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BODY WAVE ATTENUATION HERALDS INCOMING ERUPTIONS AT MOUNT ETNA
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Pasquale De Gori1, Claudio Chiarabba1 , Elisabetta Giampiccolo2 , Carmen Martinez –Arèvalo3, and
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Domenico Patanè2
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1
Istituto Nazionale di Geofisica e Vulcanologia, CNT, Roma (Italy)
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2
Istituto Nazionale di Geofisica e Vulcanologia, Catania (Italy)
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3
Departamento de Volcanología, Museo Nacional de Ciencias Naturales, CSIC, Madrid (Spain)
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ABSTRACT
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How fast and foreseeable is the magma ascent is one of the most impellent and unanswered issues of
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volcanology. The velocity of the magma upwelling depends on the local conditions of the volcanic
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conduit and rheology of the magma (Scandone et al., 2007). During magma emplacement in the
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shallow crust, transient variations of physical properties underneath active volcanoes are expected and
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in a few cases observed (Patanè et al., 2006). The predictability of such changes strongly depends on
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how fast this process is, compared to our ability to handle geophysical data and consistently resolve
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transient anomalies in the physical properties of the medium. Mt. Etna is a perfect natural laboratory to
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investigate such issues, due to the almost continuous magmatic activity and the high quality of
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seismologic and geodetic data. Here we show, for the first time, that seismic attenuation of local
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earthquakes strongly increases due to the emplacement of magma within the crust, forecasting an
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incipient eruption at Mt. Etna.
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INTRODUCTION
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Seismic tomography studies using P- and S-wave velocities at Mt. Etna (Patanè et al., 2006) have
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shown that dike intrusion before eruptions can be recognized by anomalous volumes of V P/VS in the
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shallow volcanic structure. A low VP/VS anomaly was observed on the top of the 2001 dike intrusion,
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suggesting the presence of magma enriched in gas (Patanè et al., 2002). Transient 4D VP/VS anomalies
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observed during the 2002-2003 eruption have been interpreted as the trace of fluid intrusion (magma
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rich in gas) and gas migration from the shallow magma intrusion in the cracked volume that develops
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during the eruptive period (Patanè et al., 2006). Since seismic velocity tends to be a relatively
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insensitive estimator of temperature variations in rocks (Lees, 2007), in active volcanoes, P- and S
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waves velocity models alone are often not sufficient to gain a full understanding of the physical
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processes involved. On the other hand, the inverse of attenuation, or quality factor Q of body waves, is
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a physical parameter which can be used to identify the location and extension of magma bodies, it
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being very sensitive to the saturation of rocks with fluids and partial melts, and to the thermal state of
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the crustal volume travelled by seismic waves (Evans and Zucca, 1993; Sanders et al., 1995). In
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particular, Q decreases as the homologous temperature, i.e. the difference between the material
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temperature and its solidus, increases (Berckemer et al., 1982; Kampfmann and Berckemer, 1985).
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Thus, it can be used to indirectly infer the variation of temperature underneath a volcano, as the magma
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travels within the crust.
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In this study we performed 4D attenuation tomography, i.e. a repeated 3D tomography in time, to
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detect variations in elastic parameters during the two big flank eruptions which occurred at Mt. Etna in
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2001 and 2002-2003. The dense seismic network run by INGV-CT (Istituto Nazionale di Geofisica e
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Vulcanologia–Sezione di Catania) operating on the volcano (Fig. 1) yielded high quality recordings of
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local earthquakes occurred during the studied eruptions and in the intra-eruptive periods. The overall
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geometry of the network, although complementing permanent stations with temporary deployment in
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the first period, remained unchanged during the three epochs.
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t* COMPUTATION AND ATTENUATION TOMOGRAPHY
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We used seismic data recorded from 2001 to 2003. From an initial data set of more than 3000 events,
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we selected three subsets of 232, 212, and 185 earthquakes (2.0≤MD≤3.3) located with the 3D velocity
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model (Patanè et al., 2006) respectively: a) few days before and during the 2001 eruption, b) in the
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inter-eruptive period 2001-2002, and c) during the 2002-2003 eruption. All the locations are
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characterized by a minimum number of 6 observations, final RMS less than 0.25 s, azimutal gap minor
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than 180 and formal errors within 1 km. Hypocentral depths are less than 10 km and epicenters are
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mainly distributed in the central part of the volcano, along the NE Rift and in the SE flank (Fig. 1).
