which might correspond to
a decollement plane of the portion of the volcano affected by flank instability, inducing an additional stress
testified by GPS measurements and seismic data, which led to an acceleration of the sliding flanks.

This work was funded by the Istituto Nazionale di Geofisica e
Vulcanologia and by the Dipartimento per la Protezione Civile (Italy).

Acocella, V., Neri, M., 2003. What makes flank eruptions? The 2001 Etna eruption and
its possible triggering mechanism. Bull. Volcanol. 65, 517–529. doi:10.1007/
s00445-003-0280-3.
Acocella, V., Neri, M., 2005. Structural features of an active strike-slip fault on the sliding
flank of Mt. Etna (Italy). J. Struct. Geol. 27, 343–355. doi:10.1016/j.jsg.2004.07.006.
Acocella, V., Behncke, B., Neri, M., D, Amico, S., 2003. Link between major flank slip and
eruptions at Mt. Etna (Italy). J. Geophys. Res. 30, 2286. doi:10.1029/2003GL018642.
Allard, P., Behncke, B., D, Amico, S., Neri, M., Gambino, S., 2006. Mount Etna 1993–2005:
anatomy of an evolving eruptive cycle. Earth-Sci. Rev. 78, 85–114. doi:10.1016/j.
earscirev.2006.04.002.
Andronico, D., Branca, S., Calvari, S., Burton, M., Caltabiano, T., Corsaro, R.A., Del Carlo, P.,
Garfì, G., Lodato, L., Miraglia, L., Murè, F., Neri, M., Pecora, E., Pompilio, M., Salerno,
G., Spampinato, L., 2005. A multi-disciplinary study of the 2002–03 Etna eruption:
insights into a complex plumbing system. Bull. Volcanol. 67, 314–330. doi:10.1007/
s00445-004-0372-8.
Behncke, B., Neri, M., 2003a. The July–August 2001 eruption of Mt. Etna (Sicily). Bull.
Volcanol. 65, 461–476.
Behncke, B., Neri, M., 2003b. Cycles and trends in the recent eruptive behaviour of
Mount Etna (Italy). Can. J. Earth Sci. 40, 1405–1411. doi:10.1139/E03–052.
Behncke, B., Neri, M., Carniel, R., 2003. An exceptional case of lava dome growth
spawning pyroclastic avalanches at Mt. Etna (Italy): the 1999 Bocca Nuova
eruption. J. Volcanol. Geotherm. Res. 124 (1–2), 115–128.
Behncke, B., Neri, M., Nagay, A., 2005. Lava flow hazard at Mount Etna (Italy): new data
from a GIS-based study. In: Manga, M., Ventura, G. (Eds.), Kinematics and dynamics
of lava flows. Spec. Pap. Geol. Soc. Am., vol. 396-13, pp. 187–205. doi:10.1130/
2005.2396(13).
Behncke, B., Neri, M., Pecora, E., Zanon, V., 2006. The exceptional activity and growth of
the Southeast Crater, Mount Etna (Italy), between 1996 and 2001. Bull. Volcanol. 69,
149–173. doi:10.1007/s00445-006-0061-x.
Behncke, B., Calvari, S., Giammanco, S., Neri, M., Pinkerton, H., 2008. Volcaniclastic flows
resulting from interaction of basaltic magma with hydrothermally altered rock: an
example from the 2006 summit eruptions of Mount Etna (Italy). Bull. Volcanol.
doi:10.1007/s00445-008-0200-7.
Billi, A., Acocella, V., Funiciello, R., Giordano, G., Lanzafame, G., Neri, M., 2003.
Mechanisms for ground-surface fracturing and incipient slope failure associated to
the July–August 2001 eruption of Mt. Etna, Italy: analysis of ephemeral field data.
J. Volcanol. Geotherm. Res. 122, 281–294. doi:10.1016/S0377-0273(02)00507-3.
Bonaccorso, A., Aloisi, M., Mattia, M., 2002. Dike emplacement forerunning the Etna July
2001 eruption modeled through continuous tilt and GPS data. Geophys. Res. Lett.
