acttro

and the feeding system of the volcanoes seems to be limited, likely due to the near-surface attenuation of seismic waves. It is not

sciences [1,2]. The recent Yogykarta earthquake of further focused the attention of the scientific communityon the possible link between the cited events. In theseventies, Japanese scientists first suggested a cause

Earth and Planetary Science LettersMay 27, 2006, offshore of south-central Java (ML=6.3), 2007 Elsevier B.V. All rights reserved.

Keywords: earthquakevolcano interactions; degassing; radon anomalies; magma volume; dynamic response of volcanoes

1. Introduction

The spatial and temporal relationships betweenvolcanic eruptions and regional earthquakes is currentlyone of the most intriguing and debated issue in earth

was followed by a drastic increase in the activity of thenearby erupting Merapi volcano, with the ejection ofseveral pyroclastic flows 4 km down its flanks. Thisphenomenon, coupled with the occurrence of the majorSumatraAndaman earthquake of December 24, 2004,efficient role in the triggering process.

excluded that the coupling of two or more earthquakes of similar depth and/or higher magnitude in the near field, could play a moreReceived 16 June 2006; received in revised form 6 March 2007; accepted 6 March 2007

Editor: M.L. Delaney

Available online 19 March 2007

Abstract

We investigated earthquakevolcano interactions by using a network for radon monitoring at Stromboli volcano. Radon is analpha emitting radioactive gas produced from the decay of uranium bearing rocks, soils and magmas. Its spatial and temporalvariations have been regarded as precursors of earthquakes and volcanic eruptions. Here we provide evidence of how radonemissions at Stromboli can be correlated to high magnitude (MLN5) regional earthquakes and erupted magma volumes. ThePalermo earthquake of September 6, 2002 (ML=5.6), characterised by shallow hypocentral depth (15 km) and higher number ofenergetic aftershocks, enhanced more efficiently postseismic dynamic triggering that may have contributed to triggering theeruptions of Mount Etna and Stromboli by the end of 2002. A viscoeslastic relaxation mechanism seems to be compatible with theonset of both eruptions. The rate of erupted magma volume at Stromboli is positively correlated with the rate of radon degassing,and suggests a possible link between magma chamber volume, gaseous transfer and dynamic response of the volcano to near fieldseismic triggering.

Single and isolated deep earthquakes related to active subduction, such as the Salina event (ML=5.1) of May 5, 2004, arecapable of mobilising fluids (due to the passage of seismic waves at higher depths) but their dynamic effect on the fracture networkC. Cigolini , M. Laiolo, D. Coppola

Dipartimento di Scienze Mineralogiche e Petrologiche, Universit di Torino, Via Valperga Caluso 35, 10125 Torino, ItalyEarthquakevolcano interdegassing at S Corresponding author.E-mail address: corrado.cigolini@unito.it (C. Cigolini).

0012-821X/$ - see front matter 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.epsl.2007.03.022ions detected from radonmboli (Italy)

257 (2007) 511525www.elsevier.com/locate/epsleffect correlation between tectonically-induced earth-quakes and some volcanic eruptions [3,4]. Following

tary Sthe Landers (California) earthquake (ML=7.4) of 28June 1992, Hill et al. [5] first demonstrated that dynamicstresses associated with seismic waves from a majorearthquake are capable of triggering both seismicity andvolcanic unrest at large distances from the epicentre.Several subsequent events included the Hector Mine(California) earthquake (ML=7.2) of 16 October 1999and the Denali Fault earthquake (Alaska) of 2November 2002 (ML=7.9). All of these earthquakestriggered seismicity at widely scattered sites in westernUnited States, many of which were geothermal areas orareas of young volcanism such as the Long Valleycaldera and the Yellowstone National Park [68].However, in the near-field static stresses may also beeffective in triggering eruptions [9,10]. Linde and Sacks[11] reported nearly synchronous relationships betweenthe onset of regional earthquakes and volcanic eruptionsdue to the passages of seismic waves in volcanic areas.In addition, coseismic and postseismic stress diffusionhas been regarded as a feasible triggering mechanism[12,13]. Although more earthquakevolcano interac-tions have been recently reported [14,15], the triggeringmechanism of volcanic eruptions is still strictly linked tothe problem of decoding the interplay between high-magnitude earthquakes and remotely triggered seismic-ity (not only in volcanic and/or geothermal areas). Fromthis perspective the role of fluids and pore-pressureperturbations along faults and hydrothermal clusterswithin the earth's crust seem to be a key factor ininducing rapid and/or delayed triggering [16,17]. In thispaper we will provide additional evidence of hownetworks for radon monitoring can be used to detectdiffuse degassing on sectors of active volcanoes, andhow its spatial and temporal distribution can becorrelated to regional seismicity [18,19].

