by Florence ReSeT
Nicola Casagli (Florence University, Italy) ・ Filippo Catani (Florence University, Italy) ・ Chiara Del Ventisette (Florence University, Italy) ・ Letizia Guerri (Florence University, Italy) ・ Dario Tarchi (European Commission, Italy) ・ Joaquim Fortuny (European Commission, Italy) ・ Giuseppe Antonello (European Commission, Italy) ・Davide Leva (Ellegi-LiSALab s.r.l., Italy) ・ Carlo Rivolta (Ellegi-LiSALab s.r.l., Italy)
Stromboli volcano (Italy) is characterized by a typical
“Strombolian activity” which consists of very low... more
Stromboli volcano (Italy) is characterized by a typical
“Strombolian activity” which consists of very low energy
explosions, every 10-15 minutes. In December 2002 an
eruption caused a landslide on the NW slope of the volcano
(Sciara del Fuoco slope, SdF) and produced a tsunami.
Concerns over the possibility of further slope collapses of the
SdF led to the set up of a permanent monitoring system of
ground deformations.
The ground-based radar interferometer (GB-InSAR)
system installed on the Stromboli Island (Fig. 1) was
designed by the Joint Research Centre of the European
Commission (Rudolf & Tarchi, 1999; Antonello et al., 2004);
it is continuously active since 20 February 2003 (Antonello et
al., 2003; Casagli et al., 2003 Casagli et a., 2004; Antonello et
al., 2007) and produces, on average, 120 images per day of
the area under investigation (NW flank of crater and the upper
part of the SdF), characterized by a resolution of about 2m ×
2m, with an accuracy of the measurement of less than 1 mm.
Interferograms (obtained using pairs of averaged
sequential images) contain the displacement vector along the
line-of-sight (LoS) in the time interval between two
acquisitions. Negative values of displacement indicate a
movement toward the sensor (shortening along the LoS). On
the crater area this direction of movement correspond to the
inflation of the volcanic cone while, on the SdF, this is
usually related to a local bulging or to the downslope sliding
of the volcano-clastic material accumulated on the SdF slope.
Conversely, a positive value of displacement identifies a
movement backward with respect to the sensor (lengthening
along the LoS) that on the crater area could be related to the
deflation of the volcanic cone.
In January 2007, the GB-InSAR showed a progressive
acceleration of deformation on the NE crater. The recorded
velocity progressed from 0.04 mm/h to 0.7 mm/h toward the
radar that suggest an inflation of the upper sector of the
volcanic system. The increase in the deformation rate
successively involves the portion the SdF (15 February 2007)
in which the velocity increased from 0.02 mm/h to 0.25 mm/h,
toward the sensor.
These events are related to the new eruption occurred at
the end of February 2007. The effusive phase (from 27
February to 12 April) started with an explosion to the lower
part of the crater (causing a landslide on the portion of the
crater flank) and with the opening of the effusive vent at 600
m a.s.l. Velocities in the first hours of the eruption were so
high that exceeded the capability of the GB-InSAR device.
After few hours from the effusion onset, the
interferograms returned partly coherent and showed a
complex deformation pattern characterized by:
1. a complete decorrelation on the crater flank due to the
explosion and to morphological changes related to the crater
collapse;
2. concentric interferometric fringes related to the bulging
before the vent opening at 600 m a.s.l.;
3. parallel interferometric fringes related to landslide
movements on the SdF.
During the entire effusive phase, velocity values, on the
SdF slope, constantly decreased from 30 mm/h to 0.2 mm/h
toward the sensor, with the only exception of two limited
periods related to the opening of a new vent (8- 9 March) and
to a major explosion (15 March).
In particular since 8 March the velocity recorded on the
SdF increased again with movements toward the sensor. The
interferogram highlighted a very high deformation rate (more
than 300 mm/h, Fig. 2), which again exceeds the capability of
the correct phase unwrapping. The arrangement of the
interferometric fringes is related to the bulging due to the
opening of a new vent, occurred at 14.30 UT of 9 March.
Following the method proposed by Fukuzono (1985) and
Voight (1988) it has been possible to predict in advance of
one day the opening of the 9 March vent.
After 12 April, the eruption is to be considered concluded
and the velocity recorded by GB-InSAR progressively
decreased down to the values characteristic of the normal
activity of the Stromboli volcano.
The GB-InSAR monitoring allowed us to highlight
different deformation patterns, related to the eruption and to
the associated landslides, suggesting different triggering
mechanisms of the deformation process. Furthermore the
GB-InSAR system has recorded changes in the deformation
patterns, both on the crater area and on the SdF sector, in
advance with respect to the onset of each one of the relevant
events.
The absolute values of velocity recorded are plotted in Fig.
3: the light red line represents movements toward the sensor
(negative radar displacements), while the dark blue line
represents movement backward with respect to the sensor
(positive radar displacements). In Fig. 3 the three different
phases, above mentioned, are shown. The pre-effusive phase
(from 10 January to 27 February) is represented by the first
(light) shaded area, characterized by very low, progressively
increasing, deformation rates; the effusive phase (from 27
February to 12 April) by the second (dark) shaded area,
characterized by very high velocities of deformation and the
post-effusive phase (from 12 April) by the last area in which
the deformation rates decreased down to the pre-crisis values.
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