29. doi:10.1029/2001GL014397.
Bonaccorso, A., D,Amico, S.,Mattia,M., Patanè, D., 2004. Intrusivemechanisms atMt. Etna
forerunning the July–August 2001 eruption from seismic and ground deformation
data. Pure Appl. Geophys. 161, 1469–1487. doi:10.1007/s00024-004-2515-4.
Bonaccorso, A., Bonforte, A., Guglielmino, F., Palano, M., Puglisi, G., 2006. Composite
ground deformation pattern forerunning the 2004–2005 Mount Etna eruption.
J. Geophys. Res. 111, B12207. doi:10.1029/2005JB004206.
Bonforte, A., Puglisi, G., 2003. Magma uprising and flank dynamics on Mt. Etna volcano,
studied by GPS data (1994–1995). J. Geophys. Res. 108 (B3), 2153–2162.
Bonforte, A., Puglisi, G., 2006. Dynamics of the eastern flank of Mt. Etna volcano (Italy)
investigated by a dense GPS network. J. Volcanol. Geotherm. Res. 153, 357–369.
doi:10.1016/j.jvolgeores.2005.12.005.
Bonforte, A., Guglielmino, F., Palano, M., Puglisi, G., 2004. A syn-eruptive ground
deformation episode measured by GPS, during the 2001 eruption on the upper
southern flank of Mt. Etna. Bull. Volcanol. 66, 336–341.
Bonforte, A., Branca, S., Palano, M., 2007a. Geometric and kinematic variations along the
active Pernicana fault: Implication for the dynamics of Mount Etna NE flank (Italy).
J. Volcanol. Geotherm. Res. doi:10.1016/j.jvolgeores.2006.08.009.
Bonforte, A., Carbone, D., Greco, F., Palano, M., 2007b. Intrusive mechanism of the 2002
NE-rift eruption at Mt. Etna (Italy) modelled using GPS and gravity data. Geophys.
J. Int. doi:10.1111/j.1365-246X.2006.03249.x.
Bonforte, A., Gambino, S., Guglielmino, F., Obrizzo, F., Palano, M., Puglisi, G., 2007c.
Ground deformation modeling of flank dynamics prior to the 2002 eruption of Mt.
Etna. Bull. Volcanol. doi:10.1007/s00445-006-0106-1.
Bonforte, A., Bonaccorso, A., Guglielmino, F., Palano, M., Puglisi, G., 2008. Feeding system
and magma storage beneath Mt. Etna as revealed by recent inflation/deflation
cycles. J. Geophys. Res. 113, B05406. doi:10.1029/2007JB005334.
Borgia, A., Ferrari, L., Pasquarè, G., 1992. Importance of gravitational spreading in the
tectonic and volcanic evolution of Mt. Etna. Nature 357, 231–235.
Burton, M., Neri, M., Andronico, D., Branca, S., Caltabiano, T., Calvari, S., Corsaro, R.A., Del
Carlo, P., Lanzafame, G., Lodato, L., Miraglia, L., Salerno, G., Spampinato, L., 2005.
Etna 2004–2005: an archetype for geodynamically-controlled effusive eruptions.
Geophys. Res. Lett. 32, L09303. doi:10.1029/2005GL022527.
Calvari, S., Tanner, L.H., Groppelli, G., 1998. Debris-avalanche deposits of the Milo Lahar
sequence and the opening of the Valle del Bove on Etna volcano (Italy). J. Volcanol.
Geotherm. Res. 87 (1–4), 193–209. doi:10.1016/S0377-0273(98)00089-4.
Calvari, S., Neri, M., Pinkerton, H., 2003. Effusion rate estimation during the 1999 summit
eruption onMt. Etna, and growth of two distinct lava flowfields. J.Volcanol. Geotherm.
Res. 119 (1–4), 107–123.