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A variety of methods can be used to estimate the attenuation of body waves either in time or in
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frequency domain (Evans and Zucca, 1993; Eberhart-Phillips and Chadwick, 2002). The most widely
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applied methods estimate the high frequency decay (t* operator) of body-wave displacement or
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velocity spectra (see Rietbrock, 2001). For the selected events, we computed the t* operator, which
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quantifies the attenuation along the ray paths (1/QP). Spectra were fitted by an iterative damped least
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squares procedure (see the GSA Data Repository) on the basis of a ω-2 source model, under the
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assumption of point like sources and a frequency independent QP within the 1-30 Hz frequency band,
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these choices being consistent with De Gori et al., (2005). Data were inverted for the 3D QP model and
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near sites attenuation structure using the Simulps13q code (Rietbrock, 2001). For the three epochs, the
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medium was parameterized with an irregular 3D grid of nodes and velocity values reported in (Patanè
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et al. 2006), assuming an initial QP equal to 75 (see GSA Data Repository). Both the earthquake
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hypocenters and the velocity structure are kept fixed during the inversion.
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Since we are comparing tomographic images of different epochs, the consistency in model resolution
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is crucial. We follow the current literature, computing the spread function (SF) and averaging vectors
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of the entire resolution matrix (Michelini and McEvilly, 1991; De Gori et al., 2005). Furthermore we
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perform synthetic tests that mimic different geometries of anomalous bodies beneath the volcano.
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Resolution analysis and synthetic tests (see the GSA Data Repository) demonstrate that the anomalies
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located between -1 and 2 km depth are highly reliable and we are confident that the time changes in QP
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are well constrained.
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Attenuation images which present a satisfactory resolution (SF≤2) clearly show that the 2001 and the
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2002-2003 eruptive periods are characterized by low QP anomalies (QP values between 30 and 50)
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between -1 and 2 km b.s.l. (Fig. 2). Such anomalies are located in correspondence with the fracture
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system active during both eruptions at the same place where geodetic data modelled the dike intrusions
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(Bonaccorso et al., 2002; Patanè et al., 2005).
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TIME VARIABILITY OF t*
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The transient QP anomalies depicted by time repeated tomography are an expression of a temporal
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variation of the medium properties traveled by seismic waves. The increase of attenuation before the
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2001 and 2002-2003 eruptions is efficiently revealed by time changes of QP, directly obtained by fitting
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the spectral decay and without performing the tomographic inversion (Fig. 3).
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For each event, we use t* and standard errors, as obtained by spectral fit, to compute QP and standard
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deviation along the ray paths traced in the known 3D models. The event QP is computed as the
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weighted average of QP at each station. We use the inverse of QP standard error at each station as the
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weight to be used in computing the average. To show the time changes, a moving average is run on the
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temporal series using a 80 samples window with a 1 sample step (green line in Fig. 3a). As the
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eruptions approach, a sharp QP decrease (up to 70 %) is well identified (Fig. 3a). The progressive
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reduction of the high frequency content of the P-waves before the 2002-2003 eruption (QP decreasing
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from 380 to 45) is clearly visible on seismograms and relative spectra recorded at the summit station
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ERS for two events, with similar magnitude and located within 1 km from each other (stars in Fig. 1).
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By comparing the spectra at ERS with those at stations located around the summit area, we exclude that
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the low QP values are due to source characteristics.
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DISCUSSION AND CONCLUSION
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The availability of high quality data recorded in the period preceding the 2002-2003 eruption allowed
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us to compute the QP variations (dQP) between the eruptive and pre-eruptive periods. We observe a
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consistent decrease in QP (dQP of about 30 – 50) along a N-S trending channel beneath the central
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crater (Fig. 4).
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If we express the QP values by temperature, using the relationship T=1160-150log(QP) valid for
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magmatic basic rocks (Kampfmann and Berckemer, 1985), we hypothesize a temperature increase of
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70-80 °C in the volume around the dike intruded in the 2002-2003 eruption, the axial low Qp anomaly
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located below the eruptive fissures (see Fig. 4). From the Qp timeseries, a temperature increase of 30-
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40° before the two eruptions is estimated (Fig. 3), this value being smaller since the Qp is an average
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over the entire volcano.
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We note a difference in the rapidity of the attenuation increase for the two eruptions (Figs. 3b and 3c).