Radon gas, essentially represented by the isotope222Rn (with a half life of 3.82 days), is an alpha emittingradioactive gas produced from the decay of uraniumbearing materials. Its spatial and temporal variationshave been regarded as precursors of earthquakes andvolcanic eruptions. Radon anomalies have been ob-served before, during and after the onset of regionalseismic events (e.g., [2022]). In addition, positiveanomalies in radon emissions have been associated withchanges in volcanic activity and volcanically-relatedearthquakes on Hawaii [23,24]. During a recent radonsurvey at SommaVesuvius (Italy), Cigolini et al. [18]were able to discriminate radon anomalies due toregional earthquakes from local volcanic seismicity.Recently, Burton et al. [25] used radon anomalies toinfer the geometry of a hidden fault during the seismic

512 C. Cigolini et al. / Earth and Planecrises of October, 2002 at Mount Etna.Radon anomalies have also been regarded asprecursors to volcanic eruptions [26,27]. Su and Huh[28] recorded increased contents of 210Po (a daughterproduct of 222Rn) deposited by the plume of Mayonvolcano prior its last eruption. Cigolini et al. [19]showed that major eruptive events at Stromboli volcanooccurred when summit stations reached and/or exceededthe threshold values of 20,000 Bq/m3 (lasting for atleast 3 days) 12 to 14 days before the 20022003paroxysmal eruptions. However, these critical valueswere recorded in absence of major regional earthquakes.

2. The Aeolian islands and Stromboli volcano

The Aeolian islands, located in the Southern Tyr-rhenian Sea (Fig. 1), were built in the last 1.3 m.y. [29].Erupted lavas and tephra are subduction-related calcalka-line, HK-calcalkaline, shoshonitic and potassic suites[30,31]. Subduction of the Ionian plate beneath theCalabrian arc ceased about 1 m.y. ago, when a generaluplift (0.50.7 m.y.), associated with extensional tecton-ics, affected Southern Italy [32,33]. Uplift occurred in theforearc region, and was due to the rebound of the upperplate (Calabrian Arc and part of the Ionian lithosphere)decoupled from the main Ionian plate [34,35]. Thisprocess has been ascribed to the post-10.7 m.y. rollbackof the slab. Plate decoupling was associated with mantleupwelling that was first controlled by a mainWNWESEstriking system of faults (Fig. 1). From Pleistocene to thepresent, the southern propagation of the Tyrrhenian riftingand the westernmargin of the roll-backing crust generatedthe NNWSSE striking TindariLetojanni (TL) fault[36,37]. Mount Etna and the central cluster of the AeolianIslands are located on this major structure (Fig. 1).

Stromboli is the north-eastern island of the Aeolianarc (Fig. 1) and is located on a NESE strike-slip fault:the StromboliPanarea (SA) alignment (i.e., a branchconnected to the TL fault). The cone of the volcano rises924 m above sea level and was formed during the last100 kyr [29]. Volcanic activity is strombolian, withcontinuous explosions and eruptions of scoriae, lapilli,ash and bombs [38] at three summit vents located inthe upper part of Sciara del Fuoco, a collapsed sectordelimited by a horseshoe-shaped scarp opening north-westward [39]. The persistent strombolian activity maybe interrupted by lava effusions, major explosions andparoxysms with the generation of tsunamis that maythreaten the West-Central Mediterranean [40]. The mostrecent major eruption started December 28, 2002 withthe eruption of a hot avalanche from the NE crater thatpreceded the emplacement a lava flow onto Sciara del

cience Letters 257 (2007) 511525Fuoco. This event was followed by a composite slump on

tary SC. Cigolini et al. / Earth and Planethe Sciara del Fuoco (December 30, 2002): flank failureincluded also slices of the submerged part of the volcanoand generated a tsunami that affected the northern coastof the island [41]. The almost continuous effusion of lavapersisted until July 21, 2003 [42] and was interrupted bythe major explosion of April 5, 2003, with the ejection ofa column 1 km high [43]. By the end of July 2003,typical mild Strombolian activity has resumed at thesummit craters.

The hydrothermal system at Stromboli is rathercomplex and consists of an upper portion, surrounding

Fig. 1. Structural setting of the Southern Tyrrhenian region and the Aeolianfocal mechanisms as reported by INGV (inset shows the epicenter of the Algiregion: small rectangle ); b) detailed map of the Aeolian Islands with the ma513cience Letters 257 (2007) 511525the conduit and the crater area, and a lower sector locatedat the base of the cone where several thermal waters arefound in the village of Stromboli [44,45]. Recently,Cigolini et al. [19] outlined the areas of major degassingon the NE flank, that coincide with the geometry of themost active sectors of the hydrothermal system.