Clocchiatti, R., Condomines, M., Guénot, N., Tanguy, J.-C., 2004. Magma changes at Mount
Etna: the 2001 and 2002–2003 eruptions. Earth Planet. Sci. Lett. 226, 397–414.
doi:10.1016/j.epsl.2004.07.039.
Cocina, O., Neri, G., Privitera, E., Spampinato, S., 1997. Stress tensor computations in the
Mount Etna area (Southern Sicily) and tectonic implications. J. Geodyn. 23,109–127.
Corsaro, R.A., Miraglia, L., 2005. Dynamics of 2004–2005 Mt. Etna effusive eruption as
inferred from petrologic monitoring. Geophys. Res. Lett. 32, L13302. doi:10.1029/
2005GL022347.
Froger, J., Merle, O., Briole, P., 2001. Active spreading and regional extension at Mount
Etna imaged by SAR interferometry. Earth Planet. Sci. Lett. 187, 245–258.
Gambino, S., 2004. Continuous dynamic response along a pre-existing structural
discontinuity induced by the 2001 eruption at Mt Etna. Earth Planets Space 56,
447–456.
Gambino, S., Mostaccio, A., Patanè, D., Scarfì, L., Ursino, A., 2004. High-precision
locations of the microseismicity preceding the 2002–2003 Mt. Etna eruption.
Geophys. Res. Lett. 31 (18), L18604 10.1029/2004GL020499.
Harris, A.J.L., Neri, M., 2002. Volumetric observations during paroxysmal eruptions
at Mount Etna: pressurized drainage of a shallow chamber or pulsed supply?
J. Volcanol. Geotherm. Res. 116, 79–95.
Houlié, N., Briole, P., Bonforte, A., Puglisi, G., 2006. Large scale ground deformation of
Etna observed by GPS between 1994 and 2001. Geophys. Res. Lett. 33, L02309.
doi:10.1029/2005GL024414.
Lanzafame, G., Neri, M., Acocella, V., Billi, A., Funiciello, R., Giordano, G., 2003. July–
August 2001 Etna eruption: deformative pattern and its significance. J. Geol. Soc.
London 160, 531–544.
Lundgren, P., Casu, F., Manzo, M., Pepe, M., Berardino, P., Sansosti, E., Lanari, R., 2004.
Gravity and magma induced spreading of Mount Etna volcano revealed by satellite
radar interferometry. Geophys. Res. Lett. 31, L04602. doi:10.1029/003GL018736.
Métrich, N., Allard, P., Spilliaert, N., Andronico, D., Burton, M., 2004. 2001 flank eruption
of the alkali- and volatile-rich primitive basalt responsible for Mount Etna's
evolution in the last three decades. Earth Planet. Sci. Lett. 228, 1–17. doi:10.1016/j.
epsl.2004.09.036.
Neri, M., Acocella, V., 2006. The 2004–05 Etna eruption: implications for flank
deformation and structural behaviour of the volcano. J. Volcanol. Geotherm. Res.
158, 195–206. doi:10.1016/j.jvolgeores.2006.04.022.
Neri, M., Acocella, V., Behncke, B., 2004. The role of the Pernicana fault system in the
spreading of Mt. Etna (Italy) during the 2002–2003 eruption. Bull. Volcanol. 66,
417–430. doi:10.1007/s00445-003-0322-x.
Neri, M., Acocella, V., Behncke, B., Maiolino, V., Ursino, A., Velardita, R., 2005. Contrasting
triggering mechanisms of the 2001 and 2002–2003 eruptions of Mount Etna (Italy).
J. Volcanol. Geotherm. Res. 144, 235–255. doi:10.1016/j.jvolgeores.2004.11.025.
Neri, M., Behncke, B., Burton, M., Giammanco, S., Pecora, E., Privitera, E., Reitano, D.,
2006. Continuous soil radon monitoring during the July 2006 Etna eruption.
Geophys. Res. Lett. 33, L24316. doi:10.1029/2006GL028394.
Neri, M., Guglielmino, F., Rust, D., 2007. Flank instability on Mount Etna: radon, radar
interferometry and geodetic data from the southern boundary of the unstable
sector. J. Geophys. Res. 112. doi:10.1029/2006JB004756.