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It is in the order of days for the 2001 eruption, whilst, on the other hand, it is very rapid, in the order of
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hours, during the 2002-2003 eruption, in agreement with the short duration of the precursor seismic
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swarm and the fast and sudden deformation of the summit area (Neri et al., 2005; Bonforte et al.,
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2007). The different timescale can be explained by the amount of time needed by the uprising magma
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to break the shallow-most part of the plumbing system. In 2001, the intruding dike, whose position is
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assumed to be coincident with the shallower-most seismicity (see Fig. 2 and Patanè et al., 2002) as
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suggested by ground deformation modelling (Bonaccorso et al., 2002), encountered a high QP solidified
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sill at 1 km b.s.l., (Fig. 2). The dike needed to break this sill and pass through it. Thus, the progressive
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attenuation decrease was slower and the final stage of magma ascent required a few days to break
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through the shallow part of the edifice. Conversely, in 2002-2003 the medium was already strongly
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fractured and the shallow plumbing system was relatively hot and the dike intrusion required only few
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hours to rise up to the surface. The reliability of this signal is definitively demonstrated by the gradual
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variation of the frequency content of waveforms from close earthquakes recorded at a common station
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(see Fig. 3). For the two events the frequency content and the attenuation operator QP are remarkably
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different testifying to a sudden change in the medium properties.
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On the basis of the geophysical evidence presented here, we may assert that shallow intrusion of
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magma heralding eruptions at Mt. Etna can be forecast by the thermal increase of the host rock volume.
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The final stage of magma ascent can be fast or slow, depending on the pre-existing thermal conditions
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of the system and how efficiently the magmatic fluids increase the temperature in the host rock. The
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drastically different timescale of QP changes might account for the time that magma needs to break
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through the shallow part of the plumbing system. This rapid and sudden variation is efficiently revealed
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by the strong and progressive attenuation of local earthquake body waves, which yields the clearest
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picture to date of incoming eruptions. The shorter timescale of the precursory period, i.e., hours, would
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require a very fast real time data processing to make seismic attenuation a viable tool for eruption
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forecast.
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ACKNOWLEDGMENTS
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This work was supported by grants from the European Union VOLUME FP6-2004-Global-3 and
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INGV–Department of Civil Protection V3/6 projects. We thanks the Editor Anne Patience Cowie and
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two anonymous reviewers for very constructive criticisms that improved the quality of the manuscript.
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FIGURES CAPTIONS
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Figure 1. Map of the Mount Etna area showing the seismic stations of the permanent network operating
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during the study periods. The tomographic grid nodes is marked by crosses. It also shows the
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distribution of the selected earthquakes for the 2001 eruption (red circles), in the intra-eruptive period
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(green circles) and during the 2002-2003 eruption (yellow circles). The purple stars are the two close
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events used in Fig. 3. The black heavy lines are the 1000 m and 2000 m elevation isolines. The right
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panel shows an E-W cross section (see trace on the map) of the events within +/- 5 km from the
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projection plane. Grid nodes are indicated by crosses.
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Figure 2. QP models in the well-resolved layers for 2001 eruption (a), intra-eruptive period (b), and
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2002-2003 eruption (c). The thick black contours indicate the regions of the model with SF values ≤ 2.
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The black heavy lines are the 2000 m elevation isoline. The black dots are the earthquakes relocated
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with the 3D velocity model (Patanè et al., 2002; 2006). The W-E cross sections crossing the QP eruptive
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models are also reported (strikes are the dashed lines in the layers at 1km depth). The white rectangles
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indicate the position of intruding dike modeled by geodetic data (Bonaccorso et al., 2002; Patanè et al.,
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2005)
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Figure 3. Moving average of QP vs. time (green trend, see right Y-axis), since 1 July 2001, as directly
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obtained by fitting the spectral decay, for the whole 2001-2003 period (a), and local zoom on the 2001
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(b), and 2002-2003 (c) eruptive periods. The purple trend in (a) is the moving average of the Qp-
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derived temperature vs time (see right Y-axis). The red arrow in (a) indicates a phase of magma re-
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injection into the rift zones, started on April 2002 as evidenced by geochemical observations and
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ground deformation measurements (Patanè et al., 2005).
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In the top panels, waveforms and spectra recorded at station ERS for two close earthquakes (see
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locations in Fig. 1), which occurred in the pre-eruptive period (left panel) and during the 2002-2003
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eruption (right panel). QP values are obtained by spectral fitting. See also details in the Fig. DR1 of
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GSA Data Repository.
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Figure 4. QP variations (dQP) between the eruptive (2002-2003) and pre-eruptive periods in layers (left
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and centre, the black heavy lines are the 2000 m elevation isoline) and W-E vertical cross section
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(right). The trace of section is reported in the layer at -1 km depth (black horizontal lines). The thick
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black contours indicate the regions where both models are well resolved (SF values ≤ 2, see also Fig.
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2).