3. Methods

We started our radon surveys at Stromboli in May2002. Initially, we planned to measure monthly and

Islands; a) locations of recent major earthquakes, their magnitude andersBoumerdes earthquake and the location of the Southern Tyrrhenianjor tectonic units of the region.

a diameter of 12 cm, which were set to a depth of about60 cm) isolated by a cap to minimise condensation. Theycan both coexist within a single station and can be sampledindependently within a single day. Gervino et al. [48] haverecently shown that these dosimeters are essentiallyunaffected by daily temperature or soil humidity variations.Moreover, the long-term exposure of track-etch detectors(LR115) minimizes the effects of short-term fluctuations inradon emissions, essentially due to variations in atmo-spheric pressure and microseismicity. Therefore, thesedetectors give a reliable integrated measurement of radonactivity over their time of exposure, and we may easilyrecalculate monthly radon emissions by opportunelyrecombining the results obtained for each station duringa single campaign (since their exposure time not always

514 C. Cigolini et al. / Earth and Planetary Science Letters 257 (2007) 511525seasonal radon variations by means of a network of25 stations (Fig. 2) using track-etch detectors (LR115)finely calibrated for alpha-particle beams [46], whichwere exposed from 2 to 5 weeks. During our periodicsurveys we also utilised E-PERM electretes [47] whichwere exposed from one to 4 days. Following the onset of anew major eruptive cycle, that started on December 28,2002, we performed repeated surveys by using E-PERMelectretes to be able to correlate radon emissions withshort-term variations in volcanic activity. Therefore,track-etch detectors were exposed permanently, whereas

Fig. 2. The radon monitoring network at Stromboli. Sampling stationsare numbered and subdivided into three classes: summit stations(triangles), lower stations (squares) and other stations (dots).E-PERM electretes were used (several times during asingle campaign) when we were physically present on theisland for our periodic surveys. Both detectors wereplaced in subsurface pipe-like samplers (1.20 m long with

Table 1Summary of geologic events triggered by the Palermo earthquake of Septembprior the 20022003 eruption of Stromboli

Site Date Phen

North-Western Sicily September 6, 2006few hours after thePalermo earthquake

Chansprin

Etna September 22, 2002 MarkOctober 26November 5, 2002 Viole

and vflows

November 15, 2002 to January 28, 2003 ExplPanarea November 2December, 2002 Anom

of Padue t

January, 2003 Restoof th

References: 1. Agnesi et al. [49]; 2. Acocella et al. [50]; 3. Andronico et al. [coincides with the end or the beginning of the month).The use of the above detectors contemporaneously for

all the stations of the network, gave us the opportunity tobetter discriminate the effects of regional seismicity onradon degassing from those related to variations in vol-canic activity (cf., [19]).

4. Major earthquakes and related processes

The locations of major regional seismic events thataffected the Mediterranean region in 20022004 areshown in Fig. 1. Seismic parameters and focal mech-anisms are those reported by the seismic catalogues of theItalian National Institute for Geophysics and Volcanol-ogy (INGV) at http://www.ingv.it.

The Palermo earthquake of September 6, 2002, withML=5.6 and a hypocentral depth of 15 km, was gen-erated in the Western sector of the Southern Tyrrhenianbasin. Focal mechanism resolution indicates a an ob-lique (strike-slip/reverse) solution with a subhorizontal

er 6, 2002, in North-Western Sicily, Atnean region and Panarea island,

omena Reference

ges in the flow rates and temperatures of hydrothermalgs and triggering of the major Cerda landslide

[1]

ed increase in seismicity in the Aetnean region [2, 3, 4]nt phreatomagmatic explosions, lava fountainsiolent strombolian explosions followed by lavafrom eruptive fissures on the NE and S flanks

[3]

osive activity and lava flows onto the S flank [3]alous degassing from the sea bottom off the coastnarea related to a submerged geothermal system,o the input of magmatic fluids

[5, 6]

ration of the mild steady-state degassing typicalis submerged geothermal depression

[5, 6]51]; 4. Patan et al. [52]; 5. Capaccioni et al. [53]; 6. Caliro et al. [54].

compressive P axis striking NWSE, coupled with arelative NESW extensional motion [37]). This eventwas followed by 8 aftershocks with MLN4, and 25 withMLN3 coupled with a dramatic increase in the regionalseismicity of the Southern Tyrrhenian domain. Severalgeologic processes followed this seismic crisis affectingmainland Sicily and the Aeolian islands (cf., Table 1).The immediate response to the Palermo earthquake wasthe triggering of a major landslide near the village ofCerda in North-Western Sicily (about 30 km inland fromPalermo) that occurred just one hour after the majorseismic event. This episode was followed by an inincrease in the flow rates and temperatures of severalhydrothermal springs in the surrounding area [49]. In thefollowing days, the Palermo earthquake triggered minorseismicity in the Aetnean region that culminated, on

September 22, 2002, with the onset of a strongerearthquake (of ML=3.7) along the Pernicana Fault,located on the Eastern sector of Mount Etna [5052].This event was followed by the moderate seismicity inthe surrounding region coupled with microseismicswarms below the volcanic edifice leading to theeruption of Mount Etna on October 26, 2002 [52].The eruption persisted until January 28, 2003 (Table 1).