Okada, Y., 1985. Surface deformation due to shear and tensile faults in a half-space.
J. Geophys. Res. 75, 1135–1154.
Pareschi, M.T., Boschi, E., Favalli, M., 2006. Lost tsunami. Geophys. Res. Lett. 33, L22608.
doi:10.1029/2006GL027790.
Patanè, D., Chiarabba, C., De Gori, P., Bonaccorso, A., 2003a. Magma ascent and the
pressurization of Mt. Etna's volcanic system. Science 299, 2061–2063.
Patanè, D., Privitera, E., Gresta, G., Akinci, A., Alparone, A., Barberi, G., Chiaraluce, L.,
Cocina, O., D, Amico, S., De Gori, P., Di Grazia, G., Falsaperla, S., Ferrari, F., Gambino,
S., Giapiccolo, E., Langer, H., Maiolino, V., Moretti, M., Mostaccio, A., Musumeci, C.,
Piccinini, D., Reitano, D., Scarfì, L., Spampanato, S., Ursino, A., Zuccarello, L., 2003b.
Seismological constraints for the dike emplacement of July–August 2001 lateral
eruption at Mt. Etna volcano, Italy. Ann. Geophys. 46, 599–608.
Puglisi, G., Bonforte, A., 2004. Dynamics of Mount Etna volcano inferred from static
and kinematic GPS measurements. J. Geophys. Res. 109 (B11), B11404. doi:10.1029/
2003JB002878.
Puglisi, G., Bonforte, A., Maugeri, S.R., 2001. Ground deformation patterns on Mt. Etna,
between 1992 and 1994, inferred from GPS data. Bull. Volcanol. 62, 371–384.
Puglisi, G., Bonforte, A., Ferretti, A., Guglielmino, F., Palano, M., Prati, C., 2008. Dynamics
of Mount Etna before, during, and after the July-August 2001 eruption inferred from
GPS and differential synthetic aperture radar interferometry data. J. Geophys. Res.
113, B06405. doi:10.1029/2006JB004811.
Rittmann, A., 1973. Structure and evolution of Mount Etna. Phil. Trans. Radnors. Soc.,
London 274 A, 5–16.
Rust, D., Behncke, B., Neri, M., Ciocanel, A., 2005. Nested zones of instability in the Mount
Etna volcanic edifice, Sicily. J. Volcanol. Geotherm. Res. 144, 137–153. doi:10.1016/j.
jvolgeores.2004.11.021.
Thurber, C.H., 1993. Local earthquake tomography: velocity and VP/VS-theory. In: Iyer,
H.M., Hirahara, K. (Eds.), Seismic Tomography: Theory and Practice. Chapman and
Hall, London, pp. 563–583.
Van Wyk de Vries, B., Self, S., Francis, P.W., Keszthelyi, L., 2001. A gravitational spreading
origin for the Socompa debris avalanche. J. Volcanol. Geotherm. Res. 105, 225–247.
Voight, B., Glicken, H., Janda, R.J., Douglas, P.M., 1981. Catastrophic rockslide avalanche
of May 18. In: Lipman, P.W., Mullineaux, D.R. (Eds.), The 1980 Eruptions of Mount St.
Helens, Washington. Geol Surv Prof Pap, vol. 1250, pp. 347–377.
Waldhauser, F., 2001. HypoDD: A computer program to compute double-difference
earthquake locations. U.S. Geol. Surv. Open-File Report , 01-113.Menlo Park, California.
Waldhauser, F., Ellsworth, W.L., 2000. A double-difference earthquake location
algorithm: method and application to the northern Hayward fault. Bull. Seismol.
Soc. Am. 90, 1353–1368.
Walter, T.R., Acocella, V., Neri, M., Amelung, F., 2005. Feedback processes between
magmatism and E-flank movement at Mt. Etna (Italy) during the 2002–2003
eruption. J. Geophys. Res. 110, B10205. doi:10.1029/2005JB003688.