On November 2, 2002, anomalous strong degassinghas been observed from the sea bottom off the coast ofPanarea, one of the Aeolian island situated at about20 km SE of Stromboli and located along the StromboliPanarea structural alignment (Fig. 1b). The anomalousgas flux was released from a submerged geothermalsystem, normally active in this area (Table 1). This phe-nomenon has been related to the input of magmatic fluids

the Sies hao earegion

515C. Cigolini et al. / Earth and Planetary Science Letters 257 (2007) 511525Fig. 3. Histograms of the monthly number of earthquakes that affectedwith the seismic energy released in those sectors of Southern Italy. Energof earthquakes and seismic energy are related to the onset of the PalermMay 2004 can be clearly observed; c, d) Histograms for the Aetnean r

during September 2002. Most of the earthquakes (with magnitude ML2) oeruption at Mount Etna.outhern Tyrrhenian and Aetnean regions during 20022004 comparedve been calculated according toMcGarr, [58]; a, b) Peaks in the numberthquake. The contribution of the Salina event to the energy released inalso show peaks, both for number of earthquakes and seismic energy

ccurred during October 27 and 28, 2002, thus overlapping the onset of

Fig. 4. a) Normalised cumulative-step curves for the seismic energies released within the Southern Tyrrhenian and Aetnean regions compared thecumulative curve for radon emissions at the summit stations of Stromboli (Fig. 2). Variations in volcanic activity during 20022004 are alsoindicated. Radon data have been collected by means of -track-etches (LR115): single measurements have an average error of 12% [46].b) Histograms of monthly radon emissions at Sromboli volcano (measured by -track-etches, LR115 with an average error of 12% on singlemeasurements; cf., [46]) with the indications of major earthquakes and major eruptive events at Etna and Stromboli volcanoes during 20022004.Histograms were obtained by summing the radon activity measured at each stations for each single class: lower stations, summit stations and otherstations (cf. Fig. 2). The sum of the three represents the total monthly emissions of 222Rn monitored at Stromboli.

516 C. Cigolini et al. / Earth and Planetary Science Letters 257 (2007) 511525

from a deeper source, likely stored in the subvolcanicenvironment [53,54].

The AlgiersBoumerdes earthquake of May 21,2003 (ML=6.7), offshore the Algerian coast and15 km deep, is among the largest events that has oc-curred in the western Mediterranean over the past25 years. Rupture occurred on a reverse fault trendingENEWSW with its upper edge 6 km offshore andlower edge 4 km inland [55]. Solutions indicates pure-reverse faulting with a subhorizontal P axis strikingNWSE. The major seismic event was followed by8 aftershocks with ML4.3, two of them with ML5.2. Intense microseismicity was persistent throughoutJune 2003 [56]. This event was not accompanied by anincrease in seismicity in the Southern Tyrrhenian region.

The Salina earthquake ofMay 5, 2004 is a deep event(250 km in depth) essentially associated with activesubduction with ML=5.1 [57]. Focal mechanism solu-

In Fig. 3 we report the histograms of the monthlynumber of earthquakes (with ML2) that affected theSouthern Tyrrhenian and Aetnean regions during 20022004 (the full catalogue for the Italian earthquakes isavailable at http://www.ingv.it; a synthesis consisting ofone table, for the data of our regions of interest, isavailable as supplemental material) compared with theseismic energy released in those sectors of SouthernItaly. Energies have been calculated according toMcGarr, [58]. It must be emphasised that the peaks innumber of earthquakes and seismic energy for theSouthern Tyrrhenian area are essentially related to theonset of the Palermo earthquake and related aftershocks.They both decrease exponentially until March 2003,then gradually increase finally and reach their minimaduring October 2003. It can also be noticed that theSalina event released a relatively high energy but waspreceded and followed by a minor number of earth-

ationsh an a

517C. Cigolini et al. / Earth and Planetary Science Letters 257 (2007) 511525tions by INGVare consistent with an oblique-transitionalfrom strike-slip to normal fault with a marked plunge(52) for the P axis striking NS. The earthquake waspreceded by two events withML of 2.7 and 3.2, and wasfollowed by two aftershocks with ML3. All the latterevents were located at a depth of 58 km.

5. Radon monitoring and seismicity

Systematic radon monitoring at Stromboli started inMay, 2002. The network for radon monitoring is de-ployed on the NE edge of the Sciara del Fuoco, thesummit area surrounding the craters, and the Easternsector of the volcano (Fig. 2).

Fig. 5. Nomalised cumulative curves for radon emissions at summit stcycle. Radon measurements were performed with LR115 detectors, wit

magma volume was estimated from average effusion rates obtained from MOreported by Calvari et al. [43] and Ripepe et al. [42].quakes. Data for the Aetnean region also show peaks,both for number of earthquakes and seismic energy,during September 2002. In this case the most of theearthquakes (with magnitude ML2) occurred duringOctober 27 and 28, 2002, and were seismic eventscaused by the eruption as a dike intruded to the surface.

In Fig. 4a we compare the normalised cumulativecurves for seismic energy in the above sectors of SouthernItaly with the normalised cumulative curve for radondegassing at the summit stations of Stromboli. The curvefor the Southern Tyrrhenian region shows a major stepdue to the Palermo seismic crisis, and then moderatelyincreased during 2003 and 2004. The relatively flat trendin the cumulative curve is due to the change from the

and cumulative magma volume at Stromboli during the last effusiveverage error of 12% on single measurements (cf., [46]). Cumulative

DIS images according to Coppola [69], being consistent with the data

s eartsinglasis o5, and

518 C. Cigolini et al. / Earth and Planetary Science Letters 257 (2007) 511525logarithmic scale to the normalised scale. Anyhow, arather similar and delayed trend is represented byseismicity in the Aetnean region, which is associated withthe activation of the southern sector of the TL fault systemalong a zone of crustal weakness where the volcanic

Fig. 6. Precursory radon anomalies before the AlgiersBeaumerde(measurements by E-PERM detectors with an average error of7% oncalculated for single stations according to Hernandez et al. [70] on the bperformed on a set of 222Rn data collected from May 2002 to April 200edifice of Etna was built. The cumulative curve for radonshows that degassing at the summit of Stromboliincreased after the Palermo earthquake (September andOctober 2003). It further increased during the onset of theeffusive eruption, peaked in February and then slightly

Fig. 7. Sequence of 222Rn emissions before, during and after the Salina seismseismic foreshocks and aftershocks in the area surrounding Salina. Horizontuncertainties on single measurements performed with E-PERM detectors, wdecreased during March 2003: i.e., before the majorexplosion of April 5, 2003. After this event the radoncurve follows a rather uniform slope until the onset of theSalina earthquake when both radon and seismicity in theSouthern Tyrrhenian region dramatically increased. In the

hquake recorded at some lower stations of the Stromboli networke measurements; [47]). Background values are also reported. They weref the principles outlined by Sinclair [71]. Background calculations wereinvolve: 160 measurements for st. 2; 130 for st. 24; and 153 for st. 25.case of the Palermo earthquake, anomalous radonemissions are definitely postseismic.

Histograms in Fig. 4b give a better clue for variationsin radon degassing. During September and October2002 we observed a remarkable growth in volcanic

ic event of May 5, 2004. Radon emissions (squares) are compared withal error bars define the exposure time. Vertical error bars reflect 2ith a mean error of 7% on single measurements [47].

activity and, by the end of October 2002 the magmacolumn reached the floor of the NE crater and explo-sions were particularly vigorous. The relative maximumin 222Rn emissions of February 2003 was related to thepersistent effusive activity coupled with the opening ofthe fracture network [19]. The bulk trend then shows a222Rn decrease, with the exception of August 2003,when the Strombolian activity at summit vents replacedthe lava flow. The maximum peak in radon degassing is

reached in May 2004, due to the seismic crisis thatculminated in the Salina earthquake. In addition, Fig. 5shows a very good correlation between the cumulativeradon emissions with the cumulative magma volumeproduced during the last effusive cycle.

The AlgiersBoumerdes earthquake (May 21, 2003)did not particularly affect bulk radon emissions duringthat month. However, we were able to detect theprecursory radon anomalies with E-PERM detectors

secto

519C. Cigolini et al. / Earth and Planetary Science Letters 257 (2007) 511525Fig. 8. Topographic DEM images of radon anomalies onto the NE

measurement sites. Radon measurements were performed with E-PERM deteSee text for details.r of Stromboli during the Salina seismic sequence. Dots represent

ctors (which have a mean error of7% on single measurements; [47]).

(Fig. 6). Anomalies recorded at single stations are notsynchronous, likely due to structural discontinuities andlocal variations in rock-soil porosities and permeabilities.The presence of water is also critical in modulating radonmigration in porous media [59]. The sites where thesemonitoring stations are located are underlain by an aquiferconnected to an active hydrothermal system extendedthroughout the NE sector of the island [19,60]. Therefore,a complex interplay among all these factors may havecontributed to produce the observed radon anomalies.Following the AlgiersBoumerdes earthquake, regionalseismicity and bulk radon emissions at Stromboli havebeen relatively moderate until JulyAugust 2003, whenthe lava flow ceased and the typically mild Strombolianactivity at summit craters was resumed (Fig. 4b). DuringAugust 2003, summit stations reached a relative peak dueto the increased strombolian activity.

As previously mentioned, the Salina earthquake ofMay 5, 2004 with ML=5.1 occurred at a relatively highdepth (250 km) for the Aeolian islands and may berelated with active subduction. In this case we could notdiscriminate precursory signals since E-PERM detec-

the first sector affected by a drastic increase in radonemissions is just north of the crater area (Fig. 8a) alongthe main direction of dyking and fracturing (cf., [39]).From May 7 to May 10 radon anomalies (Fig. 8b) arespread onto the whole crater area and the lower edge ofSciara del Fuoco, likely related to degassing of asubmerged portion of the hydrothermal system that wasinvolved in the activation of fractures during the sub-marine slumping of December 30, 2002 [19]. Over thistime period, the area exhibiting the May 3May 7anomaly shifted to very low Rn-emissions. During May12 to May 14 (Fig. 8c) the area with the higher anomalyis located above the N60E fracture zone detected byFinizola and Sortino [61]. At this time, degassing at thesummit as well as at lower edge of Sciara del Fuocodrastically decreases. The sequence of images showsthat once each of the single sector has degassed (andshowed higher Rn-anomalies), radon emissions reachtheir minima, and so on throughout the mapped area. ByMay 16 the residual radon anomaly is concentrated inthe crater area where radon emissions are normallyhigher and persistent (Fig. 8d). For the Salina seismic

withme fo

520 C. Cigolini et al. / Earth and Planetary Science Letters 257 (2007) 511525tors were exposed fromMay 3 to May 7, and throughoutthe whole seismic crisis. However, the higher Rnmonthly anomaly that reaches the maximum peak isdue to the Salina event, with radon activities up to97,000 Bq/m3 at summit stations (Fig. 7). Radon anom-alies projected onto topographic DEM images show that

Fig. 9. Weighted average best fit for regional seismic energies releasedstations of Stromboli. Following the Palermo earthquake the delay in ti

particularly evident. For the Salina event radon degassing peaks during the evwere performed by E-PERM detectors, with an average error of 7% on sevent we cannot say, due to the exposure time of radondetectors (i.e., 4 days that overlap the earthquake time),if the earliest anomaly is coseismic (Figs. 79). How-ever, it is clear that in this specific case the response ofthe volcanic edifice in terms of radon degassing occur-red relatively rapidly.

in the Southern Tyrrhenian region and radon emissions at the summitr the radon curve in respect to the one of the released seismic energy is

olution of the sesimic crisis (see text for details). Radon measurementsingle measurements [47].

tary S6. Discussion

Cumulative energy releases for the Southern Tyrrhe-nian region during 20022003 suggest a causeeffectrelationship for the triggering of seismicity within theAetnean sector. In particular, the seismic sequence asso-ciated with the Palermo earthquake (September 6, 2002)remotely triggered seismicity along the southern part of theTindariLetojanni fault system and prepared the groundfor the eruption ofMount Etna onOctober 26, 2002. Focalmechanism solutions for this major seismic event areconsistent with a NWSE compression coupled with aNESW extensional component that, in turn, likelyaffected the strike-slip motion of the TL structuralalignment. The dynamic response of the area surroundingMount Etna has been particularly efficient, likely due to thepresence of the hydrothermal clusterwithin the volcanicedifice coupled with the complex fault and fracturenetwork of the area (cf., [50]). This further supports theidea that pore-pressure perturbations are key factors in thetriggering mechanism of remote seismicity and mayeventually be involved in the onset of volcanic eruptions.Essentially on geotectonic grounds, we are attempted tospeculate that the increase in seismicity along the TindariLetojanni fault system contributed to postseismic stressdiffusion (cf., [12,13]) along the StromboliPanareaalignment and led to an increase in radon emissions atStromboli during September and October 2002 (Fig. 9),followed by the anomalous degassing at Panarea (fromNovember 2002 to January 2003). 222Rn degassing atStromboli further grew before and during the onset of thelast major eruptive cycle (December 28, 2002), peaked inFebruary and then slightly decreased during March 2003(Fig. 9): i.e., before the major explosion of April 5, 2003.The correlation between the release of seismic energy andradon emissions reported in Fig. 9, gives us an idea of thetime-delay for the dynamic response of the volcano to thePalermo earthquake, whereas the response to the Salinaevent has been relatively rapid. Therefore, we tested analternative hypothesis that establishes a direct linkbetween the Palermo earthquake and the eruptions ofboth Etna and Stromboli. It is well known that seismictransients directly affect magma chambers inducing over-pressures, essentially related to the viscoelastic response ofthe wall rocks [e.g., [6264]]. Bubble nucleation andgrowth also play a significant role in promoting eruptions(e.g., [1,2]; and references therein).

A rough estimate for a viscoelastic response of amagma chamber may be obtained by introducing theMaxwell time relationship =wr /E, which relates theeffective wall rock viscosity ( ) to the elastic modulus

C. Cigolini et al. / Earth and Planewr

(E). By substituting the value 1017 Pa s for viscosity,which is consistent with Borgia [65] for the roots zoneof basaltic volcanic edifices, and 1010 Pa for E, whichhas been shown to be appropriate for crustal rocks (e.g.,[64]; among others), we obtain =116 days. This esti-mate scales well with the time span occurred betweenthe Palermo earthquake on the onset of the majoreruption at Stromboli, i.e. 113 days. For Etna, we haveto reduce the effective viscosity to wr=510

16 Pa s(which is still compatible with wall rock viscosities of aporous medium affected by microcracking and fluidmigration, cf., [66]), or alternatively to set E=21010 Pa,to get 58 days, which is only 8 days in delay from thebeginning of the 20022003 eruption (the effective timebeing 50 days since the Palermo earthquake). Since thedistance of the volcanoes from the epicentre is nearlyequal, being 138.8 and 136.4 km for Stromboli and Etna,respectively, and the effective and the estimated timeratios are2, wemay infer that magma chamber volumesmay be involved inmodulating the response time for theseeruptions. Directivity of seismic wave trains, modulatedby structural discontinuities, may also have played a rolein promoting eruption at the two sites. An alternativehypothesis is that bubble nucleation and growth are key-factors in generating the critical overpressures necessaryto activate eruption. In this specific case, Etna may haveerupted first because of the increased deep supply ofundegassedmagma since February 2002 [67]. This is alsoconsistent with the observation that the system waspressurised [52], and microcracking likely induced adecrease in wall rocks' viscosity. Another possibility isthat that focal mechanism of the Palermo earthquakepromoted shear along the StromboliPanarea alignmentand compression along the TindariLetojanni faultsystem, which resulted in the local anisotropy in thestrain distribution within the two tectonic domains.

Precursory radon anomalies have been recorded for theAlgiersBoumerdes earthquake of May 21, 2003. Thesewere not associated with a growth in the bulk radondegassing in Stromboli's eruption rate due to the fact thatthe epicenter was very far from Stromboli (1120 km),and the event was not accompanied by a marked andpersisting increase in seismicity in the Southern Tyr-rhenian region. Precursory radon anomalies to majortectonic earthquakes have been recognised by severalauthors (e.g., [2022]; among others). The most strikingobservations have been presented by Fleischer andMogroCampero [20] who reported precursory anoma-lies for seismic events located over 1000's of km fromtheir sites of measurements. Although Planici et al. [22]provided, essentially on statistical grounds, a causeeffectrelationship between the two phenomena, additional

521cience Letters 257 (2007) 511525geological evidences are needed to better refine this

tary Scorrelation. A feasible explanation is that stress accumu-lation within the crust would induce gaseous transferbefore rupture may occur also very far from the site ofmeasurement. In their experiments on the emission ofthoron associatedwith rocks fracturing, Hishimuma et al.[68] have experimentally shown that fluid release wastaking place right before and during incipient fracturing,definitely before rock failure. To minimise the scaleeffects related to their laboratory experiments, they haveused thoron (220Ra), a very short lived isotope of theradon family with a half-life of 55.6 s. Their results areparticularly significant because they throw light on thebehaviour of radon as a stress tracer. However, definiteanswers could only be given by analysing the dynamicresponse of large-scale faults to regional earthquakes bymeans of radon networks, with automated stationsoperating continuously and contemporaneously.

The peak in bulk radon emissions of May 2004 is dueto the Salina seismic event of May 5, 2005, and relatedaftershocks. In this case we detected the dynamic re-sponse of the volcanic edifice to the release of seismicenergy. Data indicate that radon emissions along fracturezones are activated first, and are followed by anomaliesrelated to degassing through the hydrothermal system.

We thus suggest that major earthquakes with shallowhypocentral depths (1525 km) and higher number ofenergetic aftershocks would perturb more efficiently thefeeding systems of open-conduit volcanoes and maylead to volcanic eruptions. The role of fluids is critical inreducing fault friction thus promoting failure intectonically active areas, as well as inducing pore-pressure perturbations within the hydrothermal systemsof active volcanoes. However, relatively isolated deepearthquakes due to active subducion, such as the Salinaevent, are capable of mobilising fluids (due to thepassage of seismic waves at higher depths) but theirdynamic effect on the fracture network seems to belimited and less pervasive, due to the near-surfaceattenuation of seismic waves. In this particular case, thepeak in radon emissions is due to the dynamic effect ofthe passage of seismic waves that would mobilise fluidscirculating within the active hydrothermal system ofStromboli, thus producing higher and more extendedanomalies. However, anomalies will persists at summitstations since the crater area is affected by the con-tinuous and deeper degassing of the magmatic system.

7. Conclusions

We attempted to investigate earthquakevolcano inter-actions by assessing the dynamic response of Stromboli to

522 C. Cigolini et al. / Earth and Planethree major tectonic earthquakes by analysing radonemissions. Networks for radon monitoring are shown tobe a very powerful tool for discriminating anomalies due toregional earthquakes from those induced by the variationsin volcanic activity. In particular, the Palermo earthquake(ML=5.6) characterised by shallow hypocentral depth(15 km) and higher number of energetic aftershocks,enhanced more efficiently postseismic dynamic triggeringthat led to the eruptions of Mount Etna and Stromboli bythe end of 2002. A viscoeslastic relaxation mechanismseems to be feasible for explaining the onset of botheruptions and is basically consistent with the onset anddelay of radon anomalies recorded at Stromboli. Inaddition, bubble nucleation and growth within themagma chamber contributed to generate the critical over-pressure to promote eruptions. The rate of erupted magmavolume at Stromboli positively correlates with the rate ofradon degassing and suggest a possible link betweenmagma chamber volumes, gaseous transfer and dynamicresponse of the volcanoes to seismic triggering in near field.

The geodynamic setting of the Southern Tyrrhenianregion, coupled with the following sequence of events:Palermo earthquake (September 6, 2002) Etna erup-tion (September 26, 2002) anomalous degassing inthe submerged geothermal field of Panarea (November2, 2002) onset of the major eruption of Stromboli(December 28, 2002), does not exclude that stressdiffusion might have played a minor role in the trig-gering process.

In contrast, the subduction-related single and deepearthquake of Salina (ML=5.1) of May 5, 2004, wascapable of mobilising fluids (due to the passage of seis-mic waves) which generated marked radon anomalies,but its dynamic effect on the fracture network and thefeeding system of the volcano seems to be limited, likelydue to the near-surface attenuation of seismic waves.However, it is not excluded that the coupling of two ormore earthquakes of similar, and/or higher magnitudeand depth in the near field, could play a more efficientrole in the triggering process. In this perspective, futurework should take into account the combined effects of thetwo regional seismic events (with MLN5) that affectedthe southern Tyrrenhian region during the end of 2006,and preceded the onset of the current effusive cycle atStromboli (that started on February 27, 2007).

Precursory radon anomalies have been recorded for theAlgiersBoumerdes earthquake ofMay 21, 2003 (ML=6.7)but were not followed by significant and extended radonanomalies at Stromboli that, in turn, did not change itstypical eruptive style. In this case the release of seismicenergy occurred very far from Stromboli (1120 km), andthe event was not followed by an increase in remotely

cience Letters 257 (2007) 511525triggered seismicity within the Tyrrhenian region.

523C. Cigolini et al. / Earth and Planetary Science Letters 257 (2007) 511525We may conclude by stressing that a network ofautomated stations for radon monitoring, that will allowreal-time continuous and systematic measurements, is akey-factor for optimising surveillance on active volca-noes, and will surely lead to a better understanding ofthe complexity of these dynamic systems.

Acknowledgements

The research was funded by the National Institute forGeophysics and Volcanology (INGV, Project V2), andthe Italian Civil Defence. Additional funds for MIURare gratefully acknowledged. We are also particularlygrateful to the staff of the Italian Civil Defense fortheir logistic assistance, in particular to C. Cardaci andA. Scalzo. M. Ripepe and K. Cashman provided valuedand stimulating suggestions. We thank A. Finizola for thetopographic DEM image of Stromboli. The criticisms oftwo anonymous referees to an earlier draft of the paperwere verymuch appreciated. Special thanks toG.Gervinoand R. Bonetti for some earlier measurements. M. Zaia,M. Pruiti and L. Russo helped us in the field.

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at doi:10.1016/j.epsl.2007.03.022.

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525C. Cigolini et al. / Earth and Planetary Science Letters 257 (2007) 511525

Earthquakevolcano interactions detected from radon degassing at Stromboli (Italy)IntroductionThe Aeolian islands and Stromboli volcanoMethodsMajor earthquakes and related processesRadon monitoring and seismicityDiscussionConclusionsAcknowledgementsSupplementary dataReferences